precision linear positioning with a ball detent 1

[subsearobot]

Hi!
I am looking at feasibility and execution of using a ball detent for precision manual linear positioning in a small, lightweight handheld consumer product.

Will need .001" (or better) accuracy over a range of ~1" travel, with about 30 individual steps. likely using ultem for the main body of the system.

I do not think of a ball detent as a precision system, but in theory, I can not see why not. please discuss, and ask questions as necessary!

thanks

 

[btrueblood]

I wouldn't think .001" accuracy possible even in hard metal, due to the "slop" required to allow the ball to move into/out of the detent groove/pocket. Never mind the rounding and wear that will tend to distort the position over time.

 

[MintJulep]

I'd be inclined to try intermeshing washboard teeth - tiny ones.

One fixed and one that can be lifted to allow them to disengage and allow movement.

 

[subsearobot]

MintJulep, good idea. we are considering interlocking teeth instead of a using a ball. in eithercase, I wonder about backlash and wear. I have not done much gear-type design, and this will be made in a production environment.

 

[racookpe1978]

I don't think youd ever be able to get 0.001 accuracy or precision with a ball-headed detent device going into to drilled hole.

The clearance left-right is going to need "some" clearance anyway, so that requires the hole be slightly oversize anyway. Then, even if the left-right accuracy is 100.0000%, the x-x linear position is going to also be variable by the same hole size. If you don't use a full-depth hole, then the whole assembly could move both left-right and linearly (x-x) sicne only a part of the ball detent would go up into the partial-depth hole.

 

[hendersdc]

You could try something like this, Link, either molded/machined in or mounted so it is adjustable.

The cone shape should deal well with wear as long as the mated distance between the two parts is not important. What I mean by that is the parts are constrained in the x-y plane by the cup and cone and have allowable slop in the z direction to allow for changes in mated height due to wear.

Still, 0.001" repeatable accuracy could be very difficult with the small features that would be necessary for 1/30" steps. Could you share more about you application and why you need the high degree of accuracy?

 

[subsearobot]

it is a precision measuring device. We are in R&D, pre-IP protection, so i cant share too much about the product. for this type of instrument, typically the user input is done by rotating fine pitch threads, but the range of motion requires many turns, so a linear slide is desirable. but the input needs a high degree of precision.

 

[MintJulep]

That sort of thing is sometimes done with a two-stage slide.

The first stage has course pitch to get you in the neighborhood quickly, then the second stage had finer pitch for accurate positioning.

 

[MikeHalloran]

I think you get down into single-thou territory with a ball detent using a pretty good ball and coined triangular holes in the stationary part and coined v-troughs in the traveling part, or conversely.




Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

The problem with the ball detent is that it needs to have a stable grip on both parts; a tapered fit on each side and a spring load. I'm thinking one part has a little diving board with a cone recess and the other part has v-groove teeth. This leaves the ball with point loading This suggests using an undulating semi-circular shape on the teeth as it ratchets, so the plastic and the preload need to be low enough to withstand it. If possible, a pre-load relief for when the user is positioning the item.



Are you trying to measure a distance or save a distance? If it is measuring, I'd look at using a moire pattern; basically an optical vernier.

 

[subsearobot]

we are looking to have a repeatable displacement- save a distance. we have made a prototype or two that is very repeatable, but has backlash (and no cycle life info yet). the final product will need minimal backlash, and adequite cycle life... not sure how many cycles, but likely 50-100k.

 

[3DDave]

100k inches is about 1.6 miles. Long walk for a little part. Most calipers have a smooth slide and a thumbscrew to save the distance. Peculiar to need .03 steps +/- .001 or better. The final item better be good. Or at least interesting.