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Avoiding Lathe Chuck Workholding Marks

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macardoso

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Open ended question for you guys!

I have been making some servo motor mounts for my CNC. They are roughly 3.5" x 3.5" x 4" and machined from 6061 billet. I like using my lathe to square stock as I can set it up very quickly and really move some metal, although I have certainly squared stock on my CNC mill. I also needed to hold this part in a 4 jaw chuck to bore the various internal features to this part. As a result, I have some fairly deep witness marks from the jaws of the chuck that remained through to the finished exterior of the part. This won't be a huge deal as I was planning on anodizing or painting the part, but I'd like some idea for keeping the surface from getting mucked up.

Here are some pictures, look at the jaw marks on the center of each edge of the part.
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dpb

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I cut some copper sheet into T-shapes. The leg of the T covers the jaw surface. The top of the T is bent to wrap around the outer surface of the jaw. They don’t fall off, and so far, haven’t marked anything.
 

Dave Paine

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I did the same as dpb suggests. I have seen these called soft jaws.

I have rare earth magnet glues into a piece of scrap wood which is then glued to the copper, just so the jaws do not fall off before I have tightneed the chuck.

These protect the work from marks, but may add a small amount of runout due to thickness inconsistencies or one piece of copper being compressing more than another.

Soft_jaws_test_fit_6072.jpg
 

projectnut

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If you have the American Standard jaw configuration you can purchase several different types of aluminum soft top jaws. I have several sets for my 8' and 10" 3 and 4 jaw chucks.

There are several manufacturers online. Monster Jaws comes to mind.:
http://monsterjaws.com/american-stnd-tongue-groove/

A set of 10" jaws usually runs between $40.00 and $55.00. A set of 8" jaws is usually a bit less expensive
 

gjmontll

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As needed, I cut up aluminum cans for this. Rather than wrestle with 3 or 4 snippets, I cut a strip longer than the circumference of the work (if possible); the excess acting as a handle for the protective strip while setting up the work..
 

Norseman C.B.

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I use pieces of copper bus bar out of old electric panels
they are usually about a quarter inch thick by 3/4" or 1"wide
 

macardoso

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Thanks for all the advice. I'll be picking up some copper or brass sheets to do this.

Dave, I like all your magnets:)
 

P. Waller

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I use polycarbonate between the jaws and aluminum parts that are already anodized such as boring these extrusions, works well.
We have a lot of scrap Lexan/Makrolon lying about.

As mentioned above soft jaws also work well, I have milled pockets 1/8" deep in these, I do a good deal of thin parts and these make it a breeze to keep thin parts square to the spindle axis. Also the shape of the part requires using mixed jaws in one setup. Do not turn such an unbalanced setup at 5000 Rpms (-:
 
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TomS

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For square, rectangular, hex or round stock larger than 1" diameter I use pieces of 1/8" x 1/2" aluminum of the appropriate length. For round stock 1" diameter and smaller I have a collection of shaft collars that I've sawed a split in.

Like this:

Screen Shot 08-04-18 at 07.28 AM.PNG
 

Cadillac

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I also use copper pieces. I have various scrap of copper water pipe. Just slit and wrap the part. If part is bigger like yours I just cut small pieces to cover the jaws. I’ve also used the cardboard covers from note book paper. A little thicker to get a smoosh effect. Good job on parts.
 

savarin

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I use brass shim stock.
As an aside I've also used a few layers of masking tape or electricians tape to wrap knurled parts with.
The tape compresses into the voids and the jaws just kiss the points without marking them.
 
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kvt

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Had not thought about business cards. But have enough of them around and cheaper than shim stock.
 
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