First operation issues - SOLVED + 8mm ACME calculation

Hello fellow X-Carvers,

First time connecting machine - had some issues with Easel but managed somehow to do the machine setup.

Machine moves OK - all directions, Homing and soft limit for now - all OK . My machine is 1000mm on X axis and 500 on Y.

Moving the X and Y axis - all good - however Z axis behaves strangely - seems to do one inch when I tried to move it 1 mm. . . Went to UGS - all good there too - however Z axis is the same.

Tried to calibrate the Z axis as advised by forum members - and the best I could do is about 50 steps/mm which is way off the X-carve setting 188.976. . . ???

My machine is not original X-carve - but made from all the parts on the list. My Z microstepping jumper on the Gshield was unchanged (2) - I have the 8 mm ACME rod with GT2 20mm pulleys on motor and ACME rod. My delrin bits are OK with ACME rod.

Did not change the pots on the Gshield from what it came as default - I am happy with it the way it is.

Attaching my Grbl file - perhaps someone can see what I am doing wrong.

Many thanks

Isaac

I would suggest tweaking the Z pot up a bit, at least that’s how I solved this problem when I built my machine.
-Kelly

Can you double check the Z axis’s microstep jumper?
Can you post a picture of the threaded rod you are using?
And hopefully a part number / location of where you got the rod?

Thanks Guys,

Jumper looks good to me.

ACME rod - got it from China:

Tried to rev up the Z axis pot - no change in steps but more noise - so put it back where it was.

Honestly - I don’t mind it to stay this way since I get very good results with the micrometer when moving 1.0 mm - just wondered why it has such a big gap between my settings and X-Carve ones.
(50 step/mm Vs 188.976)

I guess it has something to do with the ACME rod pitch ???

Thanks again.

Isaac

EDIT: - Checked my pulleys - they are 20 teeth 2 mm pitch (GT2) . Also checked my rod pitch by measuring the Inventables rod drawings - it looks pretty much the same pitch - no idea ???

Thanks guys for jumping in.

@AngusMcleod - the ACME rod on the instruction pages call for M8 rod - the thinner start on this is only for the nut on top. As I understand this - the pulley is sitting on the rod itself since the spec calls for an 8 mm bore in the pulley.

12 turns per inch is about 2.11 mm - even this will not justify such a big gap. If the motors are the same, and so does the pulleys - and the pitch is close to 2 mm (12 turns per inch) - so it should be close to 40-50 steps/mm - where did the 188.976 com from ???)

Is the step/mm for the ACME rod 188.976 or this is for the M8 rod ?

@RobertCanning - Yes I did it twice and it was the ACME rod option.

Cheers

Page w/ formulae for this: http://www.shapeoko.com/wiki/index.php/Grbl_Configuration

@AngusMcleod - BINGO - you hit the nail on it’s head ! - Thanks.

What we got is an M8 ACME thread rod - so it’s outside diameter is exactly 8mm. We have assumed the rod will be M8 because of the required bore - our mistake. But on the other hand we did not need to mill the rod down which is a pain.

Now it all makes sense.

@WillAdams - Thanks for the formulas - I will have to check my rod and try to figure out the exact numbers. The last calculation there calls for 47.244 which is pretty close to what I had set it now = 50 steps/mm .

All in all - thanks for your help guys - this forum is the best !!!

Cheers

Isaac

EDIT - for those who have got 8mm ACME rods - my calculations are:

Motor 200 steps/rev
Driver 2 microsteps/step
Screw pitch 12.7 threads/inch
Screw lead 4 threads/rev
Conversion 25.4 mm/inch
2 microstep/step * 200 step/rev * 1/4 rev/thread * 12.7 thread/inch * 1/25.4 inch/mm = 50.000 microstep/mm.

Glad you got it figured out.
That is the same rod I am using.
My Z axis is set to 49.7 steps / mm.

@JeremyJohnstone - Thanks mate - I figured out it really does 4 threads per revolution - so I found out by rotating the rod it is doing almost exactly 13 threads in 26 mm. Calculated for 25.4, that’s where I got the 12.7 threads per inch.

Perhaps there is a better way to calculate this - I could not find the correct TPI for this rod - so I did it this way.

I will put your one too when I will commence carving - will check it out. BTW - How did you come up with 49.7 ?

Best.

Isaac

Measured it with a dial indicator, and re-calibrated the steps per mm.
50 may be exactly correct. I’m not sure what percentage a 0.3 step per mm difference is.
I imagine it’s quite small, and I probably can measure better than that!

Actually its easy to calculate…
At 10mm its only a 0.06mm difference.
A width of paper…
I will try 50 someday and see if I notice anything.

OK - I think I have it now:

http://openbuildspartstore.com/8mm-metric-acme-lead-screw/

If pitch is 2 mm and 4 mm per revolution - then lead would be 8 mm per rev

If we put the data in the Prusa steps per mm calculator:

You are right of course - it is not a big gap anyway. I also tested my 50.00 with a dial guage few times - it was almost on the spot every time. So I guess we are good to go with either.

Many thanks.

Isaac