Part 11C Cylinder HeadsThere has recently been lots of steam engine stuff posted on the forum. I thought it was about time to post another 'fix' for the petrol head junkies out there.
The next items to be created were the four cylinder heads. Each head contained four cylinders, each of which were fitted with 4 valves inclined at an included angle of 56 degrees to form a penthouse combustion chamber. Each combustion chamber being surrounded by a water jacket. Apparently the 56 degree valve angle was the maximum which would allow the individual valves to removed and replaced from the combined welded cylinder/ combustion chamber.
Four 8" x 4" x 3" aluminium billets were faced off and the central cleft between the inlet and exhaust valves machined. On each head, two 3mm diameter holes were drilled the full length of each embryo cylinder head. The holes were drilled 4" deep from either end of the billet using the method described in earlier installments. Again, luck was with me and all the holes lined up perfectly. These long holes interconnect the various parts of the cylinder heads water pockets. I decided to make these drillings at an early stage before much work was undertaken. It would be easier to redo the drillings at this stage if there had been a problem.
The interface with the previously made water jacket was machined next and trial fitted
This is a preview of how the cylinder blocks will eventually look.
The next stage of machining the cylinder interface is a most important stage to get right. In the full size Mercedes engine the individual cylinders are welded to the individual cylinder heads. The model engine will have the individual cylinders screwed into the combined cylinder head. The accuracy of the cylinder screw threads was of the utmost importance so I decided to try a technique which was completely new to me. I would try thread milling with a single point tool on my LinuxCNC controlled mill.
The four thread pockets were machined with an undercut to accommodate the end turns of the thread.
Here is the toolpath for the cutter. The cutter was made from a 3/8" x 32 TPI tap. All the teeth except one were ground off. The remaining cutting tooth looked quite pathetic but it cut all the threads perfectly, if fact some of the nicest threads I have seen. The single point cutter spins like a normal mill cutter and will cut a V grove of the correct angle for the thread. The cutter is then moved in a circular motion at the correct diameter while simultaneously being withdrawn by a distance equivalent to the required thread pitch. The item being machined remains clamped to the mill table, while the cutting tip moves in a continuous helix.
It all sounds very hairy, but in fact these was probably the easiest threads I have ever cut, much easier and cleaner than on a lathe. The cylinder threads were 1" x 32 TPI
Here we see all four cylinder heads plus the two test pieces I experimented with before committing to the model parts. So far , so good
Next, I pocked out the water spaces around each combustion chamber. If you look closely you can see where I intersected the 3mm diameter holes I drilled earlier.
The next stage was to machine the interior of the combustion chamber pent house shape. The V of the pent house accomodates four valves with a 56 degree included angle. I made up precision angle plates for the fixture plate at 28 degrees half angle.
The cylinder heads were attached to the fixture plate with clamps and bolts passing through two spark plug holes. A ball mill was used to form one side of the combustion chambers.
Each cylinder head was then flipped over to complete the second side of the combustion chambers pent house roof
The next stage was to machine the inlet and exhaust valve pockets. With four valves per cylinder and four cylinders per head, there were a lot of holes to mill. I chose to thread mill the 1/2" x 32 TPI threads using the same single point thread mill is used earlier on the cylinder threads.
Here we can see the four embryo cylinder head blocks with all the internal machining completed.
The next installment from Vixen's Den will describe the fun and games of machining the outside of the cylinder heads.
Stay tuned
Mike