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Offline Tef9

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Mill cutting too big
« on: October 31, 2009, 11:43:01 AM »
Hello,

I have recently converted a chineese mill into a cnc mill.

I have set the config up in mach 3 to run as metric.

The part I designed was done in bobcad, this is inteded to be a watch face.

as I cut the part I could see that it was not comming out as a 28.8 mm diameter but a 72mm diameter.

Can anyone help, I posted the problem in the bobcad forum and they reckoned there was somthing up with mach 3.

attached is my xml file of the mill.

Any ideas what could be wrong?

Thanks,

Andy

Offline Hood

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Re: Mill cutting too big
« Reply #1 on: October 31, 2009, 12:12:14 PM »
If you command a 10mm move does it move 10mm or  25mm?
If it moves 25mm then you have your steps per unit set incorrectly.

Hood

Offline Tef9

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Re: Mill cutting too big
« Reply #2 on: November 01, 2009, 03:04:30 PM »
Thanks hood,

I have now set them to 400 which seems to be fine as when I move 10mm and measure it with calapers 10 mm has been moved (I havespent a whole day doing this)

However I ran the cut circle wizard and set the diameter to 12mm and guess what... it cuts at 24mm  any ideas?

Or are my steps per not so good?

thanks,

Andy

Offline Hood

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Re: Mill cutting too big
« Reply #3 on: November 01, 2009, 03:07:48 PM »
Is it steppers you have? If so what kind of drives are you using and if they have adjustable microsteps what have you used?
If servos what is the line count of the encoder?
Do you have any reduction between motor and ballscrew, if so what is it?
What is the pitch of the ballscrews?

With that info the steps per unit can be worked out.
Hood

Offline Tef9

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Re: Mill cutting too big
« Reply #4 on: November 01, 2009, 04:11:08 PM »
Hi Hood,

They are steppers, I got them from DiyCNC.co.uk with the drv50 driver board.

it says they have a 1.8 degree and that I took to mean 200 as it states in the manual.

The leadscrew is a 20mm diameter with a 5mm pitch.  No gearing the motor connects via a coupler to the leadscrew.

I used 10 as a microstepping figure again as the manual suggests.

I got 400 due to trail and error with the auto config tool.

Hope the info helps.

Thanks,

Andy








Re: Mill cutting too big
« Reply #5 on: November 01, 2009, 04:43:37 PM »
How about posting the gcode file ?

Phil
The Good Thing About Mach3, Is It's very Configurable

The Bad Thing About Mach3, Is It's Too Configurable

Offline Hood

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Re: Mill cutting too big
« Reply #6 on: November 01, 2009, 05:09:03 PM »
Yes, 400 it is, 200 steps per rev of motor x 10 microsteps = 200 steps per motor rev, No gearing so 1 motor rev = 1 ballscrew rev so that is a distance of 5mm. So 2000/5 = 400 steps per mm.

It could well be the Gcode as Phil has eluded to, but I am wondering if you were using a 6mm cutter? If you were then its likely you chose to do an outside circle.

Hood
Re: Mill cutting too big
« Reply #7 on: November 01, 2009, 05:14:03 PM »
Hood

Sorry should have mentioned that steps seems to be set right.

So it could be Bobcad  ;D

Phil
The Good Thing About Mach3, Is It's very Configurable

The Bad Thing About Mach3, Is It's Too Configurable

Offline Tef9

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Re: Mill cutting too big
« Reply #8 on: November 01, 2009, 05:17:52 PM »
hi There,

phew thanks for the result of 400, it seems happy there and I did spend a day to get that result.

I am using a V carve engraving bit.  I used .2 as the tool diameter.

I will have to check the gcode, The test I ran was generated by the 'cut circle wizard' that ships with mach 3, I would be supprised if the wizard was wrong more likley it could be my key presses or lack of an enter.

Phil - Will post tommorrow.

Thanks to you both,

Andy

Offline Hood

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Re: Mill cutting too big
« Reply #9 on: November 01, 2009, 05:22:05 PM »
In the wizard you have to choose which side of the cutter you wish it to cut, if for example you have a 6mm dia cutter and told it you wanted to cut outside of 12mm then the inner dia of your cut would be 12mm but the OD of it would be 24mm. If you told it to cut inside the OD would be 12mm and there would be no ID as you would effectively have cut a pocket.
Hood