Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: Keith on June 21, 2014, 04:04:53 PM
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I haven't used my Sherline Rotary Table in some time and since did a lot of re-installed. And I know this topic has been covered before but from what I read and apply, I can't get my rotary table to move in degrees properly.
I understand that the worm gear is a 72 turns to get 360 degrees of movement on the table top.
I believe it's doing half stepping, so 400 steps per stepper revolution (.9 degree per half step).
I don't think the velocity and acceleration numbers effect the ability to input degrees and get the table to move to the inputted degree.
I'm just forgetting the magic number in 'steps per' in motor tuning that I had and that worked before.
I'm coming up with the number 28800 but that doesn't work correctly.
So if anybody has that number that allows to put in a G01 A5 and the table goes to 5 degrees from zero, I'd appreciate it very much.
Thanks, Keith
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I may be wrong but I believe steps per for angular is. Steps*microsteps*Gear Ratio/360
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Keith,
Sample rotary table calc for 1.8 degree stepper:
- 72 handle turns per one table revolution x 200 stepper pulses per stepper rotation = 14400 pulses per one table revolution ie; 360 degrees
- 14400 pulses x 1/360 degrees = 40 pulses per degree
If you are micro stepping say 10x.....then 10 x 40 = 400 pulses
With a .9 degree stepper------- 72 x 400 (1/360) = 80 pulses per degree, then adjust for based on your drive step setting
- Note that you should set your max velocity and accel from trial and error so it doesn't skip.
BTW, there is a calculator in Members Docs.
RICH
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Keith,
Just some more info for what it's worth as this pertains to resolution.
RESOLUTION
Resolution is just an indication of how fine the A axis system will turn the spindle per step.
Typical stepper is 1.8 degree ( 200 pulses for 1 rev ), most folks micro step ( say a value of 10 ),
and then it is a matter of how much gear reduction takes place ( lets use 72:1 ).
Steps in one revolution= 200 motor pulses X 10 micro steps X 72 = 144000 steps per revolution
Steps in one degree = 144000steps per rev / 360 degrees per rev = 400 steps per degree
Theoretically with just a single pulse sent to the motor you would have the following:
Resolution = 1 / 400 = .0025 degrees ( which is equal to .15 minutes = 9 seconds )
Practically speaking, due to backlash, variances in the gears / belts / motor or whatever, you will not get that kind of movement in one pulse / step.
The following is an example of different combinations:
+ - RESOLUTION
# MOTOR MICRO STEPS GEAR RED STEPS / DEGREE DEG = MIN = SEC
1 200 -NA- NONE .5555 1.8
2 200 10 NONE 5.555 0.18 10.8
3 200 10 72 400 0.0025 0.15 9
4 200 10 135 750 0.00133 0.08 4.8
5 200 10 200 1111.1 0.0009 0.054 3.24
Here are some angular to linear relationships:
ANGLE PER INCH PER 3" PER FOOT
1 SEC .000005" .000014" .00006"
30 SEC .00014" .00044" .0017"
1 MIN .00029 .00087" .0035"
Here is just one aspect of resolution to linear positioning and can be envisioned by the following example.
Lets assume that you are going to index the spindle and drill some holes out 3” from the center of the plate.
So if you were to drill a hole out from the center 3", then index 3 min, and drill another hole,
the distance you have between the holes would be approx 0.006” for no gear reduction.
RICH
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Thanks so much Rich. It looks like (since I'm half stepping) that my steps per=28800 is correct from what you wrote.
And so if I entered G01 A80, I should get 1 degree movement.
However, many years ago I set this up so that if I typed G01 A90, it would go to 90 degrees (from 0 degrees) and don't recall why it worked.
I do have the box checked in Mach for angular on the A-Axis I'm using.
Let me ask you, do you know do I have to do anything special with my Smoothstepper configuration that might have changed things since back when, I was using the parallel port to get the above results.
Thanks again, Keith
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Keith,
G01 A80 and you should get the table to turn 80 degrees.
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There are some general config settings in Mach that you should select.
Rot 360 rollover – if checked the A axis DRO will display from 0 to 360 degrees and then start over at 0. If not checked the A axis DRO will be additive such that 2 revolutions will display as 720 degrees.
