Square broach pilot size larger than broach?

[wildo]

I recently bought a 3/8" square broach for a project in which I need to broach 144 holes for carriage bolts. I was surprised when it arrived and stated that the pilot hole was 25/64" or .3906" which is nearly .016" larger than 3/8". Taking some micrometer readings confirmed that the square area was about .375" and the pilot area was in fact .390".

It seems odd to me that the starter hole is larger than the finished square, and in fact the finished square does still have some circular sides. This will be fine for my application as it's just some carriage bolts, but I'm wondering if this is common. It seems totally counterintuitive to me that you'd want this...

 

[BigMo]

Doesnt seem right....lets think about that for a min....pilot hole is round....vrs the broach will be square....theres a correlation

 

[dave_r_1]

While the products I've bought that use carriage bolts have been manufactured with square holes (no round dimples in the side), they are also likely made with a different technique (likely a hydraulic one-shot make a square hole here, then here and then here setup).

But if the tool makes the square of the right size, and without excessive play for catching the corners of the carriage bolts, does it really matter? It probably would take a longer, more expensive broad to start with a smaller pilot hole so you are left with a plain square hole.

 

[BigMo]

Yep

 

[intjonmiller]

He said it doesn't matter to him for this application. His question is regarding typical broach sizing and resulting form.

 

[kd4gij]

All square broaches call for over size pilot hole to reduce chip load. Look at the hole in a 3/8 or 1/2 inch socket.

 

[wildo]

All square broaches call for over size pilot hole to reduce chip load. Look at the hole in a 3/8 or 1/2 inch socket.

That's surprising to me, but a seemingly good answer. This thing was EXPENSIVE to produce such an aesthetically poor hole though! $144 for that damn thing, and your left with "kind of" a square hole. Strange!

 

[intjonmiller]

Here's a method that doesn't require an oversize pilot hole (or an expensive broach!). Your results may vary.

 

[savarin]

Neat, thanks

 

[kd4gij]

That's surprising to me, but a seemingly good answer. This thing was EXPENSIVE to produce such an aesthetically poor hole though! $144 for that damn thing, and your left with "kind of" a square hole. Strange!

I think a broached hole looks much better than a broken $144.00 dollar broach. Just sayen.

But the first time I used a square broach I did ask the same question.