Rotary broaching tool.

[thehomeengineer]

Hi All
i don’t normally make a tool if I haven’t got a job for it but I am fascinated by rotary broaching. Anyway cutting a long story short I have made one and am very pleased with the results so thought I would share with you the build. I bought the bearings, tab washer, locking nut and two arbours (R8 and a 3MT).
Before I get on to the build I will try and explain how rotary broaching works and what it is used for.
Rotary broaching is a form cutting process and both internally and externally forms can be produced using this technique. (I am interested in internal forms) Examples of forms are squares, hexagons and splines which can all be produced.
The broaching tool body has the internal shaft supported on bearings angled at 1 degree angle which obviously allows the tool to run off centre (a bit like a drill that hasn’t been correctly placed in a chuck and then runs out). As the cutting tool is in a housing supported on bearings the tool, when it comes in to contact with the work piece stops rotating with the spindle but moves in a oscillating motion and this action allows the corner of the tool to cut the form of the cutter in a pre-drilled hole.
A tool needs to be made with the desired shape and size to be cut. When grinding the cutting tool flanked clearance is needed at approximately 2 degrees for clearance and the bottom of the tool relieved to give a sharp cutting edge.
One drawback is the cutting tool needs to be positioned at a known distance from the centre of the broaching body centre line to allow the cutter to produce the correct size.
I hope the photos will clarify my explanation on how the tool works.

Material and purchased items


centring work piece


Main body set at 1 degree






machining location tongue






Tool shaft




Truing the outer body in the lathe so this runs concentric to the main spindle. The internal bearings and tool shaft maintain the 1degree angle to allow too to oscillate.


machining shanks for c-spanner


Shaft screw cut for purchased bearing locking nut.


Internal shaft thread and machined to accept locking tab washer.
The thin bee-blocks make it easy to clock to find the centre. Link to vee-blocks Story of the little discarded beat up Vee block



grease hole for bearings sunken in to body to avoid nipple protruding outer body.


completed parts ready for assembly






Completed body


Cutting tools from old slot drills




3/16” hexagon in steel


Aluminium

Thank you for viewing
The Home Engineer

[DIYer]

Thanks thehomeengineer! We've added your Rotary Broaching Tool to our Machining category,
as well as to your builder page: thehomeengineer's Homemade Tools. Your receipt:

Rotary Broaching Tool by thehomeengineer

tags: broach, rotary tool

[celsoari]

nice working

[Crusty]

You can easily add rake to the cutting edges by mounting your tool in the lathe chuck and a Dremel on center in your tool post with a cutting wheel attached. Bring the wheel edge on up to the spinning cutting tool, ease into it and you will surprisingly quickly grind out a hemispherical depression in the end of the cutting tool, providing a small amount of rake to your cutting edge. They broach better with some rake.

[thehomeengineer]

I have added a short video of the tool working

[wizard69]

I have added a short video of the tool working

Thee are many approaches to these homemade rotary broaches and this one highlights an important idea, use pre-made arbors. Why I never considered that I don't know but it makes perfect sense, especially if you might be moving the broach between a mill and drill press.

I have to ask do you have info on the bearings used in this build? One looks like a Timken style bearing.

[thehomeengineer]

Hi Wizard69
My intention is to make the drawings available and free on this site but unfortunately this won’t be any time soon as the drawings are at work and is lockdown. However the bearings are 30203 and 6003 and locking nut KM3 and tab washer MB3. The arbours are 1” 1/2 x 18TPI. R8 and No. 3 morse taper and a couple of 6mm grub screws and M4 cap head screws, grease nipple. These are the purchased items roughly £45 in total. Two pieces of 70mm diameter steel and 20/55 long and a piece 30mm diameter 50mm long.
hope this helps you at the moment
The Home Engineer