X3/X4 Z tower bending compensation for only 3KG
I had a problem, taking a single pass very light cut on stainless steel.
The tool: a 10mm, 3 flute solid carbide end mill.
The cut: 11.5mm wide. Up one way, then back the other.
On pass 1, in one direction, the Z axis load is about 1.2KG causing the tower to deflect back a bit, meaning my cut was shallow by about 3-4 microns.
On the return pass, offset sideways by about 1.5mm the cutter load I estimate is about 180-200 grams, and now the tower leans towards the cut and it is 3-4 microns too deep, causing too much material to be removed, and uneven cutting.
Observing this, on the first pass I pressed on the head with my fingers (about 1.2KG) and released on the return pass. PERFECT CUT!
Not to be out done, and having no time to make the tiny tower stiff I reasoned that if I added about 1KG to the head during pass 1 and removed it on the return journey, the problem is fixed. My wife got tired fingers!
Solution: Use my 4th axis driver.
I mounted an old printer mechanism, by it's carriage to the top of the tower. With 400mm travel, the stepper motor being the movable weight I was able to measure 5 microns (0.005mm=0.0002") deflection when the carriage was fully extended over the head.
Now during pass one, the stepper is extended about 300mm giving 3 microns bend, equivalent to the extra cutter load on pass one.
On change of direction, the stepper retracts, to about 30mm and the CUT IS PERFECT.
I did not even consider having a 5 micron higher first pass as the 'HEAD BOB' inherent in this machine (about 15-20 microns + another 50-100 from friction etc) would just make matters worse.
Cutter life has gone from 1 to 2 hours, on this job which has a 2 minute cycle.
No big structures, concrete walls or expense (except for the new 4th axis plug - and some extra lines of code) has been incurred.
4 hours work and it works almost perfectly.
I can also add weight at the stepper motor end to shorten the transit time (about 1 second full travel). It sticks out less for the same weight.
The A axis is calibrated 0 to 50 giving 0 to 5 microns compensation.
After a few test cuts, it just gets better. If you get it wrong, it no worse than it not being there. Only just like putting a 1KG weight on the head.
I will post a video/pictures when I get time to do it.
Looks a bit strange, but I am sure it will cause some questions from, visitors.
Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.