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Mach Discussion => General Mach Discussion => Topic started by: motopreserve on June 20, 2017, 01:40:33 PM

Title: Incorrect Z-Axis zero/home position
Post by: motopreserve on June 20, 2017, 01:40:33 PM
Hey folks,

First post here on the forum.  New to Mach3 and had no luck with a search - although I've had great results researching here in the past.  Hoping someone might point me in a direction for checking the issue I'm having.

I am running a DIY built machine with THK linear rails/trucks, ballscrews and Nema 23 motors.  I have measured backlash to be in the vicinity of about .02mm (no compensation applied in Mach3).  The machine is set up with homing switches on the X,Y & Z axis.  When I first start the machine, I home it using the "ref all" button, with all axis traveling to the home switches and backing off correctly.  I then jog to a position and set/zero the work coordinate G53 position (Z-axis done with a touch plate).   

The problem I have is when I jog away from the WCS zero, then hit "Go to Zero" button the Z-axis returns to a location slightly lower than it should (Go to Zero move always includes the Z-axis traveling up to the machine zero first, then back down to the WCS zero).  I narrowed down the issue by doing this repeatedly - each time hitting the "Go to Zero" button - triggering the Z-axis to travel to the machine zero and then back to the WCS zero.  Eventually I noticed that each time it returned to the WCS zero, it was a little lower than the last time.

I tested the issue by measuring the discrepancy of distance traveled at both the bottom (by locating to WCS zero) and top (by locating to machine zero) of the Z-axis with a dial indicator.  I made sure to remove any backlash before beginning to travel up to the machine zero.  It appears to be incrementally off by about .07mm each pass.  By repeatedly locating to the work zero and then MDI G28 Z0, back and forth, the axis is clearly not reaching the same machine zero location at the top, always stopping shy of the previous location that the machine zeroed to during the Homing operation.  So it seems that traveling between the WCS zero and the machine zero is pretty consistently the same distance, but always offset by a bit.  This puts the cutter too low - and even lower each time the machine raises up and back down to the WCS zero.  Hope this makes sense. 

If you need clarification, please let me know.  Any thoughts on what to test next?  Is this a software or hardware issue?

Thanks in advance for any help, tips and/or tricks,

Scott
Title: Re: Incorrect Z-Axis zero/home position
Post by: RICH on June 20, 2017, 05:50:06 PM
Check / confirm current axes calibration using the Calibration feature under ALT C  tab. Maybe off.......

RICH
Title: Re: Incorrect Z-Axis zero/home position
Post by: motopreserve on June 20, 2017, 06:05:41 PM
Thanks so much Rich.

When I thought more about it after posting, I thought the same thing - why do I always think of these things right after I walk out the door??? :).

I did test calibration this morning and made some very slight adjustments - but only using travel of 1.5" which may be leaving some errors in the longer throw I have on the Z-Axis.

I will do a more thorough calibration tomorrow when I get back to the shop. It does seem like the most logical cause.

Appreciate the reply. I'll post back any results I get.
Title: Re: Incorrect Z-Axis zero/home position
Post by: RICH on June 20, 2017, 06:18:51 PM
When calibrating an axis you should use the LONGEST move possible. That way any error would  be small.
Make sure you remove the backlash before calibrating the axis.

Quote
why do you always think of these things right after you walk out the door???


It's caused by Adult CNC ADD or how you trained the thought process when mom told you something OR wife said something.
Research has shown it can also be caused by other things, but, only you would know them! ;) ::)

We try to answer all questions .....some are challenging and require additional info........in your case lets not go there.... ;D

RICH
Title: Re: Incorrect Z-Axis zero/home position
Post by: motopreserve on June 20, 2017, 07:24:55 PM
Ha!  Seems like I have some recovering to do from CNC ADD :)

I did originally do the X & Y axis with the longest controlled measurement I could get. The Z was done early on as well - and the short check today was just to make sure it wasn't terribly off. But it looks like I need to get back on that Z-axis with a longer measurement to rule that out. Let's hope it's something that simple.
Title: Re: Incorrect Z-Axis zero/home position
Post by: motopreserve on June 21, 2017, 09:27:31 AM
Well, I was hoping for an easy fix, but after redoing the Z-axis calibration it is still doing the exact same thing.  The calibration is now spot on for 4 and 5 inches (the longest travel I could measure safely).  I made sure to have backlash removed, and repeated the calibration tests several times to ensure it was correct. 

