Over cutting corners problem.

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James E
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Over cutting corners problem.

Post by James E »

I am cutting letters out of 3mm acrylic, using a 1/8" dia by 1/64" tip radius 10degree (20 included) bit. I am using profile toolpath, with sharp external and sharp internal corners selected, machining outside.
I find that the internal corners are always overcut. The bit lifts as though to make a sharp corner with the point of the tool, but the cut is as though the fatter part of the tool has not only cut it out, but gone into the corner a little extra as well.
I suspect the geometry of the tool may be half a degree out - 20.5 not 20, and I am cutting .25mm below the 3mm, so I get a clean edge on the bottom, not a rounded one. Could these be the cause?

They may be factors that are vastly outweighed by some other stupid error I have made, but I can't think where to look.

thanks for reading

Jim

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Tony Mac
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Post by Tony Mac »

Hi Jim,

You say you are cutting 0.25mm below the 3mm - are you programming the
toolpath with a Depth = 3.25mm or simply setting the Z0 slightly lower?

Tony

James E
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Post by James E »

Hi Tony, fast reply as always.

I am cutting with a depth = 3.25.

Jim

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Tony Mac
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Post by Tony Mac »

Any chance you could take a photo of what you are seeing?

This would help.

Tony

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TReischl
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Post by TReischl »

I had a similar problem over a month ago. In my case it was the machine. Not even close to being rigid enough. Just finished my new machine and do not have the problem. If all else fails, you might want to take a look at your setup on the machine.
"If you see a good fight, get in it." Dr. Vernon Johns

James E
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Post by James E »

Tony

I'll continue the week long search for the camera charger,and get one to you.

TReischl

Some kind of whiplash effect? It's possible. I'll slow right down and see if that alters it
Thanks

Jim

James E
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Post by James E »

No, even at a crawl, to tkae out any momentum, the little scallops are still there.

Now, where haven't I looked for that charger?

Jim

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TReischl
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Post by TReischl »

Even at a crawl mine did it, like at 5 inches per minute. A dead sharp tool did not help either. What I finally did to figure it out is work the math.

I used my CAD program to interpret the numbers in the G code, it was dead on. So after that, it has to be the machine or tool. I simplified things by programming a single pass cut, so I did not have to look at bunches of numbers.

Good luck, things like this are why I am bald!
"If you see a good fight, get in it." Dr. Vernon Johns

James E
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Post by James E »

Mmm. You are probably right. It won't be Vcarve, I am hoping it is some setting I have wrong, - ball end tapers aren't really supported - otherwise, it's down to the machine. - Mr Flexible, as it will come to be known.

Jim

James E
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Post by James E »

Could this be z axis backlash? anyone?

Jim

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Post by James E »

Tony has eyeballed the toolpath and says it's ok. Thanks Tony.
Here are some pics of the problem.
Attachments
cnc mc 00a.jpg
cnc mc b.jpg

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dighsx
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Post by dighsx »

This looks like a bit geometry problem to be. Like VCP thinks the bit is a vbit but the bit is round. If you look at the inner corners you can see where it pulls up but cuts a round notch. Have you tried doing this with a vbit and see if you get the same results?

If it isn't that then my next best guess would be you've got a backlash problem. But just looking at those pics I bet it's something to do with the shape of the bit and how you've got it setup in VCP.

Take a pic of the bit and a screen shot of the setup page for that bit if you can.
Take it easy.
Jay (www.cncjay.com)

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TReischl
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Post by TReischl »

tend to agree with ya on this one dig. Either that or he has one really sloppy z axis.
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Post by BradyWatson »

Looks like there's some SERIOUS bit deflection going on there...how long is the shank? The cutting path doesn't look like it was run with a v-bit.

-B
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Post by James E »

braidmeister
Your right, it's not a v bit. I think if it was it would be spot on. I will test this asap as dighsx suggests.
pic of bit setup follows.
I had a good look at the z axis this am, because I guess if there was backlash, the tool has continued into the letter thinking the z has started to rise, but it hasn't, hence it's cutting into the letter. There was some backlash, .03mm at worst. Would this cause it?
But even on the first pass .5mm, it is starting to cut a notch out

dighsx the bit is from Menlo-USA.com , tool no 02971, if you want to look at their spec.

The whole bit is only 1.5" long, so I don't think deflection is a problem
thanks for the attention
Jim

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