# Cncfusion Lms Upgrade Kit #5



## grepper (Mar 7, 2016)

I sat down to order the CNCFusion ballscrew upgrade kit, but have some questions.  I wish the CNCFusion site was more pre-sale informative.

They offer a pre-loaded ballnut service for $20 per ballnut.  CNCFusion says,

_The stock Nook ballscrews can have anywhere from .004-.014 backlash, it really just depends on the batch of ballnuts what you get._

_The nuts will come from the factory, loaded with .125" balls. All kits come with the ballnuts already installed onto the screws, ready to go. _

_When you order pre-loaded ballnuts, we remove the stock .125" balls, and load the nuts with the largest balls that will fit without binding. This is called pre-loading the nut, so that it fits tighter to the screw, eliminating pretty much all backlash. Ordering pre-loaded ballnuts ensures that you will receive ballscrews with .001 or less backlash._

Does, or how does Mach3 compensate for screw backlash?  If Mach is just counting screw rotations, it would seem that backlash could be very problematic.  Think I need to pony up and get the pre-loaded nuts?  I don't understand the ramifications or exactly what to order. 

They also offer Heli-Cal zero backlash couplings.  Is that worth it?

Thanks!


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## JimDawson (Mar 7, 2016)

IMHO, spend the $20.  Any backlash in the system will cause accuracy problems.  Mach3 can compensate for backlash in some instances but can not compensate when cutting a circle or an arc, and in many cases a straight cut.  It will compensate if doing a drill hole pattern.

As an example, I have a couple thousandths backlash in my Y axis (I need to fix that).   By it's nature, my system automatically compensates for backlash.  But when cutting a circle I can count on it to be out of round by about 0.001, in most cases this does not cause problems, but when trying to make a bearing pocket or something like that, I wind up cutting undersize and boring to size.



grepper said:


> I think I read somewhere, but now I can't find it, that it comes with the Z-axis pre-loaded, so if it's necessary I would only need to order two. Am I correct in this?



If that is true, then yes.  I looked at their website and could not find any information on that.  I think an email or a call to them might be in order.

I would go with the Heli-Cal couplings, the Lovejoy couplings are not zero lash.

I'm here to help you spend all of your money


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## Bob Korves (Mar 7, 2016)

Backlash also causes problems with rigidity of the machine, making life much more difficult for cutting tools.


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## grepper (Mar 7, 2016)

JimDawson said:


> I'm here to help you spend all of your money



Well Jim..., Take pride in the fact that you are succeeding!  Your wonderful "help" just added another $150 to my already over budget little project.


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## compsurge (Mar 17, 2016)

Did you buy Mach 3 already? If not, can you use LinuxCNC? That will save you some. I also find it much easier to use.


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## cozmogeek (Mar 18, 2016)

You measure the backlash then configure it in your software. It then knows any time it reverses directions on whatever axis it needs to move .001" further (or whatever the backlash is) to take up the slop in the screw. It is kinda weird but it seems to work. I am liking Linuxcnc as well.


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## Firebrick43 (Mar 19, 2016)

Most cheaper systems act just as Cosmo said.  It's not the machine compensating it's the software using values you provide.  More expensive systems use glass or magnetic scales and reference it absolute motion measurement to the commanded motion measured by the encoder.  It will self compensate up to a certain value and then alarm out if they are exceeded. 

The next step combines the two.  Big machines can actually compensate in different areas of the screw/travel to compensate for the inevitable  wear, imperfectly ground screws, sag/flex of long tooling and settling of the foundation.


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## grepper (Mar 19, 2016)

cozmogeek said:


> You measure the backlash then configure it in your software. It then knows any time it reverses directions on whatever axis it needs to move .001" further (or whatever the backlash is) to take up the slop in the screw. It is kinda weird but it seems to work.



Wow.  I'm surprised that works.    I would think that if the measured backlash was off by even a little, cumulative error over many operations could be significant.


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## JimDawson (Mar 19, 2016)

grepper said:


> Wow.  I'm surprised that works.    I would think that if the measured backlash was off by even a little, cumulative error over many operations could be significant.



I would think so also!


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## grepper (Mar 19, 2016)

grepper said:


> Wow. I'm surprised that works.  I would think that if the measured backlash was off by even a little, cumulative error over many operations could be significant.





JimDawson said:


> I would think so also!



Well, then, it begs the question: How well does this method pan out?  How do folks deal with cumulative error?  I would even think that backlash could vary over the length of the screw due to variation in the accuracy of the screw.  How does that pan out?

What does all this mean?


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## JimDawson (Mar 19, 2016)

Consider that you are probably not going to be doing work for NASA, so a small amount of error is not likely to significantly affect the end product.  The place where this does become a problem is in a bearing pocket or something like that.  Even with my mill that has 1 micron magnetic scales on the table and is thus self correcting, I normally pocket bearing bores undersize and bore to the final dimension.  I have to do this because I have a small amount of backlash in my Y axis, about 0.002, so I can count on a pocket to be about 0.001 out of round.

It is possible in Mach3 to ''map'' the leadscrews to correct for errors over the length, but I'm not convinced that this works very well.

The most important thing in my estimation is:  Can the machine return to 0,0,0 consistently after an operation?  If you lose your ''0'' position then you have no idea where you are at.
.
.


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## RJSakowski (Mar 19, 2016)

grepper said:


> Wow.  I'm surprised that works.    I would think that if the measured backlash was off by even a little, cumulative error over many operations could be significant.


It only compensates once at the point of each reversal in direction.  When the direction changes again, the compensation is applied in the reverse direction so it the two comps cancel.  The cumulative error will be the difference between measured and actual backlash at worst .

There is a problem with backlash compensation if the backlash is excessive though.  When a coordinated move is taking place as when cutting on a diagonal, the compensation creates a hiccup in the movement.  For this reason, it is recommended that as much backlash as possible is removed by mechanical means rather than software.


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## RonGinger (Mar 23, 2016)

Backlash is one thing I always harp on. Yes, Mach3 will turn the screw so that the backlash is accounted for. No, it does not really solve the problem. While the adjustment is happening the table  is unconstrained by the nut in either direction- it is free to move. If you are in luck all that will happen is it leaves a divot in  your part, if not lucky it grabs the work and digs in and breaks the tool. Backlash compensation is something every software guy adds, but it is less than useless in my view.

You must do everything you can to eliminate all mechanical sources of backlash. Loading the ball nuts with oversize balls can help, but I think it is more tricky to accomplish that you might think. The more common method is to place two ballnuts back to back with some adjustment between them so that the lash can be adjusted out. I did that on my Jet mill conversion and it works pretty well, but the Y nut is in a place that makes it very hard to reach and it keeps coming loose.

I used the CNCfusion kits in the 3 build class I ran at CNC workshops. They are good quality and work well.  I am now designing a CNC lathe conversion for the next CNC workshop and have found a Chinese source of some unbelievable ballscrews. I got two sets of samples and we have built the first two machines and they are proving to be amazing, and surprisingly inexpensive. I found them on Amazon, with prime shipping, from a vendor called TenHigh.


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