I have tried a few other tests; changing the safe-Z setting from machine coordinate zero (roughly 200+ mm travel for the tests) to just 10mm and 100mm.  The lesser the travel, the lesser the discrepancy.  But there IS still a discrepancy, incrementally worse each time I repeat the process.  The amount that it is off is determined by the distance traveled - the further the travel, the further it's off. 

So for example: if the Z-Axis moves up 100mm it's off by about .2mm.  If the Z-Axis only moves up by 50mm, it's off by about .02  With the 100mm travel test, for each of these runs it returns to yet another increment of .2mm off.  So after the second run of 100mm, it's off by .about .4.  Third time is off by .6 etc.  It's not 100% consistent - but often times is very close.

I also tried slowing the velocity and acceleration way down, thinking that maybe the motor or coupler was skipping/slipping - but this did not seem to help.  I have assumed that it would not be motor missed steps or the coupler slipping, because I don't think it would be so consistently off each move.   So it appears that regardless of how fast or slow it tries to get there, it always travels in the negative direction by slightly too short an amount.  And then incrementally does this each time I repeat the process.

I'm at wits end here on what to test....

Thanks for your patience.

Scott
Title: Re: Incorrect Z-Axis zero/home position
Post by: RICH on June 21, 2017, 08:55:22 PM
Post some pictures of your machine, in particular the z axis.

RICH
Title: Re: Incorrect Z-Axis zero/home position
Post by: motopreserve on June 21, 2017, 09:13:16 PM
I ended up having some odd issues with the machine today. Not sure if they're related, but I decided to reinstall the Mach3 software. I was afraid that I may have messed something up by installing, opening and then deciding against a couple different screen sets. Unfortunately, the reinstall is still showing the same issues.

Not sure if I should have started from scratch and not used my old custom XML file???

I only have a couple shots on my phone right now. I can post some more details if you want to see a specific part of the Z-Axis.

(http://i1227.photobucket.com/albums/ee427/nortstudio/machining/New%20CNC/E9F091B5-C3EE-4D73-8438-94316F06AD7D_zps80slnamj.jpg) (http://s1227.photobucket.com/user/nortstudio/media/machining/New%20CNC/E9F091B5-C3EE-4D73-8438-94316F06AD7D_zps80slnamj.jpg.html)

(http://i1227.photobucket.com/albums/ee427/nortstudio/machining/New%20CNC/C4B36961-7CC1-492C-AA97-ED8C46E01D4C_zpsnjoa7ohx.jpg) (http://s1227.photobucket.com/user/nortstudio/media/machining/New%20CNC/C4B36961-7CC1-492C-AA97-ED8C46E01D4C_zpsnjoa7ohx.jpg.html)
Title: Re: Incorrect Z-Axis zero/home position
Post by: RICH on June 21, 2017, 09:46:11 PM
Thanks, just what I wanted to see.
1.The motor shaft coupling used on the Z "may" be the problem or contribute to it. I have experienced repeatability errors with that kind
of coupling over time. I use direct coupling with a preloaded bearing to keep any force from going back into the stepper motor
( they just have Bellvue washers  internally  ). So the suggestion would be to make a rigid coupling and test to find out.
 I am thinking it is the problem.......

2.Is the upper support  bearing preloaded to eliminate play? I preloaded mine enough such that there was no play and in doing so tried to minimize the amount of torque required to just turn the screw with no play. Was easy for me as I have torque gauge to do measurements.
First do #1 above.

3. Nest comes the ball screw nut, but, think 1 or 2 will do it.........

4. Profile you Z axis to check accuracy.  Lets not go there for now.

RICH
Title: Re: Incorrect Z-Axis zero/home position
Post by: motopreserve on June 21, 2017, 10:11:20 PM
Thanks once again Rich. Very much appreciate all your help.

