Threaded rod stabilization

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Threaded rod stabilization

Postby Thekaduu » 2014-Jan-Fri-15-Jan

Mainly RetireeJay and hopefully others:

What's the measurements of your brass treaded rod and where did you get it?
I have a PB+ v2.1 and am trying to stabilize the existing threaded rod by making a part for the top of the 12mm and threaded rod with bearing. I also read here https://www.evernote.com/shard/s211/sh/701c36c4-ddd5-4669-a482-953d8924c71d/1ef992988295487c98c268dcdd2d687e that having an imperial size threaded rod and printing in mm could cause problem with certain layer heights.

PB+ v2.1 has 3/8" threaded rod. I have no information on what the pitch. Where can I get this information?

Do you all think it is better to have metric threaded rod? What parameters need to be changed other than z-calibration (steps per mm)?

Thank you for your time.
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Threaded rod stabilization

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Re: Threaded rod stabilization

Postby Lemm » 2014-Jan-Fri-15-Jan

using a metric rod is better for calculating the correct steps you can use, but it's not impossible to get a really good result with an imperial rod (jay showed it in an other thread)

This calculator helps you to find a rod or the correct layer height for your rod
http://calculator.josefprusa.cz/

for your 3/8" threaded rod you have a pitch of 1,337mm. Layer heights of 0.3mm should work well
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Re: Threaded rod stabilization

Postby Thekaduu » 2014-Jan-Fri-16-Jan

I have been getting OK results with 0.3 mm layer height. I like the site but it does not have the 3/8" threaded rod :/... Oh well. I can use the site if I end up changing the threaded rod.
So, the only think I need to change when I change size of the threaded rod is the steps per mm then?

Thank you Lemm.
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Re: Threaded rod stabilization

Postby Lemm » 2014-Jan-Fri-16-Jan

I found some different informations about the 3/8" threaded rod pitch, therefore maybe somebody with 3/8 in use can confirm this or correct me when I talked shit :D
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Re: Threaded rod stabilization

Postby RetireeJay » 2014-Jan-Fri-17-Jan

I'm a little surprised that you have 3/8" rods. I thought that Printrbot was using 5/16" - 18 threaded rod until they switched over to ACME threaded rod. The 5/16 - 18 threaded rod has 18 turns per inch. It's best to do your calculation of Z step height by starting with the native units. I wonder a little where Lemm got his information that 3/8 rod has 1.337mm per turn; that comes out to 19 turns per inch after allowing for rounding error - which seems strange to me. According to Wikipedia http://en.wikipedia.org/wiki/Unified_Thread_Standard the pitch of a standard 3/8" screw is 16 turns per inch, not 19. A 3/8" Acme rod will have only 12 turns per inch.

In any event, if you look at this post viewtopic.php?f=21&t=5983, you'll see that having Imperial threaded rods does not hurt the ability to get smooth walls.

Apparently some early versions of RepRap printers used the threaded rod both to GUIDE the moving carriage and to LIFT it. So if the threaded rod was not perfectly straight and if it was not constrained top and bottom then the carriage would be moving around laterally as it lifted. However, on Printrbots we have the nice smooth rods to perform the guiding function, and the threaded rod only provides the lift. So we have much less problem with imperfect threaded rods and virtually no need for securing the rods at the top - at least that's my experience.
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Re: Threaded rod stabilization

Postby Thekaduu » 2014-Jan-Fri-20-Jan

Thanks... I double checked and it is 3/8". I'll take your word for the good job that the smooth rods do. I guess my motor to threaded rod coupler (wood) is not straight and the rod wobbling as it is coming up. I found some parametric coupler in thingiverse (http://www.thingiverse.com/thing:15633) that I can try to make it not wobble then. This just to ease my mind. I read the topic you posted which says it does not make difference in the print quality if rod wobbles a little bit.

But why brass rods? Brass is softer than steel, therefore, it should wear sooner...
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Re: Threaded rod stabilization

Postby RetireeJay » 2014-Jan-Fri-22-Jan

Ordinary steel threaded rod from the hardware store is not made for motion control; it's made for holding things together. Low-precision, high-load things, like in building construction, or repairing agricultural equipment etc. It's zinc-plated, so it's texture is rough. Generally brass threaded rod is made to tighter tolerances - or so I'm told; I can't cite industry documents to prove this - and it's smoother. When you are turning the rods with stepper motors using microstepping, it's better to have consistent low friction than rough and inconsistent friction.

A little wear is a good thing - the threads become even smoother. It looks like it will be decades of use before my brass rods are so worn that they need replacement. :)
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Threaded rod stabilization

Postby Nickies08 » 2014-Jan-Sat-00-Jan

Have you used the method in the setup guide to properly align your threaded rods to the motors? I overlooked that step originally and later found it. I would have to occasionally re-do the alignment on my Plus V2 because of accidental bumps and stuff or if a part failed and my hotend smashed into the part. Raise your carriage almost to the top of the threaded rods and undo the coupling on one motor. The rod will then hang down and sit over the motor shaft. Reinstall the coupler, ensuring not to move the threaded rod out of place and then repeat the same process on the other side. Home your z axis and watch the movement of your threaded rods, they should be a LOT MORE stable. If not, remove the rods from your printer one at a time and roll them on a flat surface. They should roll smoothly and not have any wobble or raised spots. If they wobble or have raised spots, the rods themselves are faulty.
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Re: Threaded rod stabilization

Postby Thekaduu » 2014-Jan-Sat-03-Jan

Thanks for the brass info RetireeJay.

Nikies08: I assembled the printrbot exactly as it was instructed. I just see the threaded rod wobble just a little bit, that's all. Being the person that I am, it is unacceptable :o.
Joking aside, I think the amount of wobble is fine like RetireeJay said. It's just me.

