Broach bushings

[deeferdog]

I finally got the bullet and purchased four keyway broaches and I now want to make a range of bushes to suit. My question is, can they be made from aluminium? I don't see that they are subject to a great deal of load and they will not be used enough for wear to be a problem but I have been wrong on so many things of late that I thought I would seek the advice of the forum. I hope that the consensus will be for aluminium as they will be a lot quicker to make. The tool holders I made a while back out of aluminium have been a complete sucesss, so I'm tempted to proceed, however I'll wait your input. I appreciate your interest. Cheers, Peter

 

[Dr Jo]

I make bushes out of steel as and when I need them. There is little point in making a set when you will probably only need one or two.

My concern with Aluminium would be it is softer than Steel so could distort from the pressure of the broach passing through and get stuck in the hole. The slot for the broach will also wear quickly.

Jo

 

[Jasonb]

Also a risk of the aluminium picking up and the whole lot locking solid. I suppose you could cut the slot deeper and use an additional shim to give the broach a harder surface to run against. Like Jo I make mine from steel as and when I need one if I don't already have something that fits.

 

[natalefr]

Hi
Yes you could use aluminum but good aluminium type AU4G and good lubrication

 

[deeferdog]

Thanks for the replies, I have decided to try one in aluminium for my biggest broach which is 6mm. The thing that I am having trouble in figuring out is how much force is exerted on the side of the bush as the broach is pushed through. If, for example, one ton of downward pressure is exerted on the top of the broach by the press, am I correct in assuming that a portion of that force must be applied to the side of the bush due to the shape (wedge) of the broach? If it turns out that the side force is low then I imagine that the wear will be low if a good lubricant is used. If, however the forces are quite high and the wear rate is also high, then the next question is, are steel bushes hardened to give a longer life? Sorry if this causes eyes to glaze over with boredom but I quite enjoy this sort of stuff. I need to get out more. Cheers, Peter

 

[kf2qd]

Nope. The bushing does not need to be hardened. There is some force against the bushing, but must of the force is straight down, between teh teeth and the workpiece, though there does get to be more force against the bushing over time as the broach eventually dulls.

 

[bazmak]

There will be some force at the bottom of the bush opposite the teeth but none to the sides.Would brass be better ?

 

[deeferdog]

Bazmark, I like your suggestion, I had thought of brass myself but the cost is a bit prohibitive. My secret workshop fund is still on life support after buying the broaches.

 

[Desmo]

You could anodise the Al bush to form a hard surface layer.

 

[RM-MN]

Bazmark, I like your suggestion, I had thought of brass myself but the cost is a bit prohibitive. My secret workshop fund is still on life support after buying the broaches.

How much brass would you really need? Can you make the bushing from aluminum, then put in a wear surface on the bottom of the bushing where all the pressure is? I'd be thinking of lining it with brass shim stock.

 

[rlukens]

I don't get it. Only during the first pass would the broach contact the arbor (bushing). Like someone said, you could machine the groove deeper so that all passes require a shim(s). The shim remains fixed in the arbor and the friction is between it and the broach. Little lube wouldn't hurt either.

 

[deeferdog]

I made two bushings for the 6mm broach today and they look OK to me. I will cut a keyway and post the result, however I now think that it doesn't much matter what material they are made from as long they don't distort under load. I think that some plastics would do the job but that's just a guess as I know nothing about plastics. I think riukens is right on the money with his observations. Commercially manufactured bushings would probably be made from steel because it is cost effective? Could be, cheers, Peter.

 

[Gordon]

I am a little baffled by this. Why not just make them from steel and be done with it. It is a known fact that steel works. Steel is cheaper and easier to obtain than alum or brass. The amount of extra time to machine them out of steel instead of alum is minimal. Just go with what is know to work.

 

[don-tucker]

Yes,steel everytime

 

[KJE]

For bushings you won't use too often or once I have used bronze or leaded mild steel ...... easy to machine and stand up fairly well

 

[rklopp]

For bushings you won't use too often or once I have used bronze or leaded mild steel ...... easy to machine and stand up fairly well

I am pretty sure the commercial DuMont bushings I have are leaded steel. They certainly rust like leaded steel! Non-leaded steel needs no protection in my low-humidity California shop, but leaded steel rusts out of spite!

 

[Wizard69]

Gordon's point of view below agrees with me more than any other posted. Aluminum isn't really cheap here and many grades don't cut it as a bearing surface. Im certain the right alloy would work for a bit but that bushing might gall up at the worst time.

By the way broaching requires a lot of oil, a good grade of cutting oil. I like to coat the entire broach with oil and even add some as the broach is pressed through. The oil is likely more important than the bushing material.

Still the point remains that steel is the cheap and proven material. It is what you find in broach sets.

I am a little baffled by this. Why not just make them from steel and be done with it. It is a known fact that steel works. Steel is cheaper and easier to obtain than alum or brass. The amount of extra time to machine them out of steel instead of alum is minimal. Just go with what is know to work.