Backlash
Is there an Admin command to enter the backlash figures for the X Y Z axes?
Similar Threads:
mike sr
Re: Backlash
No!
These settings should be used with caution and are not recommended (at least not be me) unless the consequences are understood. If your machine is not well set up they can cause more harm than good. Tormach are not currently supporting this feature although obviously it is available in LinuxCNC.
Step
Sent from my iPhone using Tapatalk
Re: Backlash
Hello Step,Originally Posted by TurboStep
I tried calling Tormach, they were all busy, just to see what the ramifications were or why they dont use it. I install it manually for each axis as my probe isnt totally accurate without it or doesnt seem to be. If it causes machining problems then maybe I need to rethink this??
I am using a converted Renishaw MP3, and while setting it up I entered the backlash readings, everything seems to be right on with the backlash set. In your opinion can it cause problems elsewhere, if so I am all ears??
thanks
mike
mike sr
Re: Backlash
Hi Mike
I bought a "new" MP8 off ebay a while back. I haven't had chance to get it set up yet but it came with a test certificate from 20 Jan 1998 (the probe is however in unused condition) and the test results are amazing! From over 60 measurements only one had an error of over 0.5 microns. It would have been even more amazing if the test certificate had the same serial number as my probe!!! I've no idea yet how well my "new" probe will actually perform but I suspect if you're not using the probe electronics the errors could possibly be significantly higher.
My interpretation of backlash correction:
Backlash, as opposed to lost motion, is how much "play" is present in each axis. To make the calculations simple let's say that a particular axis has 2 thou backlash (I like to consider this to be +/- 1 thou) and the rest of the machine is almost "ideal" (stiff, low friction etc). When the axis is moved in the positive direction to a set position, say zero, the mechanics will usually lag behind the commanded position by half the backlash, or in this example -1 thou. If additional force is then applied in the direction of the previous movement the mechanics may continue to move to the other end of the available play. The actual position will now be +1 thou. The actual position can therefore be within +/- half the backlash, or +/- 1 thou maximum. Symmetrical around the requested position.
Now if we add compensation such that when the axis moves positively to zero, the position commanded by the software is actually +1 thou, the mechanics will lag 1 thou behind and stop at exactly zero - great! However, if additional force is applied, the mechanics will be able to move an additional distance equal to the backlash i.e. 2 thou. The worst case error would then become +2 thou - twice the value compared to the maximum value without the compensation! Bearing in mind that this is the tolerance for one surface. Machining 2 parallel faces could produce an error of 2 thou per side - a total of 4 thou error.
These forces can be cutting forces or, in the case of the Z axis, the weight of the head alone. The Z axis is however slightly different. Most mill probing routines set the reference while moving in the head in the downward direction. On my machine (and I presume all similar machines with properly adjusted gibs) the head will lag behind the command position under these near static conditions. However, when the spindle starts, and at the latest when the tool enters the cut, the head will experience a downward force (c.f. tool pullout). This will pull the head down to the other end of the backlash, so for the above example the head would now be 2 thou lower than expected. If all Z movements into the workpiece are downwards then correcting for backlash on the Z axis won't change this, it won't be any worse, but it won't be any better either. I can imagine that the surface finish of 3D work could suffer with backlash correction enabled, but I haven't tried it, so I can't comment.
I'm constantly amazed when people report they've been machining with backlash values of 3 to 5 thou and didn't even notice! Adding 5 thou backlash compensation to a machine like this could result in a total error of 10 thou between 2 faces! This is not simply a cure for a poorly adjusted machine.
The first priority must be to set-up the machine to reduce the backlash to within, or better than the specified limits - yes I'm aware that this can be difficult. The specified tolerance limits are more than good enough for the vast majority of Tormach users. Backlash correction then becomes irrelevant to all but a few nuts like me
Maybe you see this differently?
Regards
Step
Re: Backlash
Great on the acquisition of the MP8, you wont be dissapointed!Hi Mike
I bought a "new" MP8 off ebay a while back. I haven't had chance to get it set up yet but it came with a test certificate from 20 Jan 1998 (the probe is however in unused condition) and the test results are amazing! From over 60 measurements only one had an error of over 0.5 microns. It would have been even more amazing if the test certificate had the same serial number as my probe!!! I've no idea yet how well my "new" probe will actually perform but I suspect if you're not using the probe electronics the errors could possibly be significantly higher.
