Mechanism to move pivot point?

[loply]

Hi folks,

I have an object which needs to rotate back and forth about 45 degrees.

The only problem is that I need the pivot point for the rotation to be in a space which isn't physically possible (as something else is already there!). The required pivot point is about 15mm from the nearest space in which I can put a part.

Does anybody know of a mechanism involving levers/bars which allows you to rotate an object around a point which you don't have a component in? I'm sure such a thing exists.

Cheers,
Rich

[tekfab]

Hi Rich, I think i'm going to be the first of many who are going to ask for some pictures or sketches that give us a better idea of what you're trying to achieve.
Most things are possible and you'll probably find some existing "linkage" type that meets your requirements but please can we have some more information ?

Mike Young

[BillTodd]

there are many mechanisms that will provide a virtualised pivot point  ( have a look at your windows for instance) . A simple four bar  will provide a low accuracy joint whereas a curved slide can be as accurated as required.


Bill

[Lew_Merrick_PE]

Rich,

The first approach would be to machine two concentric arcs that can be located to your centerpoint.  The most precise way to accomplish this would be to use one pair of "fixed" cam followers that follow the most heaving loaded side of the (outer) arc's travel and use a pair of "eccentric" cam followers that will ride on the least heavily loaded side of the (outer) arc.  If the loading is light enough, you might get away with a single "eccentric" cam follower riding on the outside edge of the inner arc's travel.  If the loading to too great for that, you will need a pair of cam followers to provide proper linkage.  If there is a lifting aspect to the loading, either a shoulder or (matched) tapers on the slot and cam followers can deal with that.

Similarly, you can machine concentric arcs as above and make a mating plate with concentric arc "keys" to control alignment.

Similarly, the operation can be done using two (fixed) cam followers on the outsid arc and a single (eccentric) cam follower on the inside arc -- just be aware that the tipping to lock load on this tends to be fairly small & sensitive.

4-bar mechanisms and their derivatives can only approach circular travel.  Their default is to construct elliptical or involute paths.  It has been several years since I did this, but (as I recall) I did get a 0.02% accurate arc using thee 4-bar mechanisms in concert.  This was something I did in 1976, so my recollection of the details is a bit fuzzy...

[John Hill]

Take a peep in the household sewing machine and see how the shuttle/bobbin case travels.

A simple application may require nothing more complicated that an crank shaped lever.

[loply]

Hi folks,

Thanks for the suggestions, did some research into some of the methods here and I think I have some good options.

The mechanism is for a table-saw tilt, I'm building a saw but when the blade is tilted I wanted it to rotate at (or near to) the top surface, whereas the nearest spot for a hinge is 20mm below that.

Cheers,
Rich

[S. Heslop]

I'm a little late to the party but I made trunnions like this building a similar disc sander.


(from John Heisz' website)

[vtsteam]

Do a photo search for "tablesaw trunnion" and you'll find lots of examples.