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Hi,

Hand wheels, knobs and levers are an indispensable part of our lives at science centers and children's museums. How else are most exhibits activated?

But there is a dark side to this. They either come off or turn in ways we don't want or (just as bad) they can't be taken off when we do want to remove them to gain access to repair or clean something. 

Traditional ways of affixing these pesky items include setscrews, keyways, etc. None are really adequate and these often cause grief.

here's a simple solution that has worked well for us: Use a hexagonal shaft and a broached wheel, lever or knob.

Here's the drill:

1) Machine the end of the input shaft to a hex shape. I usually make this portion about 3/4 of an inch long but this isn't cast in stone. Hint: it's usually a good idea to make this length match the length of the hole in your handle or lever but the length should be about 0.020" shorter than the length of the hole in the handle. Also, it's good idea to broach the handle first (see below) and then try this so it just 'slips on'  nicely without much play. Don't forget to lightly break the corners of the hex with a fine file too. 

I use as a standard a 3/8ths inch hex. At the center of this shaft bore and thread a hole for a retaining screw. For the size given above I use a 10-32 NF thread about 1/2 inch deep. Obviously larger shafts with more power to be transferred should use a bigger hex and smaller ones a smaller hex but you get the idea.

2) Procure or fabricate an appropriate hand wheel. lever or similar and if need be, bore or ream a through hole that is 0.375 inches inside diameter. This should be a precisely sized hole (don't drill it! with an inside tolerance of + 0.001", -0.000"). A good example is McMaster-Carr Part Number 8515-K-4, which is a 3-inch diameter aluminum hand wheel with no handle and a finished bore of 0.375 inches. 

3) Use a 3/8th inch hex push broach and an arbor press to broach the hole to size. The right broach is made by duMont and can be acquired at J & L Industrial Supply, their part number is DMB77709M. Note: use oil on the broach and don't try to "re-broach" the hole because you'll never get it lined up right the second time!

4) It's best to machine a fairly thick retaining washer and use a flat-head socket head screw with this to hold the hand wheel in place.

Now you're ready to roll. If your need to take it off, just remove the screw and retaining washer and the handle will pull right off the end of the shaft and is easy to replace with no mess,  lost keys, damaged shafts, etc. 

Hope this helps!

--
John Bowditch
Exhibits Director
Ann Arbor Hands-On Museum
220 East Ann Street
Ann Arbor, MI 48104
734.995.5439
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pager: (734) 670-4555

The views and representations stated in this e-mail are the individual's. They do not bind the Ann Arbor Hands-On Museum or its Board, staff or volunteers, and they do not necessarily reflect the Museum's institutionally-held viewpoints, opinions or policies.

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