Precision mounting a square peg in a (probably) round hole.

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zondar

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I've been trying to figure out a good way of mounting a 1/8" square tool bit through a 1" by 36" brass rod (across the 1" diameter). The bit has to be extremely square to the length of the rod. Bits would be mounted at both ends of the rod.

I have a tiny lathe and no mill (yet). So I don't have machines capable of holding a rod that long in a precision way. I could imagine making the ends as separate pieces, say just 2" long, so they can be worked on. But that would imply that the bulk of the rod would also need to be machined, and I can't do that.

I suppose a well-drilled hole followed by a broach would be the proper way. That feels a bit daunting.

Any other ideas?

Thank you.
 
zondar said:
I've been trying to figure out a good way of mounting a 1/8" square tool bit through a 1" by 36" brass rod (across the 1" diameter). The bit has to be extremely square to the length of the rod. Bits would be mounted at both ends of the rod.

I have a tiny lathe and no mill (yet). So I don't have machines capable of holding a rod that long in a precision way. I could imagine making the ends as separate pieces, say just 2" long, so they can be worked on. But that would imply that the bulk of the rod would also need to be machined, and I can't do that.

I suppose a well-drilled hole followed by a broach would be the proper way. That feels a bit daunting.

Any other ideas?

Thank you.
Click to expand...

Hi Zondar, seems that you have a bit of a challenge on your hands but there are many ways to get around not having a mill on hand.
Not knowing your exact capabilities or equipment you may have at your disposal, I would try something along theses lines;

1; Chuck the rod in your lathe leaving enough out to drill your perpendicular hole .
2; Mount a drill to your tool post so that you can use your cross slide as the drilling axis. (Find a cheap small dia. Ø air grinder that you can mount) this should give you a small degree of accuracy for drilling.
3; Drill your hole at .135
4; I would most likely use a small square needle file to finish the hole or with a bit hand grinding you could possibly grind a 1/8 tool bit into a tapered broach.

Hope this helps..
 
Can you post a little about the final use?
Are we talking 1/8" square HSS blanks that eventually need to cut the inside diameter (ID) of something?
Some sort of line boring tool?

I have made boring bars similar to that. Started by drilling a round hole and then filing the corners for a 1/4" tool bit.
You would need a very small (and fragile) file!

Do the two square holes need to be registered to each other? (ie. the same "clocking" around the bar?)

More info would help us to help you.

Brian
 
They do make round high speed tool bits. Or one can grind broken end mills or drill bits into cutting bits. Round hole is easy to make
 
A 3 feet long tool with cutter at both ends ? can't imagine what it is for ....

Anyway, may be set up similar to this is worth trying. Basically it turns your lathe into a horizontal mill.

Mount the rod to the cross slide. Sweep the length of the rod with a DTI mounted on a plate/block to ensure that it's parallel to the lathe bed. Mill a 1/8" wide slot on the end face of the bar of depth slightly less than 1/8. Clamp the tool to the bar with a round plate and 2 ~ 4 screws next to the tool.
 

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Thanks for the replies.

First, I can't mount the bar in my lathe in any normal way because it's a cute little Sherline that's just too small for a bar that size. The spindle opening is roughly 10 mm while the bar is 1" diameter, and the bed is half the length of the rod, too.

What's it for? I have an interest in making a precision pendulum for local gravity measurements. There are several potential configurations, but this one would be a solid bar pendulum - one that is simply a uniform heavy shaft rather than a thin rod and a heavy bob (this sort of pendulum has a straight-forward formula relating g - the force of gravity - to its length and period).

Sets of knife edges (the 1/8" tool bits, forming a 90 degree knife-edge) would be mounted to both ends. The knives must be hard and sharp, and they would balance on very hard plates for low friction. The knives must be precisely square to the bar or else the pendulum's motion would wobble, destroying its accuracy if even a tiny amount is present. They should also be square to each-other, i.e. not twisted relative to each other.

Mounting it somehow like in that photo might work. So the idea is to use the lathe to mill a slot in the end into which the tool bits could be inserted? Looks tricky to support the rod given that it must be movable, but I can see it might be possible. Interesting! Thanks!
 
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Yes, sleeves like that could help! It reduces the problem to making an accurate simple hole. I'd have to machine it some, as the end that's threaded would upset the pendulum's balance, but that's manageable on my little lathe.

To elaborate: Ideally, the entire pendulum would be uniform in density and composition, and perfectly symmetric, or else the formula for calculating g (local acceleration due to gravity) loses accuracy. Even the little difference in having a bit of steel at the ends upsets the uniformity, and having one part of that sleeve be threaded and another not upsets balance and uniformity more. So adding more steel with a sleeve is making things worse than say a broach straight through the brass, but compromises are necessary and expected.

One of the best things about machining is that there's a special tool or accessory for everything, including things you've never imagined. It's also one of the worst things about it (to your pocketbook, anyway!).

Thank you.
 
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zondar said:
Even the little difference in having a bit of steel at the ends upsets the uniformity,
Click to expand...
So the square peg or knife edge will be made of brass as well ? I would wonder how long the knife edge will last
 
zondar said:
I've been trying to figure out a good way of mounting a 1/8" square tool bit through a 1" by 36" brass rod (across the 1" diameter). The bit has to be extremely square to the length of the rod.
Click to expand...

If the bit is to be near the rod END, could you drill the cross hole slightly undersize, and use a file to open up a few
corners? Then an axial drilled hole can be tapped, and fitted with a setscrew with flat bottom to align the
square tool.
For slightly better results, two setscrews, one over and one under (just have to drill/tap the axis deeper).
 
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