I just had a minor adventure making a set of hard A-Arm bushings
for a spridget. After many hours of work, and the assistance of
Dick Nyquist, I ended up with a very nice set.
To save others the bother of redesigning the whole setup,
here are the steps and dimensions.
Note that these directions are the steps that I used on
Dick Nyquists lathe. I am sure that they will need to be
modified to work with other setups.
Needed:
5/8" rod for the inner bearing. I used steel, there may
be advantages to using bronze, talk to your neighborhood
material scientist.
1.25" rod for the bushings.
Making the bearing:
Cut four metal rods about 2" long. The final dimension will be 1.75"
but you will want a little extra so you can trim it to exact length.
Face both ends.
Drill then ream it until the I.D. is .500". You will want a smooth
finish so it will rotate nicely on the bolt.
Once you have the rod bored properly, trim it down to 1.750"
and chamfer and deburr the ends.
Making the bushings:
The bushings were tricky. It turned out that the best fit was
obtained by having a cylindrical portion on the end then a conical
portion for most of it. To get the dimensions we kept turning a little
bit more of a bit of scrap Delrin until it would just barely fit
through the center of the A-Arm. We theoretically could have just
measured the smallest ID on the A-Arm, except that the calipers weren't
long enough.
To get the measurement for the taper, we started out with too steep
of an angle, and kept shallowing it out until the best fit was found.
First, cut the Delrin into 4 rods, about 4" long each.
This will give you eight ends to work on and a couple inches
scrap in the middle of each.
Drill then ream the rod to fit the metal bearing. In our case this
was 0.625". This hole will also give a support for the chuck so that
the piece is not just supported at one end.
Set the taper cutter to the proper angle. I believe that it
was 6 degrees. in any case, it was no steeper than 4.5 degrees.
Turn the end of the piece down to 0.890-0.893"
Turn it to this diameter for about 0.3".
This will be much longer than necessary, allowing you material
to trim the bearing to exact length.
Once the cylindrical end has been cut, cut the taper out at the
specified angle until the diameter is 1.015". This was about 12-13
turns of the screw on Dick's lathe.
Zero the distance meter and run the tool down until it is 0.712"
from the shoulder.
Cut the part off so that the portion of the bearing inside the A-arm
is 0.712". This should leave about 0.1" of cylindrical at the end
of the conical section.
Turn down the outer portion of the shoulder. I used the O.D. of the
stock rubber bushings which was 1.115". I suspect that it could be made
as large as 1.250".
chamfer the end of the hole so that the metal bearing can be easily inserted.
Cut the bushing off so that the shoulder is about 0.15-0.2" wide.
Final fit and finish:
Assemble the bushing, with the metal bearing, in the A-Arms.
Use a vise to compress the bushings into the A-arm as tight
as they will go.
The shoulders of the A-arms will extend past the metal bearing.
Grind the Delrin down so that it does not extend past the metal
bearing. I sculpted the bushings so that they had a slight dome
(cabachon) shape to the outside ends.
To do this work I used a 3-M scotchbrite gasket remover disk on
an electric drill.
(If you have not yet used these disks, they are one of those tools
that once you use it, you will wonder how you ever lived without
them. They make it a breeze to remove old stuck gasket material,
without also removing the metal underneath.)
When the work is done, you should be able to clasp the assembly in
a vise, the vice will grab on the metal bearings and allow the
A-arm to turn freely.
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0.893" 1.015"
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Larry Colen lrc@netcom.com
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