LinkBack URL
About LinkBacks
Mine (on average) are about the same, maybe slightly better.
Phil
Originally Posted by Mooser
| | |
Just wanted to get some feel for what you are getting.
When restarting the machine (next day or whatever) and re-homing (ref-all)
How far off are you finding your G54 co-ords to your fixtures, vice jaw or whatever?
I've been moving the axis about 3 inches from home before ref'ing them and when re-clocking I'm usually about .002-.003 on the X, closer on the Y, maybe .0005-.001
Anyone getting better? worse? tips?
Mooser
Mine (on average) are about the same, maybe slightly better.
Phil
"Home" of course has not necessarily been designed with repeatability in mind. It is also clear that there is lots of scope to improve repeatability if the need is great enough.
Tormach (Greg Jackson) commented on "Home" repeatability somewhere/sometime. Possibly it was on the Yahoo group, or maybe here.
Phil
It is determined by the action of the limit switch. My X and Y got gunked up at different times and the switch action was really slow. A few times the Y axis crashed at the physical limit and other times ate up a couple of inches travel before releasing. Replacing with optical switches would increase accuracy but you would still want to re-zero your machine relative to your main fixture as a daily habit.
Mine is about the same. Sometime I get my limit switch in X don't work very well, The machine will reach the limit switch but come back 1 or 2 inch. When it do this, I re-home... If we could get a better reference return it would of great help for me.
It would be nice to turn on the machine without having to re-clock the fixture each time.
I'm trying to think of how the nc's at work run, Their home switches must be more accurate somehow. I've got to look at the mazak and see how it works, I'm pretty sure they are mechanical.
I've actually got a set of Fagor glass scales over one the bridgeport that I've been looking at. Seems to me that Mach3 can accept a feedback signal.
Then again, my touch probe is coming along nicely that should make re-clocking quicker.
Gives a person something to think about during longer cycle times
Mooser
For what it's worth, I've been lurking on this site for a while considering a Tormach in my very near future.
It sounds like the Tormach uses a simple switch to determine home position. I know on some of the other machines I've worked on, they use a two-step process to determine home position. A limit switch is used to trigger a reversal in the direction of the ballscrew and then the controller looks for the next index pulse from the shaft encoder. These machines typically use a servo motor rather than steppers, but the concept would work for steppers so long as there is a signal equivalent (i.e. one pulse per ballscrew revolution) to an index pulse.
Obviously the controller must be programmed to reverse upon hitting the limit switch, but in theory, it's pretty simple and very repeatable.
Rick
I checked the repeatability of the X Y axis on the ref all command and both were about 0.001". When checking Home position for repeatability is the Ref All command done at the same temperature? Are the limit switches and limit stops clean and free of chips? I know in the mornings this time of year the temperature in my shop can be 40* F and by afternoon be 65*F. Remember expansion is in/in or cm/cm so the longer X axis will change more than the Y axis for the same temperature difference.
But if the limit switch triggers at + or minus 0.001" and then the controller reverses 1 step or 10 steps isn't the positional repeatability is still + or minus 0.001".
Phil
On most machines they use a combination of a home switch and the Z output on the encoders, this is a single index pulse output that happens once or on some machines a few times per revolution of the ball screw.
They look for the home switch to close and then start looking for the index pulse from the encoder. By doing this repeatability of the home position about the same as the resolution of the encoder.
Phil,
I believe that using a two-step homing process will improve the situation if the positional repeatability is due to the switch (and not backlash in the ball nut).
In the two-step process, we are actually using the index pulse of a shaft encoder to represent 'home', rather than the position of table is at when the limit switch changes state.
The assumption here is that the index pulse on an encoder is much more repeatable than a mechanical switch changing state. This process requires that the switch is repeatable within one full revolution of the shaft encoder. This is usually not a problem. So, we use the mechanical limit switch to trigger an event that reverses the stepper motor and start looking for the very next encoder index pulse. Once the index pulse is found, we call this 'home'. Everything is referenced from this location, not when the limit switch changes state.
The beauty of this approach is that you can have a fairly sloppy limit switch and it should not impact home reference repeatability.
Rick
Ya sorry, I missed the significance of the "index pulse" as opposed to a "step". I guess that obtaining the pulse on a stepper setup is a whole different ball game though.
Phil
Posting Permissions
| | |
