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newbie - help with solving backlash issues
« on: April 29, 2012, 05:19:24 AM »
Hi there

It seems I have some backlash issues with the small mill I just bought. Basically when trying to mill a PCB I get very poor results - track lines are not "closed", drill holes are misplaced etc. A member of the CNC forum has suggested it was backlash.
How do I solve this problem?
The manual I got with the mill is quite basic - see attached pdfs.
I followed the manual for the X axis. The "auxilliary nut" does not turn more than 30 degrees (the manual says "half turn"). When I put everything back together, it is markedly more difficult to manually turn the X screw.
When tackling backlash, is more resistance better? Or shall I look for the least resistance?

Any ideas of how I can solve this problem?

many thanks for your help

Thomas

Offline Hood

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Re: newbie - help with solving backlash issues
« Reply #1 on: April 29, 2012, 05:43:35 AM »
Zero backlash will have more resistance. Just downloaded the manuals and will have a look but maybe if there is too much resistance for your motors when you eliminate backlash you will have to slacken back a bit and just use backlash comp in Mach.
Hood

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Re: newbie - help with solving backlash issues
« Reply #2 on: April 29, 2012, 05:50:06 AM »
Ok looked at the manual and its leadscrews with two nuts, not ideal. Eliminating backlash on them will increase the friction and depending on your motors it may not be ideal. If the motors dont have enough torque then they might start stalling causing more problems. Trial and error will be the only way to see if your motors can handle the preload on the leadscrews.

Hood

Offline RICH

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Re: newbie - help with solving backlash issues
« Reply #3 on: April 29, 2012, 09:27:00 AM »
You will have backlash no matter what you do. Yes minimize it with the adjustment but then try moving the axis along it's full
travel to check for binding. Don't know the quality of the screw thread you have. Trial and error as Hood said.
Also check the ends of the screw and as there should be some type of bearing and that should also be preloaded since you can get addtional backlash form it. Check the backlash periodicaly to see if has changed some as time goes by.

I will point out that without some type of torque measurement you don't know how much of the motor torque is being applied to just make the axis move. As an example, say 200 in oz motor, then with a tight adjustment the required torque to move the axis goes from 20 to 60 in oz, due to microstepping the actual holding torque is reduced to 100 in oz. In the end you have used up 160 in oz and that only leaves 40 in oz as some head room.  Poor example but does put things into perspective.

RICH
Re: newbie - help with solving backlash issues
« Reply #4 on: April 29, 2012, 10:43:39 AM »
Thanks Rich,

Will try... what is "binding"?

many thanks

Thomas
Re: newbie - help with solving backlash issues
« Reply #5 on: April 29, 2012, 11:33:42 AM »
Now that I have started trying to adjust the backlash on the Y axis... I can't manage to get it right at all. Everything I do lead to the motor stalling... £$%%£
I get the nut to move easily and smoothly on the screw, then when I connect the nut to the Z-block, resistance increases a lot and stalling appears. The tighter the screws connecting the nut to the Z-block, the easier is the stalling...

any ideas?

many thanks

Thomas
Re: newbie - help with solving backlash issues
« Reply #6 on: April 29, 2012, 02:54:13 PM »
Okay - it looks like I have some binding (I am assuming it is some suddent increase in friction between the nut and the spindle - right) on some segments of the spindle... how could I fix this?

getting crazy here - just got from a mill that was not accurate to something that does not move at all!

cheers

Thomas

Offline RICH

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Re: newbie - help with solving backlash issues
« Reply #7 on: April 29, 2012, 04:50:58 PM »
By binding I was meaning that the screw may not be the same all along the length such that when the nut has very little play it will get harder to turn in a few spots.

With the stepper motor removed you should be able to turn the screw rather easily. Using your thumb and middle finger
to turn a nut  you would need to exert a lot on a big nut  to turn 100 in oz ( very rough example to relate in terms of in oz ).
With the screw free to turn you would adjust the nut and then turn the screw by hand and if you find a place where it gets
hard to turn to move the axis you need to see what is causing it. Sorry but kind of like telling someone how to tie their shoe laces..... :D

RICH

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Re: newbie - help with solving backlash issues
« Reply #8 on: April 29, 2012, 06:16:47 PM »
Hmm..... looked the third pdf you posted
Move the Z all the way up or down or close to the end of the travel and adjust the block. I assume that the screw is a little off the center travel and with a tight nut adjustment that is what is causing the binding. You'll just have to play with it.

RICH
Re: newbie - help with solving backlash issues
« Reply #9 on: April 30, 2012, 03:58:16 PM »
Rich -

You were right, I got it working now! THANK YOU!

I need to try to get the Y axis working better now, it is very unprecise. I made a Square on a PCB - it is more like 63 vs 62 mm. Also I milled a PCB with a double path, and the paths don't overlap well - the deltaY is really perceptible. Will try to have a look tomorrow...

Thanks for our help!

Thomas