Monday, May 22, 2017

Pattern Making Made Cheap!

As mentioned before, we have a problem with the #1 truck on the 309.  The bearing surfaces on both axles were turned down at some point, which would require thicker bearings.  But none of the bearings we have available are thick enough, so there's too much play between the axles and the motor mounts.  This needs to be corrected.

Here's what an axle bearing looks like.  It's in two halves: the one on the left I'll call the inner half.  It fits up against the motor casting and has no holes, but is completely solid.  The other half is the outer half, and is held in place by the axle cap, which is bolted to the motor.  There's a large oval hole where the waste rubs oil onto the bearing surfaces.  There are also holes for pins to keep the bearings from rotating relative to the motor.  There are four holes, but only two pins, so that the bearings are the same for either left or right.  The thicker end of the bearing, which is sitting on the table in this picture, butts up against the wheel or the gear and takes the stress of lateral movement by the motor.

For this particular motor, the GE 66B, the outer surface of the bearing has a diameter of 7 1/4".  The nominal axle diameter, and therefore the inner diameter of the bearing, is 6 1/2", but on this truck we're talking about, that's been turned down to as little as 6 1/8".   Therefore, we need to make two patterns for new brass castings that will be thicker.  After casting, they can then be machined to the correct diameter for each axle.  And the spare brass castings are an excellent basis for patterns.

Tim Peters pointed out that we had some leftover pieces of 6" PVC pipe.   The inner diameter is 6", of course, and it's 1/4" thick, so the outer diameter is 6 1/2".   That means it fits almost perfectly into a spare bearing.  So I took it home and started to cut it to size in an obvious fashion.

I prefer to use an ordinary hand-held hacksaw.  It won't get away from you like power tools can.

The next step will be to attach the PVC with contact cement, and carefully smooth the edges flush with the brass, using a rasp.   First, though, I want to check with the experts in the car shop before going any farther.

In any case, we should have usable patterns at basically no expense.  Pattern making is often a large part of the expense of casting, and if we didn't have these spare bearings available, it would be an extremely challenging task to make them out of wood, or whatever.

Once the patterns are done, we will get quotes on casting new bearings, then start passing the hat.  I promise to do my best to make you feel guilty.   It's the least I can do.


Anonymous said...

Although I have watched bronze being poured into a mold; I do not know how hot it is.

Will the plastic pipe withstand the heat of the hot metal?

I think it was clever of you to work this out. As you say, saving a lot of dollars!

The #309s motor truck will thank you, I am sure!

Ted Miles, always learning, I hope.

Anonymous said...

Randy, are you taking into account the pattern shrink rate? Depending on what bronze alloy you are using, the casting will be smaller than the pattern. The foundry should be able to help you out with this calculation. You need to design the pattern so that you will have enough machining stock so that you can accurately machine the casting resulting in a good part.

Randy Anderson
Kenansville NC

Randall Hicks said...

Randy: That's an excellent question. In fact, I got an email from another reader asking the same thing. I'll check in with the experts on this issue. Off hand, I would say that the axial dimension is not critical, the inside diameter will be machined to a specific diameter, and it's possible we would need a thin layer of some material around the outside of the pattern bearing.

Ted: The mold is basically made by pounding a special "sand" around the pattern, then removing the pattern and pouring the molten brass into the mold. The pattern itself does not have to withstand heat.

Anonymous said...

Randy Anderson recommended Brost Foundry in Mansfield, Ohio when we at PTM were having trouble finding a foundry to make replica Duplex wire frogs, You might want to try them.


Anonymous said...

A couple of ways of adjusting a pattern for shrink is to use thick coats of paint or bondo to increase the thickness.

In this case you could also get some thin PVC sheet and glue it onto the pipe.

Since this is a short run part, the pattern can be fairly simple.