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Don't understand number I needed to calibrate my Z axis

So I got my stock z axis motor yesterday, and hooked it up to my custom Z axis, which consists of a standard 8mm diameter, 2mm pitch lead screw. I changed the setting for pitch to be 2mm, and things started out way too fast. After doing some testing, it turns out that it comes out accurate if I have pitch to be 8mm. This is with the default of z encoder steps at 7560 per rotation. Note I have verified that there is no way the lead screw is 8mm pitch, as I measure it to be 2mm. So does anyone have any idea why I needed to use 8mm instead of 2mm for pitch?

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I have no idea :upside_down_face:

The pitch should be number of mm moved per rotation. The fact that it is off by a factor of 4 (2x4=8) makes me thing it could be confusion around quadrature encoding which means that we actually read four times more pulses than the encoder sends my watching the transition times of the signals.

I thinks we need to clarify thread “pitch” and “lead”. The course screw on the Ridgid router is a single entry screw and therefore pitch and lead is the same. However if your “custom” z-axis has acme screw or other, it may be that you have a 4 entry screw that would make your lead 4x the pitch and therefore it would move 4x the pitch with every rotation.

Picture is worth a thousand words.

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^^what he said with more pictures


IMG_20180428_010223
Those are the ones included in my kit, i believe they are quite standard lead screws

So should the setting description be changed from “pitch” to “lead”, perhaps with some explanatory text somewhere, since this is an obscure but important distinction?

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@theratchet Thanks for the info, although I can’t visualize what is happening with the 4 start lead, it seems that yes this is what it is.

Bonus is I was worried that I was going to make the z even slower than stock, going from 3.17mm pitch to 2mm pitch, but happy to know it is more than twice as fast.

Try this: take a piece of string or dental floss and wrap it around your lead screw such that it never leaves the groove it starts in. Here’s an example with the lift screw on my table saw.*

As you can see, the thread on my lead screw causes the string to come back around to the groove right next to the one I started in. In other words, the groove in my lead screw is one continuous groove. It’s a “one start” lead screw.

On a four start lead screw there are four grooves, so if I wrap a string around a four start, the string will come back around with three intervening grooves. Because of this, every 360° of the screw causes the nut to move four thread pitches.

I’m not sure if I explained that well. Did that make sense?

*Don’t ask me why my table saw is torn down so far that I can photograph my lead screw. It’s a long story in which Mattias Wandel got me curious about something and “made me” try an ill-advised experiment. Replacement parts are on order.

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Yep makes sense, mind still blown. This picture made it clearer for me also: https://upload.wikimedia.org/wikipedia/commons/2/2e/Lead_and_pitch_in_screws.png

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I read matias wendel, i press like :smiley:

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Ok! Good morning, I have a 8mm rod upgrade kit for z axis. In which part of GC I have to change the value for match the new rod?

Thanks

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In GroundControl -> Settings

Kind regards, Gero

Edit: If it turns the wrong direction, add or remove the - (minus).

Thanks!!!

And don’t miss checking the travel per rev based on how many parallel threads there are

4 start TR8-8, 8mm/rev, is great for Printrbot Z screws but I’d suspect the extra oomph (i.e. greater torgue) of 1 start TR8-2 is better for Maslows. It might be nice if somebody tested that, though