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bogstandard:
rleete,
That is a major problem nowadays, everyone thinks that hi tech machining will give perfect results, in a lot of cases yes, but under certain circumstances hand finishing is required, we would call it fitting. Now all these old codgers are gone, the art of fitting is getting very rare to find in industry any more.
I remember visiting Dowty Rotol in the late 60's, and they employed disabled ex military fitters to flatten platens that went into powered controls for aircraft. They used a combination of three things, a 2ft diameter DTI, a polished oblong of glass and the Radio Times newspaper. The glass was placed on the top of the platten and a light shone from one end, this generated internal rays of light, similar to Newton rings, only straight, that if it sat over a high spot would 'bend' the beams of light. The shown high spot on the platen was gently rubbed on the paper and the results were checked again under the glass block. The DTI was the rough check, it was the bending light beams that gave the super accuracy. They reckoned that the Radio Times had the perfect abrasive surface for what they were doing, all others were tried, but gave mediocre results. Those people were working, in my estimation, to millionths of an inch.
These plattens were made to have the hand fitted surfaces sliding against each other, and when up to 2,500psi of hydraulics were applied, like a slide valve in a steam engine, no sideways leakage occured, but still allowed the plattens to ride face to face.
Such skill is a dying art form.
John
transcendentalnumber:
This is a pretty interesting book: http://www.machinetoolpublications.com .
Bernd:
John,
The company I used to work for used carbide scrappers. The blades were about an inch wide and about one and a half inches long. They fit in a holder that looked like a very large file. The carbide was held in place just as a carbide tool would be on the lathe tool. My Dad's got one if and when I find it I'll take a pic. The carbide was ground on a diamond wheel grinder with the table set at about 1 or 2 degree. This gave two very sharp cornered scrapping surfaces per carbide scrapper.
Also what you described was what we called an "optical flat". That was mainly used in the tool room were my Dad worked and was reserved for lapping components totaly flat.
Bernd
bogstandard:
Thanks for that Bernd.
There is always someone who can give you the correct information if you just ask. I have never seen carbide scrapers, hence the request if they did exist.
Thanks
John
kellswaterri:
Hello all,...a little off topic, but the reference to ''scrapeing'' brought to mind this...does any one know what was meant by the terms ''spring'' and ''draw'' when fitting and scrapeing main shell bearings to a crankshaft on full size engines...now That was Real ''scrapeing''
All the best for now,
John.
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