I build 2 set of the threaded rod to motor coupler and installed it already. It minimized the wobble a little bit. I also ordered 2 of 3/8" ID x 7/8" OD bearing.
I will use them to hold the barely wobbling threaded rod at the top. I already designed the part and just build one; it literally finished as I was typing this.
Attached will be the part.

Thank you all again for your comments and valuable information.
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Re: Threaded rod stabilization

Postby Lemm » 2014-Jan-Sat-05-Jan

RetireeJay wrote:I'm a little surprised that you have 3/8" rods. I thought that Printrbot was using 5/16" - 18 threaded rod until they switched over to ACME threaded rod. The 5/16 - 18 threaded rod has 18 turns per inch. It's best to do your calculation of Z step height by starting with the native units. I wonder a little where Lemm got his information that 3/8 rod has 1.337mm per turn; that comes out to 19 turns per inch after allowing for rounding error - which seems strange to me. According to Wikipedia http://en.wikipedia.org/wiki/Unified_Thread_Standard the pitch of a standard 3/8" screw is 16 turns per inch, not 19. A 3/8" Acme rod will have only 12 turns per inch.


yeah I found different information about the pitch. First this site http://www.riegler.de/page/info-tabellen/gewinde.html and then the wiki article you mentioned therefore I was a bit confused
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Re: Threaded rod stabilization

Postby RetireeJay » 2014-Jan-Sat-09-Jan

:o What?!? There's information on the Internet that's wrong?!? Who would have thought? :D
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Re: Threaded rod stabilization

Postby Lemm » 2014-Jan-Sat-16-Jan

I don't believe the information is wrong, I just think I had misunderstood it ;)
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Re: Threaded rod stabilization

Postby RetireeJay » 2014-Jan-Sat-18-Jan

Well, I don't speak German, but usually I can read tables.
WrongInfoTable.JPG

It SEEMS like the next-to-last column would be giving thread per inch (it has something to do with inches, that's for sure).

What is it really?
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Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
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E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
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Re: Threaded rod stabilization

Postby Thekaduu » 2014-Jan-Sat-21-Jan

If I want to purchase a brass threaded rod and nut, what would be the specs of both?
I thin the rod is 3/8" 19 thread per inch with 1.337 pitch... How about the nut?

Also, how about M8 rod and nut specs? If I switch to metric rod, what parameters will need to be changed?

Thank you for your time.
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Re: Threaded rod stabilization

Postby ei8htohms » 2014-Jan-Sun-03-Jan

Have you been reading the same thread as I've been reading? There aren't really 19 TPI rods around (or please find us another reference to such a thing).

Get M5 or M6 rods in brass with stainless steel nuts (or vice versa, you don't want the rods and nuts to be the same material). Or better yet, get proper trapezoidal threaded rods (and nuts).
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Re: Threaded rod stabilization

Postby Lemm » 2014-Jan-Sun-03-Jan

The informations are correct but they are for a British Standard Pipe that was my mistake. Not for a threaded rod but for a water pipe :D.
And for that 3/8" BSP the pitch is 1.337mm 19tpi.

The pitch for a M8 rod is 1.25mm
http://de.wikipedia.org/wiki/Metrisches_ISO-Gewinde

You can calculate your steps with the prusa calculator
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Re: Threaded rod stabilization

Postby wd5gnr » 2014-Jan-Mon-08-Jan

The 2.1 has PB's ACME rod which is 3/8" and the correct Z value for it is 2015.75.
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Re: Threaded rod stabilization

Postby Thekaduu » 2014-Jan-Thu-20-Jan

wd5gnr wrote:The 2.1 has PB's ACME rod which is 3/8" and the correct Z value for it is 2015.75.


My z value is 1517.00... It came from Printrbot with this value. I have been printing 5mm cube calibration object just fine. Wouldn't changing the value to what you are saying make the Z height bigger, thus, making the height of the cubes greater than 5mm?
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Re: Threaded rod stabilization

Postby Jon Lawrence » 2014-Jan-Fri-13-Jan

Thekaduu wrote:
wd5gnr wrote:The 2.1 has PB's ACME rod which is 3/8" and the correct Z value for it is 2015.75.


My z value is 1517.00... It came from Printrbot with this value. I have been printing 5mm cube calibration object just fine. Wouldn't changing the value to what you are saying make the Z height bigger, thus, making the height of the cubes greater than 5mm?

According to my Machinery's Handbook, 3/8" ACME threaded rod has 12 threads per inch.

ACME Threads.PNG

US standard threaded rod is either 3/8"-16 or 3/8"-24 (16 threads per inch or 24 threads per inch).

UNF-UNC Threads.PNG

12 threads / inch = 1511.81 microsteps/mm (ACME)
16 threads / inch = 2015.75 microsteps/mm (UNC)
24 threads / inch = 3023.62 microsteps/mm (UNF)

You probably can't tell the difference between 1517.00 and 1511.18, around 0.3% difference. No idea why they shipped with 1517.

Does this help to clarify?
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Re: Threaded rod stabilization

Postby Jon Lawrence » 2014-Jan-Fri-13-Jan

Some calculations of various threads, long hand, if anybody is interested:

Microsteps.PNG
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Re: Threaded rod stabilization

Postby Thekaduu » 2014-Feb-Sun-00-Feb

Thanks Jon. You seem to have book on that one :) i will set the z at 1511.81 to be exact. Also, if I chose to change the rod, I now have enough information to get the corrct one.

Thanks all for your help in this discussion.
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Re: Threaded rod stabilization

Postby wd5gnr » 2014-Feb-Wed-17-Feb

Interesting. I have 3 different sets of rods... maybe these aren't the ones from PB ;-)
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