Making A Rotary Broach, By Ulma Doctor

A couple years ago, I made a very quick and dirty rotary broach for a project i was working on in my normal job

Shop Made Rotary Broach, Down And Dirty!

Shop Made Rotary Broach i have a job that requires a square hole to be pierced through a piece of machined steel. the material is too thin to be push broached for fear of fracture. i don't have pull broaches or the puller for that matter, so i'm doing the next easiest method i have seen. i...

www.hobby-machinist.com

it worked for the purpose it was intended, but i wanted to make an improved version.

the new version , i envisioned was precise, robust, and economical.

(i priced out a manufactured rotary broach similar to the size i'm constructing, it was over $500 USD)

last week, i took up the gauntlet, to make my vision a reality (and for a lot less money)

as an added challenge, i'm going to do this project ,with only the stuff i have lying around the shop


ACT 1: The Capsule

after hunting down the materials, i decided to draw a plan for the capsule

the capsule is a sub-assembly of the rotary broach.
the capsule contains the toolholder shaft, roller bearing and thrust bearing assemblies, and their spacers and retainers.
it is designed to be taken apart for easy bearing changes, as necessary.
enjoy!



i had [3] 6202-2RSC316MM ball bearings, and a needle thrust bearing in the shop, so the capsule was built around those specifications.
[side note: 6202-2rsc316mm is a special bearing, the outside and thickness are metric dimensions, the ID is imperial]{also, the bearings are 316 stainless}

the broach holder was constructed from .875" hexagonal unknownium steel stock to a finished length of 2.750"
3 grooves were added to the broach holder for bearing retention by 5/8" external snap rings
i constructed 3 spacers from aluminum to provide extra clearance between the ball bearings



the main part of the broach holder shaft was turned to .625"
the distal end of the broach holder was turned to metric dimensions 14.9mm for a distance of 4.7mm, to accept the needle thrust bearing (27.8x15x4)
the proximal end was drilled to 7.5mm and then reamed to 8mm
then the broach retainer screw was added, i drilled and tapped for a 1/4-20 x 1/2" set screw

here is the capsule fully assembled




Stay Tuned for Act 2...... (if you dare)

 

A bit off-topic, but out of curiosity - has anyone built a punch-broach?
Kinda wondering if they can be used on an arbor press. All the examples (e.g. the slatertools video) show the lathe tailstock in a CNC machine being used as a shaper.

Hi ThinWoodsman,
i have not built a punch broach, but i had some 3/16 holes punched in 1/4" steel before.
i got to watch the last sets of holes because i was early to pick up the project pieces.
the press they were using was big hydraulic press and it punched 4 holes at one time (lots of force being applied)

i'm not sure if O1 is the best material for press work, but i'm sure it could do short runs- you may need pilot holes

 

i got to watch the last sets of holes because i was early to pick up the project pieces.
the press they were using was big hydraulic press and it punched 4 holes at one time (lots of force being applied)
i'm not sure if O1 is the best material for press work, but i'm sure it could do short runs- you may need pilot holes

I have a few projects stacked up that are waiting for me to figure out how to hex-broach a blind hole. Now that the shaper is up and running, I might whip up a thin tool and mount the work in a collet block. Tricky, but it should work if I drill an overly-deep hole and only cut the bottom flat of the hex socket.

Ultimately, I'll probably make a rotary broach myself. So, keenly interested in this thread

 

ACT 2: THE CARRIER

i found the materials i'm gonna use!

Now that the capsule is made, i have the next operation,
to design the carrier for the capsule.
the carrier will house the capsule and the capsule will be retained by a 1.250" internal snap ring
here is the drawing i made for the concept

NOTE: the drawing was incorrect for the total lengths of the actual bore-


the internal bore will be 35.02 mm for a distance of 2" to complete the first internal step
i used a pre-flattened 1" drill from a guy who ground it like a 2 flute endmill, and used it to make the pocket
to the final depth of 2.250"
i then bored the pocket to 1.14"(29mm) to accept the thrust needle bearing
i added 15mm x .010" depth pocket for end clearance (not shown in drawing) Making the total dept of the bore to 2.260"
i made an internal snap ring groove at 2" from the first step, to hold the thing together!


stay tight more to come!

 

IMHO using aluminum for the spacers is just fine. All the radial load is going to the three bearings. Probably will go to more like one or two of them. And all of the thrust load will go to the bearing at the back end. The thrust bearing at the back will probably get the most of the work out/load forces
when broaching. When rotary broaching you want to let the cutter do its thing. You can feel it throbbing in the handwheel. There is no great pressure/force going on if you do it right! And what a nice tool/capability for the Hobby Shop! I can’t say that enough! Good Job UD...Dave

 

UD your CAD drawings look way better than my CAD drawings LOL. Where is your offset going to coming from? The very end of the broach or the cutting points of the business end are going to rotate about dead center/datum center, isn’t it?

 

UD your CAD drawings look way better than my CAD drawings LOL. Where is your offset going to coming from? The very end of the broach or the cutting points of the business end are going to rotate about dead center/datum center, isn’t it?

Thanks Chips!
you can barely see in my drawing, but the end of the carrier will be milled to 1° angle

if it all works like i want it to,the broach will stay stationary, the rest will spin.
the wobble produced makes(cuts) the bore when downward force is applied
but i have been wrong before!

 

I was able to finish the basic machining on the carrier



and made the test fit of the capsule.
it fits like silk







the next order was to part off the carrier to just over 2.750"



i liked the finish on the carrier!




i used a 1.250" internal snap ring to retain the capsule in the carrier, to test fit.

that was rewarding to accomplish the capsule assembly for the first time




coming soon ACT 3: Drilling, Tapping, & Milling the Carrier

 

Thanks Chips!
you can barely see in my drawing, but the end of the carrier will be milled to 1° angle

if it all works like i want it to,the broach will stay stationary, the rest will spin.
the wobble produced makes(cuts) the bore when downward force is applied
but i have been wrong before!

Yes UD, correct, the broach/cutting part will not be turning. But will the broach tip of the cutting part be on center with the mounting tail of the rotary broach body? Or will the broach tip be pointing somewhere off center with a 1° angle?

 

I think you will have to mill the 1 deg angle on the base and then have an additional offset to get the tip of the cutter on center. This will be an issue if you have cutters of different lengths. No?
Robert

 

Yes UD, correct, the broach/cutting part will not be turning. But will the broach tip of the cutting part be on center with the mounting tail of the rotary broach body? Or will the broach tip be pointing somewhere off center with a 1° angle?

Hi Chips!
the broach will be off center with a 1°