Suggestion to secure a crankcase for fly cutting

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Bill Shields
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Re: Suggestion to secure a crankcase for fly cutting

Post by Bill Shields »

charkmandler wrote: Wed May 26, 2021 3:42 am 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.
AH ...now i get it...was looking at the wrong face... :shock:
Too many things going on to bother listing them.
charkmandler
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Re: Suggestion to secure a crankcase for fly cutting

Post by charkmandler »

Glen that is the front and back of the piston which is also able to rotate around the wrist pin and yes there is friction.
Rich_Carlstedt
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Re: Suggestion to secure a crankcase for fly cutting

Post by Rich_Carlstedt »

One technique not mentioned here is Plaster of Paris. ( ie POP)
When we had tricky thin /flexable/delicate aluminum parts to mill or run on a VTL , we encapsulated it and filled some cavities with Plaster of Paris.
It sets up fast ( 10 minutes) and offers rigidity to the part for a skim or light cut.
We even packed it around Mill clamps so the clamps would not slip off a cross-section.
Its a fast setup, It stops vibration, or deadens it , and it easily removed from ALUMINUM
It has some negatives.
It will rust unprotected cast iron tables . Clamps and hold-downs were not bothered generally
We waxed the table and Tee Slots where we were putting POP .
For cleanup of the part we washed it with Vinegar or acetic acid
it's a wonderful technique for some mill work as it keeps the part in location and rigid
DO not use it in tension, only compression.
Rich
stephenc
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Re: Suggestion to secure a crankcase for fly cutting

Post by stephenc »

I have another question .. If your worried about the jug sitting crooked causing problems
Shouldn't you be measuring the bore to determine if you actually have a problem ?
And then correcting things based on that information instead of inferring there is a problem based on incomplete data .

My logic doesn't always flow in the same direction as others . but I tend to see things in absolutes .
In other words I like to be sure there's a problem before fixing it .
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Harold_V
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Re: Suggestion to secure a crankcase for fly cutting

Post by Harold_V »

stephenc wrote: Thu May 27, 2021 10:54 am I have another question .. If your worried about the jug sitting crooked causing problems
Shouldn't you be measuring the bore to determine if you actually have a problem ?
I'm not convinced measuring the bore has anything to do with the reported problem.

Here's how I see the question, although I have to admit I'm somewhat lost in the description, as well as confused by the multiple improperly rotated images. As I understand, the deck is not parallel with the crankshaft, so the wrist pin would want to tilt the piston sideways. If the error is at a right angle to the crank, it would make little difference in operation, as that just changes the angle of the cylinder, but if it's parallel, the piston would be tilted as it relates to the cylinder. That would NOT be an acceptable condition, so the surface would be in bad need of correction.

For me, it would be very useful for the reported condition to be clarified. I have resisted commenting due to my confusion.

H
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GlennW
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Re: Suggestion to secure a crankcase for fly cutting

Post by GlennW »

Harold_V wrote: Thu May 27, 2021 3:46 pm but if it's parallel, the piston would be tilted as it relates to the cylinder.
True, as long as the cylinder bore is square to the cylinder base.

Was the cylinder possibly bored while bolted to the crankcase cancelling any error in the mating surfaces?

An inspection of the cylinder may be worth the effort.
Glenn

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Bill Shields
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Re: Suggestion to secure a crankcase for fly cutting

Post by Bill Shields »

From a manufacturing standpoint I have trouble wrapping my head around the concept of boring the cylinder in place to make up for a crankcase that may be out of whack to begin with.
Too many things going on to bother listing them.
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GlennW
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Re: Suggestion to secure a crankcase for fly cutting

Post by GlennW »

Bill Shields wrote: Thu May 27, 2021 5:29 pm From a manufacturing standpoint I have trouble wrapping my head around the concept of boring the cylinder in place to make up for a crankcase that may be out of whack to begin with.
I don't disagree. My point is that I'd explore all possibilities before cutting anything as I'm used to working on things that have already been worked on previously.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
charkmandler
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Re: Suggestion to secure a crankcase for fly cutting

Post by charkmandler »

Harold, I apologise for the difficult to understand description and photos but your description is absolutely correct. A drawing would have been the best description.

Glen there is no problem with the barrel as it was bored in the correct way i.e. using the base that sits on the crankcase as the datum. The result being that the bore is square to the base of the barrel.

I now have the crankcase secured in a similar way to Timthegrims setup and have verified that the deck is .006" out of parallel with the crankshaft and will flycut the deck in one sweep.
Last edited by charkmandler on Fri May 28, 2021 2:01 am, edited 1 time in total.
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Harold_V
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Re: Suggestion to secure a crankcase for fly cutting

Post by Harold_V »

Thanks, charkmandler. I hoped I understood your question properly.
It is my opinion that the condition is not acceptable. The least of my worries would be a seized piston, but I'd certainly be concerned about the point loading of the bearing as it relates to the crank (wrist pin, too), and I suspect that the assembly would lead to premature failure unless corrected.

Luck with the fly cut. Take baby steps. Nothing wrong with taking more than one shallow pass, to ensure you take only what is necessary, and that you don't have a problem with cutting pressure.

Fly cutting, when done properly, is an extremely low pressure operation and capable of providing a superior finish. Make sure the head of the mill is dialed in correctly, so you don't create a radius.

H
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charkmandler
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Re: Suggestion to secure a crankcase for fly cutting

Post by charkmandler »

Thanks Harold. Yes, I'm going to strip down the setup and recheck the tramming of the head as a double check, redo the setup, and verify the out of square results before cutting.
roundnose
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Re: Suggestion to secure a crankcase for fly cutting

Post by roundnose »

I have not been around for a very long time, and happened to see this thread. This is something that I have done a lot of commercially.
The most important thing, the cylinder surface must be in plane with the center of the crank shaft.
How I tested that was to machine a shaft, and whatever bushings needed, to go through the main bearings, or a test bar to replace the crank shaft itself.
That will allow the use of a dial indicator, down through the cylinder opening, to true the cases main bearings to the machine.
In my experience, the above statement is critical for success, otherwise, if the cylinder surface is not true to the main bearings, the effect will be running the engine with a bent rod, resulting in short life.
If you only have to do one engine, perhaps block the cases up the best you can, but, you must true the main bearings to the machine.
Back in the days of the HD/chopper fad, I did over a hundred crankcases, often very rare engine cases, some that were really busted up and welded back together.
What I ended up doing was using a 10" HV rotary table, using it vertically, and mounting the cases to the tables center with a shaft passing through the main bearings of the crank cases.
I found to always keep in mind the stresses motorcycle crankcase often go through, and generally being a stressed member of the chassis, they also get warped and twisted from heat and crashes.
It cost a small fortune to build an engine, the cases are the foundation.
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