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Old 11-20-2019, 12:08 PM   #1
Russ/40
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Default Proper crankshaft grinding

I seem to recall discussions regarding the danger of machine shops turning model A cranks to an error center, or something like that. Could someone explain to me what ro be cautious about regarding having an A crankshaft turned. Also how to query the machinist about how they do IT?

Thanks
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Old 11-20-2019, 12:44 PM   #2
Bob Bidonde
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Default Re: Proper crankshaft grinding

Check this article
https://www.enginebuildermag.com/201...at-aapex-2019/


Watch this Model "A" crankshaft being ground
https://www.youtube.com/watch?v=-7Al0_ihWgs


Watch this video
https://www.enginebuildermag.com/201...at-aapex-2019/
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Old 11-20-2019, 12:47 PM   #3
Joe K
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Default Re: Proper crankshaft grinding

A couple of things of my experience.

1. Modern crank grinders "squeeze" the crank too hard between centers. This bows the crank and makes for an offset center. I discovered this when I found the crank couldn't be turned easily in the block, even though the line boring was the right size, the individual bearing clearances correct, and shims set up correctly. The solution was found in having the crank "straightened" which is an action watched in some trepidation and perhaps even a mild bit of terror. Oh - and magnafluxed and checked for cracks after straightening.

2. Modern crank grinders grind primarily for use with inserted bearings. The surface roughness of the ground journals is entirely too course since in insert journals the load is primarily taken by oil pressure - the crank only touches the bearings on startup (and yes, occasionally or more than occasionally on heavy loading.)

3. The "centers" of cranks (dimples at the end of the crankshaft where the centers go) start their life at "geometric center" but as the crank wears, similar to the rods albeit less, the journals get "egg shaped." You may or may not want to grind to remove the egg - although in doing so you may remove more material from the journal than is absolutely necessary to make it round. Better perhaps to use what remains of the journal, fix it in a lathe using a steady rest at journals 1 and 3 and "re-cut" the center, which is a job not too difficult with a regular lathe tool ground for clearance. (be sure to skim cut the flywheel flange and indicate flywheel after bolting in place.) Some may object to this as it brings a crank to a potentially different stroke and does introduce error to the flywheel, but the purpose of all of this is driving pleasure - and cranks and strokes CAN vary without materially affecting their function.

Best would be to leave it all in the hands of an experienced Model A engine rebuilder. He will have a routine commonly used and with which he is comfortable with.

Joe K
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Old 11-20-2019, 04:24 PM   #4
Kohnke Rebabbitting
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Default Re: Proper crankshaft grinding

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Originally Posted by Bob Bidonde View Post
The middle Video is J & M Machine, in Ms. They grind a crank Perfectly.

That is one thing we have had trouble with over the years, is crank grinding.

A Customer would have his shop grind the crank, and we would pour the babbitt, and we would have to regrind the crank, for any, or all the mistakes that was possible, to make, on a crank grinding.

Herm.
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Old 11-20-2019, 06:05 PM   #5
BRENT in 10-uh-C
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Default Re: Proper crankshaft grinding

Joe, I hope you don't get mad but I'm going to offer a little different perspective on what you have stated, -with the exception of your last paragraph which I think is true.


(1) I'm not sure how the grinder squeezed the crankshaft too much but I'm sure you would agree that a Model-A crank can/will sag just under its' own weight. That is why we all use a steady-rest when we are grinding the crankshaft. (BTW, I own a Storm Vulcan 15 grinder) This steadyrest actually picks up from the bottom side of the journal pin and places forward pressure on the pin on the opposite side of the wheel. That alone should have placed the centerline in the correct location for grinding.

If the 'Grinder' was causing mental stress for someone when he was straightening an A crankshaft, then he was doing the process incorrectly. We will even check/re-straighten a crank after it comes off of the grinder, and from my experiences, this does not affect the finish whatsoever. When an A crank is straightened properly, almost never will you find the flange needing any machine work. If it does, then IMO the machinist missed his set-up on indicating the centerline. If the measurement was greater than the indicated wear, then the crankshaft was not correctly straightened. The flange is just not in a wear area, so it really should not be having a problem.


(2) Most local engine machinists these days are setup for doing performance crank work since that is about the only market that will pay for the service. Performance RA finishes are much superior than the needs of a cast bearing finish. Since most grinders use a method called plunge grinding, there will generally be some post polishing needed. Cork belts are what most shops use now, -which again produces a better finish than what the OE surface finish was some 75+ years ago.


(3) I am not sure where the "egg shaped" is coming from but I typically don't find that as an issue on mains. The main journals surfaces do wear however the rotation forces are constant which generally makes the wear pattern consistent. Even if the journal were worn in an "egg shape" (non-concentric), we/I do not use the journal to indicate centerline on, ...so grinding on the high side first to bring the concentricity of the journal pin back within specs is no big deal. Most cranks that we grind will have an interrupted touch-off & cut for the first thou. or two anyway as this is being done. We even have the ability to grind to less than a tenth of a thou., and can place finger pressure on the steadyrest if I only need to take an additional 0.0001" or so. On a side note, I am probably going to put a DRO on my machine just so I can plunge with repeatability however even then I must contend with minute stone wear and compensate for that. If your machine is tight and you take the extra time to allow the stone to finish sparking, then you can hold very exact tolerances across the entire pin.

