Junkers CLM

[Roger B]

Thank you Per  Yes that is just the start of the piston.

The two ends were milled in the same way to get a close fit and finally the corners were rounded with a radius cutter. The piston initially wouldn't enter but a little work on the corners with a fine file solved that problem. The aspect ratio of the piston, 56mm long and 8mm thick, meant it tended to tilt and jam so I made an adaptor to try it in the bench drill. It went up and down smoothly but even sealing the base with a rubber sheet and adding some oil there was no real compression    I will try and remake the piston with a nicer grade of aluminium if I can find some. If that doesn't work I will then try and cut an O ring groove around the periphery 

<a href="https://www.youtube.com/watch?v=ePD1mSQ3sA0" target="_blank">http://www.youtube.com/watch?v=ePD1mSQ3sA0</a>

[Admiral_dk]

I had thought - 'How will he make the corners with the correct Radius without a CNC' ...?

So obvious - how could I forget 

The O-Ring option is the only one I see not making too much trouble. Your little trick above, makes me think a Router-Bit like the ones for wood, with a small bearing @ the lower end - that runs against the Object, to ensure same Depth of Cut, all the way around .... My two Cents anyway ....

Having the Piston run perfectly square to the Bore is a must (I know you know). As you make the complex Piston shape for this Engine out of more than one part - how do you plan to keep them aligned (I'm thinking a 'Pocket' milled/bored for the round part into the 'Square') ?

Per       

[Roger B]

Lots more fun to go still     

Whilst thinking about the scavenge pump I finished lapping the cylinder. It's hard to photograph but all the machining marks seem to have gone 

I decided to try and polish the inside of the scavenge cylinder and then make a new piston from a different alloy. After a few strokes with an abrasive stick a strange stipe pattern appeared all round the cylinder. There are high spots at either end and one in the middle. The surface between them is around 0.02 - 0.03 mm lower (measured with a straight edge and feel gauges). I am not sure of the cause, hard and soft spots in the cast iron, badly ground milling cutter, something else  but it would not help the sealing.

[Roger B]

While I decide how best to proceed with the scavenge pump cylinder and piston I moved on the next challenge, the scavenge pump valves. The original has 12 circular disc valves, six inlet and six outlet. There is space for a 12mm bore  down the side of the scavenge cylinder which leads to using 5mm id 10mm od  thrust washers as the valve elements and brass disc with 2mm wide segments milled out as the valve body.

The body was turned from 14mm brass bar and the valve guide was 4mm square brass turned to 5mm diameter to fit the thrust washer and give a little more flow space.

I initially tried milling out the body with it fixed to a mandrel with an M3 caphead but the head of the screw was just too big and I had doubts about it unscrewing 

Next step was to make a pot chuck. This seemed to work for the first segment, but then it started to slip which I didn't see under the piles of swarf  Maybe it was the vibration, maybe I didn't tighten the chuck enough 

[Roger B]

Looking at the body it seemed that the valve disc covered all the opening so I though I would give it a try:

https://youtube.com/shorts/_ENqPf8gxEo?si=6vKj2D2PCzyTLQ3x

I seems to work, the disc is tipping as all the openings are on one side, so I will make a new body and keep a careful eye on the pot chuck.

[Roger B]

I received the helical gears for the oil pump and governor drive. There are both LH and RH so I can choose the rotation direction of the engine. As a symmetrical two stroke it will run in either direction but the oil pump is unidirectional.

I made another air valve body and this time it did not slip so I will be able to carry out some proper tests. To try to reduce the mess I put the vacuum cleaner by the milling table. At 20 000 rpm the chips go a long way 

[Roger B]

I made a number of calculation regarding the spring force on the valve disc, based on the valve opening at around 0.1 bar. 20g seemed reasonable and a quick check showed I could reach 0.1 bar by blowing. I found an suitable offcut of brass that weighs just over 20g and put the valve together for a trial:

<a href="https://www.youtube.com/watch?v=E7FX6_-ZE1E" target="_blank">http://www.youtube.com/watch?v=E7FX6_-ZE1E</a>

That worked quite well 

[cnr6400]

 

[Kim]

Very nice work, as always, Roger.

I have YADQ (Yet Another Dumb Question): What's the air valve for?  It looks neat, and the way you tested it is pretty cool. But what does it do for the engine?

Thanks,
Kim

[Roger B]

Thank you Kim   The only dumb question is the one you didn’t ask.

As this is a two stroke engine it does not have a separate induction stroke to charge the cylinder and so needs some form of pump. The simplest two strokes use the engine crankcase as a pump, others have a rotary blower/pump. This engine uses a pump cylinder on top of the engine, which in this case is rectangular just to add to the fun, and the air valves are the inlet and outlet valves for this pump. There are 6 inlet valves and 6 outlet valves so I need to make them as simple as possible.

[Kim]

Thank you for the explanation, Roger!   

There is so much I don't know about IC engines.  As I often say, I learn something new on this forum every day!

Thanks again Roger,
Kim

[Roger B]

I have ordered some springs for the air valves and will continue the testing when I have received them.

As the next two pieces, the oil pump and the top piston yoke require turning and milling I will make them in parallel.

The yoke is made from a piece of 9mm x 14mm c45k key steel. The ends were faced and centred with a deep centre drilling to leave a bearing surface after the ends were tapped M3 for the little end bearing retainers. The gudgeon pin and oil holes were then spotted and the gudgeon pin hole was drilled 7.5 mm and reamed 8mm.

[Roger B]

I already had a suitable bronze bush which was pressed into place and a 2mm oil hole was drilled from the top.

The gears I had purchased for the oil pump had bosses which I carefully parted off and the master gear was then opened out to 6mm bore and broached with a 2mm keyway.

The stubs on the yoke for the little end bearings were turned between centres to 8mm diameter with a small 9mm step behind for the inner race to press against.

[Roger B]

After a quick assembly trial it was back to the mill to shape the clearance cut outs on the yoke.