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Offline allyd

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Problem cutting circles
« on: April 27, 2009, 05:13:41 AM »
Hey all, I think I have a strange one here, when I cut a circular pocket or circle it cuts an oval. Now normally I would think ahhh, X and Y need recalibrating but my oval is skewed at 45 deg. I cut a square pocket and it measures ok which suggests that my calibration is correct. To make it worse the circle measures correct at the X and Y (Dia. 42mm). I confused to whether I have a software or hardware issue. I have tried cutting several pockets using NFW, V-carve and the simple pocket wizard in Mach but they are all out. Any ideas are very welcome.

Offline Hood

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Re: Problem cutting circles
« Reply #1 on: April 27, 2009, 06:23:27 AM »
Sounds like it could be backlash of some sort, especially if you notice the changes are at the cardinal points.
Hood
Re: Problem cutting circles
« Reply #2 on: April 27, 2009, 07:27:01 AM »
Is it a smoth oval or is it any distinct steps in it?

Offline allyd

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Re: Problem cutting circles
« Reply #3 on: April 27, 2009, 08:26:11 AM »
Thanks for the reply fella's, it is a perfect oval no steps or irregular features. My machine does not have any backlash (linear motors). I have changed the IJ from inc to absolute, to late tonight to try cutting anything so I will try tomorrow night.

Offline RICH

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Re: Problem cutting circles
« Reply #4 on: April 27, 2009, 08:37:59 AM »
allyd,
Suggest testing out your machine some to get a flavor of just what it can accomplish.
For example, machine the outside of some round stock, 1" dia down  to .8" and also .9"
in a stepped fashion. Also machine a circular pocket of  say .75" into it. The depths only need to be
deep enough to measure accurately. Do the same for a different diameter, say 3". If you have backlash,
use  backlash compensation. You may want to leave some stock for a finish pass.

Spindle runout, how the mill head is set to the table, backlash, x & y screw accuracy, table play, gib adjustment,
how accurately your steps per unit where set,etc....... all come into to play in trying to cut that  "perfect" circle. Play around some and see what tolerance your machine can hold.

Note that a mill is not a lathe but approaches it these days.  

RICH

Re: Problem cutting circles
« Reply #5 on: April 27, 2009, 08:53:11 AM »
How much oval is it? Visible to the naked eye?

Offline allyd

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Re: Problem cutting circles
« Reply #6 on: April 27, 2009, 07:11:10 PM »
Hi Stein, cutting a 42mm pocket in MDF it measured dia. 41.6 and 42.7 +- 0.1 (non-digital vernier, no glasses) skewed at 45 deg. I will try out more cutting when I get home from work tonight, it's a wood router not a ferrous metal machine.

Offline Chaoticone

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Re: Problem cutting circles
« Reply #7 on: April 27, 2009, 07:14:37 PM »
Sounds to me like your steps per could be off on one of the axis. Can you check this with an indicator?

Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!

Offline Hood

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Re: Problem cutting circles
« Reply #8 on: April 28, 2009, 03:00:04 AM »
You say you have cut a square ok so wondering if you can cut the same but rotated to 45degrees, ie a diamond shape. Reason is I am wondering if it could have anything to do with more than one axis running at the same time.
Hood
Re: Problem cutting circles
« Reply #9 on: April 28, 2009, 05:31:29 AM »
Is it set up any backlash compensation in Mach3?