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Ken_Shea
06-02-2003, 10:18 PM
Hi,
What are some of the less obvious ways of holding small or irregular shaped parts for machining?

Do electromagnet holders like those used on surface grinders work well on mills for steel parts ?

Ken

wms
06-02-2003, 11:08 PM
Ken,
It depends on how small the parts are.
The magnetic chuck thing will work for some things depending on the chuck's pull, (some of the new ones are quite impressive) and of course the part would have to be magnitic. Although I have seen guys use metal strips to fence in non magnetic parts. But the force to hold these was light, so the cuts were also light.

A common way to hold small parts is to machine soft vice jaws that the part, after it has been machined to a certain point, can then be inserted into. By this I mean that the outside profile or some other feature, say a bore, is machined in the jaws, a mirror image, that is. Then the 1/2 done part is "flipped over and the jaws close down on the part line and hold the part while you finish machining it.

Normally the material cost are so small that it is better to use material that allows you to have .125 or .250 left at the bottom of the part to hold on to it in a set of step jaws. Then you can machine all the way down the part then flip it over in the soft jaws and machine the extra off and finish your part.

As you can see in the photo of your part, you would start with material that is .250 thicker than your part. Then you would machine it down to the orange layer, which is the real bottom of your model. Then you would flip it over into the soft jaws that would be a mirror image of your part, the green layer, and the jaw line would be split along the center of your model, and slightly less deep than your model. Then you would remove the extra material.

wms
06-02-2003, 11:40 PM
Ken, Here's a shot of a set of soft jaws, I'm sorry I took a quick look around and I can't find any better examples. This is not the best set to show ya but it will give you the general idea. The part was cut as above, with .125 material on the bottom and then installed into these jaws and the extra removed. Hope this helps.

cbcnc
06-02-2003, 11:57 PM
Hi,

That's interesting. I am assuming that the green thing is the part and that it is set into the piece of aluminum. Do you then put into a vise to hold the whole thing?

Chris

Ken_Shea
06-02-2003, 11:59 PM
Ward, you are good!
Thanks for the tip and I understand what you are talking about.

Ken

Klox
06-03-2003, 01:47 AM
You guys couldn't have put it better!
Small parts are sometimes a pain to machine.

Klox

E-Stop
06-03-2003, 08:27 AM
Talk about small parts.

There are actually 17 parts. After all the parts were made, the Micro Dozer was assembled by an experienced watchmaker.

To machine the parts, an ice chuck was used. The parts are frozen into place and machined and then the ice is melted away.

E-Stop
06-03-2003, 08:38 AM
Another option is to us LMPA ( Low Melting Point Alloy ) and build a casting box to make vise jaws that conform to your part.

Ken_Shea
06-03-2003, 09:48 AM
Great, first wms says I have to have a plan and now it appears that I have to have patience as well!, I am in a lotta trouble :)

I wish they had shown a larger picture of the bulldozer, very neat. I have used LMPA in the gun business to make chamber cast but never thought about using it as a means to fixture. This should be very useful also.
Thanks
Ken

Klox
06-03-2003, 10:42 AM
Ken_Shea,

Are you a gunsmith?

Klox

Ken_Shea
06-03-2003, 11:20 AM
Klox The word gunsmith is thrown around much like the words heavy duty or mechanic, so am I known on the who's who in the gunsmithing circle?, no one knows me there but I am in the gun business and have repaired many firearms, and screwed up a few along the way as well :(

Ken

wms
06-03-2003, 12:10 PM
Guys,
Here's another shot of some soft jaws with the part.
As you can see the part, before it would go into the soft jaws, is on the right.
Then the finished part on the left.

CB, as you can see the green and orange is all one part, not two seperate parts. the green layer is not "set" into the orange layer.

There are also some companies that sell a "tape" that is heat activated, so you lay a strip down and warm it up and "stick" your part onto it. Then the tape cools down and you machine your part, then warm up the tape again and remove your part. As you can imagine, it doesn't have a lot of holding force.

Here is a link: http://www.miteebite.com/ Then click on "Press releases" at bottom of page and then "Mitee-Grip".

There are other kinds of clamps to look at here also. I use alot of the Pit Bull clamps.

HuFlungDung
06-03-2003, 12:40 PM
Oft times, if the stock is simple bar stock, I try to figure out a way to machine the piece on the end of a bar sticking out past the end of the jaws. This involves a little bit of screwing around with the tool paths to skip the area where the cutoff will occur, leaving that until last. The final seperation of the cutoff area has to be quite thin, so I'll machine and finish the cutoff end right down to about .03" remaining thickness, and making the toolpath purposely a bit wider than my tool on the way down, so I don't get a jam up when the part cuts off. It usually swings open along the "kerf" anyways.

Sometimes, I'll have a support under the outboard end, and clamp the piece down with a top clamp just before it is cut off. Then, it can be finished completely on the cutoff pass, with leaving an uncut scallop.

SRT
06-03-2003, 04:48 PM
Small parts maybe not, but thin parts, or parts otherwise hard to hold, I use a vacuum chuck sometimes.

cncfreak
06-03-2003, 04:52 PM
You can use vacum, double sided tape, magnets, I've even used super glues. You just need to be very slow and light depth of cuts.

Ken_Shea
06-03-2003, 04:57 PM
When machining parts that are being held by a means other then clamping would you normally use the same rpm or somewhat faster along with the slow and light?

Ken

cncfreak
06-03-2003, 05:28 PM
I like to slow down the RPM about 20% and drop the feed about 35%. If you try high RPM say above 10,000 you will find the air movment the tool is producing might move your part a little.