axis scale calibration

I have found that on the z axis the depth of drill is about .020 less then it should be. I experimented with the scale which was set in the configuration as 19,600 and ended up with 20,000 drilling within .002 of a requested depth of .250. I changed this value in the .ini file under scaling. The question I have is whether this is a common setup requirement for all axis and what is the best way to configure each axis. The z axis is pretty easy but x and y calibration for exact movement seems a bit more tricky to me. Maybe there is an easy way that someone has developed.

yes adjusting the scaling is probably the bestway , their are a number of factors which come into account , type of leadscrew ,backlash , stepper tuning all have a relationship to travel moved etc .

the same procedure can apply to all axis , easy way is in gcode move the axis a specific amount say 1" then measure the movement traveled and divide into your
scaled steps per inch

What I like to do is do is set an axis to 0
Then make a negative move say g1x-.5
Then go to 0
g1x0
Set your dial indicator
Then g1x1
Measure the movement and make your adjustment from there. The negative move should take out any backlash.

After that is sorted out and the scale is correct try again and also
move back to 0
g1x0 and measure again any difference from 0 will be your backlash.

Rick G

Thanks-I will get a dial indicator and try your suggestions. One of unknowns was how to get the exact amount of movement measurements to the thousands. After I had the z axis drilling to the correct depth within a couple of thousands I tried a pocket cut of .125 with pass depths of .0625 and the cut was then .006 to deep. Maybe I'm trying to get too precise, but I think the machine should cut to at .002 accuracy. Am I wrong? I will look at the axis for backlash etc. next.

accuracy depends on a lot of factors here , ballscrews used and the tpi , stepper or servo , encoders or none , belt drive or direct drive , type of stepper driver used etc , amongst others
and thats before you bring backlash into the equation , describe your machine and perhaps we can give you an idea of the levels you can acheive ,

usualy the rule of thumb if you want the machine to have an accuracy to be used of say 0.002 as you suggest , then it would need to be built to an accuracy factor of x10 across all major parts .
including ballscrews motors , drive electronics etc , so a big tall order

Thanks-I will get a dial indicator and try your suggestions.

In the mean time try the same but for longer moves.
If you can move say 10" and measure that any scale error will be 10 times greater and easier to measure than 1".
You might also look at backlash.ngc
As cncbasher pointed out there are a lot of factors, work on getting the scale right then sort at any mechanical problems.

Rick G

The machine I am using is a Fireball V90. The motors are 280 oz in., 1.8 degree/200 steps per rev, 3 amp dual shaft. The x and y axis are 5 tpi with anti backlash nuts. The z axis is 12 tpi no anti backlash. It has a RF isolated breakout board and an 40VDC linear power supply. I am using a Bosch Colt router. I'm not sure what the runout is on this router, but there have been many suggestions about getting precision collets so I may also look into that. I am very happy with the machine overall for wood hobby projects, but for very precise machining it probably wouldn't be suitable.

Strangely enough, I just cut a project out of .250 material with the z scale adjusted as described above and on the cutout of the project set at .250 it is now cutting too deep. It would appear that I am encountering a difference in depths between drilling and cutting. I cut at a depth of .063 per pass, so it shouldn't be causing too much flexing etc. I did the drilling at a plunge rate of 2.5 also very slow.
I am using mdf as the base and spoil board, which could account for some flexing.

I just cut a project out of .250 material with the z scale adjusted as described above and on the cutout of the project set at .250 it is now cutting too deep.

How much too deep?
Have you checked the Z for binding, smooth movement and no loose parts?
With your dial indicator set to 0 make several moves with Z ... at different speeds, with and without cutting, with and without x and y moving and then return Z to 0. Does it always return to the same position?
What material are you cutting?
How are you setting tool height when you change from drill to mill?

Rick G

They know what is wrong, "I am using mdf as the base and spoil board, which could account for some flexing". MDF isn't exactly on par with granite surface plates the last I checked ...

it all depends on how or what cam program you are using to produce the gcode , there are different depths required for drills as opposed to milling cutters etc
cam programs take into account the cone angle of a drill , so the z axis becomes deeper by the depth of the cone angle .

but you dont say by how much it's cutting deeper , so we cant advise further