A tiny 3 Cylinder Holt based engine

[Ye-Ole Steam Dude]

Thanks for the great video. I viewed it on youtube and left you a comment and subscribed.

[kvom]

What design criteria do you use in creating the profile, other than the diameter of the camshaft and the lift?  I assume the tip needs to be rounded, but is the diameter there critical?

[gbritnell]

Probably the most important thing is the valve timing. This will control when the valves open and close relative to the piston position. Next would be the lift but this can be increased with the rocker arm ratio. The combination of these two numbers gives you a starting point for designing the cam. The basic diameter is established by the size of the engine and how much space you have available.
As with all the engines I have designed and built the cams have always had 'mild' cam timing. What that means is the opening and closing numbers, and therefore the duration aren't radical. These little engines are cantankerous enough without throwing a 'wild' cam in them.
gbritnell

[gbritnell]

I started on the crankshaft the other day. It's made from 1144 Stressproof steel. The journals on the blank were turned to .375 diameter. The finished size is .281 so this gave enough stock to cut 3 flats that would be used for locating each of the throws when mounted in the fixture blocks. After turning the part was mounted in my dividing head using the tailstock for support. The three indexing flats were cut then each of the throw journals were roughed leaving .03 for cleanup. I have been using this method because the width of the throw journals can be cut to size and there isn't the constant interrupted cut while trying to do it all on the lathe. 

[gbritnell]

The stroke on the crank is .625 so I made up 2 identical fixture blocks to support the cam for cutting the throws. A reamed through hole (.375 dia.) was put through each block and on one a center drilled hole was also drilled for the tailstock support. The blocks need to be as close as possible to each other so that the can be aligned when cutting in the lathe. On these blocks I used a 1/4-20 setscrew that is ground perfectly flat. This is used to locate on the flats that were milled on the shaft. The other end (tailstock end) just tightens down on the .375 shaft.

[gbritnell]

The driving block was mounted in the four jaw chuck and using a dial indicator against a dowel the block was adjusted to give a total reading of .625. After setting the offset it's necessary to indicate the sides of the block to make sure it's centered in the other direction, otherwise the crank pins are off center.
The crank blank was mounted in the head block and the set screw tightened. The tailstock block was slid onto the crank and the set screw brought up until the block was snug but able to rotate by hand. A parallel was then laid across the two blocks and held tightly while the screw was tightened.
Due to the short length of the crank there wasn't any noticeable chatter while cutting the center journal.

[gbritnell]

Once the throw journals were finished the crank was clamped in the four jaw chuck and indicated true. The live center was bought up for support and the main journal next to the live center and the first journal in were cut to the finished diameter. The crank was then rotated and the process repeated. The finished crank indicated at .001 total runout between all journals.

[b.lindsey]

As always, a great tutorial in how to do things the right way!!  The crankcase is taking shape nicely.

Bill

[gbritnell]

Gentlemen,
 I know it's been awhile but with summer in full swing it's hard to spend a lot of time in the shop.

 I finished cutting the counterweights on the crank and I made the two inner bearings. On my inline six cylinder engine I adopted a different method for making the crankshaft main bearings. Rather than try to mount bearing caps then try to drill, ream or bore through the whole block and try to get things all in line I machined square pockets into which bronze bearing blocks would be inserted and bolted. This would allow me to machine everything perfectly in line and the bearing inserts could be done in a fixture guaranteeing almost true alignment. For this engine, although much shorter in length I took a similar approach. I drilled and reamed the ends for circular bearings and machined the inner two for rectangular inserts. 

 There is no planned progression of parts so with the gear case machined I figured I would cut all the required gears. The pitch is 48 and the sizes range from .25 PD to 1.00 PD for  the camshaft gear.

[gbritnell]

 The original design of the Holt model engine has a secondary gear case that has the two smallest gears. The main set of gears has the crank gear turning the cam gear at 2:1 then the cam drives another gear which takes it back to crank speed. From there the secondary gear case goes back to 2:1 to drive the distributor and ignition timing. This in part is to get enough offset for the distributor.

[gbritnell]

 Next up is the cylinders. These will be jacketed with aluminum shells and have iron liners for the bores. The shells were turned leaving stock on the enlarged area for two eventual bosses, on high on one side and the other lower on the opposite side. These would provide enough metal to thread for a banjo type fitting to supply coolant. The full sized model has triangular flanges held on by bolts but I really don't want to get below 1MM screws on this build so this is what I came up with for the fittings.

 After the shells were turned and bored to the finished sizes they were put in the mill and the square base flange was milled and drilled for the mounting screws. (1-72)  I then made and adapter bushing that would locate the shells from the square flange and this was set up in my dividing head. The cooling bosses were trimmed to width and the extra material was removed by rotating the shell and cutting with and end mill.

 After milling I filed the extra material around the diameter until it matched the original turned diameter.

[gbritnell]

 All three shells finished and ready to have the tapped holes (3-56) put in for the cooling fittings. I used the same setup in the dividing head to put the holes in.
 
 I turned up the cast iron liners and then with a light press and Loctite inserted them into the shells. On the full sized model the head mounting bolts go through the iron liner and into the aluminum shell but for this tiny version I chose to leave extra material at the top of the liner for the head mounting screws.

 I added the quarter and scale as a size reference.

[b.lindsey]

That's a lot of progress for not much shop time George!!  That last picture with the quarter shows well that it is even smaller than I had previously thought. I am loving it though!!!

Bill

[Ye-Ole Steam Dude]

It makes me dizzy to think of the precise detail work done on such a small scale. Beautiful.

[fumopuc]

Hi George, I do like it very much to see your progress. Beautifull as always.