What's new
What's new

Guess the ratio of machining centers equipped with linear encoders vs rotary only ?

wippin' boy

Diamond
Joined
Sep 14, 2005
Location
il.
4 out of 4 of my boring mills and 4 out of 4 of my big vtl's
i think its more common everyday as it lets the mfg off the hook on backlash and other fit and finish issues
dope slap taken for no estimate of worlds total but my world is kinda small
 

adama

Diamond
Joined
Dec 28, 2004
Location
uk
Worth adding some basics, even a linear scale machine will have some kinda tach - encoder on the screw for the drive to know what the motor is actually doing. The linear scale tells the control were the slide is actually at. Its how the 2 are mixed together and what the control does with that info that really matter. Sample rates these days are high enough for some blisteringly fast moves and corrections. Just like any closed control loop though too much collective slop - lag at a given speed and it falls out of position. Disconnecting the scales might be the cheap option, but its really killed the precision on a machine were the scales were working hard to counteract screw wear. Better option is to refurbish the screws and fix the wear.

That said, IMHO its all a bit of a mute point. With modern controls, proper machine temp controls and proper calibration and recalibration intervals the loop can be closed just fine with software tweaks. Modern controls can error correct moves against known values a lot faster and with greater precision than old controls. Using optical and ball bar generated geometry for finding the errors they can correct things that the scales never see, like banana in the ways and a host of other little things. Its the advantage of modern high speed multi core processing. One core calculates the moves, the others calculating the positional error compensation values. With current processor speeds this can be done to higher resolution than the machine needs for even high speed movements. It lets you fix the errors that are a long distance away from the measured axis of the scale.
 

John_B

Cast Iron
Joined
Jun 25, 2011
Location
Georgetown, TX
Kinda funny to me that no one has mentioned that a high percentage of the widely sold Cincinnati Arrow mills had scales installed on X & Y (not so quite so many with Z scale as well) per a conversation with one of the long gone super tech's doing phone support 4 or 5 years ago.

Funny to me cause the OP is Milacron, and Cincinnati will always be Cincinnati Milacron to me no matter how many times they get bought out!
 

wippin' boy

Diamond
Joined
Sep 14, 2005
Location
il.
Vanguard :( and HNK boring mills and Youji vtls
Yeah the talk between the 2 coders becomes one of the double checks to reduce chance of bad movements going unnoticed
 

metaltech

Hot Rolled
Joined
Feb 7, 2007
Location
midcoast U.S.A.
Don's question specifically asked about machining centers, not grinders, boring mills, knee mills, VTLs, etc. Surely these machines benefit from linear scales, but they're not MCs. I agree with Don that less than 5%, and I would guess 2%, are equipped with linear scales for position feedback. I'm judging that on my customer base. If you're using a high-quality machine (read "rigid, with well-made ballscrews"), with a good control, servos, and encoders, and you do the pitch-error compensation with a laser properly, you can get point-to-point within .0002 or .0003". Is that good enough for all parts? Naw, but I bet it is for 98% of them, so 98% of the machines are just encoder-equipped.
 

Mud

Diamond
Joined
May 20, 2002
Location
South Central PA
001.jpg

Here's a page from a Monarch brochure for an early 90's VMC, a fairly good sized machine. These have glass scales. It shows positioning and repeatability for both NMTBA and JIS specs. Doesn't specify over what range that spec is measured, I guess the JIC spec calls that out? Positioning of ±.00002 and repeatability of ±.00001 with a 36" X 76" X 44" envelope and 6000 lb table capacity seems quite good, eh?

Anyone know how the JIC and NMTBA specs are measured?

96133d1389194355-guess-ratio-machining-centers-equipped-linear-encoders-vs-rotary-only-001.jpg
 

swarf_rat

Titanium
Joined
Feb 24, 2004
Location
Napa, CA
This points out the laxness in the JIS spec. Note that NMTBA is 0.0001 vs JIS at 0.00002 for the same machine (a 5x difference!). JIS is simply the average of the errors if I understand it correctly. In other words, half of your positions will fall outside the spec. Here is a link to a paper comparing the various specs. I'm not sure what the NMTBA is in modern language, probably ANSI B54.5? While JIS takes the average error, ANSI uses 2 sigma and VDI 3 sigma of the error distribution. 3 sigma means about 99.9% of positions fall within the spec.
 








 
Top