View Full Version : Motor Backlash Question...
I have 3 PK296A1A-SG3.6 motors. I plug them in unattached to the SB. With them powered up, and no pinions, I can feel a very slight movement back and forth on the shaft with when I turn the shaft back and forth with my fingers. I can actually see this very slight movement (some dirt on the shaft is visibly moving). This also makes a faint clicking noise.
2 of my motors have this slight backlash, while the third has no movement that I can feel, hear or see.
Are those 2 motors worn out or is that ever so slight backlash normal?
Do new motors have it?
Does the SB software compensate for that type of backlash?
Thanks again!
R
danhamm
12-22-2012, 10:58 PM
They have backlash right from Oriental. thats normal...
They have backlash right from Oriental. thats normal...
OK I thought as much. I read somewhere there are different gear setups that have a spring that eliminates this backlash.
Anyone familiar?
Is the Circle Resolution setting in Values the backlash setting? By default it is at .05 which I think is close to what the standard movement is.
I was reading on another site that Mach control software has a backlash setting and one guy said he measured his and Mach compensated for it and his cuts became perfect. Please let SB have this setting!
danhamm
12-23-2012, 12:19 AM
If you have a dial indicator once you have the gears installed, I would measure the movement with the dial indicator on each axis, the difference I found was as much as 14 thou. from 30 tooth gears on the 7.2 ratio motors.
Also I dont remember where the settings are or if there are any on shopbots software.
if your setting up your pr machine just live with a little slop...learn what it is and use it to your advantage...:)
if your setting up your pr machine just live with a little slop...learn what it is and use it to your advantage...:)
Gene I am setting up my pr. I have the 4g controller, I bought a 12" retro Z, I have a new 3.6 arriving soon for it, some new 25 tooth pinions, a bunch of e chain...
How much slop should I expect? Is your pr using 3.6 with 25 tooth pinions?
If you have a dial indicator once you have the gears installed, I would measure the movement with the dial indicator on each axis, the difference I found was as much as 14 thou. from 30 tooth gears on the 7.2 ratio motors.
Also I dont remember where the settings are or if there are any on shopbots software.
I don't have a dial indicator... How much are they and where can I get one?
danhamm
12-23-2012, 02:56 AM
http://www.amazon.com/Dial-Indicator-Test-Magnetic-Point/dp/B002YPDQHI
Does anyone know if SB software has a way to compensate for motor backlash, and if so is it available to set for each individual motor?
garyc
12-23-2012, 08:21 AM
Richard...
The attached doc may give you some ideas to measure and find how much backlash there is. It was originally written to determine the amount and help adjust pinion/rack lash.
Make sure that new pinions are installed and adjusted properly so that you will only be reading lash from the motor's gear train.
Thanks Gary.
Do you know if the SB control software has backlash compensation?
garyc
12-23-2012, 01:40 PM
Richard....
I do know that it doesnt.
I also think that in most cases with good equipment it is not needed. I am also not a big fan of trying to fix hardware problems with software. Mechanical items wear and those who need to keep them running at their best will maintain, adjust or replace as needed.
Richard....
I do know that it doesnt.
I also think that in most cases with good equipment it is not needed. I am also not a big fan of trying to fix hardware problems with software. Mechanical items wear and those who need to keep them running at their best will maintain, adjust or replace as needed.
The motors have backlash when they are new. No amount of maintanence will fix that...and as the motors wear it will get worse. Without software compensation the resolution of running Shopbot VS Mach with the measured backlash compensation applied, on the same machine, the Mach will be a better machine.
I am now very disappointed. The resolution of the Shopbot is only ever as tight as the combined backlash in the 4 steppers, even if there is zero backlash in all other components.
When trying to do fine lettering or inlays, accurate parts, small circles etc. without backlash compensation it will be problematic.
How do we get this feature added without switching to Mach?
OK so here is a video showing a machine before and after backlash compensation, and an explanation how it is achieving the results:
http://www.youtube.com/watch?v=A27nOkTFYDY
The other thing is when I move the shaft and see a small amount of backlash, this backlash amount to the pinion teeth will be increased because the larger the pinion the greater the backlash. The further out from the center of the shaft the more the backlash is increased. As the video shows the transition from X to Y will combine the backlash of both axis to give the greatest amount of backlash slop.
garyc
12-23-2012, 04:11 PM
Richard...
