Hi All
Hope somebody can help here. Have finally got my lathe working but accuracy is a real problem.
This lathe was set up by a UK company who had never done one before and are pretty much no help at all. So its down to me.

Lathe is an Optimum 320X930
According to the company who supplied it ,its 5mm thread ball screws on X & Z.
Z has 40 tooth pulley on the screw and 20 tooth on the motor, Stepper on Z is 8.2Nm 1:8 degree. Stepper on X is 1:8 degree 4.2Nm, they said that X  is directly connected, I can see a gear box but the motor is offset from the screw so I don't think it is direct, unless this setup is considered direct, I don't know.
Z has a belt between the screw pulley and the pulley on the motor shaft.

The engineer is Polish and speaks virtually no English so this is not easy at all. He also hasn't got a clue about inches and is a mm guy. I work in inches.
He worked out some step figures and we put them into the software, I'm currently measuring and changing numbers and getting nowhere fast.
I have a dial indicator on x and z, x is giving me a real problem as if I set the steps per, so it moves .1'' its perfect, however if I then move it .4'' its .002'' over.
I'm not sure Z is right either.
 Z .600'' cuts .606'' and X .500'' diameter cuts .484'' so it no way correct.
I have backlash turned off, mechanically everything seems solid though and the build is good.
Hoping somebody can point me in the right direction.
Thanks

Sent a reply to your PM a week or so ago asking for details of your machine, what issues you were having (as you never specified)  and to send your xml to my email address. You have provided the majority of the info I require here but would still like to see your xml so that I am not working blind.

Gearbox may not be good and introduce backlash, depends on the quality and type of gearbox.
By direct they could simply be meaning there is no reduction, ie  1:1
What make of drives do you have an what microstepping do you have them set to (if they are adjustable)

Hood

Hi Hood

we worked a lot of it out and Brett helped also.

Ill send you the Lathe xml file via email.

The motors came from Optimum in Germany:

http://www.optimum-machines.com/products/cnc-pc-controls-complete-and-adapter-kit/complete-and-cnc-add-on-kit-for-turning/index.html

But I don't know the make. On the back the Z says Step Motor Bipolar, Step angle 1,8 Degrees 8,2Nm 6A Optimum Germany

The X is the same but with 4,2Nm

They don't seem to be adjustable. But i don't have a clue what to look for.

I currently have the following numbers in Motor Tuning:

Steps per X:  2580, Vel 15, Accsel 5.6983
Steps per Z: 4071, Vel 34.998, Accel 24.9343

Ok info you sent isnt much help as all I have been able to find is a German website for the stepper drives and there doesnt seem to be info.
However from the info provided above it would kind of seem like your stepper drives are half stepping.
If the info is correct then your steps per unit for Z would likely be 4064 and X 2032, so if you give them a try and let me know if its more accurate.
Hood

Thanks for that.

I ran a cutting test and Z seems to be on the money, programmed .500'' on Z and got .500''. Not sure about X, programmed .500'' DIA and got .571''DIA. I did a G54 X0 test and it didn't look to center before the cut. I touched it to .888'' DIA and used that figure in the Part Zero DRO for X as normal.

Any ideas?

I am not really familiar with the standard lathe screenset as I dont use it but if you can do the following, move the X axis so you will be taking a light cut and jog along and take the cut. Jog back and then measure and then enter that into the DRO. Now try a few more cuts and see if the DRO corresponds to the  dia being cut.
There may be some reduction in that gearbox.
Hood

Hi Hood

Been on the phone with Optimum in Germany and they say that the X ball screws are 4mm, not 5mm as I was originally told. There looks to be a tiny line on the crest of each screw and they measure 4mm between those lines. Z ball screws measures 5mm between those lines. I hope this is  the error  as have been test cutting this morning and X is out by a mile.

Had it confirmed that they are 200 step motors but don't know a make. Gearing on X is 1:1 for sure.

Would you be able to generate a magic number with the new info?

Ok motor is 200 steps per rev then you have 1/2 stepping drives so thats 400 steps to turn the motor 1 rev. 1 rev is 4mm but you want to set your native units as imperial so that is 25.4/4 = 6.35 turns to move one inch. So 400 x 6.35 = 2540steps per inch.

Hood

Much better thanks Hood but still a bit out.

I programmed .4989'' and got .4850'', so .0139'' (.353mm)  out on the diameter.

Is there anything I can do to get this more accurate. Could it be backlash that causes the inaccuracy?

Z seems to hold about .001''

Thanks very much for your time on this btw.

Could possibly be backlash, it would be quite a lot for a new machine but if there is indeed a gearbox involved then its possible.
You could easily test for it, what to do is set up a clock (Dial Indicator)  on the bed and put the point against the cross slide, put Mach into step mode of maybe 0.001" and jog onto the point a bit then zero the X DRO and then jog back in the 0.001 steps and as soon as the needle moves stop and look at the DRO in Mach and see how much it reads, that is your backlash, or twice your backlash for Dia mode.

Backlash could be in many places, could be screw, could be gearbox, could be bearings of screw or flex in mounting brackets or even gibs needing adjusted.
Hood