Anecdotes 2

[mickParticipant @mick65121]

Back when god was a boy I was shown a method of checking very large diameters with a suitable steel bar with points turned at either end. One end was held on the bottom ID while the point on the other end was arched across until it touched one side of the ID and a chalk mark made, it was then arched to touch the opposite side of the ID and another chalk mark made. The diameter could then be calculated to a few thousandths of inch by triangulation.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

(1)

Reduction of diameter at outer ends for a cylinder typically four inches long and crowning of 6000 inches radius is about 0.7 times one thou .

This is qute a suitable value for the purpose of reducing edge stress and is not particularly difficult to achieve .

I'm sure the engineers of the day knew exactly what they were doing .

(2) Rollers from roller bearings are much flatter in curvature – typically less than two micron reduction in diameter at outer ends .

(3) At the other end of the scale are rolling mill rollers – often several feet long . These (sometimes) have substantial crowning – often ten to twenty thou . These are not commonly machined with any sophistication – the radius is usually a quite adequate three or five flat approximation .

(4) Glass test always works on smaller components but to detect crowning on bearing rollers specially cleaned optical flat may be needed . Don't need to explain why .

Michael Williams .

PS : Some calculators do not have high enough precision to do the crowning sums and give spurious answers .

[Gordon WParticipant @gordonw]

The last time I used a lathe in anger (ie. for money) was in an empty factory, they had hundreds of castings for tractor pullies for flat belts. these were nearly all chilled, had a hole thru' the middle, had to be finish machined with keyway and crowned for the belt. I put the crowning on with a big file, don't think they were ever checked, they went to S America. For interest the key was done with a pull broach about 6 ft. long. Bit different to aircraft big ends.

[Geoff SheppardParticipant @geoffsheppard46476]

Michael

I'm racking my brains to remember any operation of this sort carried out during the production process. Do you mean the wrist pins attaching the slave connecting rods to the master rod? I remember that there was a complex routine of honing these in which there was a complex routine of turning them end for end. This was entrusted (during my time in the shop, at least), to one experienced fitter, who had a little ritual to ensure that, if interrupted, he didn't start again in the wrong place. I think that he may have been nicknamed 'Sailor' and was easily distinguished because he didn't have a hair on his head and he always wore a blue boiler suit whereas most of the other fitters wore brown warehouse coats or jackets.

It's all a long time ago now (mid 1950s), so the details are vague.

Geoff

[gerry maddenParticipant @gerrymadden53711]

The other way of putting very fine radii on components (such as bearing rollers and raceways) is to plunge-grind them using a grinding wheel that has been dressed at an angle to its axis. A staight cut made this way will infact produce a parabola but it's as near as dammit a radius and was always described as such. Inspection of a surface made in this way was done by simply measuring the 'drop' at a given distance with precision gauges. No one attempted to prove it was actually a radius, or even a parabola as there weren't many other shapes it could be.

Gerry.

[frank brownParticipant @frankbrown22225]

What about if it was done by profile grinding? Final size and shape in one pass? Might even had a mechanical reduction involved somewhere. I have read somewhere that one of the Roll-Royce engines had loong spindly valves whose stems came out barrel shaped on the grinder (due to deflection), so the template was just re-cut to compensate.

Frank

[geoffParticipant @geoff]

i heard one about a guy being interviewed for a job in engineering and was asked are you familiar with mike and vernier and his reply was i can get on with anybody i can

[Ian S CParticipant @iansc]

I think it was one of my school metal work teachers who told us how the crankshafts for RR Merlin engines were sent out to be hand lapped, 2 pieces of wood, joined at one end with a leather strap for a hinge, lapping paste applied, and the shaft rotated in a lathe, he said his elderly father did it during the war, he also made parts for Sten guns. Packard in the USA were all set up to do the whole job production line style. Ian S C

[Brian WoodParticipant @brianwood45127]

I don't think I'm revealing any secrets here, but modern aero engines are built into a ring shaped support for hanging in the 'pylon' off the wing. The shape of the ring is made such that the thing distotrs to round when loaded. I don't know the values involved, but getting the ovality assembled into the right place would be crucial!! Blade tip clearances in the hot stages inside are measured in thous.

And from the nuclear industry, in pressurised water reactors with steel shells nearing 10 inches in thickness, the control rod tube holes in the head are machined to allow for the head bulging under operating pressure. This is to ensure that they all [ and there are lots of them] finish up truly vertical to be able to drop the rods into the reactor core far below in the event of a 'scram' requiring prompt shut down.

Brian

[Brian WoodParticipant @brianwood45127]

One final story from Rolls Royce comes from the time the Company put out work to sub contract for the external pipework for fuel, oil supply, air bleeds etc. that runs round the shell of aero engines.

The company involved were essentially plumbers and there were persistently unacceptable levels of rejection in the work they supplied. On questioning what was going wrong it became clear they couldn't grasp at all the concept of the precision needed in pipe bending and the complex angulations required.

RR's answer was to hire a bus to collect the entire company, from MD to cleaner, and take them to the build shops in Derby so that they could see where things went and why it was all so important.

After that the rejection rate fell to virtually zero

Brian

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