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rack and pinion system

arunkumar413

New Member
Hi Team,

I'm not from mechanical background but want to build a system that moves linearly with 0.1 mm precision. Is it possible to achieve this using rack and pinion system. If yes how to do the calculations that are required for the pitch and number to teeth. If not, can this be achieved using a stepper motor which has about 1.8 degrees movement for pulse. If lets please guide me on the calculations of the diameter of the wheel and gears required to attach to the stepper motors.

Thanks,
Arun
 

Les Jones

Well-Known Member
Although a leadscrew will give you the required precision there will be some backlash so to achieve the required accuracy you will always need to approach the required position from the same direction. Using a ballscrew would avoid this problem as they have very little backlash. You will find some on ebay.

Les.
 

JimB

Super Moderator
Most Helpful Member
Using a ballscrew would avoid this problem as they have very little backlash.
Thank you Les, I had forgotten the expression "ballscrew" and was using leadscrew as a generic expression.

JimB
 

Les Jones

Well-Known Member
I think 0.01mm precision is possible with good quality ball screws. You would need to check the specification before buying them. You will also have to consider axial play in the thrust races on the end of the ballscrews so will need carefully adjusted thrust races or preloaded angular contact bearings.

Les.
 

jpanhalt

Well-Known Member
Most Helpful Member
Pitch error with ground ballscrews is measured in microns. Moreover, the good ones come with charts showing the measured errors, so software can correct for even that tiny bit.

One "problem" with ballscrews is that they offer very little resistance to movement. Thus, force applied to the "nut" can turn the screw quite easily. In good designs, that is not a problem, but in some DIY applications it can be. One example is the retrofitting of a Bridgeport-type mill with ballscrews for hand operation (rather than stepper or servo for full CNC). People who have done that have found that while precision of positioning is initially great, during the course of a cut, the workpiece may make the table move.

0.01 mm is about "4 tenths" , i.e., 0.0004". That level of accuracy is relatively easy to attain today. Even some conventional leadscrew/bronze nut designs can achieve that level.

John
 

shortbus=

Well-Known Member
Although a leadscrew will give you the required precision there will be some backlash so to achieve the required accuracy you will always need to approach the required position from the same direction.
While the ballscrew is one way the metric Acme(trapezoidal) screw with the correct nut is a cheaper solution. Bot all ball screws are created equal too, unless they specfically state 'anti-backlash' in the data sheet, they will have some backlash in them. The 'anti-backlash' ballscrews are usually a "two start" screw, with two sets of ball tracks.

Just one of the many places selling the anti-backlash trapezoidal screw/nut sets. http://www.dumpstercnc.com/
 

MaxHeadRoom78

Well-Known Member
With ball screw for accuracy you need a precision ground, not cold rolled, and also a pre-loaded ball nut, some of the cheaper, higher quality I have found is Hiwin out of Korea.
These are CNC grade.
Max.
 

camerart

Active Member
Hi A,
Much of this depends on what you're doing e,g, machinery or lenses.
Also look up double nut screws and toothed belts.
If you use a ballscrew, then divide the thread pitch by the steps of the motor, if driven directly.
C.
 

arunkumar413

New Member
Thanks a lot guys. Most stepper motors come with a step angle of 1.8 degrees. How to calculate this angular movement to the linear motion in millimeters
 

MaxHeadRoom78

Well-Known Member
Directly relative to the resolution of the ball screw, if directly connected, if geared/reduced then the ratio has to be known.
Max.
 

Les Jones

Well-Known Member
Leadscrew pitch (In mm) * 1.8/360 mm per step (Assuming a 1:1 mechanical connection between the leadscrew and stepper motor) You will probably need a reduction drive to get your 0.01mm resolution as ballscrews tend to have quite a large pitch.

Les.
 

Colin

Active Member
"I am not from mechanical background"

What travelling item needs to be accurate to one third of a thousandth of an inch?
 

MaxHeadRoom78

Well-Known Member
this is for a pick and place machine.
8μm is quite a fine tolerance for a pick and place machine, I'm guessing IC placement?
You definitely will require a precision ground ballscrew with pre-loaded nut.
Unless gearing it very high ratio a servo would be the best choice over stepper.
Max.
 

ClydeCrashKop

Well-Known Member
Most Helpful Member
1.8 degrees per step is not a problem, it makes it easy at 200 steps per revolution.
It looks like 5 mm pitch (per revolution) is a standard for ball screws.

https://www.thomsonlinear.com/website/com/eng/products/ball_screws_and_lead_screws/ball_screws/fineline_precision_rolled.php

A 2.5 to 1 reduction with timing belts & pulleys will give you 500 steps per revolution on the screw.
5 / 500 = 0.01 mm resolution.
Microstepping will make it finer resolution than that.

http://www.sdp-si.com/products/Timing-Belt-Pulleys/index.php
 

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