For design engineers, R&D teams, and manufacturers that depend on part sourcing, precision CNC machining allows for the creation of complex parts without additional processing. In fact, precision CNC machining often makes it possible for finished parts to be made on a single machine.
If you want to learn more, please visit our website GD-HUB.
The process of machining small parts removes material and uses a wide range of cutting tools to create the final, and often highly complex, design of a part. The level of precision is enhanced through the use of computer numerical control (CNC), which is used to automate the control of the machining tools.
Using coded programming instructions, CNC precise machining allows a workpiece to be cut and shaped to specifications without manual intervention by a machine operator.
Taking a computer aided design (CAD) model provided by a customer, an expert machinist uses computer aided manufacturing software (CAM) to create the instructions for machining the part. Based on the CAD model, the software determines what tool paths are needed and generates the programming code that tells the machine:
A CNC controller then uses the programming code to control, automate, and monitor the movements of the machine.
Today, CNC is a built-in feature of a wide range of equipment, from lathes, mills, and routers to wire EDM (electrical discharge machining), laser, and plasma cutting machines. In addition to automating the machining process and enhancing precision, CNC eliminates manual tasks and frees machinists to oversee multiple machines running at the same time.
In addition, once a tool path has been designed and a machine is programmed, it can run a part any number of times. This provides a high level of precision and repeatability, which in turn makes the process highly cost effective and scalable.
Some metals that are commonly machined include aluminum, brass, bronze, copper, steel, titanium, and zinc. In addition, wood, foam, fiberglass, and plastics such as polypropylene can also be machined.
In fact, just about any material can be used with precision CNC metal machining &#; of course, depending on the application and its requirements.
For many of the small metal parts and precision metal components that are used in a wide range of manufactured products, precision CNC machining is often the fabrication method of choice.
As is true of virtually all cutting and small parts machining methods, different materials behave differently, and the size and shape of a component also have a big impact on the process. However, in general the process of precision CNC machining offers advantages over other machining methods.
That is because CNC machining is capable of delivering:
While a skilled machinist can use a manual lathe to make a quality part in quantities of 10 or 100, what happens when you need 1,000 parts? 10,000 parts? 100,000 or a million parts?
With precision CNC machining, you can get the scalability and speed needed for this type of high-volume production. In addition, the high repeatability of precision CNC machining gives you parts that are all the same from start to finish, no matter how many parts you are producing.
Get some machinists&#; tips on how to keep the production of small, complex parts cost-effective in our blog Top 5 Challenges in CNC Machining Services Explained.
In the next section, we&#;ll take a look at some of the equipment and the processes that are most frequently used in precision CNC machining.
There are some very specialized methods of CNC machining, including wire EDM (electrical discharge machining), additive machining, and 3D laser printing. For example, wire EDM uses conductive materials &#; typically metals -&#; and electrical discharges to erode a workpiece into intricate shapes.
However, here we will focus on the milling and turning processes &#; two subtractive methods that are widely available and frequently used for precision CNC machining.
Milling is a machining process that uses a rotating, cylindrical cutting tool to remove material and create shapes. Milling equipment, known as a mill or a machining center, accomplishes a universe of complex part geometries on some of the largest objects machined metal.
An important characteristic of milling is that the workpiece remains stationary while the cutting tool spins. In other words, on a mill, the rotating cutting tool moves around the workpiece, which remains fixed in place on a bed.
Turning is the process of cutting or shaping a workpiece on equipment called a lathe. Typically, the lathe spins the workpiece on a vertical or horizontal axis while a fixed cutting tool (which may or may not be spinning) moves along the programmed axis.
The tool cannot physically go around the part. The material rotates, allowing the tool to perform the programmed operations. (There is a subset of lathes in which the tools spin around a spool-fed wire, however, that is not covered here.)
In turning, unlike milling, the workpiece spins. The part stock turns on the lathe&#;s spindle and the cutting tool is brought into contact with the workpiece.
While both mills and lathes are available in manual models, CNC machines are more appropriate for purposes of small parts manufacturing &#; offering scalability and repeatability for applications requiring high volume production of tight tolerance parts.
In addition to offering simple 2-axis machines in which the tool moves in the X and Z axes, precision CNC equipment include multi-axis models in which the workpiece can also move. This is in contrast to a lathe where the workpiece is limited to spinning and the tools will move to create the desired geometry.
These multi-axis configurations allow for the production of more complex geometries in a single operation, without requiring additional work by the machine operator. This not only makes it easier to produce complex parts, but also reduces or eliminates the chance of operator error.
In addition, the use of high-pressure coolant with precision CNC machining ensures that chips do not get into the works, even when utilizing a machine with a vertically oriented spindle.
Different milling machines vary in their sizes, axis configurations, feed rates, cutting speed, the milling feed direction, and other characteristics.
However, in general, CNC mills all utilize a rotating spindle to cut away unwanted material. They are used to cut hard metals such as steel and titanium but can also be used with materials such as plastic and aluminum.
CNC mills are built for repeatability and can be used for everything from prototyping to high volume production. High-end precision CNC mills are often used for tight tolerance work such as milling fine dies and molds.