The same applies to the rod journals which can be "egg shaped" on the low side however our chucks are calibrated where we can offset them at 2.1250" and fine-tune from there. A machinist that can read a mic can measure at 90 and determine the difference (wear) and then do the math to determine the proper centerline even with an out-of-round worn or tapered journal pin. From there, all four journal pins will be ground on the same offset, and if the machinist missed the exact centerline measurement by say 4 thousands, that is a tenth of 1 percent of the overall stroke, ...which will never ever be noticed in a running engine.


I cannot speak to what Herm has experienced but I think where the biggest issues with substandard crank work comes from is, from a lack of budget and worn equipment. It is hard to find a quality crankshaft grinding machine for under $20k, -and new ones are well over $100k now. Over the years that grit from the stones wreaks havoc on ways and gibs, and bearings too which causes a machine to no longer hold tight tolerances. And, to completely restore a Model-A crankshaft takes near 3 hours to do it properly. It is hard to pay for a machine and the labor to operate it when typical customers only want to pay $150.00 to refurb a crankshaft.

Not sure if any of this answered Russ' question or not, but feel free to ask specifically if I didn't.

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Old 11-20-2019, 06:34 PM   #6
Jeff/Illinois
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Default Re: Proper crankshaft grinding

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Originally Posted by Kohnke Rebabbitting View Post
The middle Video is J & M Machine, in Ms. They grind a crank Perfectly.

Herm.
Interesting, Thanks for the video link

On the cost of re-surfacing... would a guy be money ahead with a new Scat crank or Burlington crankshaft? Just curious.
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Old 11-20-2019, 08:32 PM   #7
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Default Re: Proper crankshaft grinding

Quote:
Joe, I hope you don't get mad but I'm going to offer a little different perspective on what you have stated, -with the exception of your last paragraph which I think is true.
Not mad and it appears you have taken almost all I have said and added to it. THIS is the purpose of the board.

And it has been um, (gosh) 40 years since I went through my little trial by crankshaft.

I think back to "truing" the center in my 1906 Flather lathe and I realize I was using the flywheel flange for truing, so no skim cut required - but it did help the crankshaft straightener get a better bite and do a better job.

Generally my experience with engine mains mirrors yours - but there is this "theory" about grinding the minimal amount and to do so may require relocating the center. Now with "aftermarket cranks" fully available perhaps the need/desire is not so much?

I will admit to not doing engines every day (fortunately) and I have the ultimate respect for anyone who did - or does. Including that Knight Engineering of Pigeon Cove dude who brought me the bad news of a bent crank. He like most others in his trade did the best he could with what he was given.

And, yes, cranks can bend merely in sitting - I have one on the floor right now on its side which every day I pass and think I should do better to "keep 'er straight." But not to the tune of 0.015 like my original engine crank showed AFTER grinding. This was the product of a grinder who didn't give a $*it.

Maybe it was his bad day?

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Old 11-23-2019, 12:50 AM   #8
Kohnke Rebabbitting
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Default Re: Proper crankshaft grinding

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Originally Posted by Jeff/Illinois View Post
Interesting, Thanks for the video link

On the cost of re-surfacing... would a guy be money ahead with a new Scat crank or Burlington crankshaft? Just curious.
I don't know about money ahead, Mr. Jeff, I have put several in, of each kind. There is nothing like balance though.

I don't know about about how the Burlington is now, but they used to be shy about .052 in the rear main that had to be taken out, yet.

The Scat is made very nice, and clean looking.

We also have never had a Ford crank break. That normally happens with alignment problems. One of the best crank Breakers is trying to save old, out of Alignment old or new Babbitt.

Herm.
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Old 11-23-2019, 09:09 AM   #9
1931 flamingo
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Default Re: Proper crankshaft grinding

I believe J&M Machine is in MA.
Paul in CT
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Old 11-23-2019, 10:02 AM   #10
J and M Machine
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Default Re: Proper crankshaft grinding

The most common thing I see when we regrind a Model A crank is that the previous shop didn't indicate off the true centers of the crank; the flange and crank gear surface. Other most common thing I see is that the previous shop ground the radius off the rods and mains.
We always maintain the radi as this is the internal strength of the crankshaft.
What these pictures represent is the "As Ground" finish of the crank these aren't polished yet. Referring to Joe K, Not all grinders do a crummy job.

The crank has to be straightened before grinding as well as magnafluxed; the hammer routine "ring test" doesn't prove the crank isn't cracked.

Russ40: If you have to explain how to grind a Model A crank to a crank grinding shop I would suggest you find another shop that needs no explanation.

As with any machining operation there are many ways to achieve a good job and many to do it wrong.

If you ask the right questions which the grinder should know, is mag. straighten ,grind, maintain radius, indicate off unworn machined gear and flange surface and grind to the size you desire.

If he is a know it all then find another shop.

Yes we are located in Massachusetts.http://www.jandm-machine.com/
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Old 11-23-2019, 10:56 AM   #11
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Default Re: Proper crankshaft grinding

Thank you J&M, finally an answer I can understand.
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