Introduction of tracing methods
The CNC machining process is as good as your fixturing method! Fixtures hold the part firmly in place and reduce stiffness during machining. Proper setup is essential for locating the part to be machined. Of course, fixturing is also important for safety!
The importance of strong stretching
When machining anything, the stiffness of the entire operation is extremely important. Reduced stiffness causes vibration and a significant decrease in the accuracy and precision of the machining process. The “chain of rigidity” consists of the machine itself, the tool, the part and the fixture holding the part. If the part can move while being machined, there is no way you can maintain the tolerances set forth in the design or drawing. Even minor movement during machining results in poorer accuracy. If one tries to machine multiple parts with a poor setup, each part will differ significantly from the rest, resulting in low precision. Production effort also plays a role when considering the fixture method. First, with a poor setup you will likely have to remake the part. With a thoughtful workpiece fixturing solution, you can reduce the number of machining settings. By designing convenient features for fastening in the part, you can reduce the effort and cost of machining.
Example of a metal plate in a fixture during machining.
Finally, safety requires proper fixturing. Often the part is enclosed in an industrial CNC machine to protect people from flying objects, but what if it is not? Terrible things can happen when parts or pieces of a broken tool start flying around. In addition to safety for personnel, you obviously want to think about the safety of the machine. Accidents can occur when a part comes loose or a tool bumps into something. The part must stay in one place, even under the tremendous forces of a CNC machine. You also have to make sure the fixture doesn’t get in the way of the tool. In short, it’s worth thinking about how to keep the material in place while it’s being machined.
Things to think about for the tracking method
Certain materials are easier to machine than others. Similarly, certain features will be easier to machine. For example, brass is one of the softest metals and removing material takes much less effort than a material such as steel. The amount of material being removed is also important to think about. With knowledge of the potential load, you can begin to refine your fixturing options. If you are machining a small brass part, you may want to use a slightly less rigid but also less invasive method for holding the part. With a large steel part, you want a firmer fastener. Another important consideration is where the fastening method will hold the part. Is it a standard block with parallel edges that can be placed in a vise? Should there be multiple sides accessible to the tool to avoid an extra operation? Is the part an exotic shape that requires a creative method of securing it in the machine? These are all good questions to ask yourself when choosing a CNC workpiece fixture. Finally, you want to consider how many parts you need to make with your fixture. If it’s going to be a high-volume process, it may be worth investing a little more time and effort into making a custom fixture that can hold many parts at once. If you’re only making a few parts, you may want to stick with one of the other mounting options we’ll discuss below.
Examples of clamping methods
Vice grip (vise)
We begin with the classic machinist’s vise. A vise can be clamped onto the table of the CNC machine. However, a vise will not work for every part. There is a sweet spot in sizing because of limitations on how large a part can be – a vise can only open so far. Thickness also matters – a very thin part can warp when you tighten the vise. You can also use a vise only for parts with parallel edges. When using a vise, make sure the part is level and fully tightened. If set correctly, a vise can be extremely repeatable and accurate!
Example of a part held in a vise.
Chucks (soft jaws)
A good holding method for any part that needs to be held firmly but has an irregular form factor are soft jaws. Soft jaws refer to a set of two parts milled from aluminum. Each part is flat and straight on one side (to align with one side of the vise) and the profile of your part is machined into the other side. This way you can hold the part in a vise even if it is round or has non-parallel sides. Each soft chuck is bolted to one side of the vise once it is ready for use. Soft chucks are convenient because aluminum is quick and easy to machine. You can also machine them over and over again to work for different parts. Soft chucks can be reused when machining multiple units of the same design. However, soft chucks are not well suited for larger quantities. They tend to wear out after much use. If you make thousands of parts, you can use steel chucks, the same concept but made of steel. Steel, however, is much more difficult and time-consuming to machine than aluminum. When you move to large production volumes, it can be more efficient to make a fixture that can hold many parts in one machine. With one of these machined chucks (aluminum or steel), you are limited by the number of vices that can fit in a machine.
Machine clamps (step clamps)
Machine clamping, is a commonly used method for holding workpieces in manual milling. This clamping system uses the T-slots on the bed of a machine. They are an extremely sturdy way to hold the workpiece and are another effective solution for holding CNC workpieces. However, there are some drawbacks that make other workholding devices a better choice. Machine clamps can be time-consuming to set up, although they can be adapted to different parts (as long as you have a flat surface to clamp on. They are not as easy to use again as other workpiece fasteners. Once you loosen the clamp, it won’t stay in the same place. You then have to reconfigure the setup for the next part. Machine clamps are good for holding large sheets to be machined and for clamping fixture plates.
Machine clamps used to secure this block of metal in place.
Fixtures (mounting plates)
Mounting plates can be used to mount a wide range of parts that require CNC machining. You can also call them tool plates. They are made of aluminum or steel and have a grid of holes. Some of these holes are for dowel pins and some are threaded for bolts. Fastener plates are a very useful method for holding work because of their repeatability; each hole has a known location and will not move once the plate is installed. Fitting pin holes are useful for placing the fixture or part on the plate. The threaded holes are good for screwing down the fixture or part.
A mounting plate with evenly spaced holes.
There are a number of different ways to use a mounting plate. One method is to design bolt holes in the part. This method of fixturing work allows one to access multiple sides of the part and machine interesting geometries or surfaces. However, it requires post machining to remove the fixing holes. Another method is to design a custom fixture to hold the part. You then attach that fixture to the fixture plate. This takes advantage of the location characteristics of the fixture plate. Fixture plates also protect the table of the CNC machine from a the tool. There are many options for nuts, bolts and other tools to use with a fixture plate, so go out and explore!
Advanced applications
The CNC fixturing methods discussed so far are great for a standard 3-axis CNC milling machine. These machines have many capabilities and with creative fixturing one can machine many different geometries. However, there are some limits to what a 3-axis machine can do. You may find yourself machining something more complex on a 4- or 5-axis machine. 5th axis workpiece fixturing is an entirely different story. With the extra range of motion, the machine has much more access to the workpiece. More axes means more fun, but also more chance of crashing! Pay extra attention to allowing access to multiple surfaces. You also need to allow space around the workpiece and the machine table for the milling tools to move.
Example of advanced fixturing when a CNC machine goes beyond 3 axes.
I hope this article has at least convinced you to pay attention to your CNC fixturing solution. Your workpiece can influence the design of the part, and vice versa (after all, engineering and manufacturing go hand in hand). Clamping the part is also important for precision, accuracy and safety! So you can significantly reduce manufacturing effort if you design your part so that it can be easily clamped during machining. If you found this article interesting, feel free to share it on LinkedIn!