While CNC milling can deliver quick turnaround, as-milled finishing creates parts with visible tool marks. It may also produce parts with some sharp edges and burrs, so additional processes may be required if edges and burrs are unacceptable for those features.
Of course, deburring tools programmed into the sequence will deburr, although usually achieving 90% of the finished requirement at most, leaving some features for final hand finishing.
As for surface finish, there are tools that will produce not only an acceptable surface finish, but also a mirror-like finish on portions of the work product.
Types of CNC mills
The two basic types of milling machines are known as vertical machining centers and horizontal machining centers, where the primary difference is in the orientation of the machine spindle.
A vertical machining center is a mill in which the spindle axis is aligned in a Z-axis direction. These vertical machines can be further divided into two types:
In a horizontal machining center, the mill&#;s spindle axis is aligned in a Y-axis direction. The horizontal structure means these mills tend to take up more space on the machine shop floor; they are also generally heavier in weight and more powerful than vertical machines.
A horizontal mill is often used when a better surface finish is required; that&#;s because the orientation of the spindle means the cutting chips naturally fall away and are easily removed. (As an added benefit, efficient chip removal helps to increase tool life.)
In general, vertical machining centers are more prevalent because they can be as powerful as horizontal machining centers and can handle very small parts. In addition, vertical centers have a smaller footprint than horizontal machining centers.
Multi-axis CNC mills
Precision CNC mill centers are available with multiple axes. A 3-axis mill utilizes the X, Y, and Z axes for a wide variety of work. With a 4-axis mill, the machine can rotate on a vertical and horizontal axis and move the workpiece to allow for more continuous machining.
A 5-axis mill has three traditional axes and two additional rotary axes, enabling the workpiece to be rotated as the spindle head moves around it. This enables five sides of a workpiece to be machined without removing the workpiece and resetting the machine.
Learn more about precision CNC milling here.
A lathe &#; also called a turning center &#; has one or more spindles, and X and Z axes. The machine is used to rotate a workpiece on its axis to perform various cutting and shaping operations, applying a wide range of tools to the workpiece.
CNC lathes, which are also called live action tooling lathes, are ideal for creating symmetrical cylindrical or spherical parts. Like CNC mills, CNC lathes can handle smaller operations such prototyping but can also be set up for high repeatability, supporting high volume production.
CNC lathes can also be set up for relatively hands-free production, which makes them widely used in the automotive, electronics, aerospace, robotics, and medical device industries.
There is hands-free production &#; and then there is fully automated &#;lights out&#; production. Learn about the challenges in our blog Barriers to Lights Out Operation in Precision Machining.
How a CNC lathe works
With a CNC lathe, a blank bar of stock material is loaded into the chuck of the lathe&#;s spindle. This chuck holds the workpiece in place while the spindle rotates. When the spindle reaches the required speed, a stationary cutting tool is brought into contact with the workpiece to remove material and achieve the correct geometry.
A CNC lathe can perform a number of operations, such as drilling, threading, boring, reaming, facing, and taper turning. Different operations require tool changes and can increase cost and setup time.
When all of the required machining operations are completed, the part is cut from the stock for further processing, if needed. The CNC lathe is then ready to repeat the operation, with little or no additional setup time usually required in between.
CNC lathes can also accommodate a variety of automatic bar feeders, which reduce the amount of manual raw material handling and provide advantages such as the following:
Types of CNC lathes
There are a number of different types of lathes, but the most common are 2-axis CNC lathes and Swiss-style automatic lathes.
Most CNC Swiss lathes use one or two main spindles plus one or two back (or secondary) spindles, with rotary transfer responsible for the former. The main spindle performs the primary machining operation, with the help of a guide bushing.
In addition, some Swiss lathe machine shops come equipped with a second tool head that operates as a CNC mill.
With a CNC Swiss-style automatic lathe, the stock material is fed through a sliding head spindle into a guide bushing. This allows the tool to cut the material closer to the point where the material is supported, making the Swiss machine especially beneficial for long, slender turned parts and for micromachining.
Multi-axis CNC turning centers and Swiss-style lathes can accomplish multiple machining operations using a single machine. This makes them a cost-effective option for complex geometries that would otherwise require multiple machines or tool changes using equipment such as a traditional CNC mill.
Learn about 5- and 7-axis Swiss machining capabilities here.
Older style lathes were cam-driven, making them relatively primitive. Today&#;s Swiss-style lathe with CNC is leaps and bounds better, in both accuracy and efficiency.
On a regular chucker lathe, the part sticks out and is pushed away &#; that is, deflected &#; as you start removing material. But on a CNC Swiss machine, the material moves and the tools are stationary, so there is far less deflection.
In addition, a Swiss lathe has both a collet and a guide bushing, to further reduce deflection and machine the parts more precisely. All the action is at the edge of the guide bushing; the correct length of material is fed out, machined, and parted off, then another length of material is fed.
So, with little or no deflection in its machining process, the CNC Swiss-style screw machine provides greater accuracy, precision, and consistency. Learn more about the advantages of eliminating deflection in our blog Deflection and Precision in CNC Swiss Machining.
In addition, compared with other precision CNC machining methods, CNC Swiss-style machining:
In the next section, we&#;ll examine some of the tools and techniques used with precision CNC Swiss machining.
