Hex Broaching?

[JesseK]

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.

[NickE]

Originally Posted by JesseK

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.

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

[Cory]

Originally Posted by JesseK

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.

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.

[ChuckDickerson]

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.

[AdamHeard]

You might want to check if a local college or shop has a pull broaching machine. Cal Poly (the school I attend) has a few, and is willing to let us use them. I've yet to do so though, as we have a line of push broaches in our shop which have worked adequately for us so far.

[waialua359]

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.

[Alan Ing]

Cory,

To be specific, a .505" hex rotary broach with a 1/2" shank runs about $70 from Slater Tools, when I asked for a .500" hex rotary broach, they quoted about $80 and about 1-2 day turnaround.

There are basically two kinds of rotary broach holders, Swiss type which are non-adjustable and the regular adjustable type. Since you plan to use it in a Haas milling center, you might want to consider the Swiss type. I asked the salesperson what the advantages were between the two. The Swiss type is smaller and has more clearance (it is round) whereas the adjustable has a wide base that could cause clearance problems since you will be spinning the tool holder rather than spinning your material. Adjustable ones are nice if you have a non-standard broach with a different length. You can only use standard length broaches in Swiss type holders and are limited to a max of 1/2" shank broaches.

Last year, I tried to use it on a bridgeport type vertical mill and failed miserably. I ended up having to retram the mill and gave up. We went back to our push broach which didn't always cut perpendicular no matter how hard we tried.

Of course your Haas is quite a bit stiffer. I would not expect a problem providing you adequately clamp your work piece.

This past summer, I decided to give it another try and placed the rotary broach in the tailstock of our lathe. Surprise, it was so easy and I felt like an idiot for not trying it earlier!

According to the slater tools chart, it takes about 600-700 pounds of thrust to rotary broach a 1/2" hex in aluminum.

Because rotary broaches are driven by the work piece, the hex can have a slight twist. One work around, is to reverse the spindle half way through the piece to reverse the twist. Rotary broaches don't care about spindle direction for cutting.

I'll let you know how it works out for us.

[Andy Baker]

Originally Posted by polygon

This is a great thread, and I'm wondering if you have any more observations about the broaches. Has anyone run the broaching tools on a Bridgeport? Check out this video:
http://www.youtube.com/watch?v=uTsPXFy7Xx0
Also, you might consider Polygon Solutions as a pricing option.

Polygon,

Thank you for the information. It seems that your company's services and tools could be an asset to many FRC teams. That is a great video, as I have not seen broaching done in this way before.

Sincerely,
Andy Baker

[Alan Anderson]

Polygon, if you were a bit less faux-subtle about your motives here, you'd likely be a bit better received. Try again, and present yourself as a company with a product rather than apparently pretending to be a disinterested party. Most of us are engineers by inclination, and we like straightforwardness.

[polygon]

Well said sir. My apologies, and thank you for the advice. I hope to be more helpful in the future.

[nick_sheets]

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[nick_sheets]

We are looking to broach through 1.75" aluminum, does anyone have a recommendation on what size arbor press to get? We recently had a sposnor donate 1/2" Hex broaches.

[Cory]

Originally Posted by nick_sheets

We are looking to broach through 1.75" aluminum, does anyone have a recommendation on what size arbor press to get? We recently had a sposnor donate 1/2" Hex broaches.

The short answer is that's not going to work very well at all.

You're only supposed to go 2-2.5xD on a hex broach. Any longer than that and you'll get excessive wear and require massive amounts of force. You will need a 5 ton press and it will not be easy at all.

What is this for? Some type of hub? You probably don't need a full 1.75" of engagement and could counterbore your part to allow a more reasonable hex length.

[nick_sheets]

Originally Posted by Cory

The short answer is that's not going to work very well at all.

You're only supposed to go 2-2.5xD on a hex broach. Any longer than that and you'll get excessive wear and require massive amounts of force. You will need a 5 ton press and it will not be easy at all.

What is this for? Some type of hub? You probably don't need a full 1.75" of engagement and could counterbore your part to allow a more reasonable hex length.

Thanks COry for the reply, it would be a hub for a plastic wheel we got from our sponsor. Its about 1.5" long with a nub. Its a 1" od plug with a 1/2" bore.

Would you counterbore one side to .577" or similar and have the hex be 1.5" long? What would you suggest?

Thanks!