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The Offcial Machinist Thread: Revised 01/04/2014

A place to ask general spud cannon related questions.
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Unread postAuthor: jackssmirkingrevenge » Fri May 18, 2012 9:07 pm

jakethebeast wrote:As you may see, its not flying very straight :?


And this should be the remedy, will post it on Monday :)
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Unread postAuthor: jakethebeast » Sat May 19, 2012 3:05 pm

Nice, that'll do the trick! :)
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Unread postAuthor: DYI » Sun May 20, 2012 7:13 pm

A quick question to the machinists out there:

For a prospective project, I would need to do boring, single point threading, internal grooving, and external turning on a piece of 3" diameter cold-finished 12L14 steel at between 6" and 7" from the holding point (a 4 jaw chuck which grips the first ~1.25" of the stock).
From your experience, is this likely to go well?

Please note that it is not within my capabilities to support this workpiece at any point except the chuck.
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Unread postAuthor: velocity3x » Sun May 20, 2012 8:32 pm

DYI wrote:Please note that it is not within my capabilities to support this workpiece at any point except the chuck.


Do you have a tailstock?
If you have a tailstock you could do the boring first, then support the end of the workpiece with a Bullnose Center for the other operations.
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Unread postAuthor: inonickname » Sun May 20, 2012 9:08 pm

velocity3x wrote:
DYI wrote:Please note that it is not within my capabilities to support this workpiece at any point except the chuck.


Do you have a tailstock?
If you have a tailstock you could do the boring first, then support the end of the workpiece with a Bullnose Center for the other operations.

No he couldn't, all the work is internal.

Yes DYI, it should work fine
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Unread postAuthor: ramses » Sun May 20, 2012 9:08 pm

DYI wrote:A quick question to the machinists out there:

For a prospective project, I would need to do boring, single point threading, internal grooving, and external turning on a piece of 3" diameter cold-finished 12L14 steel at between 6" and 7" from the holding point (a 4 jaw chuck which grips the first ~1.25" of the stock).
From your experience, is this likely to go well?

Please note that it is not within my capabilities to support this workpiece at any point except the chuck.


It is within the 3d rule for workpiece deflection, but I'm not sure about deflection in the chuck. I would definitely suggest using a tailstock for everything except boring (obviously).

While boring(or turning unsupported), if the workpiece moves in the jaws, you will crash within the next 360 degrees of rotation. Internal grooving could be scary, since you're essentially using a form tool on the end of a boring bar inside of a questionably held part.

A steady rest would be highly recommended.
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Unread postAuthor: MrCrowley » Thu Jun 07, 2012 1:23 am

Crna machined this for me. Unfortunately the lovely finish is no longer as I had to shave 0.1mm off it to get it to fit better in the piston housing. I'm planning on restoring it back to its shiny finish if anyone has some tips (sandpaper finer than a couple hundred grit seems to be difficult to find at the nearest hardware stores).
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Can't wait to get this going in the piston hybrid. It weighs the same as the old piston but it's infinitely more scary because it's so dense :shock:
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Unread postAuthor: jackssmirkingrevenge » Thu Jun 07, 2012 2:36 am

I approve of the machining but not the design :) there is a LOT of weight that could have been shaved off.
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Unread postAuthor: MrCrowley » Thu Jun 07, 2012 3:02 am

Your reaction was to be expected :D

I think my original design (which had an area between the sealing face and first o-ring reduced in diameter to ~25mm) would have shaved off about 130g; which is quite significant.
I decided against it, without consulting the forum engineers, because I don't like the idea of having 'pockets' where there would be gases in front of and behind the sealing face.

Has anyone done flow simulations to see if there is a turbulence caused by a piston design like that?

I'm thinking I might need to buy another boat trailer roller to stick on the rear as part of the bumper system. I was momentarily tempted to go with an air spring when I felt how heavy and dense the piston was :wink:
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Unread postAuthor: Crna Legija » Thu Jun 07, 2012 3:25 am

good to see you got it, i think ill make one for my hybrid too.

go to a car sale shop like autobarn or supercheap auto they have fine sand paper for when your respray your car. 5-600 grit should get it back.
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Unread postAuthor: jackssmirkingrevenge » Thu Jun 07, 2012 6:01 am

MrCrowley wrote:I was momentarily tempted to go with an air spring when I felt how heavy and dense the piston was :wink:


Had you made it lighter, it wouldn't even have crossed your mind ;)

If you were concerned about turbulence, you could easily have made a load bearing structure out of ali and filled the gaps with UHMWPE which is three times less dense.
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Unread postAuthor: MrCrowley » Thu Jun 07, 2012 6:33 am

That would make a cool looking piston. If weight becomes an issue, I can get someone to take a chunk out I guess but how do you fill the gaps with UHMWPE? The only way I can think of is by making a 'donut' and cutting it in half and then epoxying it back together.
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Unread postAuthor: jackssmirkingrevenge » Thu Jun 07, 2012 6:40 am

MrCrowley wrote:The only way I can think of is by making a 'donut' and cutting it in half and then epoxying it back together.


... or make the piston in two threaded halves and sandwich the donut between them ;)
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Unread postAuthor: FighterAce » Thu Jun 07, 2012 8:00 am

I've had some bad experience with dual O rings... there seems to be a great tendency for air to get trapped in the area between the 2 O rings so the piston gets stuck in the housing.

How did you avoid this in your design MrCrowley?
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Unread postAuthor: Gun Freak » Thu Jun 07, 2012 8:04 am

I don't think it'd get stuck with these o-rings, perhaps on floating though. Air being trapped between these o-rings would not cause any outward force or create greater friction. The forces would be balanced...
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