Author Topic: ML Midge Diesel  (Read 12381 times)

Offline steve-de24

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ML Midge Diesel
« on: December 15, 2016, 06:17:05 PM »
The ML Midge is a 0.8cc compression ignition model aircraft engine that was designed about 20 years ago by Mark Lubbock. The design is fairly simple so fits my requirements as (a) it will be my first i.c. engine and (b) it should be small enough to make on my Cowells table top machine shop.

Can I say up front a big thank you to the experienced guys who have so willingly shared their expertise on this and other websites - I don't think I would have even started on a project like this without others showing how it can be done. It is not going to be a quick build because I don't get that much 'shop time' but I'm hoping that keeping up the build log will prod me into continuous, if slow, progress.

A build log needs pictures so I've just bought a camera but even the 'Quick Start Guide' runs to 14 pages; and there is nothing like the print size on a set of Japanese camera instructions to make you realise your eyesight is 65 years old!

To start I will post a couple of pictures showing my set-up to bore the crankcase for the cylinder. I did this last week but set it up again because I wanted something to take photos of to see if I can successfully post pictures on here. The idea for this set-up is taken from the Midge build log on modelenginenews.org

Of note is the small faceplate bracket I had to make and the boring bar held in a cross-slide mounted block to make it as rigid as possible. For those not familiar with the Cowells lathe the faceplate is only 90mm (3.5 inch) diameter.

Offline scroc

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Re: ML Midge Diesel
« Reply #1 on: December 15, 2016, 06:57:48 PM »
Looks like a great start Steve. Have not heard of the Cowells lathe over here but it appears to be a quality machine and the idea of tabletop size sounds good.
Keep up the good work and posts,
scroc
Onward thru the Fog !

Offline Hans

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Re: ML Midge Diesel
« Reply #2 on: December 15, 2016, 10:02:34 PM »
I look forward to this build, Steve. I enjoy following these projects which are confined to machines the size of the Cowells and Sherline.

~Hans

Offline Admiral_dk

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Re: ML Midge Diesel
« Reply #3 on: December 15, 2016, 10:13:09 PM »
Yipee - another diesel (CI) engine build  :whoohoo:

You are off to a good start - I will follow along  :popcorn:

Offline b.lindsey

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Re: ML Midge Diesel
« Reply #4 on: December 15, 2016, 10:47:10 PM »
I am looking forward to it as well Steve. I have a cowells on the way so am very interested to see your set- ups and how you go about things.

Bill

Offline steve-de24

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Re: ML Midge Diesel
« Reply #5 on: December 17, 2016, 12:48:12 PM »
Thanks for the comments guys.

Bill, you've a lot more experience than I have so don't follow my set-ups without question on the assumption that I know what I'm doing!

In my last post I forgot to mention that a 'button' is screwed to the bracket to locate the crankcase. The button is the same diameter as the cavity in the back of the crankcase and has a flat machined on it which is set parallel to the faceplate. By measuring the full diameter of the button and its 'diameter' across the flat it is easy to calculate how far the flat is from the button centreline which is also the crankcase centreline. The flat can then be used as a datum to face off the crankcase to the correct 'height' when machining the cylinder bore. Not my idea, I got it from modelenginenews.org.

Steve

Offline steve-de24

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Re: ML Midge Diesel
« Reply #6 on: December 19, 2016, 12:09:30 PM »
The only things left to do on the crankcase are to tap the threads that hold on the cylinder and backplate plus a bit of cosmetic skimming of the outer surfaces. I'm leaving this until the cylinder and backplate are finished so I can spot through to get their positioning correct. (At least that's my excuse for putting off tapping 8BA (that's about 2-56) into blind holes!

Onto the cylinder. Looking at the drawing one thing I don't like is the thinness of the cylinder wall where the transfer ports are machined into it. The nominal cylinder wall thickness is only 1mm (39 thou inch) and with a 15 thou deep transfer port that leaves the cylinder wall only 24 thou thick (see drawing). I'm thinking of reducing the cylinder bore to 0.354 inch (9mm) which will increase the minimum wall thickness to 44 thou inch.  Thoughts welcome. 

Steve

Online sco

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Re: ML Midge Diesel
« Reply #7 on: December 19, 2016, 12:25:56 PM »
Nice start Steve - will be following this closely.  Your Cowells looks very clean - is this it's first proper job ;-)

Simon.
Ars longa, vita brevis.

