Whats

What's the Big Deal?

“Work” is the stock you have and part you are creating. Workholding, or how you physically attach your work to the mill, is the most critical part of the machining process.

To create well toleranced parts, and create them safely, you need to hold your work in a manner that is rigid and well referenced.This will be gone over more in depth in the operating a mill page.

Types of Workholding

Vises

There are many types of workholding for mills, most of which can be traced back to a vise of some sort. Vises are clamped down to the table in some way, and are used themselves to clamp parts. On a mill, vises usually have removable handles to make sure they don't get hit by the machine or vibrate off.

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Parallels are pairs of ground metal used to elevate the work in a vice, typically used with standard vice jaws. The thin long edges of parallels are machined to very tight tolerances of both parallelism and flatness, so they add a minimal amount to the total tolerance stack up when elevating parts off of the vise.

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Mitee Bites

Mitee bites are an incredibly powerful tool. They allow you to grab onto very thin sections (<0.1” of material).

They clamp into the part with little spikes, which gives them provide a very strong hold for the small area they touch. This will leave indentations in the part. Be sure to either clamp onto a section of stock that will be removed later, or a section of your part which you are fine having these marks.The ones that can be used for round parts must be used thoughtfully. They strong grip, but leave marks in the parts.

Mitee bites do not hold parts concentric or parallel to the face of the jaws. It is best to use them for earlier operations, and then not used when the part it flipped over. If machining an outer circle, the outer edges will not line up nicely. If machining a top face, it will not be parallel to the bottom face They are most often used early in the machining process.

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Soft Jaws

Soft jaws are jaws that are usually made yourself out of plastic or aluminum. They are shaped to match interesting contours on a part that are not easily held by workholding methods in the shop or easily sourceablecustom machined to your part to fit odd contours on your part. They can be especially useful for second jobs on oddly shaped parts.

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Round Parts in a Mill

Although round parts are usually seen as a lathe item, there are many circumstances in which they need to be put in or made entirely on a mill. There are a few ways to hold onto round parts in a mill, including these from the previous section:

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These collets can be purchased in a wide array of standard fractional sizes, and are typically in stock in any standard machine shop. They have a very limited clamping range, so don’t try to put stock or parts in a 5C collet that is more than a few thou out of the nominal size. Even if parts are larger/smaller than that and they technically fit, the nature of the clamping within the collet means that the part may not be concentric if too far away from nominal sizeare only for use on round stock very close to the nominal size of the collet.

Collet blocks are a square or hexagonal block the collet gets inserted to and tightened against. These are blocks that are ground to tight tolerance and can be tightened in a vise for the purposes of workholding.

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V-blocks sit inside the vise and get clamped along with the work. They are ground and can hold parts very concentric. There are a few different sizes of v-blocks depending on part size. You always want to make sure the part is sitting on the flats of the v-blocks, and not so large that they only touch the outer edges. Only one v-block is needed, as long as the third point of contact is another well-defined surface.

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Rigidity in Workholding

A key aspect of workholding is ensuring that your setup is rigid enough to hit the tolerances you need.