Part six - Screwcutting - by Ramon Wilson

With the cases shaped it was time to move on to the front housing and the tricky job of screw cutting to fit it together.

First off then was to rough out the front housings and then screwcut them to act as a thread gauge. Because of the protrusion of that vertical intake these were done from a block. (The hole was not a mistake, the material (HE30 - 6082) was a 'scrapper' gift from a friend.)

The inner faces were turned and the bore for the crankshaft bearing put in (drilled, bored then reamed to ensure concentricity) and the LH 26tpi thread cut. Originally I had thought of using a constant 32 tpi but after that galling on the cylinder head it was thought a stronger, more open, thread form might hedge that potential off a little better.
They were then set on an expanding mandrel and the front ends roughed to with .5mm and the faces finished. There is some milling to do around the face - the end will be screw-cut after that op is completed.
At the same time two back plates were roughed from round bar and the inner faces screw cut - same thread - and faced to finish. (No pics).

Next up was to pop the cases back on the expanding mandrel (still sitting unmoved in the chuck) and screw-cut the internal thread for the backplates...

...and that in turn was used to hold the backplate to turn the recess (lathe running backwards, of course, so it didn't unscrew).
Ready to fit together.
I had just screwed the second one up tight as Sue arrived home. That short break in concentration and bang the thing was solid. Rapid increase in anxiety level and instant visions of having to turn this backplate out. Trying not to panic I got out some of that anti slip mat that I keep for getting a good grip but no it wasn't having any of it   Oh dear-oh dear!

Then,  the penny dropped - Left Hand Thread! 

With all the screw cutting completed without any further problems, the case parts all fitted together well. Keeping the case still on the mandrel the backplate was pulled up tight and the mounting holes drilled in.

I have a feeling that as things settle in a little the tendency will be for the  backplate to go 'over centre' slightly hence the two spotted holes. These are situated on the centre line of the thread so should that feeling prove right I will drill and ream for a couple of dowels. I don't want to do that at this stage in case that action creates galling of the thread.

With the backplate pulled up tight this was used to grip in the soft jaws for threading the front end, the nylon block acting as a stop to prevent it possibly unscrewing with the cutting forces.

There's nothing special about the tool used - just ground up from a piece of HSS on the offhand grinder to match a M&W thread gauge at 55 degrees.

Taking the last few cuts very gingerly and running through as many as four to five times on the same setting on the last cut it was taken out until the front housing would screw in without drag. Though not drawn initially the housing has a register turned that fits inside the case to ensure alignment rather than rely on the thread.

With the housings tightened fully home the case was set truly horizontal and a 4mm slot milled in on the centre line. This will aid alignment on the next op which will be carried out on the rotary table to mill the excess away and finish mill the outer profile.
It will be noticed that the internal backplate has been machined away. No, not the result of a mistake but a decision based on the assembly ahead. If the backplate remained all assembly would have to be done from the front end with the attendant risk of that thread picking up should removal be necessary. This way the engine can be assembled as normal via the rear opening.

part one    part two   part three   part four  part five   part six   part seven   part eight

part nine  part ten   part eleven   part twelve