These couplers were not my first choice, and I have been planning on making some new ones on the lathe as soon as I waded through all the other things on the list. Seems like they just jumped to the top of the list!

My plan was for Oldham style. Do you have an example of the type you made?

I'm not sure if the bearing is preloaded an adequate amount. I do not have torque gauges to check. When I assembled the axis I made sure it did not have play - but that was without scientific measurements.

Sorry is this is redundant - but I was afraid I was confusing in my earlier posts. To be clear, the discrepancy I'm experiencing seems to be a constant distance traveled (up, then back down) - but each time it happens the travel up is a set amount less than it should be. So for example: spindle rises 100mm, comes back down and rests at 100.02. The next time it travels up and back, it rests at 100.04. Next 100.06 etc. although it's moving too fast for me to measure at the top of the axis, I'm visually seeing it after many runs of this same maneuver.

If you already had that down - please ignore it :)

Title: Re: Incorrect Z-Axis zero/home position
Post by: motopreserve on June 21, 2017, 10:13:50 PM
PS: do you happen to know the calculation for determining the necessary kernel frequency?  I have search high and low and found conflicting reports.

Thanks!
Title: Re: Incorrect Z-Axis zero/home position
Post by: RICH on June 22, 2017, 07:46:14 AM
By Oldham style I assume you mean just a solid piece with a bore and set screws to
attach / lock to the shafts. That is fine and easy to do.

I can't tell what type support you have on the Z. Basically there are two types, fixed and support.
If it is just a support type then there will be axial play since the screw is NOT fixed at that end.
Do you have info on the support?

-----------------------

Remove the motor and coupling. Put an indicator on the end of the screw above the support.
Push up on the Z carriage assembly and note the movement. Make sure you are not lifting the x /y  linear slide from the rail.
If it is  a fixed type and preloaded you would have
very little movement ( ie; 0.0001" to .0004")

For now,

RICH
Title: Re: Incorrect Z-Axis zero/home position
Post by: motopreserve on June 22, 2017, 08:46:12 AM
RICH,

The Oldham style couplers have a center piece of hard plastic, I assume to allow it to give (or break away) if some catastrophe happens, but not as squirrely as the style I have on there now.  They are made up of 3 parts total.  Like  the following:

http://www.ruland.com/ps_couplings_oldham.asp?gclid=Cj0KCQjw1a3KBRCYARIsABNRnxtK8nD8gPazSOpJwEocEMnJ-va7I0dhWuq2bf0DxIQDMpQayfq9X8gaAv-9EALw_wcB#about (http://www.ruland.com/ps_couplings_oldham.asp?gclid=Cj0KCQjw1a3KBRCYARIsABNRnxtK8nD8gPazSOpJwEocEMnJ-va7I0dhWuq2bf0DxIQDMpQayfq9X8gaAv-9EALw_wcB#about).

I could make something like this fairly easily.  Just need to raw materials - don't have it all in stock.

The thought of this all being caused by mechanical play is a little odd to me (although I also want to be sure that the mechanical side is correct!) - since the discrepancy always seems so uniform and cumulative.  Also, the strange issues I started having on the machine were also on the Z-Axis.  The thing went a little crazy and began hesitating.  I thought there was binding - but confirmed by hand-turning there was not.  Then when locating to WCS the Z-Axis would travel to the Machine zero point and make a weird noise, then sorta stall there, but think it was at WCS zero.  I reinstalled the software - and although it did not fix the issue we are discussing, it does seem to have stopped this weird behavior....for now :)

The support on the Z-Axis is provided by a BK20 fixed bearing block at the top.  It has the bearings seated inside the block, and the play is taken out by the 4-sided nut with setscrew that threads onto the end of the ballscrew.  I have attached a photo below of the type of bearing block I used.  

Thanks!
Title: Re: Incorrect Z-Axis zero/home position
Post by: RICH on June 22, 2017, 11:17:06 AM
Just make a solid simple coupling to test and  eliminate that part as a culprit.

You have a fixed support, BUT, still do a check for any movement per last reply. May have some play and will know how much it
is contributing if any.

