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Topic: mm/rev not accurate to measured pitch (Read 478 times) previous topic - next topic

jtbennett

Hi - I ordered a generic linear actuator set from amazon (here) and it comes with an 8mm leadthread that has a 2mm pitch. When I used this calculator (1:1 gear ratio, 1/4 step), it says 400 steps per mm.

After testing this out, I found it to be inflated, and did some back and forth testing and came to the conclusion that 325 was a much closer estimate (which would be a 2.4615mm pitch). I'm certain it's 2mm from crest to crest, so I'm really not sure where the accuracy is being lost.

I'm not 100% sure that 325 is giving me a perfect mm per rev, just much closer - I would like to get perfect accuracy but I was hoping a calculation would do that for me...is it common for these things to be off?

Paul_KD7HB

Hi - I ordered a generic linear actuator set from amazon (here) and it comes with an 8mm leadthread that has a 2mm pitch. When I used this calculator (1:1 gear ratio, 1/4 step), it says 400 steps per mm.

After testing this out, I found it to be inflated, and did some back and forth testing and came to the conclusion that 325 was a much closer estimate (which would be a 2.4615mm pitch). I'm certain it's 2mm from crest to crest, so I'm really not sure where the accuracy is being lost.

I'm not 100% sure that 325 is giving me a perfect mm per rev, just much closer - I would like to get perfect accuracy but I was hoping a calculation would do that for me...is it common for these things to be off?
Your measurements seem to be correct, but you did not report measuring the stepper motor rotation to see if you can get 1 complete rotation using the 1/4 step mode.

Paul

jtbennett

Ahhh - good idea :}

I'll do that in an hour or so and report back, sorry for being incomplete...I'm new to calibrating these things.

Idahowalker

Ahhh - good idea :}

I'll do that in an hour or so and report back, sorry for being incomplete...I'm new to calibrating these things.
Did you read any of the feedback on Amazon to see how inaccurate this product is?

jtbennett

#4
Jun 05, 2019, 12:12 am Last Edit: Jun 05, 2019, 12:14 am by jtbennett
Did you read any of the feedback on Amazon to see how inaccurate this product is?
Well they weren't whining about the thread, just the drill points and such. They call it a T8 rod, and it is 8mm, but the thread pitch is not mentioned. I've got a 1.8deg stepper motor on it and all of it is functioning fine, just as expected...just that threading math isn't proper...

e: i'm making a prototype in my bedroom while my workshop is being transported to a new location, and I really can't be doing any real work in here, so this $30 set was really convenient as far as that goes.

jtbennett

Your measurements seem to be correct, but you did not report measuring the stepper motor rotation to see if you can get 1 complete rotation using the 1/4 step mode.

Paul
So, 800 steps gives me exactly 1 rotation, as expected on 1/4 step. I'm very confused now :\

Paul_KD7HB

i see they have a brass threaded nut assembly on the threaded rod. Just for fun, Press the nut hard toward one end and measure the distance from the nut to the end of the rod. Now, press the nut the other way and measure the distance. A good fit will show a very small difference in length. A proper nut would be a two piece nut on the shaft. one tightened against the other in order to remove the slop in the nut threads. The faces of the nuts would be sanded or shaved down until almost all of the sloppy movement disappears and the two nuts are jammed together tightly. Your assembly, from the picture does not do this. Also, the threaded rod and the nut must have grease or oil for lubrication.

Paul

jtbennett

#7
Jun 05, 2019, 12:58 am Last Edit: Jun 05, 2019, 01:00 am by jtbennett
i see they have a brass threaded nut assembly on the threaded rod. Just for fun, Press the nut hard toward one end and measure the distance from the nut to the end of the rod. Now, press the nut the other way and measure the distance. A good fit will show a very small difference in length. A proper nut would be a two piece nut on the shaft. one tightened against the other in order to remove the slop in the nut threads. The faces of the nuts would be sanded or shaved down until almost all of the sloppy movement disappears and the two nuts are jammed together tightly. Your assembly, from the picture does not do this. Also, the threaded rod and the nut must have grease or oil for lubrication.

