Anti-backlash nuts

Help for hardware problems

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daedelus
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Anti-backlash nuts

Post by daedelus »

A while ago, someone implied to me that anti-backlash nuts on the Z axis in particular are a necessity for fine grained probing and PCB work. I do have anti-backlash nuts on all axis of my own machine but I have just received an email asking something similar. He is getting a lot of play on his Z axis and wondered if anti-backlash could help here.

Since I am no expert on CNC hardware myself, I thought I would open it up here. So the question is:
Are anti-backlash nuts an absolute necessity for fine pitch probeing / autolevelling?
http://www.autoleveller.co.uk/. Software to probe and adjust a GCode file for PCB's or any probe-able surface.

http://www.autoleveller.co.uk/cnc-probe-guide/. A short guide to setting up the probe.

-James
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Re: Anti-backlash nuts

Post by Country_Bubba »

It would be my "opinion", Maybe! I am no expert either, but logic would tell me:

IF you have a fairly heavy moving assembly on a very low friction set of slides and your not doing real rapid accelerations, then maybe it would work with non backlash free nuts. After all, gravity would keep the z slide in contact with the "lower" edge of the nut thread on the way down and of course going up it would also contact the same side of the thread.

A lot also depends on how high above the board you are when you start the probe cycle. If you have a "short" distance to go and rapid accelerations, the slide "may be" floating just above the thread and of course the controller will not know exactly where it is then if it happens to make contact during this time. You also run into the same problem if you have a delay between contact and the time when the controller recognizes the contact point which again would lift the z off the thread a small bit. This is why slow probe speeds should result in more accurate results. The delay in timing could be caused by either a slow recognition speed due to control electronics or possibly "breaking" through some form of resistance layer of the probe contacting the board or if someone is relying on electrical contact through some bearing etc.

Just my random thoughts. :idea:
Art
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Re: Anti-backlash nuts

Post by daedelus »

Thanks Art, sounds logical to me too.

I wonder if anyone has done any experiments on the "recognition" speed of various controllers. It seems to me that the more "smoothing" capacitors between the probe bit and the electronic end point, the bigger the delay will be. I wonder if this delay varies much between controllers.
http://www.autoleveller.co.uk/. Software to probe and adjust a GCode file for PCB's or any probe-able surface.

http://www.autoleveller.co.uk/cnc-probe-guide/. A short guide to setting up the probe.

-James
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Country_Bubba
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Re: Anti-backlash nuts

Post by Country_Bubba »

There are several factors that come into play here. You have indicated one, the caps that some use to actually introduce a delay to minimize bouncing. Some breakout boards use horribly slow rise time opto isolators on the input lines and again, some even put optos on the lines to the parallel port! Each one of these delays can add up. Again different controllers may by design place a lower priority on the input lines which could also be an additive problem.

So yes, speed will also matter.

Again, just my thoughts.

PS: I also just remembered that some don't use a clip on the "probe" and rely on continuity through the spindle. This may have an additional lag time due to the bearings having an oil film and until this is broken, there is no contact. (I have observed this on another project that I am working on. The probe would sit there in intermittent states of contact and no contact through the bearing system in that setup.
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Re: Anti-backlash nuts

Post by booski »

Just my little bit of info, the anti-backlash nuts that come with my zen toolworks style machine are brilliant.

Only issue with them is that they used mild steel nuts on a stainless steel lead screw.

On my old machine I used HDPE which has a much lower coefficient of friction than mild steel and stronger springs allowing for less slippage with a sudden change of direction and greater cutting force before the springs react.
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zebvoo
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Re: Anti-backlash nuts

Post by zebvoo »

Thanks Art, sounds logical to me too.

I wonder if anyone has done any experiments on the "recognition" speed of various controllers. It seems to me that the more "smoothing" capacitors between the probe bit and the electronic end point, the bigger the delay will be. I wonder if this delay varies much between controllers.
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