What’s the Diff?

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The pinion placed on the differential and the casting machined with the spider gears, or beveled gears, back inside the windows on the spider of the differential. Notice the difference between the oil tube and the completed casting. Mike Murphy added 1/8-inch pipe, bent it to fit the contour of the casting and put a grease fitting in the end of the pipe.
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Left: The damaged center body of the differential, or spider – notice the holes the beveled gear shaft sat in and how egg shaped they are.
3 / 7
Right, from top: The two large center bosses; notice they are made up of two layers of 1-by. The outline made with a copping saw to make the beveled gear window on the pattern. The small blocks of wood cut and glued in segments.
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Left: A good view of how the inside radius turned out.
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Below: The completed pattern along with the finished casting that came back from the foundry.
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This is the differential gear ready to go on the engine. Mike also recessed grease fittings in the ends of the spider gear or beveled gear shafts. Sorry to all you guys that say keep it original, but they are hidden pretty well and Mike knows the grease will do a lot better job of lubrication.
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Below center: A homemade router bit.

In a Steam Traction, March/April 2006
article on casting pinion gears, I talked about triumph, trials and
tragedies. This article is no different: It’s all about turning
tragedies and trials into triumphs. The title is somewhat
misleading, although after you read it, you might say the same
thing to yourself – What’s the Diff?

Well the “Diff,” in my case is the differential. The Saxon gear
on my 22 HP Wood Bros. engine is made out of two pieces, the outer
ring gear and the center body, called the spider. The spider has
four ears and sits inside the outer ring gear. On either side of
these ears are springs. These springs are to cushion any start-up
under a heavy load and cushion any transmission of power to the
drive train. I believe Case has the same in its differentials.

The differential is one of many places on a steam engine that
will show wear. They are mostly lubricated by oil, by gravity feed.
The problem is that you have to stop the engine in a certain
position in order to oil the spider gear shafts, and the oil that
runs in will run out when you advance to oil the rest of the
shafts. This is just one reason it is a good idea to inspect the
differential when you have the chance.

The wear that showed on my Wood Bros. was in the spider itself.
My dad had an Illinois engine that showed wear in the same place.
As I remember, the beveled gears were bored and new shafts were
made for his engine.

My differential showed wear at a little different area. The wear
was in the ends of the beveled gear shafts, but not so much in the
shaft, as it was in the main casting or spider. I would have to say
this was maybe more from neglect than it was from lack of

I think what may have happened is that the pins that held in the
shafts broke or came out causing the shafts to move around in the
casting, and move around they did!

At one time the differential was taken apart and square headed
nails were used for pins that must have been missing, but it looked
like the damage had already been done. I thought if these areas
were machined and trued there wouldn’t be enough of the main
casting left and I would lose strength. So my decision was to make
a free pattern that I knew would be a low production pattern. I am
no pattern maker, but I have seen it done once or twice.

So off to Lowe’s I went. They have a nice selection of no. 1
pine. Now, patterns are normally made out of Philippine mahogany.
This wood doesn’t swell much and will not lose or change its
dimension. This is important in patterns, although it is very hard
to get and very expensive. I noticed that most of the prototype
patterns in the foundry were made out of no. 1 pine. I think you
could get away with using poplar wood if you wanted. Remember you
are only going to make one or two castings from them.

You could use no. 2 pine but the knots will deal you a fit. You
could be selective at the lumberyard and pick some boards that
don’t have too many knots. I have done that before. The main thing
is to keep down the cost, because when you hit the foundry you’ll
get the opportunity to spend your kid’s college education fund


Now at this point I was ready to start my pattern. Things I had
to have in order to make the pattern were a small wood lathe with a
36-inch bed, a 4-1/2-inch swing and 1/8-inch to a foot shrink rule,
and a 10-inch table saw. These things were a must and an 8- or
10-inch chop saw is nice, but not necessary. I remember picking the
shrink rule up at a steam show from a vendor that was selling
machinist tools. You might ask a pattern shop where to get one. The
lathe is an old Power King a person at work wanted to get rid of –
I was just helping him out.

