Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: CES on November 29, 2018, 03:33:33 PM
-
Hi Everyone,
Search is broken as are external links into the database so I'm posting this thread.
I noticed that my Grizzly Mini Mill X axis was a little off in -X and +X. So I decided to re-calibrate the Mill. Z axis gives -.01 inch over .1 inch in Up direction but is almost spot on in down direction. I can't find a setting for forward versus reverse movement in Mach3.
Can anyone drive me in the correct direction?
Thanks,
CES
-
Hi,
there is no 'steps per unit' in one direction and 'steps per unit' in the other direction.
If it takes 1000 steps to travel 1 inch upwards it will take 1000 step to travel 1 inch downwards.
What you have described is either backlash and/or missing steps.
If it is backlash the error will not accumulate but it will if you are missing steps. This suggests a test that you can make
to determine which.
MDI:
G0 Z1
Z0
Z1
Z0
Z1
Z0
Z1
Z0
Z1
Z0
This just rapids the Z axis up one inch and then down one inch. If you have backlash then the start and end point could well be different
to each other by the amount of backlash. If your missing steps the error will accumulate with each iteration of the cycle.
Report back with your results.
Craig
-
Thanks JoeAverage,
Thats what I was doing except 0.1 inch, so:
G1 G91 Z-0.1
Z0.1
Z-0.1
Z0.1
Z-0.1
The movements in negative direction are spot on but positive direction is off. Rather then 0.1, I get 0.097
So DRO's read:
-0.100
0.003 Reset to 0.000
-0.1
0.003, Reset to 0.000
...
I checked backlash, it is close to 0.0008 (about as close as I can get it to 0.0005 which is the accuracy of the dial indicator.
I'll check again but thanks for the reply.
-
Hi,
try moving further, 1 inch is my suggestion.
Do not reset between cycles and you want at least ten cycles. THEN compare the beginning and end points.
Craig
-
Hi,
additionally I notice you are using incremental moves. What is your step per unit vale for the Z axis?
Craig
-
Min step is 0.0001 from motor to bed/head.
-
sounds like a Resolution/rounding Problem.
pls post a Screenshot of your
Config -> Motor Tuning -> Z-AXIS MOTOR MOVEMENT PROFILE Screen
-
Suggest you post a picture which shows the Z axis.
RICH
-
The settings are as follows:
Steps per: 10000
Velocity (inch): 15
Acceleration (inch): 19
Step Pulse: 2
Direction Pulse: 1
G's result is 0.0492136
This gives me 1 axis motor movement. That is I push the Page Down button ones and I get the motor movement for 1 increment
-
hello CES,
you have to tell us more about your installion (Motor , Motor Connection, Gear, Belt driven, drirectly driven by Ball screw),
because you data for the Z-axis are very strange.
steps per:10000 ?? are you running a servo Motor on z-axis?
velocity 15inch/min ok i have to calculate (15x25.4 = 381mm/min, i am in metric) that is nothing
this data "seems" to be a Little bit "strange".
TPS
-
Hi TPS,
Any setting less than 10000 steps per inch and I get no movement or completely incorrect movement.
NEMA 23 steppers on all axis with 5 TPI zero backlash ball screws. Doing the math, I get 0.0017 inch per step but in Mach3 is can see 0.0001 on a 0.0005 per tick dial indicator. I should get 5 steps per tick and I do and 1 inch is close to correct.
I tried setting the steps per inch to 100 and my DRO moved 100 inches before table moved 0.01 inch showing 10000 is correct (My table is 15 inches in X, 4.3 in Y and 9 in Z).
CES
-
I guess that your acceleration is too high.
You are loosing steps because the mechanical resistances in your screw/nut combo, friction oh guides and mas of Y axis (gravity :) )
Gravity assisted going down is ok (everything falls down) but way up :)
-
ZASto,
Too low acceleration causes lost steps or sluggish steps and lost steps. The high acceleration seems good on all three axis giving around 0.0001 inch per step. When lowered, the movement is not even and is sluggish.
-
Like I asked, and what TPS reinforced in more detail,.... post a picture of the Z axis.
What size steppers ie; in oz do you have? A motor curve would even be better.
You should calculate the steps per inch or use the calibration feature of Mach to get the step setting correct.
Then you test for a reliable velocity followed by testing for a reliable accel value.
RICH
-
Hi Rich, the calibration value from Mach3 is for my X and Y axis is 10027.*********x and 10025.*********x so the Z value will be very close. I am not concerned about the 10000 value. What I am concerned about is the fact that movement in one direction is not the same as movement in the other.
-
Just rectify the rising motion :)
Obviously you are loosing steps in the Zup direction.
Probable causes: high acceleration or impotent motor :D (sorry couldnt resist) :D
-
I found that my X axis is doing the same but the cause is the X-Axis Screw nut. I guess it has become loose. The Z axis has a screw nut of sorts, supplied by the kit manufacturer with the CNC kit. The Z axis nut is very secure and can't be the issue. I'll set down the acceleration and put up with the sluggish response for the calibration of Backlash and steps per inch, as requested by the commenters. My pulse width will need to be modified to allow for the sluggishness of the axis. Can anyone post their X, Y or Z calibration screen from Mach3?
Thanks,
-
Every machine is different, so calibration screens would tell you nothing relevant.
It is YOUR machine that should be calibrated.
Nut, beeing secure does not tell you anything about the friction/binding etc.
You should first start with low accel and incrementaly increase max speed to the point when stepper stalls. Back off approx 20%. Then start increasing accel, again to the point when stepper starts loosing steps/stalls, back off and test UNDER LOAD. Your Z axis should be calibrated this way in the UP direction.
