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  #1  
Old Tue 20 June 2006, 08:59
Gerald_D
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The tape measure is metric. The MDF is 16mm. 6mm cutter. 2 passes. The straight cut with the writing on is at a slope (2-axis). SB=Shopbot, M2=Mach2.


The hole in the above pic is diamond-shaped with radiused corners. The jitter in the hole is on a face at about 45 degrees to the table.



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  #2  
Old Tue 20 June 2006, 12:34
Dirk Hazeleger
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I've never had that kind of chatter. I bet it's direct drive causing it.
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  #3  
Old Tue 20 June 2006, 13:17
Scott Worden
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Lost - M110110M Angle iron rails for X and Y axes

That's the kind of chatter I use to have when I had the direct drive motors. It's better with the gearbox ones, but still noticeable.
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  #4  
Old Tue 20 June 2006, 13:58
Gerald_D
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These photos were taken in Sept. last year, before replacing the gantry and y-car, when the old SB was in a really bad way. There were no holdowns on the y-car and the car was slightly twisted to the extent that one v-roller started to bounce up and down. The pinion gears were also worn out. Our typical quality today is a heck of a lot better, meaning that the marks are much shallower, but the spacing is still about the same.
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  #5  
Old Wed 21 June 2006, 07:39
stevelendon
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I have a standard 48x96 Alpha with 3hp spindle...

I cut a lot of mdf and single axis cuts are chatter free, curves are variable, like most of us I have tried to find the cure, set up of Y axis holddown rollers did make an improvement especially as I found a stripped thread on a bolt that fixes the roller to the alloy cross bar.

I found that cutting the green 'waterproof' mdf caused chatter easily as bad as shown in Geralds pics....yet other materials were ok ?.

Wherever I can I now use a 12mm downcut spiral and general quality is better...though one would think that a larger cutter would cause more problems with the increased forces if reduced current is the problem.

In the pics above did you try a different rpm for the bit on its second pass..did the chatter still line up?
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  #6  
Old Wed 21 June 2006, 09:00
Gerald_D
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Hi Steve, we didn't try different RPM's on that job, but I am sure that the marks always line up irrespective of spindle speeds. Playing with feed speeds may be a different matter....

As bad as those photos are, I havn't really lost much sleep over the chatters, and I havn't been on a big mission to eliminate them. My theory is that they are a result of the forced together rack&pinion and the coarse steps/ratio of direct coupling the steppers - neither of which I plan to throw any money at. So I busy myself with tuning up the rest of the system as far as possible/practical.
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  #7  
Old Thu 22 June 2006, 11:29
stevelendon
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Likewise I'm not devoting much effort to tracing this chatter problem, it's just like a nagging toothache that crops up when you least want it...

On a well set up (no noticable play or backlash) alpha running aircuts one can (at times) feel chatter frequency vibrations, if the electronics aren't causing this, then like Gerald I suspect the pinion/rack system.

Would love to have a solution, or indeed a proof though.
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  #8  
Old Sat 24 June 2006, 07:21
Dick van Randen
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There was some discussion of a "test" cut file for objectivly comparing machines. Any progress?
Would like to test to compare.

I have modified the way the y-car v-rollers are mounted but have never tested to see if the mod improved things ....it just seemed like a good idea at the time.
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  #9  
Old Sat 24 June 2006, 23:57
Mike John
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I have in the last two days been cutting 20mm beech with a two flute 6mm bit. I have been cutting very carefully, 5mm deep at a time.
I have looked very hard for chatter and couldn't find it at all, until I upcut about 0.5mm off of one edge. To me this was a result of the bit going the 'wrong way' in relation to the direction of cut. Bouncing off of the material.
Even then, I only noticed it because I was looking for it. I am not very sure it was that regular, either.
But conventional cutting shows no sign of chatter.
Its a gear box motor (so Gerald informs me), on a PRT96 with the Alpha frame, running a small 700w router, 50mmps, always taking shallow cuts.

If I am not getting it (and others are suggesting the same) does that mean we can eliminate some of the possibilities?

..........Mike
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  #10  
Old Sun 25 June 2006, 01:00
Gerald_D
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Mike, you enjoy maths, don't you?.......

If your chatter was caused by the bit bouncing, you can take;
-router speed
-number of flutes
-move speed
and calculate how many "bounces" per cm/inch you should be getting. Is it even close to what you see?

And, if it is a helical bit, the marks should be helical - no?
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  #11  
Old Sun 25 June 2006, 01:28
Mike John
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Gerald
Unfortunately it is not that a precise mark I am seeing.
I don't really know what it is I am seeing.
I guess those cuts with no chatter are more interesting.

.........Mike
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  #12  
Old Sat 29 July 2006, 08:34
Joe Crumley
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For years I put up with chatter marks and spent loads of time to correct it. Chatter is much of a problem for some work. It was critical for us. We made the corrections with sandpaper.

What causes this?
Bit deflection
Loose Gantry
Out of square frame
Etc.

I was allways suspecious the culprit was deep within the controll box. It's the pulse rate. The easiest, most cost effective solution is dump the box. That thing is slow and antiquated.

I'd still have a PRT if I had a good controlled such as the Ascension 1000.

Every Shopbotter deserves a better box.

J
www.normansignco.com
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  #13  
Old Sun 14 December 2008, 09:24
Cutter99
Just call me: Richard
 
On
Canada
I have an old shopbot and it suffers from these marks. Can someone point me in the right direction? I cannot find these new replacement controllers.


Thanks,

C99
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  #14  
Old Sun 14 December 2008, 10:53
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Richard, after these improved controllers hit the market, ShopBot hurriedly brought their own improved controller to the market, thereby sinking the 3rd party controllers.

Building your own improved controller is actually fairly easy, but then you can't run the SB software anymore.
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  #15  
Old Mon 15 December 2008, 04:07
Cutter99
Just call me: Richard
 
On
Canada
I understand the Mach 3 is a much better option?

C99
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  #16  
Old Mon 15 December 2008, 04:16
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
See this thread:

Shopbot VS Mach 3
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