Newsgroups: comp.robotics
Path: brunix!sgiblab!swrinde!cs.utexas.edu!utnut!utcsri!psych.toronto.edu!colin
From: colin@psych.toronto.edu (Colin Hinz)
Subject: Re: Two setscrews: 180 degrees or 90 degrees apart?
Message-ID: <CLHI89.uF@psych.toronto.edu>
Organization: Huh?
Date: Sat, 19 Feb 1994 18:33:44 GMT
Lines: 40

kilian@cray.com (Alan Kilian) writes:
>Well, I don't drill the two setscrews at a 90 degree angle for two reasons.
>
>1) I don't have a tap that has threads cut out to the very end of the tap
>   and so I need about 1/4 Inch deeper hole than the threads. (The first
>   1/4 Inch of the tap has gradually increasing threads) so the bottom
>   1/4 Inch of the hole cannot be used. So, I drill straight through and
>   tap right through in one go and I have two threaded holes 180 degrees
>   apart. If I drill two holes 90 degrees apart and all the way through
>   the hub, there's not much left and the wheel wobbles even with 4 screws.
>
>2) I can't ever find the EXACT size drill for a motor shaft. I even use 
>   reamers and I just can't get the EXACT size hole in a hub. This means 
>   that if I have a single setscrew or two 90 degree apart setscrews that 
>   the wheel will not be centered on the shaft. With two 180 degree screws,
>   I can tighten them alternately, and adjust them to get the wheel centered.
>
>So there is a method to my madness. What works is kept, what fails gets tossed.

I am not an expert machinist, but i am an expert with Meccano/Erector
parts. The shaft diameter with this stuff is about 0.160" (British Wire
Gauge #8) and all the gears, pulleys and the like are tapped for 180
degree set screws.

Practical experience has shown that using both set screws is far worse
than using just one. Because (as you note) the bore is always a little
larger than the shaft, if one set screw works a little loose, *both* set
screws are then loose. This will happen even with a double flatted shaft.
Eventually both sets screws will work loose enough for the wheel to spin
free.

It would make more sense if the Meccano/Erector parts were made with 90
degree set screws, but i guess one tapping operation is cheaper than four
(full and bottoming tap in each hole).

- Colin

Colin Hinz             GE/GMU: (!)d-- -p+(---) c++(++++) l u(+) 
(705)EAR-JUNK                   !e- m+^2@ s (!)n+ h+ f* g-(+++) 
colin@psych.toronto.edu     w+++ t-- r(-) g+(*)@ y+(*)@
