Hit 'n' Miss engine team build cams model engineering

Having been unwell for a few days, I finally got out into the workshop today. As
always I get a little frustrated when kept out of the workshop, and the first session
back is usually short, so I decided to have a go at a cam turning job.

I needed to make a turning fixture and bolt, then take a trial cut on a cam blank made
before I was unwell. As it turned out I overdid it (no surprise!) because I wanted to
complete the trial turning, to see what effect the method of turning made.

I have been using the Home Model Engine Machinist forum for a short while, and
recommend it to any model engineer (link on links page www.mikes-models.com) as
they are very helpful particularly to new model engineers. One of the forums ideas I
like is the team builds they have. This is where a group of people register their interest
in building a model engine and then the parts required are split up between those
involved.

continued below slide show....

I registered my interest late but they made room for me and asked if I would be
interested in making the cam and a lever arm. I was happy to have a go at the came
and may do the lever but felt that it would be better to do the cam first, then if the lever
arm wasn’t done, I could volunteer again. There are 15 engines being built and 13
members are involved, with one of the engines going to a museum and the second to
charity. Although the total engine numbers is a little confusing, as my addition to the
team build came late.

So onto the cam task itself. This the first time I have had a go at turning any cam, even
just a one lobe one like this build calls for. Therefore I decided that it would be best to
make a trial one first then work out a production methodology. The turning of the cam
blank was completed with little problems. The half methodology bringing the finished
parts to within 0.001? for all three diameters.

Making the turning fixture went smoothly as did the securing bolt. So it was over to the
lathe and the first cuts taken. Getting it to the dimension required went slowly, only
because I kept checking the diameter to ensure I did not remove more than I should.
Once the diameter size was met the plans and methodology called for the top slide to
be fixed. I achieved this by clamping but am not sure if this is correct. I will be asking
the forum for their comments.

It was then required to the move the cam, by between 5 and 10 degrees and this is
where I made a major mistake! However what I was doing is with the aid of a 360
degree protractor, the fixture was turned approximately 5 degrees and then another cut
made. So I continued moving the fixture with not the expected outcome. After four or
five cuts I decided to recheck the instructions and found where I had gone wrong. It
was the cam that needed turning. Obvious in hindsight, but thats as much about my
performance level after recovery.

To make the turning of the cam blank easier I thought if I photocopied the protractor
and then stuck that onto the fixture, with a mark on the cam blank, it would mean I
could measure the 5 degrees succesfully. I must say I was surprised when the
photocopies came out on how good they were! Well after doing all of this work I felt I
should stop before starting the new cuts. So the position I will be at for tommorow is to
be able to start cutting straight away. The cam blank is ruined because of the problems
above but there is sufficient metal to be able to find out if the system above works. If it
does, then the next stage of finding some production tips will start because there are 15
to make!

Oh how I do enjoy the exercise though!

Sunday 21st September 2008

Today the cutting of the cam commenced again with the cam turning now instead of the
fixture! It soon became apparent that it was working but to aid my visualisation I used a
large permanent marker. This allowed me to easily see the amount of metal being
removed.

I fully expected to make 36 cuts (10 degrees at a time) but in fact it was nothing like that
and the nose, which should be 50 thou of the largest diameter left uncut, soon came
around. In fact if I hadn't covered the whole blank in permanent marker, I would have
missed it!

The instruction given on the plan by David Kerzel are for the flats to be filed into shape
but I thought I would try my birthday gift of the sander with the fixture just made to sand
off the flats. After all the cam I was using was a practice one. After making a bush to fit
the fixture and the cam it was fixed onto the sander and I was worried of it being taken
around but the control I had was very good and a fine sand is achievable with a final
polish to give a good finish. So I may go that way with the proper cams.

Now that I know the method works I decided I would make the blanks first, then do the
cam cutting

I also thought it may be possible to cut several cams at the same time but in the end it
seemed easier to make three tasks first. One, turn the bar down to the 0.600" outer
diameter. Two, turn the front part of the cam down to 0.375" diameter and 0.120" wide.
Three, center drill, drill twice then ream the hole to 0.250" and finally part off with
sufficient material to turn down the other side to the same 0.375" dia but this time only
40 thou wide.

