The Cam pinion Extractor. Making one.

Here is the timing side of the engine.

The Pinions on the cams and the crank need pulling off.
There is a LH thread nut on the Cams and a RH thread nut on the crankshaft.
One way of getting the nuts off is to put the idler wheel in and jam the pinion gears with a cloth. This gave me enough stick to undo the three nuts.

Now to pull the pinons off. On a 500cc Triumph they are a very tight fit onto a keyed shaft.

The cam pinion pullers costs around £40 and to be honest, they do not look too well made.

Each of the cams has a 20 tpi thread that is there for the sole purpose of extracting the pinion. The crankshaft pinion thread is a bit rough.

As I recently inherited a Myford lathe (A ML7 with a manual gear box), it occurred to me that making the tool would be a nice exercise. I have never done anything like this before but one of the technicians at work is a good source of advice on engineering matters.

This post is aimed to give a complete beginner confidence that screw cutting on a lathe is fairly straightforward. If I can do it, so can you.

The thread profile is probably BSF but because the threads are only two or three turns long, the thread angle is unlikely to be much of an issue.

To make one of these on a thread cutting lathe, you need an internal thread cutter, an external thread tool, some stock to make the thing out of and some ordinary lathe tools. A thread chaser is good but not essential.

On a Myford lathe, the lead screw is eight threads per inch and you want 8 turns of the lead screw (1 inch travel of the lathe tool) to translate to 20 turns of the chuck.

The chuck usually has a 25 or 20 tooth gear on the end and so if you put 20 teeth on the chuck end and 50 tooth on the lead screw (with idlers of any size in between) then you have 20 turns on the chuck giving 20 x 20 = 400 teeth into the gear train. At the lead screw end you have a 50 tooth gear which in 400 teeth = 400/50 = 8 turns. In other words, the lead screw does one inch of travel carrying the tool post whilst the chuck holding the workpiece does 20 turns.

There are charts of gear settings on lathes out there and more modern lathes have gear boxes as does the Myford Super 7 which is the posher version of a Myford than mine.

An internal gear cutting tool looks like this (google images > http://www.minprom.gov.by/images/products/118209.jpg). This one is a carbide tipped one.

Having put a suitable piece of stock in the lathe, you rough it down to size and drill as big a hole as you can down the centre using a drill held in the tail stock.
You then need to cut more material away for the first inch or so until you reach sufficient depth to allow the crank shaft end in and allow the thread that you are going to cut, to engage with the crank shaft pinion.
The exhaust camshaft sticks out a bit as well but the most room is needed by the crankshaft end.

You have to cut a channel inside the tube in order to take the thread cutting tool when you reach the end of a thread cutting run.

The outer piece with thread cut.

To cut the thread,

1. Set the lathe going really slow. For a beginner this will reduce the chance of you running the tool into the bottom of the hole. 
2. Wind the tool in to where you cut an internal channel mentioned before. This is the inward end of the internal thread. Make a pencil mark on the saddle that will mark where you stop the saddle at the end of each run.
3. Set the lathe tool a bit to the right of the tube which is going to get the thread. 
4. To the right of the saddle there is an indicator numbered 1 to 4 on a myford. (and on most lathes) set the lathe away and when the indicator is spot on a number, engage the lead screw.
5. Watch the saddle and when the saddle reaches your pencil line, disengage the lead screw, the tool is now in the channel you cut earlier to take the thread cutter at the end of the travel.
6. Wind the tool inward to clear your thread and wind the saddle back out. 
7. Wind the tool back to an extra 5 or 10  thou' from where it was on the previous cut.
8. Engage the lead screw when the indicator is on a number and go again. The thread will now be a bit deeper. 

The thread depth for  BSF 20 tpi is only 30 or 40 thou' and you can double the change you make for each run because you are cutting both sides of a diameter.

Now, either you take very careful measurement or you will need to remove the work to check the thread against the pinon a few times to get the cut right. Putting the work back in the chuck is a bit of a chore to get right.

Whne you have the pinion thread done, you need an internal thread to take the central jacking rod which will pull the pinions off. I cut these to 20 tpi as well and this worked fine.

Jacking Screw that goes through the centre.

The final product is here.