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The Holly Buddy Mk2 - the build from a casting

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edholly

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The original Holly Buddy is a relatively easy build made from round and square bar stock. It is a chunky engine of 2.5 cc capacity and quite a few have been made now, and it was the subject of an article by fellow Australian Maris Dislers which was written up in the 2019 issue of AeroModeller.

As the engine was quite squarish, l wanted to make one that looked more like a production engine and had had some previous castings done by a local company here in Sydney, so I made a pattern and took it to them. The casting they produced wasn't as smooth as I had hoped or as smooth as they had done previously, so they have now done a second one which is quite acceptable. I have to accept the fact that a sand casting is never going to have the fine detail of a die casting, and in a way that means every casting will be slightly different in texture, parting lines etc which makes them unique. Subsequent ones will have a small H in a circle embossed on one side to help identify them in the future.

This first casting is the trial one to prove the pattern and the build and write up I am doing for it it in real time - so if l stuff up somewhere you will all know about it. One other small point - if successful the castings will be available, unfortunately the cost to me will be $80 and for the running around I need to do I will have to charge $100 plus postage at cost for them. If you want one - could I ask you to hold off with the request till I have finished it and proven that it works please. A lot of guys have already put up their hand for one when the price was a lot less, but the foundry I think see them as a nuisance and when I took the original one back and wanted what was I guess a better job then the price went up. So hang off till I ask for expressions of interest. ( hope there will be some ! )

Finally this thread is really a continuation of the previous Holly Buddy build and that can be found at


So here we go ...
 

edholly

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This is truing up in the lathe and taking first cuts. Using the rear boss l only had to move it once and it came up within 5 thous of run-out for the front. The first cut was 5 thou and it cleaned up all except adjacent to the join line, another 4 thous cut and that dis-appeared. This shows the casting is very accurate and i was very pleased for this. With the heat treatment CAMCAST do, the fine cut swarf came off like fairy floss and in my opinion it doesn't get better than that. whilst I had a centre I drilled a 4 mm hole down into the area where the crankcase opening is to help keep that drilling centred when the time comes.

Many years ago I had a couple of ED Baby patterns I made cast by these people and when David Owen seen them he asked if I minded if he took a small cut. He mounted the tiny casting in his Hardinge lathe and took about a 3 thou cut - he was impressed. He had used CAMCAST about 20 years previous and said their work that day was far superior to all those years previously.

It really is in the heat treatment - they do it in house and it is a very involved process.
 

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edholly

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Now the spigot at the front has been machined, time to turn it around and complete the tunnel part of the case. I always have a second thought before taking something out of the chuck, once disturbed it always runs the risk of a bit of run-out. But instead of removing from the chuck, it was parted off adjacent to the back of the case.

So around it went and first thing to do was machine the rear of the engine flush with the land on the casting. This will then be the datum for further machining.

Now to work out how deep to machine the inside for the crankshaft to bear against. But before that to decide whether to make it a single ball race engine or a plain bearing engine. My recommendation and the way I am making this engine is as a plain bearing, it would be quite a bit more difficult than with the removable nose-piece of the original Holly Buddy as the well the ball bearing fits into is quite deep inside the case making a interference fit just that bit harder whereas a plain bearing is a simple straight face.

So with that decision made, working out how deep to go was the next job. The steps were
1. measure the cylinder diameter = 25.91 mm
2. measure the distance rear of case to the cylinder = 1.75 mm
3.calculate the centre of the cylinder from the rear = 25.91 divided by 2 plus 1.75 = 14.71 mm
4. that then is the centre of the conrod.
5. conrod width 4.8 crank-disk width 5.2 (at thrust face) so that is 2.4+5.2 from centre forward.
6. therefor the thrust face has to be 14.71 + 2.4 (half rod) + 5.2 ( disc width) = 22.31 mm
7. if a ball bearing was used then add 5 mm for outer land and relieve by .2 for inner land.

The diameter for clearance on the diameter of the crankdisc is the same as the Holly Buddy as this engine is designed around those internals, and machined to 20.6 a 32 tpi thread was then cut for 6 thou (nominal) to screw the back cover into.

Lastly a 23/64th drill was run down the nose and removed to check how centred it was but not before marking the piece and the chuck to put it back in the same orientation.. Using the calipers I found it was within 3 thou of centre, whew! It was then reamed to 9.5 mm for the crankshaft. I used an expanding reamer which over the years I have found gives a consistent size and a great finish when used with plenty of cutting fluid.

So that brings it to the vertical work with the Mill. All up about 6 hours work so far.

