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  #1  
Old Mon 12 September 2011, 15:59
giggler
Just call me: giggler
 
na
United Kingdom
Incrementing error in cutting??

Hi,

I'm testing my machine and am cutting a collet holder from 18mm MDF. I have some problems with the finish.

It's going slightly diagonal in the Y axis, perhaps also in the X, I'm not sure. I can physically move the Y axis when motor is enabled, I've put locktite on the pinon and when I move it by hand, both ways move the motor spindle with no backlash.

There are a number of circular pockets in the drawing, the cut on them seem fine which got me thinking it's not backlash or other unwanted moving parts.

The motors are belt drive. The drives are made by pacific scientific, they have a reduced current mode where after a period of time, current is reduced. I have enabled this feature and after 1S of no steps, there is a noticable by eat, sound. I'd describe it as a step, so perhaps this is where my unwanted movement is comming from? It's just a stab in the dark really.

Other things, the CAM s/w produced some very small arc's and I know turbocnc has known bugs with small arcs. Could this be the accumlated error I'm seeing?

It's only in the Y i'm tempted to think it's possibly mechanical but can't prove it as the motor seems firm and repeated cuts of same gcode file produce very similar results.

See attached the nc file I used, here are image links to work:

http://i.imgur.com/Pk3x5.jpg

http://i.imgur.com/AVL8Y.jpg

Thanks in advance,

dc
Attached Files
File Type: zip 1-300-8dxf.zip (2.6 KB, 15 views)
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  #2  
Old Mon 12 September 2011, 16:00
giggler
Just call me: giggler
 
na
United Kingdom
That should read I can't physically move the Y axis when motor enabled.

dc
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  #3  
Old Mon 12 September 2011, 16:25
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
Have you checked your pinions for tightness? There have been numerous prolems attributed to this.
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  #4  
Old Mon 12 September 2011, 17:00
giggler
Just call me: giggler
 
na
United Kingdom
Hi domino11,

I think so and have seen a few posts about similar subject. I do find it strange that the circle pockets are circular as I'd of thought a loose pinon, they'd be out or at least one of them on. I tried the cut file twice, it was out in the Y axis, almost the same, well 3mm on first cut and near 4mm on 2nd attempt.

I will look at this further, perhaps my timing pully is moving when under pressure. Maybe if I mark the belts with a sharpie, i could see any movement there after running the file again.

dc
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  #5  
Old Tue 13 September 2011, 06:25
Surfcnc
Just call me: Ross #74
 
Queensland
Australia
Checked your nc code for those pockets and they line up correctly.
You probably already know this but the bit is blunt or the feed rate is too slow for your spindle speed from the images you have posted.

Regards
Ross
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  #6  
Old Tue 13 September 2011, 06:58
giggler
Just call me: giggler
 
na
United Kingdom
Hi Surfcnc,

I've tried speeding up the feedrate but it seems limited to my acceleration frequency and doesn't move faster really. It's fine on straight lines, just curves it slows to a crawl. The portercable router I am using is fixed speed and the bit was 4mm, 2flute, new. My vac isn't that powerful and doesn't always shift the MDF particles and they seem to then get hot.

Do you think that a to slow feedrate, besides the burning, could affect positional accuracy, along the lines of what I'm seeing here.

I will try and rebuild the Y axis belt drive to see if it's a slipage or other problem related to the pinons as the other poster commented. Can't think of much else to check?

dc
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  #7  
Old Tue 13 September 2011, 08:11
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
Curves crawling sounds like and setting issue between "incremental" and "absolute" on what the machine does between changes of x and y. The settings you need to look at are in Mach 3, Under Config, General Configuration, 2nd column from the left - middle of that page.
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  #8  
Old Tue 13 September 2011, 08:29
giggler
Just call me: giggler
 
na
United Kingdom
smreish,

I want to try Mach3, cost is an issue right now. Presently I use a rather elderly turbocnc. I'm waiting on delivery of my expresscard parallel port to try out EMC under Ubuntu with my laptop.

I will look for a similar setting in turbocnc, when setting up the axis there isn't to much apart from upper frequency settings, step rates etc. but there is a general config that I've not looked at in any detail.

My step movement isn't a nice round number like 0.01mm, it's a huge number of digits below the decimal point, kind of off putting and I first through that it could be loss of resolution by turbocnc after the time it's crunched it's way through all those little arc's it's out by some error, perhaps as large as 3mm. This is just an idea, I've not proved it. However I've sinced looked at the finished cuts if this was the case, you'd of thought my X axis would show a similar error, but it while there is some, it's not nearly as bad, so perhaps it's mechanical.

dc
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