Model Engine Maker
Help! => Specific Engine Help => Topic started by: Roger B on June 14, 2016, 05:56:55 PM
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As ever I am looking a couple of steps ahead to my next engine.
Starting with my current vertical design, 16mm bore and stroke (~3cc) I looked to see if I could expand it to 20mm bore and stroke (~5cc). Taking the bore out to 20mm is no problem but a 20mm stroke with split big ends in a 40mm wide crankcase is tight. No real problem, make the crankcase a bit wider, then move the cam partially into the crankcase to keep the timing gear spacing the same.
The next thought was to make a multicylinder version. With separate cylinder blocks and liners I can easily bore out the water space in the lathe but this will make a very long engine. I could just about turn a two cylinder block in the same way (65mm centre height) but a 3 or 4 cylinder block would not be possible. I have a normal boring head so I could bore out for the cylinder liners in the mill but the water spaces would be a problem. Someone on here previously mentioned using a slotting/woodruff cutter and the rotary table to solve this problem. That would probably work but would be a lot of handle turning ::)
As a 'bush engineering' alternative if I simply used my normal boring head and put on a small cut (less than 0.5mm) using the spring in the system and then started the mill would it work? I would probably try something like my small grooving tool.
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You could possibly rotate the boring head 3 or 4 turns by hand using the allen key to increase the depth of cut at the same time, then start the mill and make a cut. Keep repeating the same process until you get to depth.
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Thank you Jason, I think an experiment is required :headscratch:
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I think a Tee slot cutter/rotary table would be easier and quicker even with all the handle turning.
Jo
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I had a woodruff cutter that was the perfect depth of cut between the cutter and shank, just ran the shank around the liner boss as a guide. You can always cobble up a ring for the shank to set whatever depth of cut you desire. Freeform handle turning is kinda fun and liberating. ;)
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Does the water space actually have to be round?
Assuming 20mm bore and say 22mm liner OD.
Set boring head up to swing 21.5mm dia so it fits into the 22mm hole, lower into hole while running and then move in Y direction to required depth and make a plunge cut. Pic 1
Return cutter to top of cut and make a similar cut in the opposite Y direction. Then repeat for the two X directions. Pic 2
With all the lines removed you will end up with a water space something like this. Pic 3. Moving to say 6 or 8 positions would give a rounder space if needed.
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Thank you all for the thoughts/suggestions :ThumbsUp: The water jacket does not have to be round, it should actually break through to the water jackets of the cylinders on either side. I had also thought of boring out to the water jacket size and using a liner with a large flange on top, but this still doesn't resolve breaking through to the neighboring jackets.
Another solution would be a larger lathe, but space does not permit that option :(
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Roger, rather than a large flange you can just use a thick liner and turn a waist in the middle, but as you say will not link the water spaces. The traction engine in my avitar uses this method to allow steam to flow around the cylinder and you get the seal by pressing in the liner rather than just a flange
The method I described above could be used to join the spaces together either by just moving one side cut more than the others or if the spacing between cylinders was right they will siimply break through into the next one.
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I think I will bore a block of aluminium out to 22 or 23mm set it up in the mill and give all three techniques a try. Due to an elbow problem lots of handle tuning is not a current favorite.
As a next thought I have tried reducing the width of the big end to try and get 20mm stroke inside a 40mm wide crankcase. The original has M3 fixings and a width of 20mm (8mm crankpin). I reduced the fixings to M2.5 and got the width down to 17mm which was tight. Using two M2 bolts per side I can get it down to 16mm which will work.
According to my calculations the stress areas are as follows (mm2):
M2 x 0.4 2.07
M2.5 x 0.45 3.40
M3 x 0.5 5.03
So the strength of 2 M2 bolts (of the same material) will be somewhere between the strength of the M3 and M2.5 options.
Are my logic and calculations correct?
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Figures look about right, two bolt optoin should do the job, you can also get away with turning down the heads slightly if needed
Here is one I prepared earlier but with M3 fixings, 16mm pin, 11mm x 27 rectangle
(http://img.photobucket.com/albums/v156/jasonballamy/Engineering/Lightweight/DSC01120_zpswlis69rx.jpg)
I have done 18.5mm wide with 8mm pin using M3 and could probably get it down to 18mm
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Thank you :ThumbsUp:
Attached is my (very) rough design as it stands.
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That should be OK, 1mm between bore and thread is much the same as the M3 x 18mm wide I mentioned above. Provided the holes are tapped before the big end is bored there will be no risk of bulging into the bore.