When did gunmakers “standardized” their gun parts for all their guns to have the same specs?

Nom de Forum said:

What is your source supporting your statements? Where have you seen drawings or text about the specifications? Where have you seen in print that Ford re-drew the drawings. Are you just assuming Ford would have to create new drawings rather than consistently building to specification? Please provide documentation supporting your statements. Seems odd my source would specifically mention Packard making drawings but not mention Ford previously doing the same.

Edit: I believe stories of Rolls-Royce using a significant amount of hand fitting are exaggerations. Wish I had a more authoritative and detailed source.

Click to expand...

lysanderxiii said:

The information comes from Sir Stanley Hooker, himself, he was the chief engineer on the supercharger and later went on to be a key figure in RR jet engine development. In his autobiography, "Not Much of an Engineer," Sir Stanley relates the following anecdote:

"In my enthusiasm, I considered that Rolls-Royce designs were the ne plus ultra, until the Ford Motor Co. in Britain was invited to manufacture the Merlin in the early days of the War. A number of Ford engineers arrived in Derby, and spent some months examining and familiarizing themselves with the drawings and manufacturing methods. One day their Chief Engineer appeared in (Merlin development head Cyril Lovesey's) office, which I was then sharing, and said, 'You know, we can't make the Merlin to these drawings.'

"I replied loftily, 'I suppose that is because the drawing tolerances are too difficult for you, and you can't achieve the accuracy.'

"'On the contrary,' he replied, 'the tolerances are far too wide for us. We make motor cars far more accurately than this. Every part on our car engines has to be interchangeable with the same part on any other engine, and hence all parts have to be made with extreme accuracy, far closer than you use. That is the only way we can achieve mass production.'

"Lovesey joined in, 'Well, what do you propose now?'

"The reply was that Ford would have to redraw all of the Merlin drawings to their own standards, and this they did. It took a year or so, but was an enormous success, because, once the great Ford factory at Manchester started production, Merlins came out like shelling peas at a rate of 400 per week. And very good engines they were too.”

Total Merlin production at Ford’s Trafford Park factory was 30,428.

Ford (UK) began work in early-1940, this is about the same time that Ford (US) originally committed to build 9,000 Merlins. Ford (US) backed out a few months later, and in June 1940 a example engine and the drawings were turned over to Packard. These drawings would have been the same drawings Ford (UK) had just insulted in front of Hooker and Lovesey. Packard would have been in the same boat as Ford (UK), in that a redrawing of the R-R prints was in order.

The Trafford Park in Manchester began delivering engines about the same time as Packard, summer of 1941, even after being bombed during its first week of operation. The Merlin was built in five places, Rolls-Royce's factories at Derby (82,000 engins), Crewe (26,000) and Glasgow (23,500), Ford's plant at Trafford Park, Manchester and Packard's plant on Grand Boulevard, Detroit (55,500).

Click to expand...

lysanderxiii said:

1) Rolls adopted Ford's modern production methods after 1941, and greatly increased production rates. The three R-R plants matched Ford's production monthly numbers (All this is from Hooker's book).

2) Robert J. Neal writes in Master Motor Builders, a tome documenting Packard's non-automotive engines that "the British did not specify tolerances and fits, and Packard had to take parts from an existing engine and make measurements to determine these specifications as best as they could, using engineering judgement where necessary."

This notion is also reflected in the March 1946 issue of Flying magazine, which includes a retrospective on the powerplant by Paul H. Becker titled “Mass Producing the Merlin”:

“It took the war to prove that the aircraft engine is not that complicated micro-micro-inch construction problem peacetime talk has made. It is larger, lighter-per-horsepower and has more parts than the automobile engine. But, it can be made with the same ease, relatively cheaply, and on a similar assembly line.”

Later: “The secret of the this low-cost, high-production manufacturing is the assembly line. Rolls-Royce manufacture[d] a 'fitter’s' engine with parts being brought to the unit under construction on a bench. If the part doesn’t fit, it is machined until it does meet required specifications.

“An American assembly line reverses this procedure. A conveyor belt brings the engine to the parts which always fit for by American methods all parts are made so precisely that they are always interchangeable.”

(I have seen this type production in the UK. In assembly a gas turbine, where US production would measure and assemble a shim stack to pre-load a bearing, the Brits would take a single oversize shim over to a lathe on the assembly floor and machine a single shim to the measured dimension. When I asked about this, I was told, ". . . it's no problem, the repair shops have lathes too . . .")

3) From a British mechanic with experience with their design philosophy - "It seems the original RR Merlins were balanced and blueprinted type engines. I have worked on factory Jaguar engines which had multiple different fit pistons in cylinders that were all standard bore. They were not interchangeable. I have balanced and blueprinted engines fitting parts to the minimum clearance. Pistons were knurled for the best fit to a particular cylinder. They were not interchangeable."

4) The Hispano-Suiza HS404 20mm cannon, the 40mm Bofors, and the 20mm Oerlikon all had to have the drawing package redrawn by the US manufacturers to eliminate hand-fitting and parts serializing, with varying degrees of success. The later two worked very well, the HS404 went through four variations (M1, M2, M3, M24, and Mk 12), but never quite achieved the desired reliability. More recently (early 1970s), when the US Navy bought the Oto Melara 3in/76 Compact rapid fire mount, spare parts from Italy had to be hand fit by the Oto Melara field team, until FMC took over the contract, and began to supply replacement parts. On the flip side, when the US bought the MAG-58 as the M240 back in 1977, the drawing were redrawn in accordance with MIL-STD-100 (but still in metric units), but the tolerancing was adequate for universal interchangeability.

Click to expand...