[Emc-users] Help...troublesome Z axis positioning

[Andrew G. <dre...@op...>]

Hi Folks,
firstly, thanks for the replies regards Post Processors, they worked =
well :)

Now that im pretty much up and running with my CNC mill...i have been =
fine tuning it and have struck a little hurdle....any advice would be =
appreciated.

As i have mentioned in early posts, i have a router being driven by =
150oz/in steppers with high quality linear slides.ball screws etc.


I have been practicing some nice 3D engraving ( 5   35mm high letters) =
with a chamfer / vee tool...Using mastercam i created a toolpath that =
only used the one tool...and roughed out the part in zig zag mode then =
goes back and does the contours...Climbing in the corners to keep a nice =
sharp corner.

Given its a small tool for roughing....its probably a couple of thousand =
lines of G code that takes about 30 min to machine...The roughing part =
is XY intensive while the later part is XYZ intensive.

From what i can tell X and Y seem to be very accurate both visibly from =
the output and from doing some load tests......I can drive a 10mm cutter =
at a depth of 5+ mm around a 200mm square with a decent feed rate and =
still measure less than 0.02mm inaccuracy.

Doing single cuts in the Z are accurate to the same value.

Back to the engraving....By the time it gets to the end of the =
engraving, if i check the positiong of the tool tip, i find that it is =
about 0.8mm lower than it was before i started.

Im after advice as to where i should be looking....the first thought is =
mechanical..Missing steps etc...But im struggling with that one, coz i =
can drive the thing up and down 25 times with the MDI mode at full rapid =
over a distance of 150mm and still come back to an exact zero point =
(proving that inertia and acceleration arent causing it to miss steps). =
The actual machining load in the Z direction should be quite low, as im =
only taking 2mm cuts with very little plunging.
The Z axis is supported with an eleaborate spring system that means =
there is very little force required to move up or down. I also tried =
setting the acceleration values much lower just in case that was the =
problem, However im only feeding in at 50mm a minute. So for all these =
reasons, i dont think it is mechanically missing steps...hence im onto =
other thoughts.....

Backlash: I cant feel any, but if there was any backlash would that have =
that sort of slow and steady degradation to absolute accuracy?

Deadband: Does that have any effect on accuracy?
I have set it to 0.00045...the smallest i could make it and stop it from =
flip flopping back and fourth at the end of a rapid.=20
Are there any other EMC settings that would have a slow gradual effect =
on accuracy.

Computer speed?...I have a PII350 with 512Mb of RAM running the latest =
BDI installed to hard disk. In practice has anyone seen a slow computer =
cause any issues

Any thoughts on this one would be appreciated...whether that be a =
solution or something to test/try so i can try to eliminate some of the =
variables.

I guess i dont mind forking out for a fix, if i know what the problem =
is, i dont want to go and spend a fortune on say a higher torque stepper =
motor/driver, faster PC etc only to find it was a simple setting =
somewhere in EMC. :)
I guess one solution would be absolute linear encoders....has anyone got =
any recomendations on types, brands, costs, and how to set up linear =
encoders with EMC?

Well if any of you have read this far down...thanks for sticking with me =
:)

Cheers
Andrew