How to drill square holes??

Josh Bowman · Oct 14, 2012

I'm new at this machining, so please be gentle.
Tonight, I wanted to make an adapter to fit some of my wood coring blades. They have a tang that's a little over a 1/4" and 5/8" wide and 2" long. My first thought was to use my chinese mini mill, but found the bits to be too short as well as the head above the vise. So I turned to the SB 9A and its mill attachment. I tried to drill a hole 2" deep, no problem but which way to go after the first hole? The bit won't drill another close hole without bending and besides I'll have waste left in the hole. Any suggestions? I'm thinking of just boring a 5/8" hole and allowing the blade to lean in the hole, but was hoping for a more eloquent solution. Sorry for the bad cell phone pictures, but they'll give an idea of what I'm doing. These are the tang on the bowl coring blade:

Here's the lathe setup I have to work with:

And here's the best I have in mill bit length:

Any ideas or help would be appreciated.

Cole2534 · Oct 14, 2012

Find the longest dimension of the tang, the hypotenuse of the right triangle scribed across it, and make a hole that size. After that add 2 set-screws to engage the tang's flats.

Upon edit, why are you drilling this as such? Hold the material in the headstock and the drill in the tailstock.

If you do this carefully I think you won't even need the milling attachment.

S_W_Bausch · Oct 14, 2012

If you are hoping that someone will offer to do it for you, I guess that's a possibility.

There are plenty of time-intensive techniques, but tooling is still required.

If you can drill a series of holes to remove most of the intended hole, there are a few options:

Chisel out what you can, then clean up the hole with a selection of files. You will most likely destroy a file or two.

This is work that is classically done with a coping saw, perhaps not in metal.

If you had a reliable and accurate way of pushing an edged tool through that hole, you could take many, many passes and shave the hole to your liking.

Nowadays, I suspect this is EDM work.

wr4t · Oct 14, 2012

You could use two pieces of half round, mill in the slot with your mill drill then fasten the half rounds together. I would use oversized half rounds, or they could even be square stock and turn the assembly down to the correct diameter round after you have them fastened together.

It would be similar to a round knife handle assembly.

Josh Bowman · Oct 14, 2012

"why are you drilling this as such? Hold the material in the headstock and the drill in the tailstock" The reason is my initial idea was to drill a series of holes with the mill and figure it out from there. Now I found that won't work. So I agree the headstock with be the next approach. When you mentioned the longest dimension and then the hypotenuse, do you mean just drill a 5/8" hole? I'm not understanding.

" If you are hoping that someone will offer to do it for you, I guess that's a possibility." Nope I want to learn how to do this stuff and am looking for projects to fuel my other hobbies. I googled EDM and that's one cool machine! The parts it can make are incredible. I never knew anything like that existed.

"You could use two pieces of half round", that's a good idea, but I'm afraid I wouldn't have enough up and down handle support on the tang. Also, I forgot to mention the blade has to come out, since there are several different shapes in the set and I only want one handle. Here's what all the blades look like:

lazz · Oct 14, 2012

You might consider cutting a slot on the mill and holding the blades with a set screw.

Josh Bowman · Oct 14, 2012

You guys have got me thinking, how about if I turn 2" of the blank down to about 5/8" then mill a slot. Then take another piece of stock and drill/bore a 5/8" hole. Press the "tube" over the 5/8" milled piece, drill a couple of holes for allen screws to hold the blade and tube and holder together. I'm thinking that might work.

lazz · Oct 14, 2012

If I had only 'splaind it a little better.

Cole2534 · Oct 14, 2012

Josh- for this purpose, the longest dimension of a rectangle isnt its longest side but the length from one opposite corner to the other. If you want to fit a rectangle in a hole with clearance, that dimension will be your minimum bore.

That said, I like the groove and collar idea better.

wr4t · Oct 14, 2012

I think your idea will work, you could even put a taper on the male part and thread the collar down on it to clamp the tang in place, sort of like a collet and collet nut assembly. That would require turning some tapers maybe more than you want to try, but slick.

Tommy · Oct 14, 2012

Real machinists don't post photos of themselves wearing flip-flops.

~TW~

Ken-Bergen · Oct 14, 2012

A truly square or rectangular hole can only be done by broaching.
A shaper is the perfect machine to do this but it can be done on a lathe.
It's labour intensive as it's done with the lathe off and the spindle locked.
You run the broach in and out with the carriage or compound and move the broach "up/down/side to side" until the desired shape is achieved.

steve-l · Oct 15, 2012

Josh,
Although there are tools to do what you want, I'm pretty sure you don't own them. The easiest way to do what you want is to make these handles as two piece units, which will allow you to mill the section profile you need for the tangs. Then assemble the two halves using one of many available methods. You then can turn the assembly on the lathe to any shape you desire.
Steve

Josh Bowman · Oct 20, 2012

Well the 2 piece plan worked great to make a square hole. Thanks for all the help. Now if I can just get faster at doing this stuff

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