ball screw lead amount

[ger21]

With my drives quarter stepping is so close to perfect I cannot measure the error with a dial indicator, while 1:20 stepping is no better except for more quiet running.

That's basically what I said, isn't it? Or am I missunderstanding you?

If you're drives can be setup for 1/20 stepping, can you move, say 3 microsteps, and measure to see if it's in the right position?

If I can find the time this weekend, I'll try to dig up some of the info I'm talking about.

[ger21]

Sorry, not my native language. I don't know the difference between accuracy and precision.

Somewhere around here, I think in HomeCNC's building thread, is a huge debate about accuracy and precision. I'll try to post the link later.

[ger21]

Read this:
http://www.machinedesign.com/ASP/vie...strSite=MDSite

[ESjaavik]

That page is useful in one respect: It helps kill the common misconception that a stepper goes exactly where it's commanded and stay there. But it avoids any arguments for a better precision using microsteps, thus loosing credibility.

Yes, I can measure each microstep even in 1:20 stepping. If my drive electronics did not show a problem it would better my precision compared to 1:4 stepping and certainly to full stepping.

Let me see if I can include a picture here:

If that worked, you can see the current through one winding while stepping. It is not a sinewave! If you superimpose two of those on each other with 90 degrees phase shift you will see 4 discontinuities for each full wave. And that's exactly what I canobserve using a dial indicator. But between them the indicator shows each microstep and it is as close as a 1/100mm dial indicator is able to show. And this is including the stick slip not only of the motor but also of the >50Kg axis. I trust my dial indicator more than a webpage.

I write this not to flame you, but I believe you have been taken by the black/white debate on microstepping. I did myself before I actually tried it.

One thing I do realize though is that when cutter forces come into play, the axis will not be where it is without it. The same when there is a 50Kg lump of iron on the table. Because the motor has a "magnetic spring" in it, it will not go to the commanded position. But it will go to the commanded position +- errors. And most of those errors are not dependent upon where it is commanded to go even if it's a microposition. So assuming I can get rid of the uneven current shown I *will* get a better precision because I can command it to 0,00125mm plus/minus errors which is better than 0,0025mm plus/minus the same errors.

Now what about replicapro and the original question? Well so far my setup seems to be fine direct driving a 25mm x 5mm screw which is very close to his 16mm x 5,08mm screw. If I go in one direction and stop. Then go the other direction one step, the dial indicator will also change direction for that one (micro)step. So the backlash is as it should be: less than a good indicator can measure. Going from 10 to 5 pitch is no problem for the program, it's just a setting to change. The handwheel indication will be wrong by a factor of 2. But I assume the handwheels will not go back, so that is no problem either. So I'd say go for it. But use a ground and preloaded screw.

[arvidb]

Originally posted by ger21 Somewhere around here, I think in HomeCNC's building thread, is a huge debate about accuracy and precision. I'll try to post the link later.

Here it is: http://www.cnczone.com/forums/showth...&pagenumber=16

I cannot refrain from adding how I interpret these terms (*oops* ):

Accuracy: if your machine has no systematic errors, then it is accurate. That is, if, disregarding backlash and flex, your machine always moves X mm when commanded to move X mm, then it is accurate.

Resolution: if the smallest movement you can command is 1 micrometer (um), then you have a resolution of 1 um. This is *not* the same as accuracy, since your machine will (most likely!) not be able to move and keep position within 1 um!

Precision: the ability to return to the same position many times (= repeatability). A machine with lots of backlash or flex is imprecise.

You cannot make an accurate part (= a part that lives up to spec) unless you have enough of all of the above.

Arvid

[High Seas]

Arvid - I'll add my OOPS at the risk of doing a "restart" on the previous discussion:
System Resolution also includes effects from the drive mechanism (ie, rod in this case).

TO divert from the CNC for a moment, maybe use the CRT/digital camera or similar analogy - maybe that helps. A great video card can have excellent resolution - but you can't see the results on an 80 column green screen CRT.
Or another example; my aging digital camera won't take advantage of the video card or a HiRes screen, 'cause the system resolution is limited by the "weakest link," in this case the camera.
The controller may command and produce accurate positioning and micro step - but the tool can only reach the Resolution of the combined components, controller, motor, drive system, mechanical rigidity of the table, etc.
Sorry for so many mixed metaphors, just trying to keep it simple. In most hobby uses the results are most likey better than "good enough!" Striving for Six Sigma is important - just not always necessary.
Cheers - Jim