A way to undercut ??

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A way to undercut ??

Postby DougE » Wed Dec 30, 2015 1:39 pm

Does anyone have a way to undercut a model ?

Cut3d doesn't allow a negative machining allowance.

I would love to be able to rough-cut my models by a negative .125. (I then wish to coat this form with plaster and run the final finishing pass on it)

I haven't yet come up with a solution that is plausible.

I'll definitely take suggestions if you have any.

thank you

DougE
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Re: A way to undercut ??

Postby Rcnewcomb » Wed Dec 30, 2015 8:36 pm

You could just scale your model down and do a rough cut on the scaled down model.
Then do a finish cut using the full size model.
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Re: A way to undercut ??

Postby FixitMike » Wed Dec 30, 2015 9:46 pm

Resizing doesn't work. Holes get smaller, where undercutting would make them larger. ( I think the term overcutting would be more accurate.)
One method is to specify a tool size that is .25" smaller in diameter than the actual tool used to achieve a .125" deeper & wider cut.
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Re: A way to undercut ??

Postby DougE » Wed Dec 30, 2015 9:59 pm

Mike you are absolutely correct regarding re-sizing. (It seems like the simple solution until you realize scale is not the same as deflating.)

And yes,.. overcutting would be a better description.

So,.. if I have you correct;

If I set up roughing parameters telling it I am using 1/8 inch ball nose....
I then load a 1/4 inch ball nose into my spindle.

But, how would this make the cut 1/8 deeper in Z ?.... I think I follow what you mean as far as this compensating for x and y in the tool radius, but it wouldn't compensate for Z (would it?) *I suppose Z could be compensated for by resetting Z-zero by -.125** (shrug?... this would mean cutting .125 into the spoilboard for the rough cut.. I would thenneed to reset Z-zero +.125 for the finished cut ??)

Thank you both for your input and time.

DougE
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Re: A way to undercut ??

Postby FixitMike » Wed Dec 30, 2015 10:14 pm

The excess cut amount will be equal to the difference in radii between the two tools (1/8 - 1/16 for the 1/4 and 1/8" you mentioned). And for Z, it will be necessary to have the tip of the tool that much lower when you set the Z0. How you do this depends upon your machine and software.
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Re: A way to undercut ??

Postby DougE » Wed Dec 30, 2015 10:17 pm

Thank you Mike !

I'm just trying to think all of this through before I drop blade.

It's nice to have others to throw questions at !

Thank you again.

DougE
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Re: A way to undercut ??

Postby martin54 » Sat Jan 02, 2016 6:00 pm

You could always use a sacrificial board under your work piece rather than cut into the spoilboard. Odd bit of MDF or ply would do. Just make sure it is flat or surface it before placing you work piece on it.
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