The Swiss screw machine has, quite literally, been around for centuries and shows no sign of stopping. You can read about the evolution of the modern Swiss lathe in our blog The Swiss Machine in Today&#;s Machine Shop.
The modern precision machine shop leverages CNC Swiss-style machining with a wide range of tools to create parts with an interesting array of features and functions, described below.
Drilling is a process that is often used in high precision machining to remove material before performing finishing operations such as threading, tapping, boring, reaming, or broaching.
For Swiss-style machining, almost any drill can be attached to a screw machine tool holder, within the size limitations of the machine. The drill is then used to remove material and create features such as through holes, cross holes, and blind holes of various sizes.
The world of drills is so vast, you could write a book on it, and the availability of drills has exploded. Today, there are drills of remarkably small diameters &#; as small as 0.002&#; (50 microns or 0.051 mm). Of course, the length and diameter ratios apply, so there are limitations to how deep you can drill with ultra-small diameter drills.
Drills come in a range of sizes and with different types of flutes. Here at Metal Cutting Corporation, most of the drills we use for precision CNC machining are standard, fractional, decimal, wire, and letter sizes.
Certain drills are used for specific processes. For instance, a #7 drill is used to make a hole to tap a quarter-twenty thread inside a part.
Flutes are grooves that can vary in size, shape, and the number on the bit. The purpose of a drill&#;s flutes is to ease the exit of the chips as the material is being cut. The exception is a spade drill, which doesn&#;t have flutes because it is used for shallow hole drilling.
Drills are typically made of hardened steel or carbides, some with abrasive features. The point of a drill is typically angled between 118° and 135° (sometimes 145°), depending on the material being machined, with a 118° being the standard angle. It is used on all drills for all materials, usually following a spot drill or center drill application.
For the purposes of precision CNC Swiss machining, a thread is a symmetrical radial feature that varies in its pitch. The pitch, or angle, determines the depth of the thread.
In the machining of small metal parts, a threading process is used to create precision threads on the outside diameter (OD) or inside diameter (ID) of the part. There are four methods for producing OD threads:
In Metal Cutting&#;s world of very small diameter parts, ID threading presents a different kind of variable. That&#;s because we usually don&#;t have the luxury of a perpendicular tool, due to the extremely small IDs we are requested to tap.
However, in general precision CNC Swiss machining uses one of two methods used to produce ID threads: single point threading and tapping. For ID threads, single point threading is accomplished in basically the same way as described above for OD threading, except in this instance on the ID of the part.
Tapping creates threads using a tool called a tap, which has a specific pitch and diameter according to the threading that you want to achieve. There are three types of tapping tools:
Slotting is used for fitting parts together, either by welding or pressing, and is primarily accomplished through the use of an end mill.
Depending on the toughness of the material being machined, slotting may involve drilling a hole and then using an end mill to rough out the desired steps and dimensions. From there, the end mill does a finishing pass to side cut around the walls and complete the slotting.
This is precisely the type of operation for which end mills were created. A drill would never be used to side cut, because the drill will snap and it is just generally a bad technique. The only cutting edges on a drill are at the point of the drill, not on the side.
While Swiss machines can have live tooling where you could mount a spinning cutting wheel (which Metal Cutting knows well from our cutting business), it is usually not ideal for making a slot. And you definitely could not use a grinding wheel to make a closed slot. Because of the radius of the wheel, which wears over time, you&#;d never be able to have an inside radius of a perfect, perpendicular 90°.
Boring is anything but boring. It is actually a pretty remarkable technique, where you take an existing hole and expand it to a new, larger, precise diameter.
It begins with drilling or forming a starter hole smaller than the intended finished size. Then a tool called a boring bar is used to open the hole up to the desired finished size.
Like an end mill, a boring bar can be far more precise in these applications than a drill. For example, if you need an ultra-precise large hole &#; such as a 0.25&#; (6.35 mm) hole with a very tight tolerance of ± 0.&#; (0. mm) &#; it would be virtually impossible to drill that in one pass with a quarter inch drill. (Plus, that would be a very expensive drill.)
You could use reaming, which is much faster than boring, to make a precise hole. However, reamers wear out, creating additional issues.
With a boring bar, you have better tool life and the chances of catastrophic tool wear are small. In addition, the machine can be adjusted to ensure the correct hole size even as the tool itself experiences wear.
Reaming is done with a tool known, not surprisingly, as a reamer. As noted previously, reaming is faster than boring. However, there are definite downsides to reaming, including the fact that the hole being reamed will get smaller and smaller &#; and so, less precise &#; as the tool wears out.
In fact, reaming carries with it the risk of catastrophic tool wear, which can mean two things:
Reaming is more commonly used for requirements such as a tight tolerance straight hole that is going through a tube. Reaming will follow the hole previously made by a drill. If the drilled hole is not straight, the reamed hole will also not be straight.
And if you have challenging geometry within the hole that needs to be held to a specific diameter or dimension, boring is the preferred method over reaming.
Polygon machining is an operation used to create different geometric shapes on a part. It is a milling method that can be used to create a particular shape to mate with or tighten a threaded part.
For example, polygon machining is often used to create the hex (hexagon) shape that is very commonly used as a wrench head. End mills are an ideal tool for polygon machining and are frequently used with CNC Swiss-style automatic machines.
