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I want to move a small tool table/bed linearly, so I'm designing its mechanism. Fairly simple, it's going to have 2 rails and a lead screw.

Browsing the web I find that locally the shortest trapezoidal thread rod I can get is 1m at the price of \$10 as well as \$8 for the nut and you can imagine that's me right now:

enter image description here

So I was thinking, I have plenty of metric threaded rods and nuts laying around, and even if I had to buy new ones they cost next to nothing, but I've never seen one used as a lead screw for a mechanism that needs precise-ish movement. And looking at wikipedia I found that metric threads are also trapezoidal, just at a steeper angle, so I'm guessing thats key.

Anyway, can a rod with metric thread be used as a lead screw with precision requirements of 0.5mm at worst?

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  • $\begingroup$ I don't understand your problem and question. BTW, what is a "lead screw"? $\endgroup$
    – r13
    Sep 23 at 15:49
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    $\begingroup$ what us the purpose of the image? ... it seens unrelated to the question $\endgroup$
    – jsotola
    Sep 23 at 15:56
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    $\begingroup$ @jsotola I think this is what younger people than me call internet meme $\endgroup$
    – NMech
    Sep 23 at 17:05
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    $\begingroup$ With appropriate nut material, you can totally use a "regular" UNC/UNF or ISO thread as a drive screw. It might be done to take advantage of fine pitches for precision adjustment when loads are insignificant. Lead screws reduce the angle from 60 deg to 30 deg to make it more efficient, and the pointy end of the thread form is then cut off into a trapezoid for strength reasons. Trapezoidal is the standard because of loading. $\endgroup$
    – Pete W
    Sep 23 at 17:42
  • $\begingroup$ At the other extreme, 0 degree angle in cross section (rectangular thread form) exists too. You might see it very heavy or very old equipment. It has the benefit of being insensitive to lateral misalignment of the screw vs nut (the nut would be designed with generous radial clearance). In contrast to lead screws having an angle, where misalignment would generate side forces, friction, and possibly bending of the screw/rod $\endgroup$
    – Pete W
    Sep 23 at 17:48
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It's not exactly clear to me what you mean by trapezoidal thread rod. Like you noticed Metric threads are also trapezoidal (just like ACME with slightly difference face angels).

enter image description here

Figure : Common types of bolts (source linearmotiontips)

To my understanding (from the context of the question) the main differences that you are likely to find between the expensive thread rod and the cheap metric thread rods will be at:

  • the surface treatment of the thread rod (that includes heat treatment and treatments like polishing)
  • Some times there is coating. Sometimes this is PTFE (Polytetrafluoroethylene ) which creates a low friction surface. This is not present on Metric threads
  • The Lead angle (you can probably find lead screws with the same lead angle but the reverse is not always possible).

Regarding your question:

Anyway, can a rod with metric thread be used as a lead screw with precision requirements of 0.5mm at worst?

Yes it is possible, however there are a lot of asterisks (even the weight on the linear stage would be a factor).

The actual precision part is easy if you use gears.

The difficult part has to do with accuracy and repeatability. Problems arise there from the backlash caused from axial clearance.

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  • $\begingroup$ Indeed what I was afraid from but didn't know the proper terminology. Is there anything I can do to minimize backlash? $\endgroup$
    – php_nub_qq
    Sep 23 at 19:59
  • $\begingroup$ I wouldn't worry that much if its for positioning a bed table. If you put the intended use in perspective, you should be fine with a new threaded metric, which is properly lubricated. For the intended application, and if you get a fine thread, you should be more than fine. $\endgroup$
    – NMech
    Sep 23 at 20:45
  • $\begingroup$ Example: With an M6 threaded rod, limit switches, and, crucially, always approaching from the same direction, getting better than 10um repeatability was pretty easy (mass of a few hundred grams mounted on a linear slide, Oldham coupling to a stepper motor, leadscrew had its own bearings, and I can't recall how they were mounted but that needs to be optimised). Given that, 0.5mm precision would be easy $\endgroup$
    – Chris H
    Sep 24 at 9:20
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You can use regular threaded rod as a lead screw. The size and pitch (and grade) of the thread will determine your precision but 0.5 mm seems overkill for furniture positioning. The smaller the screw the finer the pitch and therefore precision will be, but at the cost of reduced load capacity. Just keep the rod lubricated and you shouldn't have issues.

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  • $\begingroup$ My wording was probably misleading, it is a metal plate (table) that holds a tool which has to move linearly in steps of 5mm each, where I'm assuming that a 0.5mm error at max would be acceptable. $\endgroup$
    – php_nub_qq
    Sep 23 at 19:55
  • $\begingroup$ I'm mostly concerned about backlash since iso threads were not made for this. $\endgroup$
    – php_nub_qq
    Sep 23 at 19:57
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    $\begingroup$ there's usually more clearance built into typical lead screw type threads (Acme style) than fasteners, you should be OK. $\endgroup$
    – jko
    Sep 27 at 11:45
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Well, the question has already been accepted, however. I'll mention something regardless.

First of all, it's perfectly fine to use a plain threaded rod for linear motion. You mostly simply need to consider what your basic requirements are, and your willing budget.

What I'm most surprised about, is that no-one here even considered to mention all the ways you can use ingenuity to solve the problem when money is a limiting factor. This was the common factor amoung ALL of the thousands of shitty 3d printer designs over the years, who all used threaded rod for various movements, and came up with tonnes of things to correct issues (like backlash). Some of them using software solutions or mechanical ones.

But first to list what you want:

  1. Linear movement, +/- 0.5mm precision
  2. Cheap

If you want to easily cheat, without spending any money at all, but are willing to get dirty, you simply go dumpster diving for old printers. They all have linear rails and different movement stages, some (most) have belts and pullys, some high end ones may have proper lead screws. Sometimes you can find really great parts from just old machines thrown away. There are countless backyard engineers who have made entire CNC machines using garbage they found.

This one of my favourite examples

The next option is to consider which standards you're up against. Zinc coated threaded rod is really junk...But A4 stainless steel metric rod is much better. It's tolerances are much smaller and it's quality is generally very high while still being fairly cheap..

But you say, what about the backlash?

Well bachlash is a problem regardless of which screw system you use, even leadscrews can suffer from them (why do ballscrews exist?) So how have people over the years corrected for backlash at all?

Well there are tonnes of tricks for this.

Have a look at some mechanical libraries (never invent things twice if you don't have to, mech engineering has barely changed for hundreds of years, someone has probably already invented what you want to do)

KMODDL

Digital Mech Library

Here is a search result for "backlash"

As quick tip, consider if you have 2 nuts on your rod that are tightened slightly in opposite directions on your rod....they can't have play if they are mechanically forced against that play can they?

Ofc there are problems with that...more friction, etc...but then we have oil right?

I also suggest becoming a regular reader of "hackaday.com"

There are tonnes of great ideas on there.

Some search results there

Threaded rod

Backlash

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  • $\begingroup$ Thank you for the advice and sources, it is greatly appreciated!!! $\endgroup$
    – php_nub_qq
    Sep 24 at 19:27
  • $\begingroup$ You're welcome, feel free to ask more questions around here :) $\endgroup$
    – morbo
    Sep 24 at 19:44

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