Factory Tolerances--Part Two - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 10-14-2000, 04:17 AM Thread Starter
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Factory Tolerances--Part Two

[img]/wwwthreads_images/icons/smile.gif[/img] For those of you who saw the first post on factory tolerances in machining and how it is possible to build a "lemon"[img]/wwwthreads_images/icons/wink.gif[/img] with parts which are "within tolerance";[img]/wwwthreads_images/icons/frown.gif[/img] here is another post which will probably draw out some good discussion as well.[img]/wwwthreads_images/icons/smile.gif[/img] If you have looked under the hood of a late model Ford pickup with the "Triton" Vee Engine, you will notice that the engine configuration is quite different than the historic V8 that you may be used to seeing.[img]/wwwthreads_images/icons/shocked.gif[/img] What makes it so different looking is that it is a "component" engine, for lack of a better term.[img]/wwwthreads_images/icons/blush.gif[/img] That means that it has no large cast iron or cast aluminum engine block per se, but rather a crank housing, and some cylinder sections bolted onto that.[img]/wwwthreads_images/icons/cool.gif[/img] The engine has many small pieces rather than few large pieces, and that is an advantage to manufacturing for a couple of reasons:[img]/wwwthreads_images/icons/smile.gif[/img] (1) it is easier to hold precise tolerances on smaller components than it is on larger ones. Just the weight of the part alone causes wear on the tooling and can lead to out-of-tolerance parts, and therefore scrap, so that is a big factor in controlling cost.....having fewer scrapped parts.[img]/wwwthreads_images/icons/tongue.gif[/img] (2) If you DO have an out-of-tolerance part, you are throwing away a smaller piece of the engine, instead of dumping the whole block if one or more machine operations turns out badly, so production is less affected by scrap rates.[img]/wwwthreads_images/icons/tongue.gif[/img] (3) With the machines they have now, perfect parallelism and concentricity are less of a problem to achieve, and that opens the door to building a good, "component" engine, and THAT opens the door to using common parts such as cylinders and heads to build a whole FAMILY of different engines which share the same individual components, but set them on a longer crank housing for instance.[img]/wwwthreads_images/icons/smile.gif[/img] GM rubbed up against this concept in the 1980's with the troublesome 4.1 bolted-together engine in the Caddies.[img]/wwwthreads_images/icons/frown.gif[/img] The GM 4.1 was a sort of half-step toward the engine that Ford has now. When you look at the way VW engines were constructed way back before WWII, and how aircraft engines have been built for years and years.....with the crank housing supporting bolt-on cylinders, you have to ask yourself why the auto industry has taken so long to develop the "component engine" concept?[img]/wwwthreads_images/icons/smile.gif[/img] This is a VERY interesting time in automotive engine development.[img]/wwwthreads_images/icons/tongue.gif[/img]

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post #2 of (permalink) Old 10-14-2000, 08:26 AM
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Re: Factory Tolerances--Part Two

Remember the old adage from Deming that quality can not be inspected into a product, it must be engineered in.

If this is done, and that's a big if, then quality of the final product is a given. The example most often cited was GM and it's involvement in New United Motor Manufacturing Inc, GM wanted to know why the Japanese built Toyota transmissions were not failing at the same rate the US GM built transmissions were failing... so GM proposed a test... they'd pull 10 transmissions off the assembly line in the US and send them to Japan for quality inspection... and Japan would do the same... sent 10 randomly picked transmissions to the US for GM to inspect.

Well, when the Japanese transmissions were torn down and inspected, they were all well within GM tollerances....and all the parts varied little from one another in measurment. Because of the lack of variance, the American inspectors thought the Japanese had sent them only "perfect" transmission, not ones randomly pulled off the line. They complained....

The Ameridcan transmissions were all over the tollerance map... however taken as a mean they all qualified as a quality product. Tollerances stacked, cascaded aginst one another make the Japanese doubt the quality of the Amercan puoduct. The Japanese inspectors failed every one of the transmissions.

Both the Japanese and the Americans have learned a lot since that study... but it's been a foundation for the American big 3 to improve quality.

Ain't it amazing that we backyard mechanics do just that? We look at the problem, and solve it in a quality manner and come up with a quality solution and quality fix resulting in a quality product.... and people wonder how 50 year old Jeeps just keep running... it's not just a Jeep Thing, it's a Quality thing... and you (THE BIG 3) just wouldn't understand.

Good Jeepin'

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post #3 of (permalink) Old 10-14-2000, 05:25 PM
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Re: Factory Tolerances--Part Two

Very well stated fella's. I'm an engineer in the automotive industry, and I couldn't agree more. And, yes the big 3 is getting better and so are machines. We do a lot of ford drive line work, and the last several yokes we have quoted are so tight tolerence wise that we can't even get gauges accurate enough to measure the parts, let alone pass r&r.

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post #4 of (permalink) Old 10-14-2000, 10:58 PM
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Re: Factory Tolerances--Part Two

CJDave...I believe Ford calls those engines the "Modular" design....


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post #5 of (permalink) Old 10-15-2000, 03:01 AM
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Re: Factory Tolerances--Part Two


Your comments really take me back. The first engine I ever overhauled was a 1600 cc VW four banger. Worked it over with one of those "VW Repair for the Complete Idiot" books. They were pretty damn good books, actually, and the drawings were far superior to the run of the mill manual photos or diagrams of the time. I was poor as a churchmouse and had to run parts in a borrowed Lincoln Continental Mark IV. Life was good.

My boss at the time was really understanding. I assembled it on a desktop in a spare office!

It amazed me the way that little sucker fit together, with no hint of a block whatsoever. Then I moved on the the magic of Ford 302 V8s and, of course, I never really thought about the different design philosophies there.

As usual, Dave, a thought provoking post.

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