Swiss screw machines are often called on to make small parts such as screws that will need to be moved either by a machine or by a human being using a tool such as a wrench. The top of the screw can be a Torx head, Phillips head, flat head, or many other shapes, including proprietary designs for security.
In the case of a bolt &#; unlike a screw, where the motion is transmitted on the internal portion of the screw head &#; polygon machining meets the need for an external shape with a dedicated mating wrench to expert a rotational force. In addition, polygon machining could be used to create a mating part that is an integral part of an assembly.
Broaching is mainly used to make geometric shapes on a part for special tool use. An example is a socket-head cap screw with an ID hex used with a hex wrench to drive in the screw. Other examples include the very common Phillips head, a Torx head, and various proprietary shapes designed for security purposes so that the fastener cannot be easily removed.
Broaching is done with a precision ground broaching insert. Depending on the geometry you need, you can buy or custom-make (grind) your own broaching tool.
Rotary broaching is a tool where the material is spinning and the broach spins but stops once it engages with the material. From there, the broach feeds in to remove material to the desired depth and finished geometry.
Most Swiss screw machines can be used to do deburring, an important but often overlooked process. Rather than making something, the purpose of deburring is to remove something &#; namely, to remove unwanted burrs and sharp edges on machined parts.
Deburring a part helps with fit and the accurate measurement of the part. It also aids in preventing injuries such as cuts and splinters in people who will handle or use the finished part.
Why deburr during machining?
In precision CNC Swiss machining, the machinist can program the screw machine for deburring so that there doesn&#;t need to be a separate deburring operation afterwards.
While deburring on the machine is an additional operation that will increase cycle time, it is almost always faster than doing the necessary secondary operation on the parts after machining. So overall, deburring during machining will reduce the vendor&#;s lead time for a customer to get the part.
Deburring is an operation that is done after a specific operation is finished during the machining process. This allows the part to be smooth and free of sharp edges. It can also be an intermediate step done because of chip formation that can affect subsequent tool action, such as a chip that will wrap around a drill.
Features to deburr
For the purpose of programming a tool to achieve deburring, a burr means more than just an extra bump hanging on the corner of a part. It can also be a sharp edge created by the machining process itself.
If you are looking for more details, kindly visit CNC Machining For Precision Aluminum Parts.
Machining often leaves behind sharp features that need to be deburred, such as at the top of a hole (with a corner feature) or at the bottom of a through hole. Sophisticated deburring techniques can also include ID bottom hole deburring, where you interpolate a tool to break the edge.
How to deburr
In Metal Cutting&#;s business, we typically utilize our proprietary abrasive cutting methods to cut tubing and a range of hard, soft, and specialty metals without forming a burr at all. However, for parts that are better suited to other fabrication methods, such as machining, we also offer a range of highly effective mechanical deburring techniques.
Because automatic CNC Swiss machining is not an abrasive process, the deburring technique that is used requires running a tool over the area in question. Various tools can be used, but typically the tool needs to have a cutting action &#; which means removing the burr will yield a chamfer or radius feature.
At Metal Cutting, we generally use a turning tool for deburring anything on the diameter geometry &#; programming the CNC Swiss machine to automatically do a finishing pass to add a radius or chamfer, depending on the part. (Learn more about radius corners and deburring in mass production.) For deburring holes, we use an angled carbide spot drill, chamfering tool, or other simple tool capable of breaking the edge
In the next section, we&#;ll look at some applications that benefit from the use of automatic CNC Swiss machining.
Precision CNC Swiss machining is vitally important to a wide range of manufacturing processes, including:
In particular, precision CNC Swiss machining is frequently used where the goal is to produce very small, tight tolerance parts that have complex geometries.
The growing demand for very small, tight tolerance components has ensured the continuing use of precision CNC Swiss machining in a wide range of industries that commonly source parts. This includes electronics, robotics, aerospace, and other markets where the precision of small components is critical to the form and the function of countless end products.
In the medical device industry, precision CNC machining is particularly well suited to a wide range of applications, including surgical tools, catheters, surgical needles, replacement joints, and components for respirators and ventilators.
The ability to Swiss machine a wide range of materials, including plastics as well as exotic metals and alloys, adds to the benefit of its use for the fabrication of medical device components.
The following are just some examples of parts that engineers design into medical devices and that are frequently produced using precision CNC Swiss machining.
Anchors
Also known as bone screws, anchors are used in a variety of orthopedic and orthodontic applications. Often made from titanium, their primary use is to help repair bone fractures by producing compression that holds a bone in place so it can heal more quickly. Anchors are also used in fixation of soft tissue (such as tendons) or attaching teeth or medical implants to bone.
As you would expect, bone screws are cylindrical in shape with a head and thread along the length of the body. They can also have features such as holes and steps, as well as different tips designed to aid in adhesion of the anchor to bone or other tissue. The form and features of the anchors make CNC Swiss machining the preferred method for achieving the proper head, length, diameter, pitch, and tip.
Learn more in our blog Swiss Machining of Bone Screws and Anchors.
