Sliding (Spring Loaded Fixture)

[Bob La Londe]

I am working on an idea that involves a guide rod (3 actually) sliding back
and forth in a "bushing" with some degree of accuracy. Actually the guide
rod would be in two different bushings about 2.75 inches apart. Side load
would be very light, but there would be some. (less than inch pounds in
most cases) During the course of use the guide rod might have to move 0.050
40-50 times, and have continuous movement from .001 to .009 for a 20 minute
job.

Some small amount of side play would be tolerable but experimentation is
required to determine how much. Greater for some jobs. The idea is that
the bushing can be pressed out and replaced periodically as needed.

1. If you were to do this ideally what would you use for the bushing and
the guide rod?

2. If the ideal materials where not available what would you use that is
commonly available?

3. What would you use that is commonly available and cheap?

Due to space limitations I am looking at a guide rod diameter of
approximately .25, but I could redesign for up to .375. Extra mass could
have a negative impact on the intended direction of travel, but mass could
be removed elsewhere.

I was thinking of something like this for the bushing. The guide rod will
be fixed and not rotate in the bushing. Just slide back and forth. The
guide rod will be vertical. Side load (minimal) will be in any and all
directions at one time or another.

http://tinyurl.com/yjub9ak

And something like this for the guide rod.

http://tinyurl.com/yzhmvlm

I would add that the over tool body would be aluminum (for weight) and the
guide rod would be fixed into the carriage or moving part of the tool
directly either by a press fit or possible with green Loctite bushing and
sleeve locker. This would make a stainless alloy for the guide rod
desirable.

[Geof]

I doubt you could come up with anything better than you are thinking about, sintered bronze bushings and hard stainless rods.

You will have to be very careful to get the holes you press the bushings into parallel and concentric and use a threaded rod with some washers to pull the bushings into place so they go in straight. Also you will have to make sure the fit on the bushings is not to tight so they do not compress down too small for the rod; it is difficult to ream these bushings to size.

[Bob La Londe]

I doubt you could come up with anything better than you are thinking about, sintered bronze bushings and hard stainless rods.

You will have to be very careful to get the holes you press the bushings into parallel and concentric and use a threaded rod with some washers to pull the bushings into place so they go in straight. Also you will have to make sure the fit on the bushings is not to tight so they do not compress down too small for the rod; it is difficult to ream these bushings to size.

Makes sense. How about a very easy press fit for the bushings and locking them in with sleeve locker like with the rods in the other part?

I fgured I would mill a sacrificial wood block perfectly flat epoxy my aluminum blocks in place on the wood block, and then use my CNC mill to drill all my holes. With what I have I do not see how I could do it any more accurately.

I can even mill a guide lip or fence on the wood block so I only have to adjust in one direction for zero from one piece to the next.

Had also considered stacking all my blanks and drilling each hole through all pieces at once but I fear due to the varience in plate thickness I could wind up with more inaccuracy that way and a likelihood to not be parrallel.

I figure I will get the best repeatability if I take my mill back to 0,0 relative to the piece before positioning to drill each hole.