Inch threads make no sense.

Inch threads make no sense. What is the standard thread pitch for the 5/16" threaded rods that are usually used as a replacement for M8 when people build SAE repraps? Is 5/16-18 or 5/16-24 more common? I’m working on a detailed writeup about choosing the right leadscrews, and I need an example for why you should always choose a metric pitch.

because (quoting the Zeus book I have in front of me) ISO Metric Coarse threads @ M8 have 1.25mm pitch. 4 revolutions = 5mm travel. Easy.

@Whosa_whatsis I believe the Printrbot uses 5/16-18.
@John_Ridley The issue is not entirely how hard the calculation is but rather if those step calculations you come up with are an even number of not when fed into the firmware. B/c the step rate is spec’d as steps_per_mm using a metric leadscrew over sae results in more even z movement and less banding.

5/16-18 has 18 threads per inch (TPI) so the thread pitch equals to 0.0625 inches or 1.5875 mm

5/16-24 is about 0.04167 inches or 1.05842 mm


5/16-18 is a lot more common and readily available at local hardware stores.

wrong math, John corrected me

@John_Ridley facepalm You’re right, I totally goofed on the math and used the wrong number. I had written down the pitch of my 1/4" acme rods which is 1/16" and typed that in instead.

As I mentioned in the other thread, the whole reason this is an issue is the push for thinner layers, which decreases the steps per layer and thus increases the percent error in the height of each layer. As a rule of thumb, this issue usually doesn’t become visible until you go below .1mm.

Here are the only layer heights you can use with 5/16-18 that won’t result in a rounding error (either due to this effect, or just because the layer height includes a repeating decimal): 63.5, 127, 190.5, 254, 317.5, 381, 444.5

Sorry, those were in microns.

444.5 microns is the largest you could do with a .5mm nozzle, which is why I stopped there.

It’s not that simple, given all the fractions used with the inch system. Your full-step length is 1/3600", so a 1/256" layer height would give you the same problem.

True, but the use of fractions makes all of that so much worse. M8 rods with a pitch of 1.25mm and a full-step length of 6.25 microns aren’t a whole lot better, which is why I recommend M6, which has a 1mm pitch.

FWIW the industry should pick one as the standard for all. SAE has the problem of flipping back and forth fractions/decimal and resulting confusion as demonstrated in the math above. In metric it starts as a decimal and remains that way. SAE nuts and bolts vary all over the place eg 1/4-20 or 1/4-28. In the auto industry most cars, American too are designed in metric and built that way excepting lug nuts and legacy engine blocks. As a fix my own s**t guy I curse when a bolt looks like metric and I need an SAE sized socket or vice versa.

When everything else in the DIY 3d printer universe is metric why mix in one piece of the other ?

@John_Ridley the real problem is when a newcomer builds a printer with SAE leadscrews - he’ll probably just try and use 0.3 or 0.25mm layers and those will show Z ribbing. Not so on a M8 rod, where the often-used 50micron increments will always work perfectly (unless he followed one of those great calibration guides and messed up his steps/mm).

Yep, 50microns imho is very much a waste of time. I was referring to increments, as in 150, 200, 250, 300… microns.