RICH
Title: Re: Incorrect Z-Axis zero/home position
Post by: motopreserve on June 22, 2017, 11:25:10 AM
Will do.  I'll check the play when I disassemble for installing the new/test coupler.

Should be able to bang something out quick f I am just making a rigid coupler.  I have some aluminum round bar here in stock.

Thanks again!
Title: Re: Incorrect Z-Axis zero/home position
Post by: motopreserve on June 22, 2017, 03:23:44 PM
Well RICH, I can't thank you enough!  I suppose the only thing left is to get your address so I can deliver you some whisky/wine/beer/chocolate/centerfinders.... Pick your pleasure or poison.  :)

Made a quick direct/rigid coupler, slapped it on there and the thing is repeating PERFECTLY!  Good grief I thought the stress would crush me!  Wondering if you have a suggestion for a more permanent solution for a coupler.  You said you were using custom ones with just Belleville washers as safety relief?  Do you have any photos?  I feel it's probably best to have some type of give for those "oh sh*t...." moments that may occur if Mach3 goes sideways.

I'm going to check the play even though it is working well now.  Just curious to see how she's holding up.  Forgot to do it when I pout the coupler on there - too excited to see if it fit right and helped at all. 

Really appreciate all your time and patience. 

Scott

Title: Re: Incorrect Z-Axis zero/home position
Post by: motopreserve on June 22, 2017, 05:32:42 PM
Photo attached of the quickie coupler that I made on the lathe.

Title: Re: Incorrect Z-Axis zero/home position
Post by: RICH on June 22, 2017, 07:18:39 PM
Glad you got it fixed. The coupler you made is fine and two set screws are better than one.
I would suggest that you put / file  a small flat on the motor and screw shaft
as the set screws will mark the them making it hard to pull the coupler off.
Just use the new couplerand nothing else is required to be done.

Quote
You said you were using custom ones with just Belleville washers as safety relief?

No, mine are plain Janes just like yours. The washer comment pertains to the
motor ( if you ever take an old one apart you will see the washers to keep the stater
in place.

Quote
I feel it's probably best to have some type of give for those "oh sh*t...." moments that may occur
if Mach3 goes sideways.

Any force up is taken up by the support bearing so would not worry it.



Quote
so I can deliver you some whisky/wine/beer/chocolate/centerfinders....

Thanks for the offer, but we moderators are highly compensated. ;)

RICH  
Title: Re: Incorrect Z-Axis zero/home position
Post by: motopreserve on June 22, 2017, 07:55:35 PM
Glad to hear you are well compensated - as well you should be! :). Actually, I helped out with a forum for a bit - a thankless job, so please know that I am indeed VERY thankful!

My only concern with this rigid coupler is of course any radial alignment issues that would not be compensated for. I know I shouldn't have it to begin with - but I did have some slight irregularities with the work done at the water-jetting place.

I may be able to tweak the motor alignment to ensure a perfect fit. Then I wouldn't really be too concerned. Especially since I realized that while pretty nifty, Oldham style couplers do not offer any axial support. I suppose that's only an issue when the motors are powered off.   I'll
Check what kind of play I have, as you suggested.

Any higher quality flex coupler (Bellows) is ridiculously priced. At that rate - cheaper to risk it and replace the damn motor if something wears :)

The motor shaft has a flat already - and I will try to file a small flat only the ballscrew end as well.

Now that I've got that sorted, time to get back to the list! :)
Title: Re: Incorrect Z-Axis zero/home position
Post by: RICH on June 23, 2017, 08:24:35 AM
Quote
My only concern with this rigid coupler is of course any radial alignment issues that would not be compensated for.

Idealy the Z axis components should be parrallel to the rails and perpendicular to the X Y surface.

Just need to play some with the mounting of the z components.

Motor mounted first, install coupling with support loose, ball nut loose / not attached to plate.Now tighten support. Check how
perpendicular the screw is to the X y. Rotate the screw by hand using the motors shaft (attached old coupling
to the motor shaft to make turning the screw easier). Check runout of of screw at the end. Move end of screw half of runout and tighten the screw nut. Loosen support and re tighten. Attach motor. Should be darn close to being alligned.

You can  work top down or bottom up or even work out from the middle.

