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Rotary Broaching Suggestions


Jan 3, 2020
I am looking for a machine capable of utilizing a slater tools or polygon solutions rotary broach in order to broach a 5/8 square hole into a part made of A36 mild steel at a production level. I'm not completely sure on the necessary specifications of a machine capable of this, but have heard that a 3 hp mill with enough thrust force (1500 lb thrust as provided by the manufacturer) would do. Would anyone happen to know of or have experience with any mills or lathes capable of enough thrust to accomplish this? Currently we want to do rotary broaching in order to reduce the operation to one machine for sake of production. The part is a 1.25'' diameter 1.5 '' long piece of A36 round bar for reference.
Any advice is very appreciated.
Check with the 2 suppliers you mentioned. I have never done anything larger than 5/16 hex with a rotary tool. Seems to me that a Bridgeport class mill should do it.
For what it is worth I worked as a temp at a shop that did a 1/2 square through mild steel bushings that were about an inch long. The broach was a multi tooth outfit maybe made by Dumont. I did buckets full of them, not sure if that would not be the best approach, with a rotary tool you might get a little bit of a spiral going that deep.
1500# of thrust? That is roughly a 1" twist drill. Better find a machine with at least a #4 morse spindle.

Why rotary broaching? Usually the only advantage there is doing the part while its in the lathe spindle for other work. How deep is your broached length? If its the full 1 1/2" there may be to much helix. Helix can be reduced by reversing rotation.
Rotary broaching a square is exponentially more difficult than doing a hex. The face contact area is much higher.

You require extreme radial rigidity of the body to maintain the wobble at the actual broach, rather than just move it upstream. If the body wobbles instead of the broach then you are effectively just push broaching.

For comparison, I did a low quantity production job for a couple years that required a 1/2" square to be rotary broached in 4145H. We did that in a heavy CNC lathe (think 18" chuck, turret by itself heavier than a bridgeport) with a large rotary broach holder on a 50mm shank, and it was all barely stiff enough to keep the body sufficiently still. Broach life was sufficiently poor that we abandoned the process entirely and switched to milling the squares.

Even in A36, 5/8" square is not happening on a bridgeport, or any mill with a quill really. To do what you want is 50 taper territory at the very least, fixed spindle.
Looks like you need about 2400 pound of thrust. The biggest thing to remember is buy the rotary broach holder. It holds the broach at a slight angle and really cuts down on the amount of thrust needed. We were broaching 3/4 hexes on a large boring mill with no rotation only thrust. Took a lot of pressure and tool would jump when shearing. Bought the rotary holder and we could do it on a Haas ST20 lathe with no problem and smooth cutting and finish.

The material was 4150 pre hard.
I just completed a short run of 6 drive bushings with a 7/8" square machined in each. The material was harder than A36 mild steel based on the bouncing ball bearing test, probably in the 1030 range. I thought about the rotary broach method but these bushings are 1.75" thick which is way to thick for rotary broaching plus the corners had to be precisely located. Next idea was the square broach but at nearly $1000 for a new one and 23+ inches long I didn't have the budget or the arbor press to to push it. Plus precisely locating the corners would have required a starting fixture. I ended up machining them with a slotting head on my Kearney Trecker 2H universal mill.

Each bushing was drilled in the lathe then moved to the mill. You could skip the lathe part and drill the hole and machine the square with one machine and a slotting head. This of course would depend on how many units are in a typical production run.

The actual machining of the square took 25 minutes per bushing. But I'm not a professional machinist so I was very careful, besides each bushing blank was a $130 before it got to me so I didn't want screw any of them up..My slotting head has a stroke of 2.5" so I had to use every bit of 2.1" to clear the tool and have room and time at the top of the stroke to feed. I ran the head at 35 strokes per minute and allowed at least one spring pass after each stroke.