Suggestion to secure a crankcase for fly cutting

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charkmandler
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Suggestion to secure a crankcase for fly cutting

Post by charkmandler »

I have a motorcycle crankcase that needs the deck fly cutting as it is .006" out of parallel to a mandrel through the main bearings i.e. the barrel will be leaning to one side. I have a proof of concept set up that shows I can clock up the mandrel through the main bearings on a sweep of the dti and also have room to flycut the deck. I will make some proper blocks to sit the V blocks on.
There is packing under the back part of the crankcase so that the X axis of the crankcase is parallel to the table.
To make the setup work the fixture plate (holes and tapped holes to be done) - is hanging over the back of the mill a little.

I'm looking for suggestions on how to 'bolt' down securely for fly cutting. There are holes for crankcase bolts that can be used but they are clearance for .312" spindles and there is play so using jacks under and clamping over will still allow movement of the case. I also need to clamp down evenly so there is no uneven stress tilting the cases.

Sorry some of the photos are on their side, no options to rotate them (they are correct on my PC) Any suggestions please.

Thanks, Mark.
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ctwo
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Re: Suggestion to secure a crankcase for fly cutting

Post by ctwo »

I love workholding challenges, but I'm having a hard time visualizing which surface you want to fly cut.
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GlennW
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Re: Suggestion to secure a crankcase for fly cutting

Post by GlennW »

Perhaps if you posted an image showing the surface that you want to cut, as I don't see anything that looks like a flat surface in what you have posted.

How are you sure the error isn't in your machine or your setup? (is the knee drooping?)

Using a knee type milling machine to determine square and parallelism on something like you are attempting can be a head scratcher.

Did you do the inspection work to determine the error on a proper granite surface plate?
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armscor 1
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Re: Suggestion to secure a crankcase for fly cutting

Post by armscor 1 »

Mandrel through main bearing bores.
Mandrel set on Vee blocks, that will give precise alignment for the cylinder side to side.
Front to back alignment is not that critical.
Most engine rebuilders set the block to the main bearing registers before boring or decking.
charkmandler
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Re: Suggestion to secure a crankcase for fly cutting

Post by charkmandler »

I will post a picture of the surface to cut tomorrow but its basically the top surface behind the timing chest.. Armscor, the crankcase is already set up as you mention. The main issue is how to clamp it into position.
Glenn, the mill is not a knee type. The measurement is accurate as the table is trammed true and the mandrel also trams up true. I then fit the dti to a short holder and effectively tram the top.
stephenc
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Re: Suggestion to secure a crankcase for fly cutting

Post by stephenc »

.006 ? if you have a good clean sealing surface theres no way in heck id try to chase that down .
unless your building a super high horsepower engine thats at the very limits of what the cases themselves can take id ignore it . with a timing chain or belt its no where near enough to cause issues with wear or timing accuracy , nor is it enough to cause rod bearing issues . and the cylinder itself surly wont care .

i would take a bet that if you dig deep enough and find some factory tolerances for those cases your gonna find that your well within spec at .006
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Bill Shields
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Re: Suggestion to secure a crankcase for fly cutting

Post by Bill Shields »

Agree with in tolerance concept.

How much do you think things are going to move when at operating temp vs room temp?

If you must do it then I could suggest expanding arbors up thru and into every bolt hole along the split line..anchored onto the mill table.

If the holes are too small for expanding arbors then put slip fit studs up into every hole -> get it where you want it and drop some loctite down into the fit and wait for it to set up.

If you really want to be creative you can make them adjustable from the bottom up.

This way you can secure the piece everyplace you have a bolt hole and be sure it does nor ring or move while cutting.
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TimTheGrim
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Re: Suggestion to secure a crankcase for fly cutting

Post by TimTheGrim »

I just did this 3 weeks ago and it was solid enough. The use of the 246 blocks allowed holding one in the vise but having the ability to fine tune the angle on the other using that big bolt. Then I anchored the other end of the case to the table with an angle plate and a parallel. I only had to remove .004 to get the case halves to match decks. I’m guessing that was from all the welding after divorcing the trans half.
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charkmandler
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Re: Suggestion to secure a crankcase for fly cutting

Post by charkmandler »

Thanks TimTheGrim, thats given me some ideas. I like the idea of using the vice.

Bill / Stephen, this is machining the deck that the barrel sits on (nothing to do with timing). Is the deck is out by .006" it means for the bore of this engine the top of the barrel / bore is .003" closer to the piston. For the low expansion piston being fitted to this engine that has .0035" cold clearance there is the chance of cold seizure. Also there will be a great deal more friction.
stephenc
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Re: Suggestion to secure a crankcase for fly cutting

Post by stephenc »

Ok so I'm trying to follow along here , your wanting to deck the seating surface for the jugs or barrel as you call it to the the engine case .

But here's where you realy have me confused ... Where's this cold clearance at ?
Valve / piston ? Piston skirt to the case ? Or piston to cylinder ?

I'm not a motorcycle guy , but I've been inside a whole lot of both air and water cooled engines
Both smaller and many many times larger then a motorcycle engine .
I've never heard of or seen valve to piston clearance that small so I'll assume you mean piston to cylinder .
And I'll stand by saying that if your mating surface will seal your chasing fairies .
I'd also say that if its a problem you really want to correct and again assuming you have a good mating surface on the engine case your better off correcting the issue on the jug itself then the engine case . always work on the cheapest part possible .


I'm also going to throw this out there , if your worried about .006 on the mating surface .
Maybe you should buy an extra base gasket then make a torque plate and bolt the jugs down without a cylinder head and see how things go wonky under torque .
charkmandler
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Re: Suggestion to secure a crankcase for fly cutting

Post by charkmandler »

Setup complete thanks for the setup ideas Tim.

Stephen, its the top of the crankcase, if its out with the main bearings the barrel will sit on like the leaning tower of Pisa - slight exaggeration but good to imagine what it would do to a piston going up and down it. It would rub on one side. .006" is massive and not acceptable.
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GlennW
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Re: Suggestion to secure a crankcase for fly cutting

Post by GlennW »

charkmandler wrote: Wed May 26, 2021 12:25 pm ...but good to imagine what it would do to a piston going up and down it. It would rub on one side. .006" is massive and not acceptable.
One side of the piston is already heavily loaded against the cylinder wall on the power stroke due to the connecting rod angle. It doesn't just glide up and down centered in the cylinder.

The friction you are concerned about is pretty much immeasurable in comparison.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
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