Stress relieving EN1A (?)

I'm making a Stuart Turner (type) triple expansion steam engine, and need some advice please. The crank shaft is about 9" long, and the throw is 1/2". My first attempt to manufacture out of solid (but with balance weights added separately) was with a piece of steel of unknown pedigree. Although rather difficult to machine, all went well (using very sharp tools) until I milled out the excess material between the crank webs. The shaft then took on a "bend" - I presume this is because of internal residual stresses becoming unbalanced as more material was removed from one side of the bar than the other (?). I have abandoned this piece, and have now sourced a length of 1

3/4" (42mm) pedigree EN1A. The question is, should I try to stress relieve this before I start to machine it, and if so, is "cherry red for an hour and cool in still air" an appropriate recipe? Any other comments / observations would be welcomed.

Mike D

Reply to
Mike D
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Always used to be. I'd also suggest rough machining it first, stress relieving and then finish machining.

Regards

Charles

Reply to
Charles Ping

I'd like to add:

600°C, one hour per 10mm thickness. Put the part in the oven while heating it up. Cool it down *in* the oven. 50°C drop per hour is the suggested rate.

Nick

Reply to
Nick Mueller

Yes, seems the right answer.

Composition with steel is not the whole story. If it is bright bar it has probably been drawn through a die, which cold-works the surface. This leaves residual stresses, and if you unbalance them they can distort work a little. It also means the surface is slightly tougher than the interior. These are good properties if you want to use the bar as-is, as in bending or torsion the maximum stresses occur at the surface. Any symmetrical turning operation will also be OK.

Annealing at a high enough temperature will ease these stresses out.

I wonder if black bar might be better suited to this sort of job - the black oxide suggests the last mechanical shaping was while red hot - so it is likely to have the stresses already annealed out.

Steve

Reply to
Cheshire Steve

I have made a number of crankshafts from solid bar and I always rough machined first (very rough) and left the bar for a week or so then machine again and left it another week. The stress will move the bar but will gradually even out with time. Repeat as many times as you find necessary before the crank does not move any more. Finish machining is after about 3 weeks or a month. You may not want to wait that long of course.

Reply to
Alan Marshall

Alan,

Do you use EN1 for your cranks? The reason I ask is that the couple of designs I have for IC engines (one in progress (15cc single), one in plan (9cc twin)) call for EN8 and one is much nicer to machine than the other...

Steve

Reply to
Steve W

Chalk and cheese, but for the Stuart chuffer the En1A will probably be OK. The En8 is much stronger (around 40 ton) and a completely different species being a thro' hardening medium (~0.4%) carbon steel. Altogether a very different beast.... Richard

Reply to
Richard

In my case the Stuart is planned as a display only model - occasionally operating on low pressure air, probably without a load attached, at very slow speed - so load on the crankshaft will be very low. IC engines I guess are a very different story. I chose EN1A for it's machining properties.

Mike D

Reply to
Mike D

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