Broaching a Cast Door Knob in Bronze

Lyn's thought of a rotary broach is my first thought also, typically this would be production approach, as the tooling is very expensive for a one off project. You may want to search some metal working sites for a home built rotary broach (e.g. Practical Machinist). You need a pretty good machine shop and heat treat skills.

A picture would help.

I would go down the drilled round shaft approach, and cross drill a set screw in the knob (grub screw for those that use that term). As it's a door knob, the torque can't be too great unless the knob has some very long "T" form extension. If you do this, file a flat on the mating shaft to ensure a good bite with the set screw.

So can a round shaft be used, or is the square used for a latch actuating mechanism?

This is pretty cool....if you're ok with rounded corners...

https://www.youtube.com/watch?v=L5AzbDJ7KYI

We are very good here at breaking (cast) iron bath tubs. Breaching something bronze should be much easier.

How many knobs are to be broached?

Hi there,

Try googling up "Drilling a square hole" there are utubes on the subject

yours cryogenicly

cryogenically

As a follow on from #8, use a piece of 5/16" sq HSS tool steel ground flat on both ends to do the final clean up. Here is one company that supplies lathe tools.

http://www.millerstooling.com.au/Lathe-Tools-HSS-Toolsteel.asp

Jimrat: That is quite the nice machinist website, great source of information. Thank you. I doubt, however, that for me, purchasing anything there will match prices I can get here in the States, as the shipping is halfway out of this world.

Every one of these threads gives me further insight to metal working, and I am becoming a somewhat eager learner. Too bad I didn't farm out to my uncle (WWII veteran from the US Army) many years ago. He had a barn full of heavy equipment, horizontal mill, etc. He designed and built most of his own farm equipment, strange stuff mostly, but effective in sugar sand.

One of the 'Machinist' community on YouTube, "DoubleBoost" , has just recently posted a method of cutting a keyway in an I.D. that may help . The method referenced begins around 3min. into the vid.

https://www.youtube.com/watch?v=pvV1OvxSU2w

Chuck the knob into a lathe & use a custom ground bit to 'shave' your square hole.

If the knob is not drilled you can incorporate that step into the lathe work as well.

HTH

In the old olden days, this operation may have been performed on a shaper. Today I think an EDM might be the most efficient method. Would love to learn how you solve this problem.

It would have to be a plunge type EDM, not a wire EDM, since it is a blind hole. That's a great machine for removing broken taps, as well as for creating odd shaped holes and depressions, but I suspect not many DIY'ers have one available.

It's also not very fast at removing material.

I don't know about brass, but I used to get P-20 steel cavities EDM'd for injection molds.

Brass may be faster, depending on thermal conductivity, but I doubt it.

Very true! I don't exactly remember how much of the time was setup, but as I recall, it took me over an hour the last time I removed a broken #10-24 tap. Still, it was a whole lot better than scrapping a nearly finished part!

We most often plunge EDM honeycomb in high nickel alloys. Since it's mostly empty space, it goes pretty fast.

If you were going to do a lot of them you could get a "trepanning" head ( might also be called a shaper head) for your Bridgeport (it goes on the other end of the dovetailed head mount, the big round hole is for mounting it). Nobody even knows what they are anymore, so you might find a used one cheap.

Could you please elaborate a bit! I'm familiar with trepanning, which is essentially using a hole saw without the center drill, or a fly cutter, but just in case I had missed something I did a bit of research. I never found anything that connected trepanning with making square holes.

We don't have one in our shop, but it basically is a head that goes on the other side of a Bridgeport, the whole top of the column is rotated 180 degrees, leaving the milling head on the back side and the shaper/trepanning? head over the table. The head will take any shape of cutter with a square shank, it doesn't rotate but instead it reciprocates. You would start with a drilled hole and feed the reciprocating cutter down the hole where it peels a cut off kind of like a broach. You can easily grind a lathe tool bit to the shape that you wanted, except it cuts on the nose instead of the top. It gets pushed straight down the hole. It's hard to explain how it works with out a picture, but it will make a square or even a triangular hole if you know how to use it. I may be misremembering the name of the head, it might be called a shaper head, on ebay there are some model e heads that are what I am talking about.

Sound like the shaper is the better term. I did use a true shaper a few times, about 50 years ago...

How about a Vertical shaper/slotting attachment for bridgeport type mill.

Used ones available on ebay for $1000 -$2000. Cuts square, hex, octo, wahtever shape you want with the right tool.

This type of broaching presents a problem in a BLIND hole, where does the awarf/shavings go....between the bottom of blind hole and cutting tool...jam!

Why not try casting the knob/s onto a solid Carbide former and then pulling

Carbide former from the knob/s, pretty sure the Bronze won;t bond to the Carbide

former without a lot of effort....

Agreed there could be an issue with chips.

Drill slightly under size hole, slightly deeper than required hole depth.

Don't try to cut right to dead bottom.

Feed slowly to keep chips thin and use light air - blown into hole to evac chips.

Back down table a few thousandths each time you move cut sideways to get to your square hole size. Prevents tool from hitting bottom of last cut too hard. Basically you are only cutting the corners out.

Not sure about the carbide idea - that might work.

Although I have brazed carbide onto steel but you have to use cleaning flux to bond. Don't know if the bronze would bond or not. Something to try out.

Lot of good ideas given.

We might be over thinking this, after all it's just a door knob, not part of the Space Station. Drill out he hole like JustCurious said, maybe even a little over sized and as deep as you can go without punching through. Get a 8 mm piece of tool steel and grind a hollow face on it. Press it in as deep as it will go with an arbor press. It's just brass, the chips will end up in the bottom of the hole but you should be able to go deep enough to get a grip on the connecting bar. if the chips bother you make another pass with the drill.

IF chips become a problem a small boring bar with an end that cuts a 3mm wide groove ( as for an 'o' ring ) can be used in the bottom of the hole.Jim

A lot of great ideas here. However I believe that the best solution is to EDM the holes if you have only a few knobs complete. I did EDM a few years back and there have been a lot of advancements in the art. As for the carbide mold pin, it may work as there won't be any flux on it and the molten bronze won't have enough time to heat up the carbide before the bronze solidifies there by leaving a clean molding fracture. And hopefully you have the equipment to pull it out now.

Guys,

The knobs are already cast.