Friday, July 29, 2016

Compression Shims

When I first started making the ScopeRoller product, I made the sleeve that goes onto the tripod leg by boring out a recess in a cylinder of acetal.  I tried to make them +-.001 of the diameter of the tripod leg.  As you might expect, the results were frustrating.  Not everyone measured their legs correctly, and manufacturing variances from lot to lot, especially for Meade and Celestron, were often substantial.

How do you bore these?  Start by making a hole slightly smaller than you want.  I did this by drilling a pilot hole with a drill, then used a series of progressively larger Forstner bits.  (Forstener bits are primarily intended for making smooth flatbottomed holes in wood, but they work great in acetal; not so much in aluminum--I tried once.) As a result, I have a Forstner bit set that goes from 1/8" to 2 1/8" diameter, and one monster that is 2 3/8" diameter.  The rest look like dwarves by comparison.

I put the Forstner bits in the drill press and found how slowly these cut acetal.  I started hanging weights on the bar that lowers the dell bit down.  This way, I could start it, and come back every few minutes, allowing manufacturing and blogging to operate in parallel.  Once at that diameter and depth, you put in the lathe and use a boring tool to get the precise diameter you need.

This was slow and frustrating for the reasons described above and my wife had a brilliant idea over dinner: Make sleeves that fit various similar sized legs.  In many cases, they are within .1" of each other in diameter.  So I started using round aluminum tubing for the sleeves in which the legs go.  I use hex head bolts to hold these onto the legs, which tends to get everything centered in the sleeve.

Customers were happy and I have since switched to square or rectangular tubing: easier to center and tap the holes for the hex head bolts.  But I really wanted to improve centering in the sleeves and protect the tripod legs from being marred by those hex head bolts.  The distance from interior of sleeve to leg is typically .1" or less.  So now, I ship these with what I call "compression shims," cut from .020", .032", .050" or .100" aluminum.  Four of them go between the hex head bolts and the tripod leg, both reducing space and spreading the bolt's load over more area.

How well do they work?  In the manufacturing process, I put the tube sections into a larger square tube in which I have bolts threaded to lock the sleeve in place.  Originally I did this for round tubes.  (It is not easy to clamp a round tube with a C-clamp.)  But when I ratchet down these bolts on to the tube, it leaves little circles that have to be sanded out for cosmetic reasons. Today, I used some of the .050" and .100" shim aluminum between bolts and tube being held, and no little circles on the aluminum.  So if ratcheting doesn't push damage through the shim then customers using a small wrench should not be a problem either.

So many lessons learned.

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