Electrodes
Medical electrodes are used to deliver an electrical signal for purposes of cardiac pacing, defibrillation, and neurostimulation systems. Generally made from MP35N® nickel-cobalt alloy due to its electrical properties, the electrodes are usually assembled at the end of a lead and come in contact with tissue to deliver the signal to the treatment area.
Most ring electrodes are simply cut from implant grade tubing. However, the electrodes also require various features that are produced through precision CNC swiss machining and serve purposes such as:
Learn more in our blog Swiss Machining of Medical Electrodes Made from MP35N®.
Bushings
Customs bushings are sometimes used at the tip of catheters and other medical devices where the tip must rotate at a high rotational speed. The purpose of the bushings is to reduce wear on the end of devices ranging from dental drills to atherectomy devices used to remove plaque from arteries.
Custom bushings are often made from MP35N®, Elgiloy®, or other materials that provide better wear resistance than the 304 grade stainless steel used in so many medical devices. Because tubing is not readily available in custom sizes in these materials, the tips have to be uniquely made. That means the bushings are often machined to the necessary final shape using a CNC Swiss-style lathe.
Learn more in our blog Swiss Machining of Custom Bushings for Medical Devices.
Pull Rings
Pull rings are a crucial component of steerable catheters and sheaths. Located at the tip of the device and connected via wire or cable to a control lever in the handle, the pull ring is what pulls the catheter or sheath tip in one direction or another, allowing a surgeon to guide the tip through veins or arteries.
Many pull rings are simply cut from 304 grade stainless steel tubing. However, if they have specialty features or are made from another material that is not available in standard tube sizes, precision CNC Swiss machining may be required. For instance, when a pull ring is designed with grooves for the wires, OD steps, or slots for polymer flow, it is easier to machine them from solid bar stock.
Learn more in our blog Swiss Machining of Pull Rings for Medical Devices.
With its ability to create complex and tight tolerance parts efficiently and cost effectively, it is no wonder that precision CNC machining &#; and in particular, the Swiss-style automatic lathe &#; continues to be a mainstay in many machine shops and manufacturing operations.
For a wide range of features, such as threads, holes, and steps, subtractive CNC Swiss-style machining even matches newer additive methods such as 3D laser printing in dimensional end results.
Using an evolving array of tools and techniques, along with the consistency and repeatability advantages of CNC automation, Swiss machining can enable a workpiece to be finished to precise specifications in a single operation, with little or no intervention by a machine operator.
With the ability to work with just about any material and any production volume, precision CNC machining is often the fabrication method of choice for a wide array of manufactured components.
Is precision CNC Swiss machining the right choice for your small parts requirements? Learn more in our free white paper Swiss Machine FAQs: What to Know About the Swiss Lathe and Its Advantages in Precision CNC Machining.
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CNC Precision and Accurate CNC Repeatability are important characteristics for any CNC machine. Precision (or Accuracy is another good term for it) is all about how closely we can hold tolerances. Repeatability is all about how precisely a commanded motion can be duplicated time after time.
This page contains the results of extensive research I did on the accuracy and performance of CNC machines. I have attempted to identify the key technologies and techniques involved in achieving greater accuracy and repeatability.
What Are Your Goals?Note: This article is primarily intended for DIY CNC&#;ers. If you&#;re looking at Industrial CNC Machines, they&#;ll have capabilities in excess of what this article contemplates.
Whether you&#;re evaluating CNC machines to purchase or contemplating a CNC Conversion, the first step is to determine what your goals will be. It&#;s all fine and well to do the normal male thing, grab all the spec sheets, go down all the columns, and decide yours will have the best value in each column. Just consider whether you really need all that (i.e. are you over engineering for your purpose?), whether your skills are up to those levels, and whether you can afford the expense. Also please remember the immutable laws of nature which boil down to:
If I want something twice as good it will cost me four times as much and be at least four times harder to achieve.
Your goals should be measured in several ways. First, there is the capacity question. How large will your work need to be? You&#;ve probably already made this choice in advance if you own the machine and are doing a CNC conversion on it. It will be very hard to increase the capacity of the machine beyond what it was originally designed for. Also, larger work envelopes require almost exponentially more effort to make them accurate. That&#;s why industrial CNC&#;s get so heavy as they get larger.
Second, you must determine what you want to do with the machine as far as CNC is concerned. This is somewhat a function of what your software is capable of, but that software must be chosen with your aspirations in mind too. Are you going to basically use CNC to do the same kinds of things you could already do manually? Are you going to do things in CNC, such as 3D profile milling, that are impossible to do manually? Are you prototyping, or attempting to manufacture parts efficiently? Make sure your software supports whatever you are up to. Think about the materials you will be machining. Wood requires very little precision and power, hence the prevalence of gantry-style router machines. Aluminum and plastic are much easier to cut than steel, and dictate different design tradeoffs.
Lastly, you must determine the degree of accuracy and speed you are shooting for. On this you must be brutally realistic. It sounds great to think you are going to hold to a tenth of a thousandth and have speeds in excess of 500 inches per minute, but that will not be an easy goal to reach, and do you really need it for what you are doing? Sometimes it isn&#;t obvious. If you are going to 3D profile mill a design in metal, and you want to do so efficiently from a production standpoint, tighter tolerances and higher speeds may mean a surface finish that minimizes the need for separate finishing steps in your manufacturing process.
