What's the Big Deal?
“Work” is the stock you have and part you are creating. Workholding is a , or how you physically attach your work to the mill, is the most critical part of the machining process. If your part is not held properly, it can cause damage to both the part and the tool/machine. This can be due to intense vibrations or the part flying out of the vise. If the part is not held well, it can also blow tolerances, i.e. not being held parallel.
To create well toleranced parts, and create them safely, you need to hold your work in a manner that is rigid and well referenced.
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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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The parts of the vice that physically hold the parts are called the jaws. These are usually swappable so the ideal ones can be used for each job.
Standard Vise Jaws
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These are the jaws that come with a vise, and are typically left on. They are ground to be entirely parallel. They are the most common type of jaws for holding any typical parts.
They sometimes used in conjunction with parallels. {See https://nerdocs.atlassian.net/wiki/spaces/NER/pages/166002984/Mill+Workholding#Parallels}
Parallels
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 . 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 faceor parallel to the face of the jaws. They are most often used early in the machining process.
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If you are trying to hold a part parallel or concentric, do not use Mitee Bites. They should only be used for grabbing onto small stock for initial passes. |
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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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https://nerdocs.atlassian.net/wiki/spaces/NER/pages/166002984/Workholding#Mitee-Bites https://nerdocs.atlassian.net/wiki/spaces/NER/pages/166002984/Workholding#Soft-Jaws
Collet Blocks
Collets are round sleeves with slits in them, that tighten around a circular part as the nut on the back of them is tightened. 5C collets are a specific style of collet that is extremely common in both work and tool holding. They are one of the best ways to hold parts concentrically.
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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 are 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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Table-Mounted Chuck
A way to hold larger round parts in a mill is a table mounted chuck. It is the same setup as a 3-jaw chuck on a lathe, except has the ability to be bolted down to the table in a mill. The jaws of this type of chuck can be flipped around, as to grab the part either internally or externally.
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V-Blocks
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 work holding is ensuring that your setup is rigid enough to hit the tolerances you need. Also, if your part vibrates too much under cutting, your tools teeth will dull much quicker and potentially just stop cutting all together and break.
Area of material being held
The more material being held, the more aggressive of a cut you can take as your workholding can react higher loads without allowing the part to slip out.
Amount of material stick out
The further away from the vice you cut, the more the part can vibrate (as deflection of a cantilever beam is a function of length cubed)
Generally try to reduce stick out as much as possible