Broach square holes with D-bit

Hi All

I am currently in the process of motorising my bead roller and thought I would also do the same to my slip rollers and ring roller using the same motor by simply bolting which ever tool I intend to use to a table and using vee-belts.

The issue I have is the slip rollers were made along time ago and I didn't consider ever powering them until now. There is currently a square drive which an old vice handle fits to. This doesn't allow enough round shaft to fit a vee-pulley to. So I decided to make an adaptor to drive of the square and locate of the 20 mm diameter shaft but allow enough room for a 1-3/8" 2012 taper lock bush to fit with a 1/4" keyway.

So I thought I would share with you all how I broach square holes on the mill using an indexing head and a cutter ground like a D-Bit.

The first operation is to turn the adapter and bore/Drill a hole to finished sized square this is sized across the flats (A/F).
A D-Bit is then ground from a round piece of high speed steel (HSS) to half its diameter.
The component can then be centred over the indexing head on the mill. The tool can be held in a chuck and the machine placed in low gear to prevent the spindle from rotating. Even better if you have a brake.
It is now a case of moving the quill of the machine up and down in conjunction with one axis taking very small cuts. The two photos below show the procedure for cutting the square.

Complete setup on mill
Attachment 33115

Process for cutting
Attachment 33116Attachment 33117

Cutting square
Attachment 33118

Finished hole to size
Attachment 33119 Attachment 33120

Square drive shaft and fitted adaptor
Attachment 33124 Attachment 33121 Attachment 33123

I have used this method for cutting different size squares and have made several size broaches. The large one was made from mild steel and a piece of parting off blade silver soldered to the mild steel shank . This was used to cut a square slot in cast iron on the axle boxes of my 3" scale Foden steam wagon.
Attachment 33125 Attachment 33126

Square cut and gauge piece used to check fit along with the D-Bit
Attachment 33127 Attachment 33128

This method works really well and is very accurate, Like everything there are some drawbacks The deepth of square is limited by how far you grind the tool and the longer the tool more flex so more patience is needed. Plus side very cheap to make and almost any size can be cut rather than buying expensive square broach which will only cut the size it was intended for and the shape. Using the D-bit hexagons, slots etc. can be cut.

I hope you can make use of this post and thank you for taking the time to view
The Home Engineer

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thehomeengineer,

You have done a very nice job of broaching! I have used this technique on several shapes including triangular shapes. Thanks for posting!

I'm a fan of D bits too. Need to mill an unusual shaped channel - make a D bit with the desired profile. Need to make precise holes - make a D reamer from the same shaft that will go into the hole. And now there's a D broach.

I've been using Thompson QS shafting intended for linear guide purposes because I've found that it's hard enough for D tools without a subsequent hardening process but not too hard to machine to the desired shape. Granted it will wear quicker than a properly hardened and tempered bit but for my projects it holds its dimensions long enough.

I only make one size square hole with my rotary broach (¼") using a HSS bit and it serves its purpose for that.

Attachment 33138

Crusty,

Is that shop made rotary type broach for the 1/4" square hole?

I started with an adjustable tool holder bought on Ebay and then made the rest.

https://www.homemadetools.net/forum/...590#post136275

I was intrigued by your broach, since I have an old mortising bit very similar in my scrap pile and I’ve been wondering what to do with it. Would you like it?