Paul
I don't have my calipers with me and that's what I'd need to measure this difference, it's about half a hair's width of play...I imagine it might throw things off running all the way down the line, but I'm seeing a ton of inflation on just 25mm back and forth at the suggested pulse per mm.


I'm just gonna go back and check my sketch math now, I'm pretty sure it's all sound, but no harm checking. If there's any other possible culprits, I'll entertain them all :}

Paul_KD7HB

The play is only a problem when starting a back and forth movement. It may be a problem when initially starting a movement.

When the nut is actually driving something with some resistance to moving, the thrust bearings at the ends of the threaded rod may allow movement that is a problem. Except for ball screws, all of your bearings will allow movement.

Paul

jtbennett

#9
Jun 05, 2019, 01:40 am Last Edit: Jun 05, 2019, 01:59 am by jtbennett
The play is only a problem when starting a back and forth movement. It may be a problem when initially starting a movement.

When the nut is actually driving something with some resistance to moving, the thrust bearings at the ends of the threaded rod may allow movement that is a problem. Except for ball screws, all of your bearings will allow movement.

Paul
I have a new development here actually - I am finding the steps to be exactly 4x too high...as in, my driver is on full-step mode. The thing is, it's not on full-step mode. They're TB6600s and the step control is set by configuring the switches on the side of the device, so there's no messing it up unless you don't know which way is on (I assure you I do).

Am I missing something here? Does AccelStepper somehow override my driver settings or something? I am confused still, but slightly happier possibly knowing what's going on..


e: 1 full revolution is still 800 pulses, so it's definitely in 1/4 step mode....this is mental...

groundFungus

#10
Jun 05, 2019, 02:14 am Last Edit: Jun 05, 2019, 02:19 am by groundFungus
Some lead screws have multiple "starts".  That affects the turns per inch.   See this page.

jtbennett

#11
Jun 05, 2019, 02:54 am Last Edit: Jun 05, 2019, 03:09 am by jtbennett
Some lead screws have multiple "starts".  That affects the turns per inch.   See this page.
There are 13 full threads per inch, which brings my pitch to 1.954mm and calculates to 409.42 steps per mm. I have only seen accuracy at far less than this number, so is it possible that I have 4 starts? Or more likely 3?


e: I'm not sure if I'm doing my calculations correctly now because I was under the impression that this rod was of the metric persuasion since it has an 8mm diameter. I will post everything that I know of this rod here and maybe someone can just throw a number at me :}

Rod:
--------------
metric:
-8mm diameter
-2mm thread-to-thread

imperial:
-13 TPI = 0.07692in, or 1.954mm

# of starts: 3 or 4 maybe? :|. I read the article but I didn't pick up how to check to see if there were multiple starts, only that some manufacturers do that...what in the case of unknown manufacturer specs?

Motor
--------------
-1.8°


Stepper driver
--------------
-1/4 step
-800 steps per rotation

jremington

Quote
I didn't pick up how to check to see if there were multiple starts
Just look at the end of the lead screw.

https://www.quora.com/What-is-the-difference-between-a-single-start-thread-and-multi-start-threads

Paul_KD7HB

If you indeed do have an inch lead screw, you cannot do metric properly. Just as the lead screw in my ENCO lathe is inch and I cannot do metric threads. But my other lathe at the plant does have a metric lead screw and will easily do metric threads.

Which do you want your machine to do? Metric or inch? Get  a lead screw to match.

Paul

jtbennett

Just look at the end of the lead screw.

https://www.quora.com/What-is-the-difference-between-a-single-start-thread-and-multi-start-threads
OK - definitely 4, so I suppose I was right setting it to 100 before.

If you indeed do have an inch lead screw, you cannot do metric properly. Just as the lead screw in my ENCO lathe is inch and I cannot do metric threads. But my other lathe at the plant does have a metric lead screw and will easily do metric threads.

Which do you want your machine to do? Metric or inch? Get  a lead screw to match.

Paul
The leadscrew is an 8mm diameter, so I'm assuming everything else is in metric as well.

Anyway, I seem to have finally figured it out by dividing the given step per mm by the 4 start threads - thank you everyone for putting up with me, but now I am sure I can measure any leadscrew in the future too so I can annoy you less

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