Here are some rough guidelines where accuracy is concerned:
In my case, I want to use my tools to prototype a variety of parts that may be related to hot rodding, guns, or virtually anything else. Realistically this means I have to deal with steel, and can&#;t assume machines suitable for wood, plastic, or even aluminum will be acceptable. My work envelopes are determined already by my machines.
I am shooting for 0.001&#; accuracy and a high degree of repeatability. If I can even get close to 0.001&#;, my CNC capabilities will be the equal of my old manual machining capabilities. If I actually hit 0.001&#;, I will be able to do even better with CNC than I could manually.
Note that accuracy can also be a function of scale. Due to factors like thermal expansion, tolerances may be more difficult to hold for larger parts.
I am less concerned about speeds, so will go with steppers rather than servos in all likelihood. Speed is important for production applications, but I am unlikely to use these machines for production purposes, although I may make a few small runs from time to time.
Start With RigidityA lot of the performance potential of your machine is going to be baked in by its rigidity. This is one reason why the best machine tools weigh so much&#;there simply is no other way to keep them rigid than to use a lot of structure. Cast iron is heavy, rigid, and also has good vibration dampening characteristics. This all contributes to rigidity and the performance of the machine.
Some industrial machines even use granite as part of the structure, for example the bed or column of a mill. It is extremely rigid, and has several times the vibration dampening of cast iron. I have wondered about incorporating inexpensive granite surface plates into some machine designs.
There are things you can do to your machine to increase rigidity. A common modification to Asian lathes is to replace the compound clamp with a stronger 4-bolt variety. I won&#;t spend too much time on this page dwelling on how to improve this issue. You can find plenty of that elsewhere and ultimately you can wind up remanufacturing your machine if it gets to be too much of a Holy Grail.
Beware some of the materials you may be tempted to use to obtain rigidity. Cold rolled steel, for example, warps easily if you machine the skin off one side in an attempt to make that side true. Aluminum is not as strong, but since it does not have this property a lot of CNC machine builders are using aluminum. Cast iron also does not have this property, but is sometimes expensive and can be difficult to machine.
I will leave you with a parting thought. Sometimes we can trade speed for rigidity in a CNC machine and its a good bargain. If your machine won&#;t cut 0.125&#; on a pass, it may be capable of cutting 0.&#; and doing so over the course of 10 passes. Since its automated, we can live with it when we have to. Machines that are great at hogging aluminum or plastic may need to take much more shallow cuts on steel to get where they are going.
Friction vs Accuracy vs BacklashHaving decided on the broad design goals for your CNC project, you will shortly descend through the looking glass and into the myriad of conflicting opinions and details about the design choices needed to realize those goals. Before we go there, we need to discuss a little bit about friction, accuracy, and backlash. Consider this a background of understanding needed before we can discuss the actual gadgetry with any authority.
Let&#;s start with backlash. While there are precise engineering definitions, let&#;s keep it a bit more informal. Think of it as lost motion of your machine along one axis. It can be due to many factors. An input is given to the axis that is lost, and does not move the axis.
You experience it whenever you change directions with your handwheels during manual machining. There will be a brief period when turning the handwheel does not move the axis right as you reverse direction. The distance that would have been moved by the handwheel is the backlash. On my Lathemaster lathe, this value is somewhere in the 0.004 &#; 0.006&#; or possibly even 0.008&#; region. It can be precisely measured, but let&#;s not worry about it for the moment.
Backlash comes about for a variety of reasons. On an ACME screw with a single nut, there is some inherent play between the threads on the screw and the nut engaging. The support system (bearing or bushings and ancillary components) may allow the screw to move axially back and forth as well, which adds to backlash. Backlash is often not such a problem when manually machining because we&#;re all used to taking up the slop with our handwheels well before we reach the point of cutting. As you can imagine, it&#;s necessary to take out that slop any time you reverse directions on an axis.
The manual operations one can undertake almost by definition do not involve reversing direction while cutting unless the reversal is intended to pull the tool out and stop cutting. It would be very hard to freehand cut a circle on a mill by turning the X and Y handwheels just the right amounts, but if you did, you would see glitches in the circle at the direction change points due to backlash.
For certain operations, backlash can induce chatter and other undesirable effects. Imagine that instead of cutting that circle manually, you are using a fly cutter in the mill. The forces on the cutter are very similar to the manual circle cutting as the fly cutter travels around its circle. If the tables are jittering back and forth under those forces due to backlash, the fly cutting will not go well. Most of the time, the mass of the machine together with the friction, will provide enough resistance to minimize this effect on manual machines.
Now let&#;s consider the CNC case. CNC software, such as Mach, often has backlash compensation built in. It&#;s a rule of thumb sort of thing&#;you have to measure your backlash, and the software will do something similar to a manual operator in making sure the slack is taken up before cutting proceeds. It doesn&#;t work quite as well as for the manual operator, but it isn&#;t bad. Unfortunately, the CNC software rarely can exercise the judgment and experience of that manual operator. Sometimes a tool path is generated that calls for a direction reversal that just isn&#;t accommodated well by the backlash compensation. Even worse, CNC now allows us to contemplate doing things a manual operator would be hard pressed to follow. Cutting that circle should be child&#;s play for CNC, assuming the machine is up to it and doesn&#;t choke due to backlash. Imagine some of the engraving and profiling (think sculpture-style carving) that can be done. Lots of direction reversals going on there. Just carving or engraving an alphabet makes you think how often your pen reverses direction when you write down the letters.
