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Backlash Settings
« on: July 24, 2009, 06:59:54 AM »
I'm new and I'm trying to fine tune my small wood working CNC router. I found that I had a first pass issue that caused the first pass to cut into the part by as much as .025 and then the remaining cuts would be spot on. I added Backlash to Y of .024 and this cleared the first pass issue.

I adjusted my machine so that the Y axis moves much more smoothly now. I had the V-Wheels too tight. I was able to reduce the backlash setting to .0095. I found that adding the same .0095 value to the X axis cleared up a very slight first pass issue on the X axis as well.

Is a Backlash setting of .0095 for X and Y a reasonable number for a CNC router designed for wood or should I continue to try to smooth out my machine movement? Everything looks right now including circles but it is hard to tell via the old eye ball method.

My Backlash speed is set to 100%. This seems to work but I wonder if this setting should be lower. ??? I did have an issue once during test cuts and jogging where the machine would trip. I turned Backlash off and the problem went away. I turned Backlash back on and the tripping came back, finally I changed the setting to .1 and the problem did not come back and then I changed back to .0095 and I have not experienced any more tripping issues. My machine is set to run at 60 IPM Max at the moment.

Thanks,
BillJ

Offline Dan13

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Re: Backlash Settings
« Reply #1 on: July 24, 2009, 08:00:34 AM »
Hi Bill,

Basically, the correct way to treat backlash, and what everybody on this list would say is to try and eliminate the backlash mechanically as much as practically possible. On a router (or mill), if you have backlash, the cutting forces can move the table (head) to any direction within the backlash margins. Thus, the software backlash compensation can do nothing to help in this.

Having said this, many users benefit much from using the software backlash compensation. It all depends on the type of work you do and the forces acting. In my opinion, in most of the cases that you can't afford a ZERO backlash mechanical system, a software compensation will be very satisfactory solution. If you do a finish cut of say 0.05mm (or less), most probably, the cutting forces would be smaller than the friction in the system, not being able to move the table to unwanted directions.

If you have a constant backlash along the leadscrew and you can't eliminate it mechanically, you can define the value in Mach3 to compensate for. It is desirable that the mechanical backlash be as small as possible. I think that software backlash compensation for up to few hundredths of a mm can be of a huge benefit for some systems. You can cut very precise circles (up to 0.01mm tolerance) if you do several spring passes starting each one at a different location along the circle circumference. By probability theory it eliminates most of the possible errors caused by uneven leadscrew pitch or backlash compensation.

As to your backlash, I think it is too big and you should try to reduce to at least half of that, or preferably to the 0.002" region. You shouldn't be checking the backlash by the eye. The correct way to do this is using a dial indicator - you program a move towards the dial indicator and then backwards and the difference in the dial indicator reading is your backlash.

Daniel

Offline RICH

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Re: Backlash Settings
« Reply #2 on: July 24, 2009, 10:50:42 AM »
BILLJ,
My 50-50-50 rule which you may want to consider.
50% - The max velocity is 50% of where your steppers will start to skip
50% - Set the shuttle Wheel setting in configuration to .0050  to .050 ( .5 too slow )
50% - Backlash speed
This is a good starting point and refine upward, but always leave say 30% headroom.

Wood is not metal, but if it's a read fine grained hard wood, it will machine to rather close tolerance.
( example; ebony, lictum vitae ( however you spell it ), babinga, etc ).
So it depends on what your trying to do.

BTW, try to always start your program with prep moves such that the backlash is taken out.
If you are using a SS, to bad, no backlash yet, been waiting for a over 1 1/2" years but surely it will get done
one year!  ;)

RICH
 
 
Re: Backlash Settings
« Reply #3 on: July 24, 2009, 06:22:45 PM »
Thanks for the help. I've got a much better understanding now. I probably still have my wheels just a little tight and I could tighten the belts a little as well.

I want to try the 50-50-50 rule and I found the Shuttle wheel setting under General Config and I'll change that to a value between .005 and .05 and set the Backlash speed to 50% to start with. I use steppers and I'm not sure what you mean when you say SS.

Now I know that .0095 is still a bit high, I will try to smooth out the mechanics a bit more but I'll first start with the settings and I'll eventually do another adjustment. Although my setting is a little high, the Backlash adjustment does seem to take care of my issues but I realize now that I really need to use a accurate measurement device and method to get it spot on.

One other point made was about using a prep step to eliminate the backlash before you start the job. Now that sounds like a good practice but I'm going to have to think about how to do that. I'll probably figure out a method to draw an extra tool path move that will do a mini movement above the work in the direction that I'm about to cut. Is that what you do?

I appreciate the information.
BillJ
« Last Edit: July 24, 2009, 06:40:13 PM by BJenkins »

Offline Dan13

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Re: Backlash Settings
« Reply #4 on: July 25, 2009, 12:51:41 AM »
You are correct about the prep move, but if you're using software backlash compensation you don't need to do this, as this is exactly what the software does.

Daniel
Re: Backlash Settings
« Reply #5 on: July 25, 2009, 05:39:11 AM »
OK, Thanks again. I cut some small parts tonight and a good number of them are circle cuts. I will inspect them closely tomorrow. Does backlash help or hurt when cutting circles. As I cut these circles, I wondered if I should have turned the backlash off.

BillJ

Offline Dan13

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Re: Backlash Settings
« Reply #6 on: July 25, 2009, 09:09:00 AM »
Why would you thing you need to turn off the backlash comp off? With the amount of backlash you have, if you would have turned the backlash comp off your circles would have been egg shaped  ;)

Daniel

Offline RICH

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Re: Backlash Settings
« Reply #7 on: July 25, 2009, 11:11:22 AM »
Rough cut / measure, rough /finish cut...measure, now finish cut ( maybe start the cut 180 or 90 from rough finish cut start point ) . Heck, can pocket and hand press a bearing into something with backlash on.
If you want to get picky about a particular circle. But generaly speaking, leave it on.

Unless your using a SS ( SmoothStepper ), which dosen't have backlash yet implemented.

RICH
Re: Backlash Settings
« Reply #8 on: July 25, 2009, 05:00:38 PM »
Very good points.
Thank You!
BillJ
Re: Backlash Settings
« Reply #9 on: July 28, 2009, 12:01:44 PM »
FYI   there is a good video on youtube.com about Mach 3 backlash settings    just search mach3 backlash