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Mounting trunions

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Submachine

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#1
I have a tapered cannon around 14 inches long and 3 inches wide. It has already been bored. I want to mill an area for trunions, tap the cannon and thread the trunions. I am using a x/y table.

Question.

How can I secure the cannon to table?
What is the best way to make sure the holes are 180 degrees apart?

Thanks
Sub
 

RJSakowski

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#2
The best way would have been to change your sequence of operations so that you milled your bosses for the trunions first. One of the hallmarks of a good machinist is to be able to determine the best sequence to optimize machinability and accuracy.

However you are past that point. The one dimension that you have to build on is the cannon bore. If you can add a counter sink for a center on the tail end, you could index on those two points to provide for milling in a plane parallel to the axis of the bore. In my case, I have an RT and tailstock . Padded jaws (to prevent marring the finish) on the RT mounted chuck and a center on the tailstock would work. Near the mouth of the cannon barrel, I would use a machinist's jack and clamp , both padded or better yet with a pad machined to fit the barrel to secure the work.

Once the first boss was machined, that surface could be used to reference the second surface.

I have seen where hot melt glue has been used to secure irregular workpieces. In your case, I would probably build up enough so I could turn a cylinder that could be mounted in the vice. This will give you the parallel plane for machining your boss. Other materials used for this purpose would be low melting alloys or possibly a thermoplastic like polypropylene.
 

dlane

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#3
I did mine like this , 3/16 strap with trunions welded to it 180* strap sets in a groove in barrel,
Didn’t want to drill into barrel. 529B8207-32E5-4276-BB22-244CA630E969.jpeg
 

derf

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#4
Make a pair of square blocks from hardwood, and bore a hole in them for both ends of the barrel. Make them a snug fit and split one side to compress with a screw. Once the blocks are secured well, clamp them directly to the table, and just flip to do the opposite side.
That's how I did mine, but is was a slightly larger barrel.......36" long, 2-14" bore.
 

MozamPete

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#5
but is was a slightly larger barrel.......36" long, 2-14" bore.
How can you just drop that into the conversation and not give us a photo?
 

derf

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#6
cannon.jpg
I have some more pics of the build, but they are on a floppy disc whereabouts unknown.
I didn't drill the trunnions, they were cast in the barrel and I had to machine the outside of the casting to get them round and square with each other.
 

Submachine

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#7
I am a total newbie so keep it simple for me please: : )

I am envious of the cannon pictures. That is where I want to get.

I get the hardwood and will use that method for clamping it down. I still don’t understand how to mark center 180 degrees apart. In the future I do plan to mill the trunion section before boring/trimming. I would like suggestions for how to handle this with raw stock.

Thanks everyone
 

Submachine

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#8
Make a pair of square blocks from hardwood, and bore a hole in them for both ends of the barrel. Make them a snug fit and split one side to compress with a screw. Once the blocks are secured well, clamp them directly to the table, and just flip to do the opposite side.
That's how I did mine, but is was a slightly larger barrel.......36" long, 2-14" bore.
Can you explain split one side and the screw. Thanks
 

seasicksteve

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#9
You could scribe a line on the end and use square to set it up do one side and then rotate 180* do the other. I would set it up in a super spacer and stick a tailstock in the muzzle do one side then rotate and do the other.
 

P. Waller

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#10
Place a spacer on the mill table and chuck one end. place a steady rest on the table to hold the far end.
Mill one side then rotate the part 180 Degrees and mill the other side.

If you do not have a spacer or indexer then make fixtures to hold the part.
 

derf

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#11
You don't need RT or indexer, that's what the SQUARE wood blocks are for. The hole in the square blocks is centered. Make a saw cut on one side of the block, put a screw in it to squeeze it back together around the barrel.. Now you have a barrel centered in a box.
Clamp it directly to the table, and bolt down some stops on 2 sides to locate it after you flip it 180 degrees.
 

Submachine

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#12
You don't need RT or indexer, that's what the SQUARE wood blocks are for. The hole in the square blocks is centered. Make a saw cut on one side of the block, put a screw in it to squeeze it back together around the barrel.. Now you have a barrel centered in a box.
Clamp it directly to the table, and bolt down some stops on 2 sides to locate it after you flip it 180 degrees.
Ok, I get it now. I am a little slow. Brilliant method and perfect for my situation and available tools.
 

mmcmdl

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#13
I used to make up barrel plugs when machining our Howizter barrels . We had the big lathes and mills that could handle such size . At home on smaller ones , once your bore is finished , locktite a bar into it and chuck it up . Use a tailstock center in the rear of it . You can also turn the OD with precision concentricity with this method .
 
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