How Do You Accurately Measure The "Repeatability" And "Runout" Of A Linear Stage?

idbruce

Hello Everyone

Over the past three or four years, I have built numerous linear actuators (stages) in some form or another. During this time, I have also strived for a design that was cheap and inexpensive to manufacture, but I also wanted a design that would produce highly accurate results with minimal wear during operation. While creating the Z stage (3RD design ) for my CNC PCB Drilling Machine, I had a brainstorm and decided to attempt my new idea for a linear actuator. This afternoon, I partially assembled the actuator with the pieces that I had already machined and I was really quite impressed with the outcome. The slide does not have any noticable slop, but of course there is clearance for movement, because it is not binding, and it has a nice, smooth, and fluid movement.

As indicated by the title, I would like to know how to accurately measure the repeatability and runout of a linear stage. I would imagine that the runout can be determined by simply attaching a dial indicator to the actuator and twisting the slide assembly. As for measuring the repeatability of the stage, I don't have the foggiest notion on how to go about getting this measurement. So if you have any knowledge pertaining to this subject, please feel free to share your knowledge

Thanks in advance.

Bruce

localroger

Bruce, I'd suggest running the stage to a target position at various speeds and then measuring where it actually ends up after stopping with the dial gauge or with a dial caliper braced to a fixed point. Also try it from both directions. This will reveal most of the mechanical weaknesses that also cause errors while in motion.

idbruce

@localroger

Yea, that sounds like good advice. A mounted dial indicator would probably be just the ticket, however I would be a little nervous about slamming into it

Thanks Roger

Bruce

pjv

Bruce;

You will have to be more specific on what you mean, and what precision you are looking to measure.

For example, if your stage has a screw to position it, then the linearity of the screw along its length is important, and is very difficult to measure precisely without proper tools. Also the uniformity of pitch verses rotation around EACH TURN might be important. The side-to-side slop as well as forward/backward slop might be important. I suppose these could be considered your "run-out".

Repeatability could mean the ability to return to the same spot from either direction, and is probably easiest to measure with a dial indicator as localroger pointed out.

And dont forget another important measurement, and that is the deflection due to weight or work (cutting) load. These might be negligible in trivial cases, but often overlooked.

Some of these errors you can minimize by calibrating them out.... others not so easily.

To get good precision, buy ready-made lead screws and ball nuts ! "Home made" will yield mediochre results at best.

Cheers,

Peter (pjv)

idbruce

@Peter

Repeatability could mean the ability to return to the same spot from either direction

That's the type of measurement I want to make, and yes I agree, deflection is very important and often overlooked.

The side-to-side slop as well as forward/backward slop might be important. I suppose these could be considered your "run-out".

Not including the runout of any cutting device, I would think the side to side movement would be considered runout and the backward and forward movement would be considered backlash. I am not certain, but that is how I would interpret it.

localroger

Bruce, I run into this kind of thing a lot playing with my 7x10 mini-lathe. If you have a mag mount dial indicator you don't have to worry about slamming into it because it's got its own range of motion, you just check where the needle lands. They're used on lathes to see if a mounted object is centered. If you use a vernier or dial caliper you just arrange a reference point and let the carriage do its thing, then measure the distance to the hard reference. Both of these common pretty cheap (at least from Harbor Freight) instruments are accurate to 0.01 inch or so. I know some people like to claim micrometer tolerance on their positioning, for most forming systems that's overkill even if you can achieve it.

Circuitsoft

One option might be to buy a a known good actuator and compare its movement and positions to yours.

idbruce

@Circuitsoft

In most instances, a manufacturer will specify the repeatability of a specific actuator, and I believe most also indicate backlash if screw driven, however I cannot recollect if they indicate runout or not. In any case, unless I specifically measure these tolerances on my unit, I will never know how accurate it truly is or how it compares with other commercially available units. And then you also have to consider the differences between belt, worm, rack and pinion, and screw driven actuators, when making comparisons. I am currently working on a screw driven actuator, but I also have a rack and pinion drive up next, however the rack and pinion is just a simple actuator that runs on two guide rods, which is no big deal. I have also been thinking about modifying my current design for belt drives.

Bruce