Cutting narrow deep rectangular hole.....

I'm building a model beam engine (Stuart Major) and am starting on the
connecting rod. This is a fairly faithful copy of full sized practice
with bearing shells that are held by straps onto the end of the rod. The
straps are held in place by cotters that have to pass through a
rectangular hole 3/32 x 11/32 (about 2.5 x 9mm) which passes right
through the end of the rod which is 5/8 thick. (tried to attach picture
of drawing) The question is how do I accurately cut these holes ? It as
to be fairly square with parallel sides. Milling with a slot drill would
be tricky as even long series slot drills in this size are not long
enough. I could drill a series of holes and then try to open this out
with needle files but it would be really difficult to do this neatly.
Has anyone got any ideas ?
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Seems to me that a combination of drilling holes, milling with an end mill/slot drill from both sides, and filing might do it. A custom made broach would be nice but probably too expensive.
Don Young Alabama, USA
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Don Young
And you have two more problems with the slots on the other end that are 5/64ths wide!
Options without pondering as much as you probably have already
1 Chain Drill then open to slot with "abrafile" type blade in your saw frame. Make a "hard" jig that fits around the end, to ensure that your filing is flat and square. 2 Have it wire cut 3 Come in from the end with a slitting saw then pin a slug in to fill the end.
Whatever option you finally choose, I would test out on a bit of scrap first!
Good luck!
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Richard Edwards
I would also tackle it by making the slot from the end with a slitting saw and then pin/silver solder a strip in to close the end.
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Thanks for all the suggestions, As I was writing the post the solution of using a slitting saw from the end then filling the gap with a plug occurred to me and I think this is the only practical solution to get a neat result. The Stuart supplied casting for this component seems to be made of cast iron so I don't think I can silver solder the plug in. I think I will have to pin it with some 2mm silver steel rivitted over. Hopefully this will work ! Thanks again and I hope to contibute more to the forum in the future.
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use a square electrode and die sink it.
Take it to your nearest tech college if you don't have one yourself.
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Having done this job on a similar model in the not too distant past, may I suggest you drill it from both sides as accurately as you can and then finish off with a sharp miniature file in order to square the ends of the slot. Little of this work will be seen as it's completely concealed by the strap, is it not? It really doesn't warrant too much worry!
If you do make a major pigs' of it, you can always saw off the leg below the fluted casting, drill the casting and make a new leg to be fitted with Loctite 603, or even instant glue with a little cross pin for added security if you choose.
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Chris Edwards
Me, I'd drill away as much as possible and take a couple strokes with a needle file, or shave it out a bit at a time with an end cutting tool. Possibly hold the tool in the lathe chuck, with the work on a Milling slide and use the lathe as a shaper or planer, or, if you have one, use a slide table set-up and hold the tool in a drill press and broach out the slot after drilling (or milling) most of it away.
Milling cutters as small as this are readilly available with a long enough reach, and you can make a single lip milling cutter from either a broken HSS drill shank or a bit of silver steel if you don't mind working slowly. Once the slot is milled out, it's a cake-walk to square up the corners!
No need to hack off the end of the casting!
Cheers Trevor Jones
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Trevor Jones

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