backlash compensation only way to fix this? (Resolved)

[AdamCNC]

I have the strangest thing going on with my lathe.
I dont want to call this backlash, but its almost like a "springback" on my x axis towards the front of x+ of travel.

My x axis travel is all ok in terms of backlash except right at the front of the x+ where I can turn the ballscrew without moving the lathes cross slide, instead its almost like backlash and then it "springs back".

I have uploaded a video to try and demonstrate. I marked a pencil line at the point in x+ travel this begins on the left side. That pencil mark to the limit of x+ I get this problem.

Its seems to feel like the gibs are too tight, but they are not, because its only this small portion of the travel on the cross slide.

I have no idea how to fix it at this point, its 100% mechanical. I have adjusted everything I can think of. My ball screw is mounted with a bk12 mount from automation4less, with A/C bearings installed back to back.

The problem only occurs in a small portion of travel, and only in the x+ direction.

I am wondering if someone knows what is going on, and maybe a solution on how it can be fixed. Or, is this something I will have to use the comp tables in CNC12 to address??

[Muzzer]

You've clearly got some springiness in the mechanism between the ballscrew and the cross slide. Time to get the DTI out and see what is moving when that action is occurring. If you have a 4-5mm pitch ballscrew and that movement is of the order of 3-4 degrees of rotation, you are talking in the region of around 50um (2 thousandths in old money). That should be easy to spot.

As there can't be many elements in the mechanism, it seems most likely to be the connection between the ballnut and the cross slide. The drag due to the gib is surely increasing as the cross slide comes towards you, so the effect is more noticeable there. If you can't get the DTI onto the ballnut in situ, you should remove the cross slide and test the rigidity of the ballnut yoke. You may be underestimating the force you are actually applying here.

Also be aware that misalignment of the ballscrew can generate a significant torque. If the ballscrew is supported at the front by a bearing at the (slightly) wrong height, it will have to distort as the ballnut approaches the front of the machine. It's very difficult to measure exactly where the bearing needs to be placed, which is a good reason to provide some adjustment (slop) in the bearing housing. You can then set the final bearing position with the ballnut close to the front bearing and the ballscrew perfectly horizontal before tightening up the bearing retainer and locking the bearing at the correct position. This was something I learnt on an earlier conversion (Bridgeport Z axis) and have taken into account subsequently.

With your gibs loosened off, you should be able to spin the ballscrew easily over the whole range of movement - does it do that? It looks fairly stiff, so check if that's the gibs or the ballscrew assembly to get an idea what is going on here.

[AdamCNC]

Muzzer thanks you for your reply.
Today I spent all day working on this, and I finally took out a bore scope and looked down the saddle pocket?? I dont know if thats what its called.
But the ballscrew flange nut was hitting the bottom of the pocket and pushing the cross slide upwards.
Took the x axis all apart and used a bandfile sander with 36 grit on it to clearance the pocket.
It seems to be functioning now! Ill need to reset my overall steps per rev and test with a DTI tomorrow, but I think clearancing that pocket out was all it needed. It did stump me though! Again, thanks for the response Muzzer.

[cnckeith]

once you get the major mechanical issues fixed cnc12 can correct for about up to .002" with backlash comp as long as it is consistent. backlash comp in the control is a way to tune out small amounts of lash (and not big mechanical issues) read.. https://www.centroidcnc.com/dealersuppo ... ads/37.pdf
or look at martys videos on the subject.

be sure to read viewtopic.php?f=63&t=1801
as your use of the term "overall steps per rev" has got me concerned.

[martyscncgarage]

Steps per revolution is STATIC and that value should match your DRIVE settings.
Overall Turns ratio is what you will adjust, which takes into account ball screw pitch x pulley reduction. Any fine tuning of the axis is done here.

[AdamCNC]

CNCkeith,
Thanks for that info. My lathe is set up very nice now. Its within tenths when I command a 1" move in either x+ or x-, my DTI is a 1 thou, so id have to get a better indicator to get things dialed in any better. So since CNC12 can handle up to 2 thou, I may spend some time running through and setting up backlash comp, IDK yet its looking really good as is.

Marty,
Sorry for the confusion, overall turns ratio is in fact what I meant. Thank you for correction. I have set my new overall turns ratio (using your excel file in fact) and have great results! No more than half a thou off in either x+ or x-.