Hex Broaching?
 

For the last while I have been wondering how to broach hex holes in parts inexpensively, and can't seem to find an answer. All the rotary broaching tools I can find seem like they would be quite expensive to purchase/maintain.

What method do most FRC teams use to get this job done?

Re: Hex Broaching?
 

There is a startup cost to broaching.

You have to buy an Arbor Press and Broaches.

We have a 10 ton press from harbor freight and then we bought a 3/8ths and 1/2 hex broach. Arbor was about $200 and then the broaches were donated via sponsor.

-RC

Re: Hex Broaching?
 

We started hex broaching last year. Our mentor purchased 2 hex broaches, 3/8'' and 1/2'' out of a catalog (don't know which sorry) for around $150-$250 a piece. Must say well worth it.

Re: Hex Broaching?
 

Thanks for the prompt replies.

Apparently I was searching in all the wrong places, as soon as I input into McMaster I got a quote of about $200 for a 1/2" hex broach. Is there a minimum recommended press size in order to do this kind of work (probably for aluminium applications only)?

Re: Hex Broaching?
 

It really all depends on how thick the material you plan on broaching is.

If I recall correctly, we use a 5 ton arbor press with our 1/2" and 3/8" broaches. It starts to get pretty hard to broach as you approach 1" thickness in Aluminum.

The biggest problem that we encounter with broaching is getting the broach perfectly perpendicular to the part. This season we built a fixture which is pressed onto the square ram of the press. This piece has a hole in it (about 1" deep, i believe) which is a slip fit for the broach. This ensures that the broach is held perfectly vertical in reference to the press, allowing for straight cuts.

Re: Hex Broaching?
 

We have a standard 1/2" McMaster push broach which we use in our ~3 ton arbor press in a pinch, when we need to do something in-house. It works well enough, but this is about as small of a press as I'd go for this. The pinion in the press has since let a couple teeth go.

A pull broach would be the best in terms of perpendicularity and whatnot, but these are usually found only in specialty broaching shops.

You can also broach in a very large lathe or horizontal milling machine, if you have access to one of those. And when I say very large, I mean >2000lbs. I would fear it could damage a smaller machine. A press is the definite way to go if using a broaching shop is out of the question. I think a larger arbor press would be preferable over a hydraulic press in this application, for the better feel for things. On an arbor press you'll know when something is wrong.

Just be careful to never drop the broach! Best to have a box with a pillow or something under the press. Don't trust yourself to hang onto it / catch it.

Also, remember to use lots of oil!

Re: Hex Broaching?
 

Hex broaches are expensive. Following 254's and 968's lead, we have been hex broaching much of our hubs for various things for the past 3 years. One problem that we have always had is that it is sometimes tough to get the broach to cut perpendicular. Our arbor press is a bit on the small side, I think its a 2 ton press. We ususually start the broach on a lathe using the tailstock as a ram, and then finish up on the press. Even then, sometimes the broach runs crooked. This year, we are going to try rotary broaching. The holder itself is very expensive, up to $700 new. But the broaches themselves are about $60 - $80 (half the cost of a push broach). They are usually used on cnc lathes, but can be used on manual ones. I bought a used holder on ebay sometime ago. I think it was about $150. There is a guy who makes and sells his own branded swiss type holder on ebay for $200. Might be the way to go. The nice thing about rotary broaching is that it is much more likely to be perpendicular since it is cut on the lathe. It also can broach blindholes. check out www.slatertools.com and watch the video you can just buy the broaches from them and then buy a holder on ebay to save a ton of money. I would not suggest this for smaller lathes. I consider our 13" and 14" lathes about the smallest I'd want to try this on.

I already broached a 7/16" hex in some aluminum for fun and it was surprisingly a lot easier than I thought it would be. Next week I'll try a 1/2" hex and report back if anyone is interested. The only thing I don't like, is that rotary broaches are a bit oversized. A 1/2" hex broach is actually .505" this is to account for wear, but for $10 more you can have them grind a broach to .500" if you can wait a couple of days. We will just cut our hex shafts a bit larger.

