micrometer without spinning jaw?

[Phooey]

Hi!

I'm looking into using a micrometer as very precise low torque linear positioning device, but i require that the jaw does not spin while it extends or retracts. Those micrometers which i have seen always have the end attached to the dial spin as it moves in or out. Has anyone seen something similar, but which does not spin? It would be great to get the kind of positional accuracy a mic can offer at the low price (some are $50), with which precision positioning stages cannot compete. Thanks for your time,

Bryan

[wildwestpat]

Hi Bryan

Yes linear positioning in optical and microwave labs is achieved using micrometer thimbles which are clamped to the 'bed' and the rotating anvil presses against the spring loaded carriage that rides/slides on the 'bed'.

As you are looking for platform mechanisms that use a micrometer drive I am assuming that this is a small mechanism with modest loads. Although both the micrometer and the mechanism I describe below could be up-scaled you son enter the field where ball screws are the best bet.

I have used thin shim stock between parallel plates driven by two or more rollers that are 2/3 the gap between the plates in diameter. The shim is wrapped round one roller back to the start end of the bed then wrapped round the second roller to pass over the first and then clamped to the end of the bed furthest from the start. The bed comprising of two identical ground plates (gauge plate in the lab models) with spacers at each end. The only critical part is to ensure the shim and bed plates are running parallel. This is easy to achieve using 4" ground gauge plate and 4" shim stock. I used phosphor-bronze or spring steel as the shim. Brass developed a permanent set and did not have sufficient spring to grip the two rollers. More rollers can be added for greater traction or to provide parallel motion . Reverse motion would be possible if a self closing mechanism were required.

This was for an application that required zero backlash and a very smooth transport of the platform with no 'slip stick'. Now that plastic films are available that have near zero stretch a plastic substitute for the spring shim stock might be worthy of investigation - increasing the % fill of the two rollers would be necessary as would providing some pre-tensioning of the film to ensure a full wrap on the drive roller. With spring material the tendency of the shim to go back to a flat strip provides a positive grip on the rollers. In the mechanisms I have built the rollers were ground drill stock (silver steel) running in miniature precision ball bearings.

Hope this helps. Regards - Pat

[sarahhill033]

You guys seem to know a lot about micrometers. Can you tell me which one of these I should get? I always buy from Grainger, but I don't know too much about micrometers. Here is the link: micrometers

[RICHARD ZASTROW]

Did you notice one of the Starrett depth mic's has a non-rotating blade?

Dick Z

[wildwestpat]

Hi Sarah Hill

The choice of micrometer or indeed any other measuring device is governed by accuracy required - durability and like everything else price!

I am assuming that you are setting out building a tool kit in view of your other posts. If this is not so then I apologize in advance. I am also assuming that you have tried the simple electronic caliper and found them wanting - I find they are difficult to get repeatable measurements.

My personal preference is for Mitutoyo as I have had one of their micrometers in my pocket for the last 40 years and it is still as good as the day I bought it as it lives in its case when not in use. If you do not need the super accuracy types do not be tempted as they are mode difficult to use - the vernier scale add factor and the number of turns become tedious if you only want to measure 0.001". On retirement I treated my self to a set of five micrometers of unknown Far East origin covering in 25mm (one inch) increments from zero to 125mm. These are well made but may not last as long as my trusty 0 - 25mm Mitutoyo which I bought when the UK was using inches and I got tired of converting to use inches. ( I am / was a microwave engineer by training and all the calculations are metric.) If the cheap set are of interest do an ebay search. May I also suggest you look at the Mitutoyo web site as there is a lot of information on measuring equipment.

If you can handle the micrometer/s prior to purchase test out the smoothness of the action including the adjustment as well as the repeatability of reading using the ratchet. After a bit of experience you will be turning the thimble between the thumb and forefinger whilst holding the micrometer in the palm with the remaining fingers using the other hand to hold the work and or make notes.

When it comes to the readout there are three main types. Digital electronic. - nice to read but prove to battery replacement at inconvenient times - look for auto switch off - tolerance bands are nice for production testing as is the ability to hook up to a remote indicator (this allows the mic to be used as a go no-go gauge). Digital mechanical - nice and easy to read but tend to have small figures no mental gymnastics as the reading is direct. Traditional (old fashioned but still very popular) here the thimble vernier has to be read and this entails a 360 degree view of the scale which is not always convenient - also needs care adding the vernier reading to the main scale which is easy to learn but is not required for the digital versions.

Also check when buying micrometers that do not start at zero that they are complete with their reference gauge to permit correct adjustment along with their 'c' spanner. Good to check the smooth operation of the lock - some of the low price instruments are a bit wonky in respect of the lock.

Remember to treat the mic with respect and ensure the surface of the work to be measured and the mic's anvils are clean prior to taking the measurement. I have a very sensitive electronic mic and it is interesting to see the effect of proper cleaning on the final accuracy of measurement. I always use one of the aerosol cans of cleaning fluid but may have become addicted to the solvent!

Regards - Pat