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george356
Oct 10, 2017

Potty Mill Engine advice

8 comments

I am building this engine from the plans available here :- http://www.vapeuretmodelesavapeur.com/telechargements4/h-potty-mill-engine-09-02-08.pdf

 

and would like some advice as to how to machine this component

It is the crankshaft and would appear to be turned from a piece of flat bar 18mm wide by 6mm thick. I have both lathe and milling machine but I don't fancy turning the main part of the shaft due to the interrupted cut. Should I mark it out and mill away most of the excess or just go for it?

 

Any and all advice gratefully received.

 

George

 

charliepipes
Oct 10, 2017

 

I will preface this input by saying that, like Keith, I am not a trained engineer. Just a person who is self-trained in sticking things together.

 

My first choice, based on the size of this model, would be to produce the crank as three pieces. Turning the bearing end and the main shaft from round stock on the lathe and the flat end on the milling machine.

 

It would require you change the drawing to place a hole in the side of the bearing end and the center of the flat plate along with turning a short extension to each end of the main shaft.

 

In assembling the three pieces you could use a press fit, silver solder or Loctite.

george356
Oct 10, 2017

Thanks for the response and advice, Charlie.

I have considered doing it the way you describe ( I had thought about threading the small stub extensions to prevent on-load separation) but wanted confirmation that it was the sensible way to go.

 

This engine isn't supposed to be an accurate model of an existing engine - it's a free scale interpretation of a typical mill engine and so can use any construction method I suppose.

 

Thanks again

charliepipes
Oct 10, 2017

My only concern with threading the ends would be the clocking of the threads to get the two end pieces lined up. Come to think of it, Keith does the timing with various thickness of washers to ensure his fitting line up.

charliepipes
Oct 15, 2017

So, what have you decided to do and please post photos when you do it.

george356
Oct 15, 2017

Well, I had a trial run and made a built up one - 3 pieces with the shaft socketed into the bearing and the plate.

 

Result? Horrible, in my opinion - I am ashamed to even post a picture of it. I tossed it to the back of the workbench and will probably try again tomorrow.

 

To be fair, the bearing part of it was semi OK but the whole thing just looked clumsy and disjointed.

u87project
Oct 16, 2017

George,

 

Ive attached a picture of my connecting rod for my engine....i turned the center in the lathe, the interupted cut was not that bad...my connecting rod is a little different shape, but the concept is the same.. Once the center was turned, i then machined the rest on the mill. You can see more detail pictures of my engine on my website. https://www.jag-aire.com/steam-engine

 

bencsly
Nov 6, 2017Edited: Nov 6, 2017

In my humble opinion, I would start with a length of rectangle bar and turn the ”shaft” center section on a lathe. Then i would mark out the bearing hole and bore it out. Finally, I would fit a mandril through the bearing and chuck it up in a rotary table to turn the outside diameter of the bearing end on a mill.

PS The interupted cut is no big deal if you take it nice and slow until your part is completly round. It’ll come out real nice.

Hugh Coleman
Jun 16, 2018

I’d be making it from the rectangular bar as described by bencsly. you can drill and ream your big end and small end holes accurately first. intermittent cuts are unpleasant, but sometimes necessary.

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