A note for thread cutting. I never disengage the lever once I start making a thread. I cut the initial thread. move the tool away, back it out and then cut deeper on the next cut and repeat this till I am happy with the thread. For very fine threads this is what i have found to work best for me. You have to concentrate and take it slowly but the results are worth it.
 

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NickP

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Please keep the photos and commentary coming - it’s really helpful especially for newbie engine builders like me who would like to have a go but don’t really know where to start or who don’t have the knowledge needed to get it right!
 

edholly

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The next step is to create the cylinder aperture and hold down bolt holes. I actually modified my vice so the I can clamp things down that are within the jaws, to do this element of the build. You can see how this works in the 3rd photo.

I needed to know how much to take of the "deck" as a start as this is obviously critical to the cylinder height and therefor the port positions which is the timing of the engine. It is very difficult to measure this externally, so I measured the internal cavity diameter using a telescopic gauge - this was 0.838" I then drilled a 5mm hole exactly centred on the top of the engine into the internal cavity. I then faced off the top of the engine to get a level to measure from.

Depth from top to bottom was 1.590" and the internal cavity centre is .419 from the bottom, so therefor from mid-point of what will be the crankshaft to the top is 1.171. The plans call for this height (deck height) to be 27.26 mm or 1.073" ... therefor have to remove 0.098" and as a final check, the top to bottom when done will be 1.492" Measuring it now, the casting is just tall enough by about 10 thou ... whew ! Might add another 30 thou to the top for subsequent castings. Next installment in a day or two.
 

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KenC

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The normal method for setting deck height is to fit a close fitting bar of known diameter into the crank bearing in the front of the case and measure down with a depth gauge from the top of said bar. With this method there can be no error because the deck height is related to the crank cantre.

A further point. In picture 3 you appear to be using the unmachined as-cast underside of the mounting lugs as a reference for machining the deck. I hope that this is not the case. Normally one would try to use the crank bearing as a reference then you know that the cylinder ends up square to the crank, but using the surface of an unmachined casting leaves you just hoping things are something like square. I guess your answer is that it works for you, but I suggest that this is not setting a good example to novice engine builders.
Ken
 
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edholly

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So the next step was to deck the top of the crankcase, bore the cylinder well and drill and tap the 4 x 3 mm hold down holes. The deck was machined flat as per above dimensions which in fact was "cleaned" by just 10 thou, so the casting had just enough meat to allow spec to be maintained. I had thoughts of just running a plain bearing arrangement, but when the cylinder well was machined it removed about 140 degrees of arc from the thrust face - this would probably still be ok, but in the end I decided to follow the Holly Buddy's original layout with a ball bearing as the thrust bearing.

This then meant I had to set up the case in the lathe right on centre to ensure the bearing was on the same centre as the shaft's nose piece. 3 jaw chucks are good, but usually when you remount you do get a tiny offset, mine was about 1 thou and a litle tap here and there got it right on centre using the boring bar and the cross slide as the means to that end.

I found that the depth of the thrust face was 0.8 mm to far forward, so the 5 mm bearing's hole was only made 4.2 mm deep - this then allows the shaft to be made on spec. One of the things I hate is deep hole machining as it is very difficult to measure diameters when the hole is well over an inch fro access. I used a pair of outside screw adjusting calipers and ever so carefully transferred the measurements to my very accurate digital calipers. Seems to have worked ok as the ball bearing will just enter the hole, and I am sure a little heat and it will go in and then tighten up when cool. Next srep will be to make some of the compontry, the case is basically finished now, just the extension at the front which was used to hold it in the lathe to be removed.

Photo 1 - decking the top of the crankcase.
Photo 2 - using the outside caliper to get the diameter of the bearing's hole.
Photo 3 - Obtaining the hole's diamter with the digital calipers allowing for a slight pinch fit.
Photo 4 - The almost finished crankcase.
 

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edholly

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Next installment - machining the venturi hole. Simple process, setup in the vice and ensure chocked so it can't move when jaws done up around the nose extension. Drill the 5.5 mm hole and lightly face off each side flat. Make the rear cover with an extension and screw in then holding that in the chuck and the nose into the live centre, part it off. But not before thanking it for it's good service :) in achieving many steps in the process ! That just about finishes the casing, sometime later the 4 mounting holes to be drilled in the engine mounts to finish it off.
Photo 1. Machining the venturi inlet
Photo 2. Parting off the nose extension
Photo 3. The finished case
Photo 4. ditto
Photo 5. ditto with rear cover with extension yet to be machined hollow
 

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