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.
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They sometimes used in conjunction with parallels. {See https://nerdocs.atlassian.net/wiki/spaces/NER/pages/166002984/Mill+Workholding#Parallels}
Parallels
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They clamp into the part with little spikes, which provide a strong grip, but leave marks in the parts.
Mitee bites do not hold parts concentric or parallel to the face of the jaws. They are most often used early in the machining process.
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https://nerdocs.atlassian.net/wiki/spaces/NER/pages/166002984/Mill+Workholding#Mitee-Bites https://nerdocs.atlassian.net/wiki/spaces/NER/pages/166002984/Mill+Workholding#Soft-Jaws
Collet Blocks
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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 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