Extending hex broach cut length

[RJSakowski]

Extra pushing force will not be a issue as i am using a 10 ton press. And the pushing force will not even come close to the capacity of the press.

Yes i do.

In that case, if aesthetics isn't an issue, I would consider a slightly larger through hole. A perfect broached hole 25 mm deep will ideally remove 120 mm3 of material. An 8.2 mm diameter hole 35 mm deep will theoretically remove 119 mm3 of material while an 8.15mm diameter hole will remove 131 mm3 of material. Both of these options provide more contact area than a standard 12 pt socket. Here is screen shot of an 8.15mm hole broached with an 8 mm broach.

 

[Flyinfool]

Extra pushing force will not be a issue as i am using a 10 ton press. And the pushing force will not even come close to the capacity of the press.

Yes i do.

It is not the force of the press that is at issue, it is the force on the tool. To much force will bend or break the the tool.

Drilling 6 holes near the corners will reduce cutting forces but it is so easy for a 2.5mm drill to wander that you are like to damage the area near the points where all of the torque load is happening.
Making the starter hole bigger to reduce the volume of chips will also reduce the cutting load and leave with a shape like RJ drew. The center of the flats does not contribute anything to the torque load so for torque purposes that bigger starting hole will make no difference. Your hex shaft or tool will never even touch the center of the flats unless you have already striped out the hex. In order for a hex tool or shaft to slide in there the hex hole is bigger the the hex shaft.

What is the recommended pilot hole for your broach? From there you can calculate how much material is being removed in the 25 mm max cutting length. You can then use that number to calculate what size pilot hole will give the same amount of material removal and that should make the broaching work for both force required and chip volume. Once you have this number you can also draw it up to see if the finished shape is acceptable. The recommended drill size is already usually bigger than the hex size just to reduce chip load without compromising the function of the finished part.

Another option.
There have been a number of threads on making a wobble broach (also called a rotary broach). Standard off the shelf wobble broaches have a max depth of 2 times the hole size. But if you are making your own you could make it longer. OR If you have a way to index the part and tool you could use the stock wobble broach from both ends to get the hex thru.

If you already have the push broach, you could use the wobble broach to however deep it will go (around 16mm deep) and then finish it off with the push broach. This way the push broach is only cutting 19mm of depth. If you mare making your own wobble broach you can make it just a bit undersized so that the push broach will just shave the walls to clean them up and make it all pretty.

 

[whitmore]

If the cut length limit relates to axial force, would it be feasible to cut three
of the six corners in one pass, rotate the workpiece by 60 degrees and cut the rest in a second pass?
Grinding down half the teeth of a broach would take some doing, of course.

 

[pontiac428]

I like RJ's suggestion, but if it doesn't work with your critical application, maybe two broaches stepwise is the way to go? one pass with an undersize broach, and one pass to finish. It is how we drill and bore round holes, and we all know what happens when we try to hog too much material in that context.

 

[Storm_Eagle]

So i just went for it without any extra removal of materials before broaching and it worked out ok. The broach was bending a fair amount in the beginning but that was probably because the moving carriage of the ram on my press has about 3 mm of play so it is tilting sliding around when it is starting to press on the part so it was likely not pushing straight down on the broach. I have to fix that in the future. The teeth of the broach did not take a equal amount of material off, with the first 1/3 taking almost all of the chips and the last 1/3 of the teeth having almost no chips. The finish was pretty good except some scratches in the middle of each flat that i think is from the pilot hole being slightly to big. The hex was dead straight trough the whole part with both the exit and start being right in the center. The pushing force was around 1 ton according to the dial on the press. And i was using a lot of cutting oil in the pilot hole and on the whole broach before starting the cut.

 

[Flyinfool]

Sure looks like a nice hex hole to me.