[How do I?] Cut A Blind Hexagonal Hole

[karim]

A part I'm making needs a blind hexagonal hole (think socket-cap screw). I need enough precision that I can rotate (drive) this part on its axis via a 3/8" ball Allen wrench as part of a low-speed power train.

Any ideas on how to approach this?

 

[brino]

Either counterbore and weld in the head of a socket head cap screw, or look into buying/building a rotary broach.
Search here, there are a couple threads on building one.


-brino

 

[Martin W]

Could you drill each corner with a small bit and then remove the centre on the mill. It will leave radius corners, but if you drill with a small enough bit and work your way down to smaller end mills. It would work.
Cheers
Martin W

 

[brino]

......of course if your only driving it clockwise, simply drill and tap a hole and use a socket-head cap screw

 

[Martin W]

Brino is a genius! This is why my projects take so much time, I don't think them thru

 

[karim]

Could you drill each corner with a small bit and then remove the centre on the mill. It will leave radius corners, but if you drill with a small enough bit and work your way down to smaller end mills. It would work.
Cheers
Martin W

This was my first thought.

I can't use a regular socket-head screw because of the necessary geometry of the part... It needs to socket over an existing shaft on one end with a small key-way for alignment, and then a 10/32 screw will screw through the bottom of the hex opening to secure the part to the shaft. It's a little confusing, I know. I'll post a sketch when I can.

I was thinking of drilling/boring the minimum diameter, then drilling out the corners and milling flats between with a very small end mill.

 

[Uglydog]

Brinos suggestion works well.
I've used Red Locktite with good success for future removal.

Daryl
MN

 

[brino]

two more thoughts....

Can you weld on a socket? If the part can be lathe mounted could you bore a hole to centre the socket. Perhaps one of those heavy walled, 6-point impact sockets. You could cut the socket to length and weld it in the hole.

Also, I know I have seen "off the shelf" steel tube with a hex hole thru the middle, perhaps a chunk of that could slide over two hex heads like a sleeve and couple them together.

-brino

 

[karim]

Here's a sketch:



I'll have to give the socket idea some thought... I don't have a welding rig, but there probably won't be so much stress on this that silver solder wouldn't work fine.

 

[karim]

Also, I know I have seen "off the shelf" steel tube with a hex hole thru the middle

Oh! any idea where to source such rod? That might be perfect... I could cut off a lag-screw head and silver-solder it part-way through the hex hole to form the "bottom" of the hole, then drill/bore from the shaft-side to make the necessary mating profile.

In fact, since the forces on this part will be strictly rotational, I can probably get away with using a thru-drilled rubber plug for the "bottom", expand it with the 10-32 screw, and rely on friction to prevent the collar from coming off the shaft... that axial flex might make the whole thing more resilient.