My interpretation of backlash correction:
Backlash, as opposed to lost motion, is how much "play" is present in each axis. To make the calculations simple let's say that a particular axis has 2 thou backlash (I like to consider this to be +/- 1 thou) and the rest of the machine is almost "ideal" (stiff, low friction etc). When the axis is moved in the positive direction to a set position, say zero, the mechanics will usually lag behind the commanded position by half the backlash, or in this example -1 thou. If additional force is then applied in the direction of the previous movement the mechanics may continue to move to the other end of the available play. The actual position will now be +1 thou. The actual position can therefore be within +/- half the backlash, or +/- 1 thou maximum. Symmetrical around the requested position.
Now if we add compensation such that when the axis moves positively to zero, the position commanded by the software is actually +1 thou, the mechanics will lag 1 thou behind and stop at exactly zero - great! However, if additional force is applied, the mechanics will be able to move an additional distance equal to the backlash i.e. 2 thou. The worst case error would then become +2 thou - twice the value compared to the maximum value without the compensation! Bearing in mind that this is the tolerance for one surface. Machining 2 parallel faces could produce an error of 2 thou per side - a total of 4 thou error.
These forces can be cutting forces or, in the case of the Z axis, the weight of the head alone. The Z axis is however slightly different. Most mill probing routines set the reference while moving in the head in the downward direction. On my machine (and I presume all similar machines with properly adjusted gibs) the head will lag behind the command position under these near static conditions. However, when the spindle starts, and at the latest when the tool enters the cut, the head will experience a downward force (c.f. tool pullout). This will pull the head down to the other end of the backlash, so for the above example the head would now be 2 thou lower than expected. If all Z movements into the workpiece are downwards then correcting for backlash on the Z axis won't change this, it won't be any worse, but it won't be any better either. I can imagine that the surface finish of 3D work could suffer with backlash correction enabled, but I haven't tried it, so I can't comment.
I'm constantly amazed when people report they've been machining with backlash values of 3 to 5 thou and didn't even notice! Adding 5 thou backlash compensation to a machine like this could result in a total error of 10 thou between 2 faces! This is not simply a cure for a poorly adjusted machine.
The first priority must be to set-up the machine to reduce the backlash to within, or better than the specified limits - yes I'm aware that this can be difficult. The specified tolerance limits are more than good enough for the vast majority of Tormach users. Backlash correction then becomes irrelevant to all but a few nuts like me
Maybe you see this differently?
Regards
Step
I think what I am getting out of this is that backlash should be set under a no load condition which is what I have done. Steel is a spring of sorts and extra pressure can cause more lost motion etc. I also was curious about the effect of it but in my 3d carb venturi's the finish seems to be the same and in the other 3d parts the finish seems to be the same at least visually. I do have a problem with machined holes not being perfectly round etc. I took the backlash figures out and tried cutting the same hole and had about the same error. I also cut a hex and had the same result, the flats were off by about .001-.0015.
For my probe to be accurate, I needed the backlsh readings set for X and Y. I can probe a hole then move the machine off in X and Y, then re-probe the hole and install the cutter and it will drop back into the same hole,that was probed previously there is probably still some measured error but visually it looks to be right on... close enough for me.
I think you will really like the probe, the interface box would make it even more accurate from what I have read, mine is usually right on or off by maybe a tenth.0001 at times.
I really like the fact that you can drop the probe into the hole and one button push and it will locate center, I usually do this twice, the second time the probe is on center and doesnt contact X on an angle other than 90*.
mike sr
Re: Backlash
Mike, what's it take to convert the MP3 for a Tormach?
Re: Backlash
Hello John,
I used the same MP3 that Brian Bone Doc used, the one that has an inductive arm on it, I removed the arm and connected the Tormach leads to the socket, reasssembled it and installed the DIN plug that matches the Tormach, basically thats it. There wa a center section that had the sensor circuitry in it, just remove that.
I removed and machined the K&T holder down to a .750 shank so I could use it with my TTS.
The probe is just basically a sensitive make break switch, there are no electronics in the probe head itself. I use mine directly connected to the Tormach probe input, there are interface boxes that these are used with on the large machines, I am not using that.
I think all this is documented with pics in the "Renishaw probe" post, from a few months back. I absolutely love the thing! I used it today to indicate a small engine carbruetor fixture this evening. I mostly indicate holes and bosses so its just a one button push, it goes thru the routine and in a few seconds it locates center and zero's the axes in X and Y.
I did need to put the backlash figures in for the XYZ axes to make it as accurate as possible, those are in the Tormach_1100-3.ini file, it works fine without this but is not as accurate by a few tenths.
If you buy one from Ebay, question the condition of the rubber boot and if it works or not and if it can be returned if it doesnt work. The used ones in most of our hobbist price ranges can have ruptured boots eaten up by some types of coolants or other chemicals etc.
In my opinion its well worth the effort involved then some!
Last edited by popspipes; 11-26-2017 at 10:36 PM.
mike sr
Reply with Quote
