Square hole broaching

[makila]

Hi,
I am about to use a 3/8 square push broach to cut a square hole in 1" mild steel. Last time I tried this I broke the broach due to excessive bending. Before I broach, has anyone got any tips? I am not feeling too confident.
Looking at the tool, there will be about 4 teeth engaged, possibly 5. These things take a lot of force to push through.
Steve

 

[kvom]

The more the corners are milled in advance the easier it is.

 

[Jasonb]

Also make sure you use the right drill size, there are the usual sq broaches that need an oversize hole 25/64" in the case of a 3/8" broach. You can also get "full square" broaches that use the nominal size as pilot and don't leave a slight scalloped area on the flats.

So if its a standard one make sure you are pushing it through the right size hole.

 

[makila]

Thanks for the advice, some good points here. I thought that the slight relief of the hole might work, using a 1/8 cutter and scalloping a few thou off the corners to a depth of .2 at less than .125 from the centers of the sides looks good. The reason I will go to .2 is to not affect the pilot hole as this hole needs to concentric and accurate as its a pair of crankshaft cheeks.
Yes I did wonder about the pilot hole being 25/64 or .390 which is over size to .375, there is an oil way going through here and I am scratching my head on this one.
I have made a new push bar on my press to maintain a closer fit to the guide as the standard push bar was a bit sloppy. Thanks for the advice, I hope to do this later today, but I think the odds are that it will not be a successful outcome, I am not fond of square hole broaching, especially in hard stuff.
Steve

 

[Blogwitch]

There is an easy way if you are scared of breaking your broach.

Buy a set of diamond files and do it by hand.

By the time you have set it all up and relieved it for broaching you could most probably have done it by hand anyway.

This reminds me of the time the carpenters had to down tools because the battery in his drill needed charging as he was putting some screws in, what's wrong with using a normal screwdriver?

Are we getting too reliant on modern day tooling?

John

 

[tornitore45]

what's wrong with using a normal screwdriver?

In my lifetime I wound and unwound several miles of screw, some easy and some hellish. It gets old pretty soon, on the wrist and on the arthritic fingers.

As much as hate batteries, thanks God for electric screw driver.

 

[MachineTom]

I have found when broaching every inch or so let up the pressure at let the broach center itself. In most presses, nothing is really square, Table to Vertical, Ram to Table etc. So letting off the force, allows the broach to straighten out. I've not bent a broach in years following that procedure.

 

[makila]

Well, today was a success.
I took all of the advice here and set about broaching a 3/8 hole in 1" of steel. Before the broaching was carried out, I had to make a new shaft that was a close fit to the guide and spend some time ensuring the shaft was perpendicular with the press table. I then started the broaching, releasing pressure, as suggested here and also to adjust the table in increments as the broach is about 12" long!
The previous attempt at broaching was with an unbranded tool from our Eastern friends, it bowed significantly and did not cut straight, eventually snapping, this broach was a US made Dupont, an excellent tool, it did not bow at all.
I was a bit apprehensive and went very slow feeling for feedback of binding and was a bit careful as each tooth popped out of the bottom of the work. But as the broach got shorter, it went quicker and with plenty of pressure releases to check for bowing etc, the cut was complete. The square is perfectly concentric and did not wander of course one bit, as you can see by the witness marks left by the guide hole, each side was exactly the same.
Thanks to all here who offered their valuable time to respond, I recommend buying tools that are not from the cheapest source, the Dupont broach, as far as broaches go, is of supreme quality and if it is treated with respect, this broach will cut 1" thick steel without tears!
I have posted a couple of pictures, those with a keen eye will note the work is a pair of crankshaft cheeks for a Hodgson 9 cylinder radial. I will post work in progress soon, but after Terry's excellent thread, I think you are all a bit bored with radials.
Steve

 

[makila]

That's the job done, here is what the broached hole was for. The alignment between the two sides is as good as it will get so hopefully there will no crankshaft binding.
I will not add more now as the Q & A is done as the broaching was a success.
Steve