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Need Help - All Dimensions coming out small
« on: February 19, 2011, 09:22:31 AM »
I'm having a problem with my wood router running Mach3.  When I make my cuts everything is coming out smaller than it should be.  If I make a 2"X2" square it actually is coming out 1.994x1.994.  I first thought that maybe my cutting tool size was off so I then tried a 2"x2" pocket.  That came out 1.988 x 1.988.  This is really confusing me.  If my tool was too big then the OD on the profile would be undersize and the ID on the pocket would be oversize.  They are both coming out undersize.  The pockets appear to be 2x as far off as the profiles for finish dimension.  Any ideas on what could be causing this?  I am running probotix drivers and powermax II stepper motors and 1/2-10 acme 1 start screws.  My motor tuning has steps set to 2000 per inch.  The machine doesn't have any measurable backlash that I can find.  I put a dial indicator on each axis and jogged it 1" forward and 1" back and it came right back to zero.  There doesn't appear to be any tool offsets setup in Mach3.  Any ideas what else to check?

Keith

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Re: Need Help - All Dimensions coming out small
« Reply #1 on: February 19, 2011, 10:24:36 AM »
If your steps per are 2000 like you say then that means your drives must be in full step mode, can you not set the drive to some microstepping as I am sure it will make for a smoother motion even if it doesnt cure the problem.
Few things to check
how accurate are your screws?
Do you have the step/ Dir active setting correct for your drives?
Do you have a wide enough pulse for your drives?

Hood
Re: Need Help - All Dimensions coming out small
« Reply #2 on: February 19, 2011, 10:53:41 AM »
I can go up to 1/8th microstepping on my controllers but have kept it at single step just for simplicity for now.  I will try upping it and see if it helps.  I am pretty sure I have the step and direction active settings correct.  When I jog the machine it goes in the correct direction.  What would setting the step active high vs low do?  Will it still work in either one or would the motor not turn if I had it wrong?  The pulse width is set to 5.  It doesn't seem to be missing steps since the dimensional error is repeatable.

Keith

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Re: Need Help - All Dimensions coming out small
« Reply #3 on: February 19, 2011, 11:01:50 AM »
The motor direction does not really relate to the correct step and dir states, yes they will reverse if you change the Dir state but it is really your drives specs that will determine what you need. If you set them to what your drives require and the motors are going the wrong way you can change the motors direction from Config menu then Homing and Limits page.

Hood

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Re: Need Help - All Dimensions coming out small
« Reply #4 on: February 19, 2011, 01:38:27 PM »
One other thing, can you try cutting your square in the other direction and see if its the same.
Hood
Re: Need Help - All Dimensions coming out small
« Reply #5 on: February 19, 2011, 01:59:15 PM »
Hi, what was the DOC and diameter of the tool, material?

Could it be that the tool flex under load.

If you make a finishing pass, around 0.005 is it the same dimension?

This will eliminate that possibility.


Jeff
Re: Need Help - All Dimensions coming out small
« Reply #6 on: February 19, 2011, 04:03:38 PM »
Looking at the driver mfg website it looks like I may have the step active wrong.  It should be active high and I think it may be active low.  I will also try cutting in the other direction tonight to see what happens. 

The material is just 1/2" mdf and I am using a 1/4" upcut spiral bit.  I already have a finishing path programed.  It takes .020 on the last pass.  With the soft material I doubt that it is tool flex that is causing the issue.  I don't really expect to get .001 accuracy with the machine but it is really throwing me why both inside and outside cuts are coming up small.  Things like runout, tool diameter, flexing, etc should all make the inside and outside cut sizes opposite of each other.  If one is big the other should be small.  Seeing how they are both small I was thinking that maybe it is the software or controller that is causing the issue.  It could be the active high/low problem but I would think that I would only be off by one or two steps with each move.

Keith
Re: Need Help - All Dimensions coming out small
« Reply #7 on: February 19, 2011, 05:54:29 PM »
If you cut a square and the inside is small so will be the outside.
With your dial indicator move a distance you can read on the indicator, say .02 then again in the same direction, then again in the opposite direction.
This will tell you what your backlash is. If it's not backlash check that the amount you move is accurate.
Re: Need Help - All Dimensions coming out small
« Reply #8 on: February 19, 2011, 06:01:48 PM »
I think you misunderstood what I meant about inside and outside.  If I cut a pocket that is supposed to be 2" x 2" the material removed is actaully 1.990 x 1.990.  If I write another program and make a profile that is 2" x 2" the material left is 1.990 x 1.990.  If my cutter was too big or my runout was large then the pocket should be bigger than 2" and the profile less than 2".  If my tool was undersize then the pocket should be less than 2" and the profile larger than 2".  Since they are both small it just doesn't make any sense to me. 

I tried playing around with backlash.  I put a dial indicator on each axis.  Jogged it .100 in one direction and then .100 back to zero.  It goes right back exactly to zero.  If I had backlash it would have undershot the zero and not made it all the way back.

One of the other posts mentioned maybe inaccurate leadscrews.  The ones I bought were the 1/2 -10 single start from Enco.  They aren't the best but aren't hardware store threaded rod either.  How would I go about checking them for accuracy?

Keith
Re: Need Help - All Dimensions coming out small
« Reply #9 on: February 19, 2011, 06:18:08 PM »
First paragraph.
I agree with you, but what if your tool is the correct size and run out is acceptable. Both inside and out side would be small.

Second paragraph.
This will work if your backlash has already been taken out by first moving toward the indicator.
Try moving away from the indicator then slide the indicator into position then move away again then back.
This way the backlash will show itself.

Third paragraph.
A 1 inch travel dial indicator or even dial calipers can be used, but I think that backlash is probably the problem.

Best of luck, you have to admit this is fun, even with the minor problems.