Z-Axis thread rod pitch ?

I have a mechanical background.
I have a lathe, I have thread gauges and I cut my own threads on a lathe.

Yet I can’t understand what I am seeing referenced about these Z- axis threads

My machine, GeeeTech I 3 C Pro has the factory installed rods.

20 revolutions moves 25mm. So that is a 1.25mm thread pitch
8mm on the major diameter

Yet I keep seeing z-Axis lead screw sold as 8mm pitch

Is that correct 8mm ? So one revolution moves the gantry 8mm vertically ?
Seems very inaccurate . Surely not ?

Can anyone explain

Thank you

Your lead screw is a likly to be a single start

many 8mm are 4 start and yes they really move 8mm

inaccurate?

most are connected to 200 step revolution steppers at 1/16 microstpping so 3200 steps a revolution.
That is 8/3200 = 0.0025 mm per step.

I popped the drivers off yesterday and checked the jumpers. yes all connected so 1/16 stepping

Theoretically yes a good accuracy, but mechanically a lift of 8mm over one revolution as opposed to 1.25mm per one revolution Far finer control with the 1.25mm...but with 4 lead, then yes, a more 'square pull' of the threaded collar up the rod.

I believe the previous owner did try some other rods that were a greater pitch, no doubt these 8mm ones but he put them back to the 1.25mm ones as the comment he made was that the steppers did not have the torque to drive that steep a pitch. But I am yet to try. But with enabling the Z2 and two drivers, that should help with that.

I will see if he still has the rods . otherwise will bu some more. any recommendations as to quality...or makes suppliers to avoid?

He just got back to me
He has the two new rods, he fitted but removed as he could not get them to work




Edited 1 time(s). Last edit at 06/21/2023 06:56AM by NeilP.

Thread pitch is the spacing between adjacent threads on the screw. Lead is the distance the nut will travel in one rev of the screw. With single-start screws, pitch = lead. With 2 start screws, lead = 2x pitch, etc. You can tell how many starts a screw has by looking at the end of the screw and counting the notches.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Yep, thanks
I had not considered them being multi start threads initially, so a thread pitch of 8mm just sounded way off when looking at it ( the 8mm diameter set screw)
8mm metric course is 1.25mm, , metric Fine 1.0

So looking at the pictures, of what was being sold as 8mm pitch did not compute.. the threads just looked too close.. but I was not considering the multi start

I had vaguely thought about the multi start threads, but not to any degree until you guys put me right.

Cheers

What I was trying to say was that the 8mm pitch number you saw was not actually pitch, but 8mm lead with 1.25 mm pitch.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I was thinking lead = pitch, because I was only thinking of single spiral / start threads.. so it does move 8mm per turn, even though distance between threads is 1.25 mm

I had not heard of the term ‘lead’ before. To me pitch was the same.. but I get it now

So a 4 start rod, with 8mm lead would have 2mm pitch ?

Yes.

That's why they're called "lead screws", not "pitch screws".

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Ha! I’d have just called it threaded rod !

Learn something new every day