It may be as I get further into it that a better way springs to mind but after mucking about
trying t be clever, it seemed that the method above would be best.

One point of interest that came up, related to the measuring all the dimensions. I am very
focused on making sure I don't let the other team members down so I want the cams to
be as near as possible to the correct dimensions. With this in mind I was using my
micrometer. When checking the depth or width of the front recess the digital caliper was
used. Just to make sure I then measured other parts of the blank only to be dissapointed
to find the part 3 thou under size! I then checked another blank and it was the same. All
surfaces of the digital caliper were cleaned and the same 3 thou undersize measured. So
I cleaned off the micrometer and all were spot on. I then got out a digital micrometer I
have thats brand new, another digital caliper and then another free caliper that came with
my birthday gift. They were all used to measure the same part and the results can be
seen in the photo below. So how can the home model engineer achieve fine tolerances
when the measuring equipment is so variable? Imagine sending parts to 13 other builders
who have part made by others all believing they are to size but each persons measuring
equipment having variations which the engineer thinks he has done a good job by getting
to that 1 thou allowance only to find they don't fit!

I was very surprised by the difference with one caliper in particular which was 3 thou out
compared to 'most' others.....certainly food for thought.....

The next session will continue with making more blanks. However I may decide that the
second (turning the blank to finished size) task is done on those already made, for a
change. Time will tell. One things for sure though, it keeps me sane!

Saturday 15th November 2008

Well the best laid plans amd all that ensured it wasn't until today that I got back to the cams. I am
running two projects at the same time at the moment. The Fowler traction engine and the cams so I
today I alternated between two tasks.

The cams blanks (first phase) were completed and a fixture was used to start to do the remainder of
machining before the cam shape is formed. By using the home made designed and produced fixture i
have the beniefit of batch production of finished sizes.

The cam blank consists of an outer diameter 0.600" with a step down to 0.375" diameter and 0.120"
wide. Thats the part that has been done. Using the fixture they all have to be machined so that the cam
itself is 0.140" wide with another step down, slightly smaller than the front one.

Once this part has been finished it will then need to be profiled by using the fixture that forms an
eccentric and a compass photocopy at the back. Allowing me to move the can 5 deg at a time for the
next cut. if tomorrow I can get it ready to start profiling on Monday then that will be great! Time as
always, will tell..........

Sunday 16th November 2008.

Well things didn''t go the way I planned (not unusual..) and it took me longer to bring the fisrt blank to
overall finished size. In part this was caused by the fixture I made to hold the blanks. When checking
the size of the central web, which gets profiled, I found a couple of thou difference around its
circumference. This fault was traced back to the fixture which served its purpose for one stage but the
method I have developed requires a greater degree of acuracy. I decided that it would be better to
use brass with a clean cut inset area. I also thought it woulld be usefull if the fixture helped in getting
the blank to size. Therefore it was bored to size and the depth set accurately.

Once this was fine tuned, a methodology was developed and a trial carried out. I found that the 1
thousand of an inch (0.001") tolerance was not going to work! I found that it was easy to get +
0.003" because the blank has three facets. Each one adding to the other. So after getting the central
part (0.140") correct the overall width had to be achived with very fine cleaning up. A much more
time consuming process than I considered. Another learning point!!!!

After completing the first blank succesfully (but time consuming..) I thought I would try a second to
see how long it would take now that both a process and the correct fixture, were in place. Well I
should have stopped really. I was getting tired and I KNEW I should stop....yep, I took off too much
and it finished - 0.003thou somehow so I did the right thing and packed up for the day.

So I now have one very close to its correct size (+0.0005") and one )(*^^% up. I hope this doesn't
carry on as a 50% failure rate won't be acceptable...........I think though it will be ok, I was just tired
.....Onwards and ever upwards though.........