This has been, since the beginning, one of the problems with keeping "affordable CNC" affordable. You can replace your gearmotors with planetary style gearboxes or belt reduction, both have little or no measureable backlash. You could also upgrade to the tapered hob gearmotors, which should have less lash than the straight gear version that you list above.
You may have to upgrade your controller also to take advantage of their features. (newer style motors) There are literally thousands of users that have found a way to employ toolpath stategies to cut millions of parts using all versions of the ShopBot drive combinations, which have been continuously upgraded. Adding a few thousand dollars to the price may have kept hundreds of them out of the CNC world.
My replies to your 4 comments in order:
The SG motors have lash when new. So do the TH versions, albeit much less. Have you tried your cutting on a current late model PRSalpha? The compensation in MACH is a nice feature, but I would be hard pressed to see how it may help a worn tool. Especially when one of the 2 X motors is worn. Especially when one is worn and the other not. There are a number of ShopBots that are running on MACH and instructional threads where changeover steps are shown. This may be a viable option for you if the backlash compensation is required for your setup.
I am sorry that you are dissappointed, but this is the way it has always been, and is not specific to SHopBot machines. And I also note that I would not expect the combined lash of the mechanical parts to be zero either. These also vary with ShopBot model, decreasing from older models to the newer.
Without trying to be repetitive, I know many that are accomplishing just that, possibly with later model machines than yourself.
That would need to be proposed to the SB software developers. It would also have to be agreed by them as neccessary and then implemented and tested prior to release. At best, a number of years down the road.
bob_dodd
12-23-2012, 04:44 PM
Richard , I use PK296A1A-SG3.6 on all axis , when I do small parts , I attach a bungie cord to the X axis to the machine to apply a preload , to hold the axis in 1 direction , this removes the backlash . I do the same thing to the Y axis . Make sure bungie is long enough for the travel you are doing .
The SG motors have lash when new. So do the TH versions, albeit much less. Have you tried your cutting on a current late model PRSalpha? The compensation in MACH is a nice feature, but I would be hard pressed to see how it may help a worn tool. Especially when one of the 2 X motors is worn. Especially when one is worn and the other not. There are a number of ShopBots that are running on MACH and instructional threads where changeover steps are shown. This may be a viable option for you if the backlash compensation is required for your setup.
I am sorry that you are dissappointed, but this is the way it has always been, and is not specific to SHopBot machines. And I also note that I would not expect the combined lash of the mechanical parts to be zero either. These also vary with ShopBot model, decreasing from older models to the newer.
Hi Gary,
I have never used other machines...I have seen some at trade shows over the years though...
At this point probably switching to Mach is not my 1st priority. Getting the best possible cuts I can get is though.
Maybe someone with skills could figure out what is going on with the Mach backlash compensation and it could be added at the top of each file to take up the slack like in the video.
Thanks for the bungee tip Bob. I will definitely give it a try.
Gary as far as the compensation for X, there would simply be a spot for X1 and X2 motor compensation.
Apparently too much backlash compensation will cause the steppers to gain steps. Would need to carefully measure...
Steve M
12-24-2012, 09:00 AM
I've been using Mach3 to run my ShopBot since new and have never used the backlash comp except for initial experimenting. I'm not expecting aerospace tolerances from a ShopBot, but it's perfect for the type of parts that I cut.
There are cutting strategies that result in very good accuracy.
If you want sub .001" tolerances on small parts, a Taig micro CNC mill is very affordable. I have 2.
I've been using Mach3 to run my ShopBot since new and have never used the backlash comp except for initial experimenting. I'm not expecting aerospace tolerances from a ShopBot, but it's perfect for the type of parts that I cut.
There are cutting strategies that result in very good accuracy.
If you want sub .001" tolerances on small parts, a Taig micro CNC mill is very affordable. I have 2.
No I just want motor run-out to be compensated in the software. I would like to be able to cut near perfect circles and small lettering.
If you watch that video I posted you will now be able to use the backlash compensation successfully and improve your cutting. I'm jealous.
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