Re: Hex Broaching?
 

it might be expensive, and depending on what you are broaching, a arbor press might be "light weight"... we use a hydraulic press most of the time.....

we have used an edm to do it before, (mill a hex out of brass and chuck it in a sinker edm)


as long as you drill the clearance hole to the correct size, you shouldn't have any problems with your broach running straight......

if you plan on broaching thick material, it helps to mill (cnc) a hex to start with, then use the broach to square the corners.... thats what we do for our wheels, it cuts broaching time in 1/8!!!!

Re: Hex Broaching?
 

Alan,

This is great info. Would love to hear more about your results. We're just about ready to pull the trigger on a Somma rotary broach for our mill so we can do everything in the machine in one operation and eliminate all secondary processing.. Good to know that custom sizes are only $10 more-that's less than I was expecting.

Re: Hex Broaching?
 

This is all great info -- the kids did an offseason drive train design on their own from scratch (sans help), and if they went down the hex-shaft route I was going to buy the team a 1/2" hex broach for Christmas.

How long does it take to broach a hex through a typical aluminum sprocket (5/8" wide)? We have both a 3-ton press and a (decent) manual lathe available.

Re: Hex Broaching?
 

Probably anywhere from 30 seconds to 5 minutes depending on how much care you put into it.

Re: Hex Broaching?
 

We too got our 1/2" broach donated about 5 years ago. If you want to save yourself some dough, contact a DuMont Sales Rep and beg. We agreed to put their name on our robot and sent them a nice picture plaque at the end of the season.

Re: Hex Broaching?
 

I would recommend against using the lathe if at all possible. Depending on the size of your lathe and the force you're applying to broach your part you could damage the spindle bearings.

We've winged it a couple times when broaching small parts and stuck the broach in the spindle of our mill and pressed it through with the knee, when we wanted to make sure it was absolutely perfectly perpendicular, but it's really a very bad idea. You never want to apply axial force to bearings that aren't spinning.

Re: Hex Broaching?
 

In my experience Enco has the best prices on broaches. McMaster-Carr the worst. Check Ebay as well. I have gotten some really outstanding deals on broaches on there but it is hit or miss as with anything on Ebay. Watch out for dull chipped teeth on EBay broaches.

You can buy import quality broaches or first class USA made ones. I have some of both. The imports are cheaper and from my experience work reasonable well in aluminum but I don’t think they will last as long as a good American broach like a Dumont. I have never even tried an import broach on anything other than aluminum and only keyway broaches at that. I have seen import hex and square broaches but don’t own any so I can’t speak to them. Maybe someone here can better speak to the durability of the imports. I would probably just recommend spending the extra money and getting USA made broaches like Dumont if at all possible.

I never really like using a hydraulic press when broaching because I can’t “feel” the broach as it goes through the material as well. I much prefer a manual arbor press of appropriate size. When deciding on an arbor press not only keep in mind the ton rating but also the physical size and weight of the press. A typical 3 ton arbor press is about as small as you can get and still fit a ½” hex broach under and still have any working room for your material being broached. IIRC the length of a ½” hex broach is around a foot long. Make sure you have plenty of clearance in any arbor press you are considering for the size(s) of broaches you are going to need and of course the material you are going to be broaching. I would also recommend a ratcheting arbor press over a non-ratcheting type. It is just more comfortable to use to me. Don’t underestimate the shear size and weight of an arbor press. It just isn’t something you are going to want to move around any more than you have to. I have seen teams with big 3 and even 5 ton arbor presses in their pits and I have always wondered why? Then I feel sorry for the poor freshmen that they made bring that thing in. Maybe it’s just me but I figure if you think you need a giant arbor press in your pits maybe you should have thought about a different design back during build season or at least made some spare parts or something.

Whatever you do NEVER EVER even think about smacking an expensive broach through your part with a hammer. You are just asking to snap your expensive broach in half or ruin your part or both.

Re: Hex Broaching?
 

Similar to other teams, we use an arbor press, 4 tons I think. We made 1/2" hex holes for our west coast drive system the past several years.
We make the initial hole 7/16" slightly smaller , then use a 1/1000 reamer slightly smaller to make it square (perpendicular), then use the 1/2" broach with the press.