[4]

Making A Rotary Broach, By Ulma Doctor

[3]
[10] Like what you see?
Click here to donate to this forum and upgrade your account!
A couple years ago, I made a very quick and dirty rotary broach for a project i was working on in my normal job

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 :grin:


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!

290968

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

290969

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

290971290970
290972290973

Stay Tuned for Act 2...... (if you dare)
 
Last edited:
Nice project, Have a question, why us AL on the spacers, My though would be using something harder that could stand up better, Just seams AL would be to soft. Again Nice project.
 
I'll be watching your project with great interest Mike. I built a rotary broach a couple of years ago and it's mediocre at best, so I'd really like to build a better one. Keep up the good work.

Ted
 
I like Ted, built one, not as robust as I wanted so I bought a commercial model and I make my own broaches which is easy if you have a surface grinder and time.
 
UD, do you have a source of 8mm HSS or something? Remember when you make the broaches the across the flat measurement is one thing but the point to point is a lot bigger. Where is the broach angle coming from? Or should I just sit back, wait and eat more popcorn…Dave
 
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.
 
Nice project, Have a question, why us AL on the spacers, My though would be using something harder that could stand up better, Just seams AL would be to soft. Again Nice project.
Hi Ken,
i used 7075 aluminum.
although not as strong as steel, i'm sure it will suffice for the intermittent service this tool will see.
but if they do wear too quickly, i'll be sure to upgrade the materials ;)
thank you very much for your kind words! :)
 
I like Ted, built one, not as robust as I wanted so I bought a commercial model and I make my own broaches which is easy if you have a surface grinder and time.
Hi FLguy,
i'll have to try making the broaches on the surface grinder, thanks for the idea!!!:grin:
 
UD, do you have a source of 8mm HSS or something? Remember when you make the broaches the across the flat measurement is one thing but the point to point is a lot bigger. Where is the broach angle coming from? Or should I just sit back, wait and eat more popcorn…Dave
Hi Chips!
yes sir, i have a 2ft length of 8mm O1 that is itching to become broaches :grin:
( i could use commercially manufactured 8mm broaches too)
yes sir, thank you for pointing out the point to point measurement-
the thought for the 8mm capsule, is to broach 3mm to 7mm hex holes [or other square holes] for upcoming projects
i also have a .500" version of the capsule in the plans for the near future too :cool 3:

:immersed: you'll see, in the next act, the 1° broach angle necessary for operation, i promise!!!
 
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-
291118

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!
 
Last edited:
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
 
Last edited:
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! :grin:
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!:grin big:
 
I was able to finish the basic machining on the carrier

291212

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

291213



291214

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

291216

i liked the finish on the carrier!

291217


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 :grin:

291218


coming soon ACT 3: Drilling, Tapping, & Milling the Carrier
 
Last edited:
Thanks Chips! :grin:
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!:grin big:
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°
 
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
i'l be interested to see if cutter length is critical.
the thought was to make the cutters to the same general length for grinding purposes,
but i had not considered the ramifications for length yet :bang head:
 
ACT 3: Drilling,Tapping & Milling

Now, the capsule fits like a glove into the carrier,
but i need to drive the carrier and put a 1° angle on the carrier
and i need to drill and tap for 3) 1/4-20 socket head capscrews in a 1.5" bolt circle

i chucked up and indicated the carrier, gave it a scratch at 1.5" to aid in bolt center location

291275

i needed to make the driveshaft to make the whole thing work, so i sketched out the idea-
then i made it! :big grin:


291277

i transferred the bolt circle onto both the driveshaft and the carrier
and used a pair of dividers to walk out 3 equidistant points on the carrier, on the 1.5" mark
i punched the first divot and walked out the other 2 points, until all 3 were the same distance and on the 1.5 mark.
i went through the same process on the driveshaft.
it was quick work and i didn't have to use a lot of tools get there

the pilot holes for both the carrier and driveshaft were drilled then punched out to .185" in the old South Bend drill press
the carrier was drilled to .201" to a depth of .395"

291276

next came the milling of the carrier!

i set the carrier into the Bridgeport's vise with a v block and a 1° angle block under the work.
i milled the carrier approximately .030" to ensure the 1° was transferred across the whole face of the carrier.