Backlash compensation really can&#;t compensate for cutting that involves a direction reversal. There&#;s just no way to take out the slack fast enough without moving the cutter for it to be practical. If you want to do these kinds of operations, you will have to minimize the backlash in your machine.
Lathes have it a little better than mills because the profiling operations that reverse direction seem to be less common there. Unless you are making nozzles or chess sets with flowing curves, most shaft work can probably avoid direction reversal. For the mill, backlash is a hard problem. Based on what I read in the forums, if you want nice 3D profiling, you had better be able to get down to 0.001&#; or less backlash.
Now let&#;s get back to the friction and accuracy issues. We&#;ve already mentioned that friction can be helping to hold things in place and fight chatter. It dampens errant motions, in other words. Unfortunately, friction is bad in most other respects. It&#;s a crude force that has to be overcome. You can imagine on a tiny scale that as the machine pushes against the force of friction, the axis will suddenly break free of the friction and start moving. Anyone who has ever played with friction understands this stick/slip phenomenon and it isn&#;t helpful to precise CNC operations. It can make very slow precise motions jerky, and in the worst case, can be a source of chatter.
CNC has very little means of sensing what&#;s really going on (we&#;ll talk about encoders and limit switches in a minute, but they are no match for a human operator&#;s senses, or even a good DRO!). Because they lack this fine feedback (even servo systems with encoders to an extent), they depend on the machine always doing the same thing if they issue the same commands to it. This insensitivity of the computer (frustratingly literal devices that they are), has been dealt with largely by dramatically increasing the precision of the machines, which also involves lowering their friction. Ballscrews and linear slides, much beloved arcana of the CNC community, are all about increasing accuracy and reducing friction.
Now for the ugly secret that you must have surmised by now: low friction requires zero backlash! Without friction, backlash is left free to wreak maximum havoc on our operations. The tool cutter can potentially jitter around on every cut along every axis within the backlash spec if we let it. That would be very bad! Other forms of errant motion must also be precisely controlled if we eliminate the damping effects of friction. Tormach, for example, argues that very low friction linear bearings are best used either for small CNC machines cutting wood and plastic, or massive industrial CNC machines that have rigidity and don&#;t need the damping. They argue that for a medium sized case (most hobby CNC conversions fall here), if you want to cut metal, you will have high cutting forces and will benefit from a little bit of damping.
Another aspect of accuracy is the accuracy of the screws themselves, which we&#;ll call Lead Accuracy. The threads will not move the nut exactly the same distance per turn on all places on the screw. This is another case (much like backlash!) where the CNC control commanded an input to the axis and it didn&#;t wind up where it was expected to. Lastly, too much friction results in having to apply a lot of force to the axis, which may in turn deform the screw or some other part of the machining&#;another change in positions that the CNC control did not ask for and cannot compensate for. Let me say it loudly and clearly, lowering friction and backlash almost always improve accuracy.
Okay, so now we understand approximately the relationship between backlash, friction, and accuracy. What we can conclude is that our worst enemy is backlash. It is never good, always causes trouble, and can only be compensated for in a limited number of circumstances and then not necessarily very well. They used to say when you buy a stereo, spend most of the money on the speakers. I would say that if you are building a machine tool spend most of your money getting backlash under control. Note that I said, &#;under control&#; and not &#;eliminated&#;. Your backlash needs to be less than the accuracy you are striving for, potentially a lot less. Cutting wood to an accuracy of 0.010&#; can obviously live with a lot more backlash than cutting steel to 0.001&#;. Following the backlash, our next enemy is lead accuracy, and then perhaps friction.
ACME Leadscrews vs Ground Ballscrews vs Rolled BallscrewsNow that we&#;ve educated ourselves a little bit on the vagaries of friction, backlash, and accuracy, let&#;s delve into one of our first design choices for our CNC conversion. Specifically, do we want to use standard ACME leadscrews (probably already on our machine in the event of conversion or much cheaper to purchase if building from scratch), rolled ballscrews, or ground ballscrews (in approximate increasing order of cost and desirability)?
This is an important question with respect to cost as precision ground ballscrews can be extremely expensive, even when scrounged on eBay. In addition, the effort required to convert a machine from the leadscrews that came with it to a set of ballscrews properly mounted can be very large as well. We had better not set off in search of precision ground ballscrews out of sure desire to have bragging rights!
The differences in these choices boil down to some of our old friends: efficiency (aka friction), accuracy, and backlash. What a surprise! Let&#;s summarize these choices:
Screw Type Efficiency/Friction Accuracy (Lead Error) Backlash ACME Leadscrew 25-35% Efficient 0.003 to 0.004&#; ErrorPrecision ACME screws available to 0.&#;, but they&#;re expensive and high friction!
