mills 1.3cc diesel mikes-models

27th February 2008 session 13

Well we have gotten past a big hurdle, and not without problems I may say. The
cutting of the threads in the rear of the crankcase has been something of a block
point in this first construction of a model diesel engine for me. I left it yesterday
so that it was all set up to go when I would be fresh. I double checked every
setting and finally bit the bullet and started the first pass. It all goes pretty quickly
at 25 threads per inch! However the tool cut pretty good and reversing the
carriage direction I carried on making 2 thou cuts per pass with no further
advance on the outward travel. Making 10 passes in all and the end result was
excellent!

Because I wanted to make sure the threads were clean I decided to make an
extra two or three passes without advancing the cut. Imagine my horror then
when looking at the threads and seeing that had almost all gone! If I had a dog
to kick it would have had a sore bum then! Gathering my thoughts before
scrapping over 24 hours of work I tried taking a light cut, and the threads
started to reappear. I was worried about how much metal was there when I
stopped and looked at the previous instructions. It would be close but there
could be sufficient metal for nearly the 20 thou (thou = thousandth of an inch)
again, and I could only try it really. So on I went, only taking a thou at a time
and seeing as each pass was completed the threads becoming more
pronounced. The last two thou were extremely nervous and I stopped at 18
thou deep. I cannot see that it will effect the running of the engine, and I will
make the screw in back plate 18 thou larger to compensate. So I believe I have
gotten away with it. The depth of thread is very close to what it should be and
the crank case will live on but it was a very dodgy moment! What I still don't
understand is how it happened. If some kind soul out there could enlighten me I
would appreciate it. I will be looking at the process in my books and manuals
but one thing for sure is I will not do that again!

I spent the rest of the morning tidying up and changing the 4 jaw chuck for the
three jaw. Isn't it a fuss on these boss type lathes. The Myford system is
certainly a lot quicker that's for sure. Next stage will be to create a BMS plug as
described in Ron's article but the main issue regarding the dreaded internal
thread cutting is over! I must just make sure I don't make any serious errors on
the last few machining operations before the crankcase is finished. The positive
though was the performance of the home made screw thread tool. It was much
better than the trial cuts with 'cheap' bought sets of tools. Whilst it is certainly
not perfect, and definitely not exhibition standard, it is the best I can do with my
level of experience and that's good enough for me (for the moment........),

It seems I have been stuck on this stage for so long it will be a pleasure to get
back to more 'normal' workshop tasks.....

Time spent today 1 hour 30 minutes.Total to date 26 hours.