I could not find much information about making broaches and broaching square holes. I’ve written up my first attempt. Please share your experiences. Comments and suggestions are welcome.
Designing and making a square broach.The number of teeth.The round hole in the hammer to be broached is 5,2 mm in diameter
I want the finished hole to be 5,2mm x5,2 mm. The cutting part of the broach is square in both ends.
The square at the start must fit in the round hole so:
5.2
2= X
2+X
2 => 27.04= 2x
2 => X= 3.7mm
The starting end must be 3.7mm X 3.7mm square and it’s diagonal 5,2mm
The finishing end must 5.2mm x 5.2mm square and it’s diagonal X
2=5.2
2+5.2
2=> X= 7.35mm
The amount of metal each broach side must remove is 7.35mm – 5.2mm = 2.15mm
I want each tooth to remove 0.05mm(0.002").
Number of teeth pr broach side= 2.15/0.05= 43
The length of the cutting part of the broach and size of teeth.The depth of the broached hole is 6.5mm and I want at least two teeth for that length.
I chose 2mm pr tooth which gives 2mm x 43 = 86mm, which I rounded up to 90mm
Taper of the cutting part of the broach:I drew the broach full sized in Qcad an measured the taper angle to 0.48
o.
This was used later to make jig for milling the taper in the blank. Hopefully this will be clearer when I show pictures of the milling process.
Milling the broachI start with a 12cm long piece of 12mm drill rod and turn down the ends to 5.2mm.
Next, I turn down the inner part of the front end to 3.7mm.
I make the end blocks from two short pieces of 12mm drill rod. These are center drilled with 5.2mm and the ends turned down to 8,4mm. They are then glued to the blank. Notice the gap between the blank and the end pieces.
Place level in the vice and mill down to the 8,4mm ends.
Rotate and mill square.
Look at the gaps and mill down to 5.2mm square.
The tapering jig is two steel plates milled to hold the blank on a 0.48
0 slant.
Mill in the taper to the bottoms of the gaps on both ends.
Place the blank the tapering jig. Rotate the vice 10
0 so the teeth are cut on a slant. The teeth will then start cutting at two diagonally opposite corners. The teeth are cut with a 60
0dovetail mill. The milling head is tilted 28.5
0, which cuts the teeth with a 1.5
0 rake. Spacing is done by eye.
Rotate the blank and mount in the tapering jig.
Rotate the vice 10
0in the opposite direction. Cut the teeth lining up the corners by eye. Repeat for the two remaining sides.
Heat to 200
0 Celsius and pull off the end pieces. Sharpen the cutting edges with a diamond stone.
DiscussionI made three broaches with minor variations. These are my experiences.
Cutting depth of the teeth.When broaching, the chips cut is trapped in between teeth. When this space is full, the broach stalled abruptly. Teeth cut 0.5mm deep stalled after 1 tooth cutting. Teeth cut 0.6mm deep stalled after 1.5 – 2 teeth cutting. Teeth cut 0.7 mm stalled after pushing 4 teeth through the hole.
Slant of the vice.There was no discernable difference in needed force to push the broach with a 20
0 or 10
0 slant.
The broach with a 5
0 required quite a bit more force, but this could because it had fewer teeth and cut thicker chips. One thing is clear, the larger the slant, the harder it is to line up the corners by eye.
Number of teeth.More is better. More teeth = smaller chips= less force needed to broach.
No discernable difference between 45 and 40 teeth. 30 teeth need a lot more force.
Spacing of the teeth.The cutting edge of a tooth is made of the side of the tapered blank and the rake cut by the dovetail mill. If you space the teeth to close, some will end up to low and not cut. Look at the picture. Two of the teeth have no green on them and are too low to cut. The closest I could space was 1,5mm.
Heat treating the broachI made the broach out 0f 12mm Sandvik 20 AP steel:
https://www.materials.sandvik/en/materials-center/material-datasheets/wire/exera-20ap-free-cutting-medical-wire/It’s annealed and easy to machine or file. I coated the broach in Brownells anti scaling compound before treatment and let it dry for 24 hours.
Hardening I harden all three broaches in an oven at 810
0celsius.
The first one I soaked for 30minutes, the next at 10minutes, the last at 5 minutes with no discernable difference in results in hardness and grain size.
Quenching.Tried it on scraps. Without anti scaling compound the steel hardens in water , oil+water and in oil alone. With anti-scaling compound oil alone does not work.
Tempering.Have not found any solution I’m happy with. I tempered the first broach at 300 Celsius for 30 minutes in a preheated oven. It broke after broaching 1,5 holes. The second broach, I tempered at 300 Celsius for 45 minutes in a preheated oven. It broke after broaching 4,5 holes. The last broach, I preheated the oven to 200
0 and soaked there for 10minutes before heating it slowly up to 350
0Celsius and soaking it there for 45minutes. It broke halfway through one hole.
The broaches don't break while cutting the hole. They beak when tapping them gently out of the hole with a brass hammer. It’s the tip with the cylinder that broke off on all three.
Broaching a hole.Holding the broach. Used my mill to broach the holes. The two first broaches were held in a 6mm collet, the idea that a collet was less likely to slip holding the broach. The second broach broke again when tapping the collet out of the mill.
The last broach was held in a drill chuck. This did slip and the jaws of the chuck were pushing against the end of the cutting portion of the broach. Aside from marring the end of the broach, this work fine and was a lot quicker to use than the collet. Next, I’ll fit a mild steel washer on the broach to avoid marring.
The hammer to be broached rests on an “anvil” and a square “washer”. The “anvil” is a piece of 2” axel with 8mm hole drilled through. The “washer” is 10mm sheet stock with a 6,5mm hole. The washer keeps the hammer head off the anvil.
After the broach is pushed as fare as it goes, its remove from the mill and tapped out, supported by the washer.
The chips are cleaned out of the broach. The broach rotated a quarter of a turn, placed in the hole, mounted in the mill and the process repeated four times. This ensures that every side of the square hole is cut by every tooth on the broach. This evens out the differences between the teeth.
After a full rotation, flip the hammer over and broach from the other side. It took four full rotations (two from each side), finish the square hole. The whole process took 1.5 hours.
Here is a couple of pictures of the finished broached hole.
Ideas on the next attempt at broach design.The purpose of the 5,2mm cylinder at the front end of the broach, was to center it in the hammer hole before starting the cut. This proved unnecessary, the broaches with broken tips are self-centering and function fine as long the tip protrudes out of the hole before starting to broach.
I broached this hole starting with the longest broken broach and switched to the shorter broach when the square hole was big enough. The shorter broach cuts better. This worked fine.
It should be possible to make a set of broaches with one “starting broach” and different “finishing broaches” for the different sizes of square holes.
Lengthening the “starting broach” would reduce the taper needed on the blank and increase the number of cutting teeth. This reduces force need to broach and the risk of breaking the broach. The same goes for the “finishing broach”. Both broach blanks would be milled with the same taper and overlap with 10mm in length.
At the moment I ‘ve broached all the hammers I need, so it will be a while before I try this out.
Best regards
Rolf