0.005 to 0.025&#;Can Be Low Or No Backlash With Special Nuts But It Drives Friction Even Higher. Wear Can Become A Huge Problem
Rolled Ballscrew 90% Efficient 0.003 to 0.004&#; Error 0.003 to 0.010&#;Can Be Low Or No Backlash With Preloaded Nut or Dual Nuts
Ground Ballscrew 90% Efficient < 0.&#; Error 0!No Backlash
Clearly, if you can afford them, ground ballscrews are the superior choice. Yes, you can get very high precision ACME&#;s, but they will have extremely high friction and will need an anti-backlash nut that adds even more friction. All of that will lead to increased wear. That wear is going to stress your machine and it will be uneven, introducing varying amounts of error across the range of travel that are hard to compensate out.
I can&#;t see the benefit to the ACME&#;s when you can get rolled ballscrews at a decent price unless you actually want the friction for some reason, or already have the ACME&#;s and are trying to decide if you can &#;live with them.&#; Living with them has to be a function of how much your application is susceptible to backlash problems, what accuracies you hope to achieve, and so on.
I can imagine some scenarios where living with an ACME screw makes sense. We&#;ve already talked about how reversing direction is a prime problem area for backlash, and how mills are probably much more sensitive to backlash than lathes. If you are willing to forgo profiling operations, you can also worry less. I think also plasma tables and router tables probably care less either because they are working in wood and don&#;t need high precision (though some applications will) or because plasma cutting isn&#;t a high resolution operation. Lastly, there may be a preload situation on an axis that squeezes out the backlash automatically. Some folks have even attached springs for this purpose. This is easy to do, for example, on a lathe cross slide, where a spring may be used to force the tool against the work piece creating a bias against the backlash. It may be that the Z-Axis for some tools would be fine with some backlash because gravity will drag a heavy spindle down against the backlash. The head on my Industrial Hobbies mill weighs over 200 lbs, for example! OTOH, most recommend counterbalancing these tools will improve their performance.
I&#;m going to try out the ACME approach on my CNC lathe conversion and see how it goes. It&#;s easy to drive the existing screws and hard to fit ballscrews. For reasons described above, I believe lathes are less subject to the backlash morass than mills and the worse case is I will do a conversion later. For my mill conversion, I cannot see even starting out with ACME screws. It&#;s down to the issue of rolled versus ground ballscrews. So what are the pros and cons of those two approaches?
If we can get ground ballscrews at a reasonable price, we have the best of all worlds. What if we can&#;t? Let&#;s explore how well we can do with the rolled ballscrews. Rolled versions can be had fairly cost effectively from a variety of sources. They have a track record of successful use as well. One fellow on CNC Zone uses a Bridgeport retrofitted with rolled ballscrews to do custom CNC cams for engines. He reports they have achieved nearly 0 backlash with the rolled screws. His error on direction changes for circles is 0.&#;. As you can imagine, machining cam lobes requires precision! He achieved this through a combination of preloaded ballnuts (more on these in a moment) to get to 0.&#; and careful tuning for the rest. We can see from this that it is possible to create a rolled screw configuration that does away with all but imperceptible amounts of backlash.
I&#;ll get on to the techniques needed to reduce backlash on rolled screws in a bit, but first, what about the accuracy of rolled ballscrews? The accuracy refers to the fact that the threads on the screw may not precisely move the nut according to specification. In fact, one turn of the screw may move the nut different distances depending on the starting point of the nut on the screw. We&#;ve discussed how backlash compensation doesn&#;t work as a panacea. It appears that leadscrew mapping has the potential to be much more successful in dealing with these accuracy problems. What&#;s done is to create a map of the inaccuracies and let your cnc software use the map to compensate for errors.
Creating these maps involves varying degrees of difficulty. One could use a high precision DRO if you have one available. A fellow on one of the boards was using 4&#; job blocks and a tenths indicator to laboriously check each 4&#; of travel. The pros use a laser system to setup commercial CNC machines and measure to very exacting tolerances in a very short time. With some judicious tuning of the compensation map, you can keep your lead errors to 0.001&#; or less. I have also heard of cases where it made sense to focus the compensation on a small portion of the center of the work envelope in order to achieve very high accuracy at the expense of the extremes. I&#;m not clear how those trade-offs work, but I note as something to consider for further research.
Anti-Backlash NutsFor those who want to stick with ACME leadscrews and wonder about backlash compensating nuts, this section is for you. For those who have rolled ballscrews because the ground screws were too expensive or hard to find, this section is also for you.
Section not finished.
Ballscrew MountingSection not finished.
Linear Rails vs Dovetail WaysSection not finished.
Steppers vs ServosSection not finished.
Encoders and Closing the LoopSection not finished.
Machine AdjustmentsSection not finished.
How Good Are the &#;Pro&#; Machines?The figure I see quoted most often is that shops are comfortable that a good machine will hold 0.&#; all day long without too much trouble. With special care, they may do better. Here are some anecdotes I&#;ve collected:
&#; Makino VMC with boring head holds 0.&#; for press fit bushings.
&#; A Toyoda horizontal mill cut bearing bores in cast iron all day long to 0.&#;.
&#; Mori Seiki SL15 manual lathe holds 0.&#; all day long except for tool wear and will turn to that accuracy a 6&#; long 2&#; diameter cylinder with no center. Okuma CNC lathes will do this, but Haas will not, according to these posts.
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