291278

here is a shot after milling
the carrier was tapped to 1/4-20 to .375" depth
291279

i reassembled the carrier and enlarged the driveshaft thru holes to 7mm for clearance

291280


act 4, coming soon..... same bat time, same bat station
 
UD, do you have that 3/4” tail shaft on center???? And the 1° on the back of the body??? And everything else is made to be on center??? Sorry UD, to me things don’t look right??? Unless I’m missing something the design will have a problem. VERY IMPORTANT BELOW.

“When everything is made and you have the rotary broach set-up to use………The broach tip (cutting part) MUST BE ON CENTER. And yes, broach length is critical when and if you interchange them.”
 
I assumed you were going to use this in the lathe tailstock with the work rotating. I believe that could work if you get the cutter on center. If you use this in the mill I feel like the cutter will just move in a circle and never be on center? I have never used one of these personally. Am I missing something?
Robert
 
UD, do you have that 3/4” tail shaft on center???? And the 1° on the back of the body??? And everything else is made to be on center??? Sorry UD, to me things don’t look right??? Unless I’m missing something the design will have a problem. VERY IMPORTANT BELOW.

“When everything is made and you have the rotary broach set-up to use………The broach tip (cutting part) MUST BE ON CENTER. And yes, broach length is critical when and if you interchange them.”
Hi Chips,
to recap...
the driveshaft was turned and faced after it was constructed in a ER32 collet system in the spindle of my lathe (sub .001" indicated runout)
the 1.5" bolt circle was inscribed with a boring tool
the carrier was bored internally in a 4 jaw independent chuck (indicated sub .001" runout) and i turned the od in the same set up
the carrier was reversed and re-indicated and faced.
the 1.5" bolt circle was inscribed on the face of the carrier
after locating, drilling, & tapping the 3 mounting holes, the rear of the carrier was milled at 1°
the driveshaft was drilled to 7mm on the 3 bolt holes, to allow clearance and minor adjustment, if truing is necessary.
considerations have been made to enlarge the mounting holes if the carrier needed it.
 
FINAL ACT: ASSEMBLY
i finished assembly and took some pictures...

IMG-3316.JPG

IMG-3315.JPG


View attachment IMG-3323.MOV
















there is no tool in the holder, i substituted a 3/8" ball bearing for testing purposes
this is only a test!

when i made my down and dirty broach, i made no attempt at accuracy in any sense of the word.
i banged it out in less than an hour.
nothing was on center, nothing was Book of Hoyle about it, there was no consideration of tool length
the tool was hacked out of a new tool blank to provide about 10° side clearance (poorly made at that)
it was an abomination that got the job done.
from the experiments of the former, i learned not to fear the rotary broach
nor the construction of a better one.
for right or wrong, here is the rotary broach as i made it.

rotary broach drawing.jpg

i'll do testing and broach fabrication.
i'll share my findings as i go.

thank you for reading, watching and being a part of the experience.
 
FINAL ACT: ASSEMBLY
i finished assembly and took some pictures...

View attachment 291357

View attachment 291359


View attachment 291360
















there is no tool in the holder, i substituted a 3/8" ball bearing for testing purposes
this is only a test!

when i made my down and dirty broach, i made no attempt at accuracy in any sense of the word.
i banged it out in less than an hour.
nothing was on center, nothing was Book of Hoyle about it, there was no consideration of tool length
the tool was hacked out of a new tool blank to provide about 10° side clearance (poorly made at that)
it was an abomination that got the job done.
from the experiments of the former, i learned not to fear the rotary broach
nor the construction of a better one.
for right or wrong, here is the rotary broach as i made it.

View attachment 291361

i'll do testing and broach fabrication.
i'll share my findings as i go.

thank you for reading, watching and being a part of the experience.
Looking forward to seeing test cuts, results and photos of the parts made! Nice job Ulma!


Sent from my iPhone using Tapatalk
 
OK UD, if everything is on center and you have induced a 1° angle. And if you have no provisions for an offset. Then sorry, but the broach cutting end will run in a solid angle pattern and not remain stationary as it should be…Dave.
 
When the cutter is stationary. The 1° angle makes the stationary cutter end walk around on a small angle causing a minut shear action every rotation. If the cutter end is not stationary but instead doing a solid angle pattern thing. I have no idea, probably wild stuff will happen, please be careful.
 
[6]
[5] [7]
Top