Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: cturc on November 07, 2010, 09:51:25 PM
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Hello.
I'm new on this forom and on CNC world.Also I,m a french Canadian and sorry for my bad English !!!
I modify a metal lathe to turn wooden piece with step motor...good.All run great but I have a .008 inch of backlash on Y axis and I have a problem when I use the backlash fonction on Mach3. I expland...The backlash function operate well separately but when run a program the function didn't run because stop and start X axis with not slope programed...???
If anything have an idea for resolve my problem I appreciate.
Thanks
Claude Turcotte
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Claude,
Do you mean the X axis has backlash?
Lathe is usualy X and Z axis ( no Y axis ).
RICH
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Hello Rich,
Thanks for your response,effectively lathe use X and Z but I use Machmill for this machine because with Machturn the drawing is not translate correct.
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Claude,
My 50-50-50 rule for backlash setting which you may want to consider.
50% - The max velocity is 50% of where your steppers will start to skip ( in Motor tuning )
50% - Set the shuttle Wheel setting in configuration to .0050 to .050 ( .5 too slow ) ( in Config )
50% - Backlash speed % of max ( in config>backlash )
This is a good starting point and refine upward.
RICH
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Hello Rich,
The problem is not with axis with backlash but the other...I try to explain with my bad english...;-)))
When X axis get back the backlash, stop for a fraction of time the Y axis and when restart the motor go skip even if the backlash speed is 5%...???
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Understand the language problem, we'll just work through the problem very slowly......
Smile :) ......you French ......me Lithuanian.....here we go..... ;)
Do you mean the following movement:
X and Y are moving
X changes direction, X backlash is implemented, and then the Y axis stops for a short time.
When the Y axis tries to move, it skips.
Please correct motion described above,
RICH
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Yes it's my problem
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What is your velocity settings in motor tuning for the X and Y axis?
Post your xml file.
RICH
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Hello Rich,
The lathe is not here but the velocity is approx X=22 IPM and Y is is 30 IPM
Because it's an older metal lathe the carriage is very heavy (approx 200lbs) and it's hard to move also the slope for start and stop is approx 1.2sec
thanks
Claude
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Here is what i am thinking, could be wrong. I think the Y axis is trying to adjust for the backlash. ie; With a heavy load like that the Y can't stop or accel instantly. Have you tried running at even slower speeds like 5 ipm just to see what happens. I thought you may have been running at 100 at 150 ipm. Try the slower speed and see what happens.
I get a chance, will play with the lathe here and refresh my backlash memory.....
The trouble is the whole lathe is little more than your carriage weighs.
RICH
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Thanks Rich,
But yes I already try to running at slower speed and the problem is always here because at my knowledge when the X make a backlash adjust the Y seem not use slope for stop and start .Also any speed I enter do not change the result...motor skip when restart...;-(((
Claude T.
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use Machmill for this machine because with Machturn the drawing is not translate correct
Curious about the above. Can you post the dxf drawing.
What program are you using to generate the code?
RICH
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On other lathe I use Deskcnc before and now use Mach3 and the drawing are import from deskcnc and all work fine on other lathe but this lathe have no backlash and a very light carriage
Thanks for help Rich
Claude
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Claude,
I have exaclty the same issue with my mill. I'm using the latest lockdown vn...40. I am cutting a profile using X axis and Z axis with a round part in the A axis. The X axis moves along the part and Z goes up and down to give the right profile on the part. A then rotates half a degree for the next part of the profile and x moves back the other way.
I have 0.05mm backlash in X and Z that is not normally an issue but for this part it is, so I turned on backlash comp and set up each axis distance, shuttle time and velocity and all is fine with each axis.
BUT, as you say, when one axis is moving along smoothly (x) at 0.3M/min and then the other (z) changes direction the X axis suddenly stops to wait for the Z axis to complete its compensation, then suddenly X carries on at the same rate.