-
Sorry, if you can't post what was asked or take some simple advice won't be able to help you.
Have Happy Holiday and good luck,
RICH
-
Hi,
additionally I notice you are using incremental moves. What is your step per unit vale for the Z axis?
Craig
JoeAverage,
My single motor movement is 0.0001 inch on all axis. That is the best I can get out of this machine and seems good enough for the parts I make.
I compromised on the calibration of my Z axis. Going up can be 0.0003 different than doing down as long as the down direction is < 0.0002. All axis are not calibrated to less than 1/5 of by 0.0005 dial. 1 inch or 0.015 inch, the return is to zero.
The up direction has to move the weight of the head, spindle, motor, Collette and bit so I see how it would put a lot of stress on parts that would not have the same stress in down direction. I do not have a zero weight Head.
Thanks, all how commented.
p.s. my machine is slow for a CNC. In the next week, I'll turn up the acceleration.
-
Sorry, if you can't post what was asked or take some simple advice won't be able to help you.
Have Happy Holiday and good luck,
RICH
Hi Rich,
I have posted all requested information except a video of the movement (I don't know how to post VERY large files).
Have a great holiday season yourself.
CES
-
To check backlash, here is a macro by CNCNutz
Attached metric and imperial. Edit the macro as it suits your machine (https://www.youtube.com/watch?v=CS2rosB_ce0)
-
CES,
Don't need a video. Just need to see how the stepper is mounted, attached to the ball screw, how the ball screw is mounted.
Also info on the steppers in oz rating. If you have info fine, a link to info on them.
RICH
-
Hi Rich,
I have Nema 24, 3.1Nm Steppers on all three axis connected to spider couplings and 5 terns per inch ball screws. I have attached pictures of the Coupling on the Z axis but all axis are the same. I've also attached a picture of my Mill, a Grizzly G8689 CNC controlled using a GECKO G540 4 axis controller and Mach3 licensed.
The flood is under construction... also the home switches for X and Y are under construction.
I've searched for the original manufacturer of the parts but can't find them. Out-of-Business or my migraines are getting in the way.
BTW - I have all axis calibrated to less then 0.0005 in all directions except Z in Up direction (I am now thinking the Z is under a heavy load due to the weight of the head). The direction of Z that makes a difference for me is Down which is good. Up is not nearly as good, something like 10X the down accuracy.
-
Not sure why the picture was blown up to such a degree. Select it and you will see a smaller view.
-
BTW - I have all axis calibrated to less then 0.0005 in all directions except Z in Up direction
that is one of the bad sides if you are living in the imperial world, in the metric world it is more than you want in the Hobby world.
sorry that i have sayed this
-
CES,
How exactly is the stepper mounted to the ball screw.
ie;
Stepper flange bolted / anchored to fixed bracket - inside the bracket is the coupling which connects the stepper shaft to the ball screw.
Are there any bearings in the above description? I can't tell from the pictures.
If there are bearings are they mounted and or how were they preloaded?
What make is the coupling?
It would helpful to know more about the motors. ie; 3.1 nm ( 438 in oz ) motors but at what amperage and voltage. If memory serves me
the G540 is limited to 3 amps for an axis. How many wires do the motors have.?
Your steps per unit of 10000 may need to be tweaked some. More on that later .........
Believe your accel is too high. We'll get into refining your velocity and accel later.......
For now just post the info on how the Z stepper is connected.
RICH
-
RICH,
The 24V, 4 wire motors are connected to the ball screw with a spider coupler ($15 from Amazon) but mine came in a kit. The ball screw is a Thomson and the spider coupling is red or orange but I cant find them on the web and I can't read the text on the mounted units but a picture is attached.
When I calibrated the axis, the 10000 came down to 9999 and change. They are as accurate as I can get them. Much less than 0.0005 inch over 1 inch movement. The Z axis is the worst at 0.0004 in Up direction and 0.0002 in down direction over 1 inch. I think the up direction is pulling a lot of weight as the ball screw in on the side of the column.
The X and Y axis both calibrated to 0.0001 in both directions.
Calibrations of X, Y and Z were done using Mach3 calibrate function over 1.000 inch. The Backlash is set to very low values to give the correct response.
-
Oh, I forgot, 3.1Nm motors.
-
CES,
You have 0.010" of Z backlash according to your posting.
I suggest you try to minimise the backlash. You will have backlash no matter what you do it's just
a matter of how much! Can't see how the screw is attached, so in the picture, I point to what I think is
a support bearing.
Now consider doing the following:
You should have a bearing / bearings ( depends on how you want to do it) which anchors the upper end of the screw. You preload the bearing support such that the torque required to turn the screw is minimized. With the anchor fixed you can use a solid coupling and get rid of the spider coupling. Consider increasing the torque ( don't think you actual have 438 in oz with 24 V) and also consider providing for a 2:1 ratio or so for increased resolution and motor torque.
If done properly you "MAY" only have a 1/2 to 1 thou free play due to the bearing. The ball screw is rolled so don't be suprised if the there is a slight diference in the up or down accuracy of the screw. In the end, the major contributor to backlash will be in the nut.
Total backlash is the sum of each axis component non-movement.
RICH
-
hi CES,
you are gving the answeres by yourselve
the ball screw with a spider coupler ($15 from Amazon)
you ae talking (dreaming) about 0.005 (doe's not mater if it is imperial or metric) with a rubber loaded Spider coupler (ok that is only one part of the dream).
ok again 0.005 (doe's not mater if it is imperial or metric) some "ideas"
-machine air condituioed?
-spindles rolled/grinded
-stepper Motors with Feedback (angle error??)
your Tuning expiriances 9999 or 10000 say everything.
sorry i have sayed this.
Tom