Beng's Laura Stirling engine

Now I have "free time" I am starting to build the Beng Laura engine.

I think I will be able to manage most things (I hope to use a Seig SX2 mill and a Seig SC3 lathe) but one part has me struggling.

The aluminium piston has to have a bullet shape machined into it to fit a glass test tube cylinder. The radius tool for the lathe is not available and I simply cannot afford the expensive tools.

Can anyone suggest a simple way of machining the shape.

I was bought the lathe as a retirement present having always wanted to build a diesel engine since I was introduced to lathe work in the 60's at school. Many years have passed since then but the desire to build something that moves is still with me.

Thanks, and I hope everyone keeps safe and fit in these sad times.

Peter Armstrong

Hi welcome to the forum. where are you based, it helps with local supliers and help. This engine has been done on here and it can help with your prodject. At the top of the page, in the search box put Stirling engine Laura it shouldgive you a lot of information on machining and other tips.

David

There are 2 or 3 ways forward. One obvious option is to build your own radius tool but it'd take time and more metal to buy. Another option is to build a hand rest and freeform the thing with a hand held tool. A little simpler is to draw the shape on graph paper and work out a number of steps with their coordinates for position and depth of cut - the more steps the longer it takes but the smoother the preliminary result before you then file the final shape.

I did something similar with this shaped base. It was a bit quicker with a DRO but doable just with hand-wheels. Sorry for the blurry dodgy pics when i did it.

Final option is to grind a form tool with the risks of chatter if cutting too wide a final surface or make more than one form tool and finish off with files again.

If you haven't been given dimensions for the bullet shape then it may help to rough shape some putty/clay/filo/plastecine around a bit of tube (to avoid suction), grease or oil the inside of the test-tube end to stop it sticking and create the shape you need as a pattern to work from.

pgk

Another option is to use a file freehand with the lathe running to create the shape. This is the displacer so it doesn't need to be a particularly accurate fit as it has to have an airspace all round. Some don't like the idea of having a file and hands near to the rotating chuck but if careful there should be no difficulty. Make sure though that the file has a well fitted handle just in case it catches on something, an unprotected file tang can cause a nasty injury.

John

have you looked on Bengs website? there are build sequences (with pictures) for most of the models Bengs does.

try this link https://www.bengs-modellbau.de/magazin/ you might need to use google translate.

You may find this web page from www.meades.org useful, it has a build thread with pictures.

John