The dreaded..........."Backlash"

[pjl83]

Hi,

Maybe a FAQ but I can't find an answer. There are dozens of mentions of backlash and anti-backlash options but I can't find an explanation of what "Backlash" actually is with regards to a cnc machine.

Is there a general explanation so I can understand the potential problem? Then I might be able to understand the importance of it and the solutions that I read about will make more sense :)

TIA
Paul

[Robin Hewitt]

Backlash is how far you can turn the handle when you shift direction before the slide moves.

Backlash happens in the screw mounts and in the nut.

The lower the backlash in a nut the more expensive it becomes.

Zero backlash is achieved by springing two nuts together, you get zero backlash up to the preload.

You can compensate for backlash in the software but only if your slides are tight, if the tool pressure can move the slide...

[pjl83]

Thanks for that. By "springing" 2 nuts together, is that as obvious as it sounds? I've got visions of 2 nuts on the threaded bar in opposite directions and a spring between the 2 pushing them away from each other. Or is "springing" a technical term for something completely different?

TIA
Paul

[Robin Hewitt]

Thanks for that. By "springing" 2 nuts together, is that as obvious as it sounds?

Hi Paul

Put two ball nuts close together on the same screw, arrange it so neither can turn but one can slide. Add springs to push them together or apart.

I use Belleville washers and preload them to about a quarter of a ton, so I get zero backlash up to that point.

Obviously this won't work with ordinary nuts, they'd just seize, has to be a rolling contact.

I also spring the thrust races holding the ball screws in place. It's complicated to do but solid as a rock :smile:

Robin

[pjl83]

Thanks.

So let's just say that I was going to use threaded bar :redface: I know it's not ideal and it won't be as accurate as lead screws but it's the way I'm going for mk1. Would putting 2 lock nuts on the bar with the 2 flat ends facing each other, backed with washers and a strong spring in between the 2 actually help at all? Obviously the nuts will be captive in the bracket of the sliding part and the ends will be held with bearings, but would it be worth taking the time to fashion a bracket where I could spring in between the 2 nuts?

I'm not looking for a highly accurate machine. If I can stay within a mm over the length (A4 size) then I'll be happy. I'm using mk1 as an exercise in getting the thing to move and working well with the PC.

I've got my tin hat on ready so fire at will! :naughty:

[BillTodd]

So let's just say that I was going to use threaded bar :redface: I know it's not ideal and it won't be as accurate as lead screws but it's the way I'm going for mk1.

That's unlikely to work with steel nuts, the friction will be too high, unless the spring is very weak and, since spring force should be much greater that your cutting force, that will limit the machines ability somewhat ;)

For a simple cheap job use a stainless steel rolled treaded rod with a tight delrin (or PTFE) nut (I think Irving has a description of this somewhere on this forum).

Bill

[irving2008]

You can use two delrin nuts pushed apart by springs on a carbon steel or Stainless trapezoidal screw to get very low backlash - at least as long as the nut doesnt wear too much (which it well eventually, probably within 6-9mo if used regualrly). The friction between delrin and steel is low enough for it to work with a reasonably strong spring, but only for small scale, clearly its not going to work for machining steel or heavy cuts in alloy. Using threaded rod instead of a proper trapezoidal screw is a false economy, unless its SS the surface finsh of a zinc plated threaded rod is very rough and high friction and no anti-backlash nut is going to work well. You are better off to buy trapezoidal, and an extra 150mm to make a tap for your delrin nut(s).

[pjl83]

I've only ever considered stainless rod. I'll have to have a play with it and see what I think. I've seen lots of home build's around the net that use threaded bar with good enough results for what I'll be looking for. Although I agree that there is a little bit of false economy in going this way, but if the funds aren't there then I'd rather get it going on threaded bar and see how we go than waiting until I can afford the better options.

The construction will be easy to disassemble and modify in the future.

Thanks again
Paul

[Robin Hewitt]

Sounds like a good idea, get something cutting then worry about backlash.
It may turn out to be the least of your problems :beer:

[der_fisherman]

What I have done is to use "long nuts" (I don't know the right name sorry) with a cut/split through one side between the two holes....also I found that threaded rod is not very accurate, so I recut all the lengths of threaded rod with a standard DIN Die nut. This got me a thread that was the same along its whole length....

The "split" nut was then slightly squashed in a vice, till it was a firm fit on the rod. I then used metal polish and ran the nut up and down about 10 times with an electric drill, to "lap" the two together. Clean well and lubricate with a light grease or oil, best quality.......I use two hose clamps to keep the gap where it is....

This is not as good as proper threads for CNC of course, but gives a surprisingly good system that can make pcbs quite accurately......I measure about 0.02 to 0.04mm back lash....or about 0.00158" Max.

I would use a good silicone oil if I could find one, but its not as easy as I though....I have not repolished yet, but eventually I expect I will need to do that.....I have used the vice once to close up the split again....

Its a cheap way, costs very little, but proper screws and nuts are always better....I also use skateboard bearings for the radial loads and axial bearings for the axial loads.

This site gave me my inspiration:-

http://buildyourcnc.com/default.aspx

regards
Andy