What's the Diff?

Making a Wood Bros. Differential Gear Out of a Free Pattern
By Mike Murphy
June 2007

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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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 lubrication.

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 there!

PATTERN MAKING

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 showing.

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 centered.

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 first.

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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