From what I have read in other threads, this sudden stop is possibly because the backlash comp is not within the Mach3 "trajectory planner" - it just stops motion, adds the compensation regarding the adjustments set and starts it up again. This is very hard for stepper motors as, with a heavy X axis, it will possibly/probably loose steps. There is no respect for the (de)acceleration or any other setting for the axis that is having to wait for the other axis compensation.
So, it is possible to adjust the way the compensated axis behaves (as Rich explains well) and to get that smooth and not loose steps but other axis are abruptly interupted. Luckily my mill is small and it just about copes with it, but it is painful to watch and I'm sure will loose steps under heavy load one day.
Rich, Hood, Brian, Art, may I ask if my understanding is correct or have I missed a magic setting somewhere? If correct, are there any plans to get backlash inside the trajectory planner so it can work with all axes smoothly and be more stepper friendly?
Or, any advise to improve the situation for the "not compensated" axis motion?
Thanks!
Woody.
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When using backlash comp every axis has to stop when any axis is changing directions. If you don't do that then your just changing the position of the backlash error. Think about travel around a circle counter clockwise (anti-clockwise for any Brits) as you hit the 3 O'Clock position the X-axis will need to reverse, if Y does not stop you will have a small straight line while Y keeps moving and X is trying to 'catch up' by taking out the backlash.
open loop backlash comp is not a cure for a loose machine, it can help in some cases but will cause problems in others...
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Jeff,
Yes, completely agree, but there are different ways of stopping. ;)
It knows it has to stop the non-affected axis. It should ramp down to a stop, then the other axis gets compensated, then they both should start off with acceleration. Of course, this means CV is not possible, but that may be better than loosing steps?
At the moment Mach just bangs the unaffected axis to a stop, compensates the problem axis gently then and bangs the other back to the velocity again.
Unless I'm wrong, reading other threads it seems there is a solution being considered and I'm wondering if / when that will be implemented.
Woody.
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Woody,
I have seen the "banging" you describe. My friend was showing me how screwed up backlash was ( he has about 1/8" on one axis )and i even posted that there was a bug ( THERE WASN'T). He was runnning the axes at 100 IPM and doing reverse movements with one of the axes during two axis moves, and yes, there was some banging going on, but no missed steps.
What do you expect to happen at those velocities......!
You can't really see what's happening at fast velocities other than the banging. If one would start with slow settings and gradualy increase them ( backlash settings and velocity ) you will find a point where the stopping of one axis and backlash implementation
creates the situation. Since weight of the table comes into play you could start to loose steps because the motor is not capable
for the axis travel. ( there are programs that evaluate motor capabilites )
So what is one to do?
Well i would experiment some and find the operating parameters that are appriate for your running condition and use of backlash. I may add that if there was no backlash and you changed direction that quickly you still have forces acting on the system
and the "unheard banging" is pounding the crap out of your machine elsewhere.
RICH
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Rich,
Yea, its painful and as Mach is, unless the uncompensated axis goes really slowly, there is no way to avoid the sudden stop in that axis. I've tried to draw what I mean. Does this make sense to you?
(http://)
1) without backlash, X axis almost constant speed, slows a bit for the Z axis to get up to speed
2) with Mach3 backlash, when Z changes direction, X suddenly stops whilst Z changes and does comp, the X suddenly starts again
3) how it could be, X predicts it has to stop (look ahead) so ramps down, Z does its thing, X ramps up again. All done together so exact profile is followed and no instant acels required.
Woody.
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1) without backlash, X axis almost constant speed, slows a bit for the Z axis to get up to speed
Maybe, i can't really see it when jogging on my lathe???? but , not during a G0 or G01 move that's active and i am not sure if compensation takes place if you do a G02 or G03 move greater than 90 degrees as I would consider that a continous move. Would need to test since i don't recall.
2) with Mach3 backlash, when Z changes direction, X suddenly stops whilst Z changes and does comp, the X suddenly starts again
If it's an X & Z move the X is intended to stop at the same point / time as Z, as soon as the backlash is implemented ( and how fast that is done depends on the backlash settings), X should accelerate per your settings and if that next move is an X & Z then move rate is governed by the slower of the axes.