Ang Short Rot on G0 – The axis will move in the shortest possible move to a new position. So if at 0 degrees and you jogged to 359 deg then it would just rotate -1 degree.
Rotational Soft Limits – if checked will apply software limit switches to the rotary axis.
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I have not used my SS in a few years so don't remember if you had to do anything speacial for a rotary.
RICH
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Okay and thanks Rich. Well here's an example of what I get which is of course way off but if I type in G01 A.05 (that's point zero five) the table moves a bit more than 5 degrees.
Does this hint to where I've gone wrong? Again, I have the steps per at 28800 (half stepping mode, also box checked in ports and pins for Sherline half pulse mode)
-Keith
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So now with a little fiddling, at 28,800 steps per, G01 A.5 goes to 90 degrees, A1 goes to 180, A1.5 goes to 270, and A2 brings me to 360. Weird.
And A0 brings me back exactly to 0. How strange.-Keith
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Keith,
Your rotary table based on info you posted as follows:
Rotary table calc for .9 degree stepper:
- 72 handle turns per one table revolution x 400 stepper pulses per stepper rotation = 28800 pulses per one table revolution ie; 360 degrees
- 28800 pulses x 1/360 degrees = 80 pulses per degree
Since you are 1/2 stepping ..... 2 x 80 = 160 pulses
So your steps per unit should be 160 for the A axis.
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Link to the stepper calculator:
http://www.machsupport.com/forum/index.php/topic,16315.0.html
See attached pic.
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Now if you wanted to, you could always use the axis calibration in Mach > Settings (ALT-6 ) tab to set the steps per unit for the A axis.
Click on Set steps per unit button ( it's just above the Reset button) it will ask you to select the axis and how much you want it to move.
Then based on the actual DRO value displayed, you would put that number in and Mach would calculate what the steps should be. So if you accept the steps per unit will be changed.
RICH
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Thanks Rich. I'll try that tomorrow.
However, that axis calibration intrigues me. So when the program asks: "how much to move", what unit are they asking for (in other words, knowing what you do about my setup, what part of the equation are they referring to?
Thanks for all your help. I'll get there.
160 does seem to be a very familiar number I used in the past.
-Keith
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Keith,
It should be degrees. Now if the the rotary was configured as linear then it would be inches.
Try this, tell it 360 with your current settings, see what the A axis dro actualy says, then put that dro reading as how far it actualy moved, then it will calculate and tell you what the steps per unit should be.
RICH
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That was right on, Rich. And thanks again. Used the calibrate and indeed it changed steps per to 160.
Now I'm just having to find a reasonable velocity and acceleration. When I use the 'tab' for the MPG mode and set the jog rate for 100% and use hotkeys to move, all is well at some good velocities. But when I do an A0 move or an A01 move, I get a clunk when it starts out; worse at different velocities and accelerations. Might just be a mechanical thing since I refurbished the rotary recently. But still it's odd that it happens only in the manual screen for moves.
But anyway, thanks for all your correspondence. -Keith
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Keith,
A rotary table can have have backlash and you should check to see how much yours has. One should take out the backlash before indexing and
then move in one direction until finished indexing for a task.
A0 and there should be no movement if you are at A0, but depends on the your configuration ( reply #5 ). Not sure but A01 may be interperted as 1 degree.
For acceleration adjust the velocity until it skips, then lower by say 50 %, then start increasing the accel until it skips and reduce that by 50%.
Then refine both by rotating from different starting locations about the rotation in both directions. Just need to play around.
The clunk probably is just the backlash being taken up and as the velocity / accel increase so will the force of engagemant of the screw and gear, and also the start and stop of the stepper. Just jog the rotary in very small increments and you will hear the stepper clunk / step even though the
table is not moving.
RICH
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Thanks Rich. Took all your advice and since there's some wiggle room adjustment of where the worm gear 'sits', I very slightly loosened the bolts that allow for that and very slowly rotated in both directions so it could find it's own place to sit without any lash and low and behold, I'm in action. Works beautifully.
You've been a great help. Thanks, Keith
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Oh and that was a typo (A01) I meant G01 A0 as opposed to a G0 A0.