One of the first things I did was to measure across the outside
of the bore to determine the size of boss it would take to make the
center hub. Remember, measure with a regular tape measure and lay
out with shrink rule. I made two large center bosses. This was to
give me the right height, but more importantly, each layer is made
of pie sections. This way there will not be any end grain

It didn’t really matter for this pattern, but for a production
pattern, end grain is something you don’t want to show. It will
take on moisture more readily and swell more quickly, plus become
rough and hard to draw from the molding sand. So the rule of thumb
is to hide end grain. You might say, “What’s the Diff?” And you are
right, you are only going to make one or two castings so don’t
worry about it.

Once I made two center hub bosses on the lathe it was time to
make the center body with the beveled gear windows. I glued boards
together until I had the right thickness and enough for four
pieces. I put a centerline down both sides of each of the four
pieces; I set the chop saw at 45 degrees and cut from the
centerline to the edge of the board. I did this to all four pieces
so they all came to a point at one end. I then transferred
centerlines on both bosses. On one of the bosses I transferred the
centerline to the backside of the boss. I lined all pointed ends of
the four pieces to the centerlines on the back of the boss, and
glued and screwed them into position. I glued and screwed the
remaining hub to the back of the four pieces aligning the
centerlines. This would insure the hub on the backside would be

Out of the scrap pieces, I cut at a 45 degree angle and made
gussets that would make the outer half of the beveled gear window.
After that, I extended the centerlines through the gussets. Next, I
made a template of the beveled gear window with a center-line on
it. I aligned the centerlines and transferred the outline of the
template on the pattern. I cut the outline with a copping saw to
make the beveled gear window on the pattern.

It was time to sand the outside diameter to final dimension. I
used a beam compass to lay out the final dimension, put a sanding
disc on my 10-inch table saw and sanded to final circumference.
This brought me to the final challenge (or trial). On the casting
there is a flange that runs around the outside on both sides. I
thought about how I was going to make this. Remember, my lathe is
only a 4-1/2-inch swing and 9 inches would be the maximum diameter
I would be able to turn. So I glued pieces of wood together to the
right height and cut them into small segments. I glued the pieces
to the outer perimeter of the pattern on both sides.

Now, I knew I could sand these blocks to the final diameter with
the sanding disc that could be put back into the table saw. The
real question was, how was I going to cut a nice radius on the
inside of these small blocks? I looked around the shop and spied my
router mounted to a small router table. I thought this would be the
right implement, but guess what? I don’t have a router bit that big
and I know they don’t make one.

Well, I made up my mind that I’d make one. I looked around the
shop and my eyes landed on a 11/2-inch paddle bit. I took a piece
of paper folded in half like a Valentine heart and cut a radius I
thought would work. I transferred the pattern to the drill bit and
ground to the line. I determined which way the router turned the
bit, and with the belt sander ground back an angle on the bit’s
cutting edge. I cut the shank to length and ground it to the
correct diameter. I thought I better try the bit in case it had a
mind not to work. I put the bit into the router, tightened it as
much as I could, stood back and turned the router on.

Much to my amazement it turned without a lot of vibration. I
knew I was in business, but I knew I couldn’t use the traditional
router fence. I also knew I couldn’t router the inside of the
pattern rim without some kind of fence. On top of it all, it was
going to be a blind cut no matter how I did it. Out of a piece of
2-by-6 I cut a radius that matched the outside diameter of the
pattern and clamped it to the router table at about the right
distance from the bit. I ran a piece of scrap wood through

Next, I sanded blocks to half rounds and glued them at each end
of the beveled gear windows. This was to increase the bosses for
the beveled gear shafts. The only thing left was to attach the ears
on center-line at the outside perimeter of the pattern, fill and
sand all radiuses with car body putty and attach core prints to the
center of the inside hub on both sides.

Just remember, a good differential makes all the Diff! Well,
that’s all the triumphs, trials and tragedies I have for you fellas
out in Steam Engine Land. Remember to keep your water and steam up,
your hand on the throttle, your eye on the gauge, and Jesus Christ
close to your heart.

Mike Murphy, 7115 W. Bleck Road, Michigan City, IN
46360; (219) 879-4082; (219) 405-9113;
e-mail: m.p.murphy@hotmail.com

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