3) how it could be, X predicts it has to stop (look ahead) so ramps down, Z does its thing, X ramps up again. All done together so exact profile is followed and no instant acels required.
Would still be the same motion as in 2) there is an interruption in movement although at higher speeds it may seem instantaneous.
The Maunual states " Mach3 is not able to fully honor the axis acceleration parameters when compensating for backlash. So stepper
systems will generaly have to be detuned to avoid risk of lost steps". Which brings me back to what i posted before.
"Well i would experiment some and find the operating parameters that are appropriate for your running condition and use of backlash."
Just a user like yourself, ;)
RICH
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Rich and Woodycam,
Thanks for good explication of my problem, it's exactly my problem.
It's possible find a software patch for ""fully honor the axis acceleration parameters when compensating for backlash""
At my sense why include this parameter (backlash) if not operate with other ?
Thanks
Cturc
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I am having issues with this as well, but didn't have them in version 1.84, which I just upgraded from. Hood sent me to this thread, so I thought I might ask here. My lathe does some funny stuff, but if it involves backlash then I don't know what. I have 0.002" in the X axis, and 0.020" in the Z axis. In the Z axis, it accelerates about 0.200", then slows down to almost a dead stop, then accelerates up again. I had changed nothing mechanically since the upgrade, and the XML transferred over fine. I tried not using "enhanced pulsing" on the general config page, no difference there. I noticed that the backlash setting page shows the backlash as in "units", would that be the same as the actual measured amount or?
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Did you change the shuttle setting on the General Config page? If I remember you had 1 there and according to what Rich has said it should be more in the order of 0.050 to 0.005.
Hood
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Shuttle wheel setting you mean? I thought that was for a jog wheel.
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Yes thats it, try setting to 0.05 and see or maybe even going down to 0.005
Hood
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I set it at zero and it stopped it, nice. But when I went back and looked again, Mach had changed it to 0.001, so I guess you can't have zero. Wondering why it is labelled "shuttle WHEEL setting", which is what made me think it was for a jog wheel that some had used a few years ago.
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This should explain it :)
Hood
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Makes sense, thanks Mr Hood. maybe it should be relabelled as something like "Jog wheel/ jog accel" or something like that to avoid confusion.
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Cant do that, it would mean I wouldnt be able to post pretty pictures as every one whould know ;D
Hood
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It's interesting to have a read of all the different manuals as all of them are similar but may add a few additional words about backlash.
In the lathe manual it states backlash will prevent the effective operation of CV.
In the Mach3 software installation and configuration manual it says"Using it will generally disable the CV features at corners and Mach cannot fully honor
axis acceleration parameters when compensating for backlash and detune the stepper to avoid lost steps.
Then you have the Shuttle Accel comment from Hood posted ( it's not mentioned as such in the Using Mach3 Mill manual).
So each wording gives rise to a little different understanding to the user.
So responses to problems are use exact stop, adjust motor tuning and backlash settings. No exact answers when one considers all the differences in machines.
It falls on the user to find what works for them.
The Shuttle Wheel Accel is in seconds. How exactly is the" time" applied to the backlash "cycle"? Well I don't really know. ??? It's internal to Mach. ;)
By observing the difference in how the motor reacts to different settings you will see that it affects the overall time it takes
for Mach to take up the backlash and at say a value of 5 you will find that over five seconds the handwheel / shaft will turn rather slowly.
At 0.005 sec its almost instantaneous. In the real slow setting, and say you jog a distance of .1" you will find there seems to be almost 3 distinct
motor responses in the cycle,....... a small rotation then deceleration...... then backlash taken up over a time...and then accel and move to finish the distance.
So to get of flavor of it you need to start slow, observe, listen to see what is happening over an operating range. Work your way up in settings.
RICH
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The jog shuttle thing is much better now, but it started losing steps while threading, but not any other time. I have had to go down about 25% for the Z axis speed and have the accel down too, from where it was with 1.84. This is only at 325 rpm, and a fine pitch thread at 24 TPI. Should I bump the shuttle time setting up, so I can go up again with the rapid speeds?