Help with circles

[sandman92k]

these are the settings i used


I have been testing my cnc to make sure it cuts correctly to measurements in cad.

I have a home built cnc 3 axis, x and y axis are belt driven the z axis is on a lead screw.. I have the machine set really close..

I use Linuxcnc to run the machine and use vcarve destop for drawings...

today i did two sample cuts, they are done with a v bit, a cut depth of .030..

I need some help with identifying the issues with the circle cuts, I have circled them in red..

second photo

in this photo you can see two different size circles cut, i have circled in red an area that looks like entry ad exit of the tool bit, is there a way to correct this in vcarve, or is this something else?

first cut

in this first photo you can see i cut several squares, and everything was great all the measurements were excact.

[IslaWW]

What is shown in your circles picture is most assuredly a machine deflection issue. Nothing with VCarve, could prevent or fix this issue other than possibly a spiral ramp which would distribute the deflection gradually and make it less noticeable

[sandman92k]

how do you eliminate machine deflection?

[Adrian]

Depends on the machine. If you're lucky there is something you can tighten up/adjust to eliminate it but with a lot of lower end machines (and a few more expensive ones) they just aren't rigid enough in their construction to take the loads that are imposed on them.

As said you can try spiral ramps and also more passes all of which will reduce the sideways load on the machine.

The machine that I learnt on was a homemade plywood job that couldn't cut more than a 1/16" at a time without deflection but I learnt a huge amount from it.

[Adrian]

Forgot to mention feed rates as well. If you're pushing too fast or slow the loads can increase. Looking at your toolpath again you're already doing a very light pass as it is. My guess would be the Z has some sideways movement as that wouldn't be so apparent in the squares.

[ger21]

how do you eliminate machine deflection?

Make it as rigid as possible, with the highest quality components, and drive it with very expensive ground ballscrews. It's almost impossible to eliminate all deflection, though.

Got any pictures of your machine?
If I had to guess, I'd say the belt drives may be the biggest issue.

[sandman92k]

i know i am new at this and this question probably will show that.. would the flex on the machine show in the circle as the machine cuts the circle and not at the start and end point, the mark on the wood is where the circle was started and ended

[Adrian]

Movement will often show on the plunge and retract points as the machine goes from being unloaded to loaded and back again. A plunge rate that is too high can magnify the effect.

[martin54]

The deflection is showing in the circle by the fact that the start & end points don't align, the deflection is just not enough to see in the cirlce by eye
If you post some pictures as ger21 suggested then there are people on the forum who may be able to suggest ways you can make improvements to the machine to help reduce this.

[joeporter]

I just wonder if this has something to do with your choice of "OUTSIDE" for the profile cut. The deflections look to be where a node would be located and if the circle wasn't closed completely the bit may transfer from one side to the other. Just a thought....joe

[sandman92k]

thanks everyone i appreciate all the info, i will try and get a photo of the machine up later

[sandman92k]

here is the machine

[martin54]

What size is it? Looks like you have a very high gantry which doesn't help when cutting thin materials

[highpockets]

From the picture it looks like a fairly lightweight machine. I'd recommend adding a cable chain to reduce the drag on the Y axis also the vacuum hose is large for a lightweight gantry. It too will cause a lot of drag on the X and Z axis. The gantry has weak spots on both ends in the way they are connected to the gantry upright posts.

With this being said, have you tried taking a very light .03" or so circle cut just to test it's deflection or something else.

Have you checked the backlash on all three axis?

[dwilli9013]

From the picture supplied my observation immediately went to the router mounting clamp. Do you have room to add another clamp about an inch down from the existing one? Or maybe thicken the existing one.
As previously suggested I would slow down my feed rate a bit for your test cuts.
Your machine has quite a few areas that could be beefed up a bit. Amazing what a few well placed gussets can do.
Hey that's what this forum is all about good folks helping each other improve their craft.
Keep pegging at it. You will get there and then its game on.