Workholding lessons

Buying a CNC machine is easy. Simply go down to the local distributer, hand over a few bills and become the proud owner of a new machine. Things get a bit more difficult when it comes to programming a CNC machine, but that’s still an easy task to learn. I can take the average person off the street and have them making simple programs in a day or two. Give me a skilled machinist, and that time usually drops to a few hours. Obviously, proficiency and skill are directly proportional to experience and dedication, but programming a CNC machine is not very hard.

I have always been surprised by the number of people who are intimidated by CNC programming when the hard part is getting a workpiece firmly fixed to the machine so the machining can take place. Seriously, the most difficult part of machining (CNC and conventional) is holding the workpiece so that the cutting tool can access the work zone.

I have learned some important lessons over the last 31 years.

Chucks

Keep it simple and make every effort to utilize something that is mass produced and not custom.

If it is a round part, then I try to use a chuck of some kind. Three-jaw chucks, for example, are not just for lathes; they can be used on machining centers, grinders and other machines. A three-jaw chuck can accommodate a range of sizes and is relatively inexpensive and readily available from countless suppliers. When three jaws do not fit the job, chucks also come in two-, six- and four-jaw versions, although these are less common.

Collets

There are also a large variety of devices that use collets for gripping. There is an abundance of collet styles available on the market, and they can be used to grip a large range of diameters as well as non-round geometries like hexagonal and square shapes. Most of the collets
on the market hold an infinite number of sizes inside of their working range. Collets are one the most versatile workholding tools that a shop can purchase.

Vises

Vises are almost as versatile as collets and chucks. Unlike a chuck, a vise does not work well for turning applications, although I’m sure someone has probably mounted one to a faceplate in the past. While they are configured to hold rectilinear parts, a creative machinist can hold almost anything with them. They can be used alone or combined with other vises to hold multiple parts on a machine. Or you could use multiple vises to hold one large part.

Magnetic Chucks

Magnetic chucks typically found on surface grinders are another good choice for workholding. Although they do not grip parts as robustly as the other tools mentioned, they are versatile and can be machined to accept odd shapes. For light milling or turning they are an alternative, especially for small parts that are difficult to hold.

Chucks, vises and collets all share a similar feature that gives them universal appeal. That is the ability to machine the jaws or collet so that they can accept almost any workpiece geometry. It is common to see a machinist preparing soft jaws for a lathe — especially a CNC lathe — but vises also can be outfitted with soft jaws. Vise makers have a large selection of soft jaws that can be machined to hold parts that do not work well with the hard flat jaws that come standard on a vise. Soft machinable collets are also available for the various collet chucks. These are an ideal way to hold extrusions and other odd shapes in a lathe, mill or multi-axis machine.

When a Custom Solution is Needed

Sometimes vises, chucks, collets and other off-the-shelf workholding will not suffice, and custom, dedicated fixtures are necessary. There are important considerations for these instances.

Like off-the-shelf workholding, keep it simple. When designing custom fixtures, focus on minimizing the number of components. Where possible use off-the-shelf components like locators and clamps rather than designing custom components. This reduces the cost of acquisition and maintenance.

Design fixtures that are screwed and pinned together and avoid welded construction. Replacing welded components is troublesome; it is much easier to unscrew the old part and screw on the new one. Pinning the components also prevents movement during use and ensures the new components are located correctly when installed.

Where possible, include poke yoke or error-proofing features to help ensure quality. These might be pins that fit snuggly in an important hole or having features that nest together. There are countless ways to poke yoke a fixture when a little creativity is applied.

A good fixture should be easy to use and not require extra labor when, for example, indicating to get a part aligned. I have seen and used fixtures that require the machinist to indicate, measure and adjust the part to ensure proper alignment in the machine. While this is necessary in rare situations, often with expensive materials, it should be avoided whenever possible.

The best advice I can give is to imitate and copy. While I am not suggesting that you steal intellectual property, there are a lot of great ideas that can be borrowed from seasoned machinists. Observe how others make fixtures. Keep the good ideas and reuse them. Of course, the internet is a great source for ideas, and trade shows are also an excellent place to learn.

Programming a machine is easy, especially with modern software. Holding parts so that the machining operation is efficient is far more difficult. Keep things simple, borrow good ideas and simplify. If you do these things, your workholding will be successful.