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Back Spot Facing Tool

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charkmandler

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I am currently building a McOnie Oscillating Engine and have a requirement to spot face the backs of the cylinder faces where the end caps bolt through. This needs a spot facing tool that can be pulled through the 8BA clearance hole and run in a drill (or some method).
Most of my machining is with bought cutters as the model making is an off shoot from vintage motorcycle restoration and brass musical instrument repair. Can some one instruct how to make such a cutter or point me the direction of a tutorial?
Thanks, Mark.
 

Charles Lamont

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I have never made one that small, and it is not going to be easy but I think I would do this:
  • Take a length of silver steel (drill rod if you are in the USA) and, if necessary, turn a short distance to the required seating diameter.
  • Drill and tap the end 8-BA.
  • File two (or three) teeth on the end, like an end mill, but remembering you want it left handed.
  • Part off.
  • Heat to bright red supended in such a way that you can quickly dunk it in water to harden.
  • Heat to 200°C or a bit more for some minutes in the domestic oven to temper.
  • In your drill, lathe, or mill, chuck an 8-BA stud, and pass it through the hole in question.
  • Screw on your cutter.
  • Cut.
 

charkmandler

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Thanks Charles, that was the route I was thinking of taking. Just wondering if there is a machining way of cutting the teeth as filing that small could be hit or miss.
 

simonbirt

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I made such a thing for my Stuart boiler feed pump. I will find some pictures and post them later. Not difficult To make.
 

Steamchick

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Brilliant!
Not knowing the part that needs machining, I guess the cylinder needs a spot face on one side that is absolutely perpendicular to the axis of the cylinder bore? My method is to mount the cylinder on a shaft (I have used the parallel shank of a reamer) mounted on v-blocks and clamped to the bed of the table of the miller-driller.... then the central threaded hole can be made and a spot face achieved with an end mill. (By end mill I mean a milling cutter that can mill on the end face - which may be called a "slot-drill"?)- but I am not an expert so maybe I am doing the wrong set-up for this problem?
K2
 

Charles Lamont

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The OP is for back spot facing. Sometimes you need to spotface a seating for a nut in a place where you cannot get at it from the nut side because there are other bits of the casting in the way, so the cutter has to be driven from the opposite side of the flange to where you want the seating.
 

simonbirt

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Here is (a not very good) picture of the one I made to spot face the body of my Stuart feed pump. Simple enough to make with end mill to create the clearance angles. Hardened and tempered in the usual way. One thing, because the stem is so thin it has a tendency to emulate a banana during the hardening stage.
5B32F01E-01D5-4E1D-AC23-156E4315F0C8.jpeg
 

simonbirt

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E34E872F-05AF-44C5-ACC6-621E2734B316.jpeg

The cutter was used to spot face the frame for the cylinder mounting nuts shown here. I threaded the cutter through the pilot hole and then used an electric drill.
 

Ramon

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Mark - I have made the McOnie but didn't spotface the cylinder bolt holes - used studs instead.

Here's a reverse spotfacer made recently - The drive is the pin held inplace with the blob of blue tack!




I have a sometime made a reverse countersink for something just loctited to an 8BA bolt but can't find a picture of it. Only thing I would add to the advice from others is, unlike the one above, to make sure you cut the teeth opposite to the standard way (unless you can run your machine in reverse)

Regards - Ramon (Tug)
 

charkmandler

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Sorry for such a late reply, I have been solid with work since posting this and not had a chance to do any more modelling. Thanks Ramon, thats the sort of cutter I made except I did it with one 'tooth' and threaded the centre of the cutter. This worked very well for one side of the cylinder and then I got a bit heavy handed and the cutter snapped so I had to make another one - the thread made it weak. Next time I will not use a thread and use a pin as you suggested.
The whole cylinder assembly is now finished and I'm moving on to the frame as soon as some bench space is found in my workshop - restoration of a Vincent Comet and other work. I will post some pictures of the build when I get a chance.
Thanks again.
 

Ramon

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Hello Mark - I remember it well - work that is, and how it could get in the way of more 'important' things ;)

If you made your cutter from silver steel did you quench it in water ? (as you are normally supposed to) I ask as I find it's much better to do it in oil and not to temper it - very hard cutter but not so brittle.

Found the countersink I mentioned previously and had recourse to make another just this week. Only had to do one hole so it was Loctited to the shank of a 4mm bolt.

One bit a time Mark - it's always moving forward no matter what the pace 👍

Regards - Tug
 
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