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.

Re: Hex Broaching?
 

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.

Re: Hex Broaching?
 

Picture please? Sounds like a great idea!

Re: Hex Broaching?
 

It's pretty simple, imagine making a new end to your press that has a hole 1" deep in it that the end of the broach sits in.

Re: Hex Broaching?
 

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.

Re: Hex Broaching?
 

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

Re: Hex Broaching?
 

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.

Re: Hex Broaching?
 

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

Re: Hex Broaching?
 

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Re: Hex Broaching?
 

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.

Re: Hex Broaching?
 

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.

Re: Hex Broaching?
 

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!

Re: Hex Broaching?
 

We made our own 1/2 hex broach years ago and it works just fine.

Re: Hex Broaching?
 

That sounds pretty cool. Have any pictures/details?

-Brando

Re: Hex Broaching?
 

Hi guys,

Our team is making our FIRST (haha get it!) westcoast drive. We bought an arbor press and broaches a long time ago, but we can't seem to get a perfect broach... We have had this problem buying from FIRST suppliers as well. What is the best way to get perfectly hexd parts? When I mean perfect, I mean no wobble?

Nick!

Re: Hex Broaching?
 

Start it in a lathe, finish it in an arbor press.

Re: Hex Broaching?
 

Obviously you need to ensure that the broach is kept perpendicular to the part's face for the entirety of the broaching process. Hex broaches take some patience to get through, but taking your time will go a long way in helping out.

If you have a lathe- you can simply start the part there and move it to the arbor press to finish to get a nice straight broach. If you do not have open access to a lathe, you can try this:

I've had success making jigs and guides for broaches. Essentially I just machined a part that had a large face to mate against the part I was broaching. I drill and broach the guide to same size I was broaching my new part. I carefully feed the broach through the guide, lined up the hex properly, and put the whole stack together on the press and then began broaching my new part slowly, especially in the first few teeth.


Also make sure you are liberally lubricating the part you are broaching as well as the broach itself.

-Brando

Re: Hex Broaching?
 

Start it on a lathe if you can. If you can't, just constantly take 5 or so steps back and check how straight it looks from a distance (from multiple view points as well), and adjust as necessary.

Also, don't stack multiple parts and broach them together. It may work sometimes, but we had a bad experience when a machinist in the NASA trailer at CMP tried it. They broached all 4 gears at the same time, they ended up crooked, and we had to have them weld it shut and try again.

Re: Hex Broaching?
 

Similar to Brandon's post, I've had success with a jig that holds the broach colinear with the arbor press ram. Here's a simple mockup of this idea:



The square inset in the top fits over the end of the ram. The small hole on the front face is for a 1/4-20 or similar size set screw to hold the adapter onto the ram (you can also use a thumbscrew or similar fastener to mount/remove it).

The through hole in the bottom is a reamed hole that provides a very tight clearance fit around the shank of the broach. The tighter this clearance fit, the more accurate the broach alignment will be. When properly machined with tight tolerances, this adapter can be used to quickly and accurately hex broach objects in one operation (no need to start on a lathe).