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Milling setup question

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John281

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#1
I have a question about how to best setup a workpiece on the milling table or rotary table when I want to drill through or cut around the periphery. What material or method should be used to elevate the workpiece off the table to avoid cutting into the table? I've been using thin plywood that is slightly smaller than the workpiece so I can get the clamps close to the workpiece. I'm sure there's a better way.
 

BROCKWOOD

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#2
If parallel & perpendicular are of any importance to your operation, you will not be using plywood. Blocks from a clamping set or even a few scraps that you have milled to an equal height will put you miles ahead in set up time & accuracy.
 

Asm109

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#3
Second brockwoods suggestions.
Wide parallels.
If you want to stay with wood products, mdf would be a better choice than plywood.
 

RJSakowski

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#5
I use plywood or similar material as a sacrificial backing material with the understanding that it is compressible and not necessarily uniform thickness. If I am profiling a thin piece, any variation in z is usually not significant enough to distort the path. When the z dimension is critical, I would use something like aluminum plate for the sacrificial backing. Being the cheapskate that I am, I don't like to sacrifice good aluminum stock so this isn't done very often.

I use two sacrificial backing plates, one 1/2" PVC and the other 3/8" aluminum. I will resurface them from trime to time and discard them when they start to look like Swiss cheese.

One workaround is to leave something like .01" is stock at the bottom and cut through the final when the poart is removed and dress with a file. If your stock is thicker than your final workpiece, you can do the same but flip it over and face to final thickness.

As far as work holding thin pieces goes. Using superglue and painters masking tape has a rapodly growing fan club I use double sided carpet tap for printed circuit board milling.
 

John281

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#6
Glad I asked. Thanks for the feedback. Looks like I'll have to add "sacrificial setup blocks" to my To Do list.
 

TomS

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#7
When I need to profile the entire height of a part I use material that is thicker than the finished dimension. As an example my finished height is 1". I use 1-1/4" thick material and profile to a depth slightly more than 1", say 1.015". When done profiling I flip the part and machine the height to 1".

This doesn't work 100% of the time but does work well in most situations.

Edit - Oops! Jjust saw that RJSakowski mentioned this method in his post above.
 

RJSakowski

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#8
Sometimes we don't have the freedom to set dimensions or to select stock. For me, at least, when designing something I have an overwhelmin tendency to deisgn a dimension to a nice number so I would pick a height of .5" I would then select stock of 1/2". Why would I buy stock with 5/8" just to throw away the excess 1/8"? A better approach would be to design the part with a thickness of .45" so I can leave matreail for fixturing. The same applies to length and width. If we leave additional material for clamping, it can make our lives much easier.

One of the most overlooked lessons in machining is optimizing order of operations. For accuracy's sake it is desirable to complete all machining operations with as few setups as possible. Leaving yourself a handle or set of handles to hold the work and parting them off as a final operation makes for easier and more accurate machining.

As an example, I had a part which required machining on five faces, using 20 tool changes The smallest feature was .008" so accuracy was important. The stock was welded to a stub mounted to an RT oriented vertically. The stock could now be rotated to expose four faces for machining with the fifth face needing a facing cut. The 70+ operations were all done with the same setup with the final operation being parting from the stub.

The work was done on a mill/drill with a freshly installed DRO and the part, which mated up with a similar part, was successful. All the optical, fluidic, and dowel pin features aligned perfectly.
 
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