A tiny 3 Cylinder Holt based engine

[steamer]

Watching along George!    Great Start!

Dave

[fumopuc]

Another thread to follow. George, thanks for showing the details.

[gbritnell]

Gentlemen,
An update to the progress on the mini 3 cylinder.
The gearcase is next. I started with a rectangular block of aluminum the required thickness. I touched it off to get my zeros and then drilled all the mounting and shaft holes. To create the radii in the corners of the screw bosses I drilled them out. The shaft and gear pockets were opened up with end mills prior to boring them.

[gbritnell]

The piece was flipped over to mill the rear surface to thickness while leaving metal for a boss that the upper gear train will mount to as well as the crankshaft boss. These were cut with the boring bar inserted backwards and the spindle turning in reverse.
The part was then roughly cut out on the bandsaw and mounted on a fixture plate to cut the outside shapes to size.
The rotary table was set up and indicated. The fixture plate with the gearcase was then centered under the spindle with a tapered brass alignment pin. For shaping parts I find that this will get me within a couple of thousands which is close enough for contours.

[gbritnell]

Here's the gear case attached to the front of the engine.

[gbritnell]

The next piece is the cover for the gear case. I find that it's often times easier to sacrifice some extra metal to create a part. With small parts like this it's not a lot of metal so this is the direction I took. I figured out how much extra metal I needed to create a picture frame around the part and the stock was cut and milled square. I picked up the crank center which is where all my ordinate dimensions are taken from and as with the gear case put in all the needed holes. I left an island of stock which would become the boss over the end of the camshaft. The cam gear will need to be held in place with a 2-56 screw so I needed a recess on the backside and the boss on the front. The boss was then created using the reverse boring bar method like was used on the gear case. I have a dedicated cutter made from an old end mill that is ground up with a radius on the corner.

[gbritnell]

The piece was then flipped over just to put the counterbore in for the cam end. I indicated the crank hole and then moved to the cam location and using an end mill made the pocket.
I then gave myself some rough cut lines so the part could be cut free on the bandsaw. The part then deburred and mounted to the gear case so I could scribe a witness line to file to. Once the part was filed to the line it was put back on the gear case for the final sizing.

[gbritnell]

Not being familiar with 3 cylinder engines I did a little searching on the net to see what the cranks and timing looked like. There's not many options with a 3 cylinder engine for timing, 1-2-3 or 3-2-1. I looked into making a 120 degree crank but the firing sequence would leave a big gap from when the third cylinder fired until it got back to the first so I settled on a flat plane crank (180 degree) the end two cylinder up and the center down. This would shorten the space between the last firing and the first. With that settled I drew up the camshaft. It would be an easy job, just a matter of creating a step-off chart to mill too. As with all of my engines I make the cams from W-1 drill rod. I mill the lobe profiles then file and polish. The lifters will be hardened and polished so that there won't be two like surfaced running together.
The cam blank was cut with my small lathe creating the lobes and shaft diameters.

[gbritnell]

The plan was to clamp the chuck on the nearest cam lobes (.25 diameter) and work my way out to eliminate any vibration while cutting. The issue would be that when the blank was extended for the next set of lobes I would lose my register so I made up a bushing with a flat milled on it and mounted it on the end of the camshaft. That way every time I advanced the shaft I could re-indicate the bushing to get back to my original starting point.
I set up my H/V rotary table and indicated it true. I set the table to -0-, inserted the blank and indicated the bushing. For added rigidity I supported the end with my home-made tailstock.

[gbritnell]

The first set of lobes were cut then the shaft was extended and indicated like the start. The step-off chart starts from the first lobe on the first cylinder as -0- and each successive lobe is cut using the calculated numbers. Each lobe never restarts at -0-.
This thing was a piece of cake compared to doing a V-8 camshaft.
Once the lobes were milled they were filed and polished.
gbritnell

[90LX_Notch]

Amazing work as always George.  I always learn someting from your builds.

-Bob

[b.lindsey]

That is a lot of progress George!! Interesting as to the cylinder timing for three cylinders, but it makes sense once you think about it. Great looking parts as always!!

Bill

[Admiral_dk]

There's not many options with a 3 cylinder engine for timing, 1-2-3 or 3-2-1. I looked into making a 120 degree crank but the firing sequence would leave a big gap from when the third cylinder fired until it got back to the first

Well that is the reason I asked you in an earlier post if you where going to fire 240, 240 and 240 degrees apart or 120, 120, 120 and coast 360 degrees before starting on one again - both schemes are normal for 120 degree crank. Laverda is the only 180 degree crank 3 cylinder engine I know off - see :
http://www.motorcycleclassics.com/classic-italian-motorcycles/180-120-degree-zm0z13mjzbea

[gbritnell]

Thank you for the link. After reading the pros and cons of the two types I decided to go with the 240 degree firing order. With the small displacement of this engine I don't know how well it will run with the gap in firing. That being said I made a new cam today to suit the new firing sequence.
gbritnell

[gbritnell]

Gentlemen,
While machining the new camshaft I decided to make a short video of how I do it. I'm not the best movie maker but I think it gets the idea across.
gbritnell
<a href="https://www.youtube.com/watch?v=ysuUhHU6MvA" target="_blank">http://www.youtube.com/watch?v=ysuUhHU6MvA</a>