Question about Broaching 1

[metalman8357]

Can someone please explain to me the broaching process for putting a hexagonal recess into a screw driver shaft. I've been asked to figure out why our shafts keep breaking, and I want to understand more about the broaching process to see what parameters can be optimized. It appears that the surfaces of the hex recess are rounded, is this normal? Ideally I would want these faces to be flat in order to impart an equal amount of stress on each face, but if it's an artifact of broaching then maybe there's no way around it. I'm assuming there's a pre-drilling step which makes it so these surfaces aren't completely flat, but is there a way to broach these surfaces and keep that face flat? (without any severe increase in cost). The hex flat to flat dimension is 0.192". Also, what type of tolerances can be kept with this process?

 

[mfgenggear]

metalman

this type of broaching is "push broaching", it is done concurrently in the cnc lathe, cnc screw machine , or cnc bar loader with a collet.
the machine will center drill, drill or bore"pilot dia" & punch the hex. it is a very common machining process.
the limitation is in the lbs force & rigidity of the machine.

the second question is can the drill diameter be completely removed the answer is yes.
it will dictate close manipulation with the tooling that is available.
pre drill the hole size so the the push broach will completely remove the pilot dia.
if this part is mfg by a supplier. contact them & discuss these issues with them.
there is a limitation of how much stock can be removed without damaging the tool.
it is easier to to remove least amount of stock.

there is a limitation how hard the core hardness can be & the depth required.
if the length of the hex is more than I will take a guess at 3 x more than the pilot dia.
optimum core hardness should be not more than 40 HRc, after that EDM should be used.
which has it's own problems.

HTH
Mfgenggear