Offline steve-de24

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Re: ML Midge Diesel
« Reply #8 on: December 19, 2016, 03:53:47 PM »
Hi Simon, good to have you on board. I think I was very lucky getting my Cowells lathe and mill second hand about 6 years ago. From the paperwork that came with them they were new in ~2000 and spent the first 10 years of their lives making parts for a gauge 0 railway, its loco's and rolling stock. They've always been kept in the warm and dry and are in good condition. I haven't used them very much, there always seems to be something else taking priority, but I've decided I need to get off my @rse and make something, hence this build log.
Steve

Offline steve-de24

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Re: ML Midge Diesel
« Reply #9 on: December 19, 2016, 11:11:07 PM »
Made a start on the cylinder. I decided to go with a bore smaller than that shown in the drawing because I am worried about the out-of-round stiffness that could result, especially if the tolerances stack up the wrong way.
A piece of 5/8 inch bar was faced off, centre drilled, and then drilled through 4, 6, 8 and 9mm. For the two larger sizes I had to use stub drills because of the lack of space along this tiny lathe bed.
A small boring tool was then used to clean up the bore - I think it ended up at about 9.2mm but I don't have a good way of measuring bores. I don't really care because the piston will be made to match. The resulting minimum cylinder wall thickness increases from 0.6mm (0.024 inch) to 1.0mm (0.039 inch) a very useful increase in bending stiffness.
Next the outer surface was measured for the locating flange and a start made on reducing the outer diameter that will fit in the crankcase (I've left the swarf on the lathe in this shot just for Simon).

Steve



Offline steve-de24

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Re: ML Midge Diesel
« Reply #10 on: December 20, 2016, 03:31:47 PM »
A quick 20 minutes at the lathe enabled me to finish the diameter on the cylinder that locates in the crankcase. I wish I was more confident doing this sort of job - perhaps then I'd be faster - I seem to be constantly worried that I'm going to go from 'oversize and won't fit' to 'undersize, wobbly and unacceptable'. If anyone out there has a fool proof method then please let me know. However this time I got lucky and managed a push, wobble free, fit.
Steve

Online sco

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Re: ML Midge Diesel
« Reply #11 on: December 20, 2016, 06:30:44 PM »
Nice looking swarf!  ;)
Ars longa, vita brevis.

Offline Admiral_dk

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Re: ML Midge Diesel
« Reply #12 on: December 20, 2016, 11:07:32 PM »
Looking good so far.

As to the reduced boring diameter - I read many places that going below 1cc swept volume increases the difficulties with starting and running a small CI (diesel) engine by a order of magnitude, and if this is correct, then any decrease in boring will increase the demand on the quality off your work ....

If you keep all other dimensions the same, you might be able to increase the boring later if needed ....

The original dimensions are a proven design - not that I'm saying do not change anything as this is a hobby and we all experiment  :thinking:

Best wishes

Per

Offline steve-de24

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Re: ML Midge Diesel
« Reply #13 on: December 21, 2016, 01:18:41 PM »
Per, thanks for your comments. Last night I was looking at the late Ron Chernich's excellent modelenginenews.org website. Regarding the Midge he shows the first page of Mark Lubbock's original 1996 article from Aviation Modeller International (but unfortunately just the one page!). In Mark's introduction to the project he explains that the engine was originally designed with an 8mm bore and 0.5cc capacity and he reports that it was an easy starting, nice running engine. Mark uses his engines for flying model aircraft and it was in the search for more power that he increased the bore to 10mm. The AVI article actually presents both the 0.5 and 0.8cc versions of the engine. From this I think I will be ok with my 9.2mm bore - if nothing else I might have the only 0.66cc Midge in the world. Time will tell.
Steve

Offline steve-de24

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Re: ML Midge Diesel
« Reply #14 on: December 23, 2016, 04:41:40 PM »
I managed a bit more shop time this afternoon. The chuck and cylinder now need to be transferred to my simple indexing fixture on the mill so that the ports can be drilled and transfer passages milled. Last time I drilled using this kind of set-up I wasn't happy with the result and came to the conclusion it was because the work piece was not rigidly enough supported. So this afternoon's job has been to make a simple machine jack from bits in the odds and ends box. The result is shown in the picture.

I was concerned that there would be insufficient space when using the small milling cutter so a trial set-up was made. If I avoid milling in line with the chuck jaws I can do it with about 3mm clearance between mill spindle and chuck. Fortunately the small milling cutter I've got has a 35mm long shank (most of the others are 25mm) so I'm able to have 25mm in the collet and 10 sticking out to give me clearance. Drilling the holes shouldn't have a problem with clearance.

However it all has to wait because duty calls and I've got to go shopping - oh whoopee.

Steve


 

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