Writing this is like try to tell someone how to tie their shoes!

-------------------------------------------------------------------------------

How I do it for a short rather rigid axis alignnment:

I use  torque indicators and they are very accurate.

1.Preload the support bearing. Check torque with nut finger tighting of nut.
  Continue tightening to add preload and check torque. As preload increases the
  torque required to turn the bearing increases. If the motor is say 100 in/oz
  you don't want to use up 50% of you motor torque. You want to minimize the torque
  and also have min axial play. Axial play results in backlash.
  There is no such thing as no backlash.
  Total backlash is the sum of each components backlash.
  If you isolate a component you can measure it's backlash.
  Note both the torque setting and the backlash at that setting.

2. Measure torque required to turn the nut. Note it and measure any backlash.
 

2.The support bearing is installed / mounted  and rather closely alligned to other axes.

3.The nut is installed / mounted.
  You will find the torque ( nut + support) is very close when the nut is located at the
  end, middle, and close to the support if installed correctly. Note any increase in torque.
  The increase will be due to axial misalignment and whatever attached load is being moved
  due to friction and weight.

4.Install the end support when the nut is close to to the end support. If fixed type do step
 #1 for it,but, usualy it is not.
 
5.Check total torque when axis is near fixed support, middle, and at end of longest  travel
  near the other support. All torques should add up to individual torques.

6. Re calibrate axis after motor is attached.

Now nothing is ideal, for instance, the screw may be not be straight. So you need to tweak
each component along the axis travel.In the case of a vertical installation it may just right
but the axis travel is not parrallel to another axis.ETC!

In the end one will have minimised backlash and the torque to move the axis having a
a positive affect relative to velocity, acceleration, and repeatability and component wear.

There are other way to do things.......and surely I missed something......

FWIW  ;)

RICH
Title: Re: Incorrect Z-Axis zero/home position
Post by: motopreserve on June 23, 2017, 09:06:27 AM
RICH,

Once again, thanks for the great info.  Much of the concepts are familiar to me from research, and also my experiences with tweaking my 60 year old manual machines to work at their "best."  

I love the idea of checking torque.  Can you tell me the type of indicator you use?  I only have torque wrenches for the motorcycles, which I use often and may be able to rig something up that can at least measure the differences in torque required - but likely not accurate measurements of the actual torque (any adapters will throw off proper calibrated measurement).  But I think the torque differences would be the telling measurement while reassembling the axis to recognize the misalignment etc.

This is a brand new build, and the parts I had cut before this Z issue developed were plenty accurate enough for my needs.  None of the parts we make for motorcycles are mission critical (you are not going to crash due to failure/tolerances of anything we make).  But getting the machine to be as accurate as possible - and more importantly repeatable, is something I am more than willing to spend the energy on.  It's time well spent.  

You have made that job easier with your help and thorough explanations.

Attached is an example of a part cut on this machine.  The satisfaction of cutting it, and then mounting it perfectly to the bike is amazing! :)

Thanks again!

Scott

Title: Re: Incorrect Z-Axis zero/home position
Post by: RICH on June 25, 2017, 09:17:57 PM
Quote
Can you tell me the type of indicator you use?

Made by TOHNICHI - model BTG120Z-S - 10-120 IN OZ.
Has 3 jaw chuck,easy to use and listed  for $550 some time ago.There are probable others,
but, I got it for cheap. You can use a torque wrench, beam type ( for ounces ), or small 1/4 drive
torque wrenches ( in pounds) for low torque values but they are not as sensitive as an actual measuring
gauge and everything should be calibrated.

A few of the tings things made for bikes in the past.

http://www.machsupport.com/forum/index.php/topic,6188.0.html
http://www.machsupport.com/forum/index.php/topic,6189.0.html
http://www.machsupport.com/forum/index.php/topic,6187.0.html

Have Fun,

RICH
Title: Re: Incorrect Z-Axis zero/home position
Post by: motopreserve on June 25, 2017, 09:30:03 PM
That's some great looking work!  My clients lean a little less toward the grim reaper/skull & bones type of biker style, but I know good craftsmanship when I see it :)

Thanks for the info and links!