Dimensional accuracy of T8 lead screw threads

I got a “ReliaBot 400mm T8 T8x4” lead screw from Amazon for about $14. This is a 2-start thread and one rotation moves the nut 4 mm. I don’t have any kind of CNC machine yet, I’m just playing around with things so far. The first thing I looked at was the dimensional accuracy of the thread itself (unrelated to the nut or any other mechanism) with some digital calipers and I was surprised to see the measured pitch is about 0.3% shorter than claimed. For example, a distance of 128.00 mm should cover exactly 64 thread forms, but in fact this thread measures 127.61 and I’m pretty sure my Mitutoyo calipers are not that far off (esp. when another brand of digital calipers agrees exactly). Is this dimensional accuracy typical for hobby-grade leadscrews?

Welcome to the forum.

Its like anything else, the more you pay - the better you get.

It doesnt really matter though. Assuming you are building a CNC or 3D Printer, you’ll be calibrating the steps per MM.

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Very true! My thought was making some sort of CNC engraver eventually. I’m so far disconnected from the current state of production that I somehow imagined that lead screws for positioning were generally precision-ground parts. I had not considered that the cheap ones would likely be formed by thread rolling, with the looser tolerances accordingly.

“As a general industry rule, lead screw accuracy is defined using a 300 mm (V=300) or 12-inch length of lead screw. This method works fine for lengths that are over 300 mm, however, this does not provide accurate tolerances for smaller travel applications.”
Lead Screw Applications for Medical Devices and Lab Automation.

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Boy, I wonder if this is a manufacturing thing or something else. Sounds like it shouldn’t be a problem when you calibrate steps/mm but if two purchased are different I would start to look at how accurate that is across the length of the lead screw.

I’m not terribly surprised that the pitch is that far off on a $14 screw. As noted in other comments, with CNC you can compensate for the unexpected pitch, however a bigger problem would be inconsistent pitch along the screw.

If the pitch distance changes along the screw, that’s not something that can be compensated for without direct measurement of movement (e.g. by a glass scale) and a lookup table recording where the screw is off. Then it takes smart software to use the lookup table when calculating step counts during motion. This isn’t a normal feature of hobby machines or even factory machines unless they’re holding the tightest of tolerances.