Wobble and Morse taper

A simple question: Would a rotary broach be likely to loosen a 2MT taper when fitted in a tailstock?

I am planning on making one and wish to use a 2MT blank end arbor as a drive - most seem to use a parallel shaft. Clearly no problem in the spindle of the mill, or lathe, as it can be secured with a drawbar, but predictions (or experience) of one fitted without a drawbar, please, as it’s a plunge cut, but cutting on several corners cyclicly.

I made one using a MT2 soft blank, and it works ok for small sizes up to 3/8". It does flex a bit but I never noticed any loosening when in use. If I remember, I will take some photos of it on Saturday. The MT does not use a drawbar, It had never crossed my mind that it might need one.

Edited By old mart on 14/11/2019 20:28:58

Should be OK, as it 'pushes' into the work with no way to grab like a drill or milling cutter.

Neil

Thanks, Neil. That is what I thought/hoped but I was also thinking of when the pressure is off and withdrawing the tool - don’t want the MT to loosen. I suppose that stopping the machine, before withdrawal of the cutter, would largely negate that possibility.

OM, I don’t envisage anything larger than 3/8”. 3/8” would already be wider than the 10mm silver steel or old end mill shanks I anticipate using for the broaches. First job for it will be making a tapered screw for a grip-tru chuck, I acquired recently, so they all nearly match (not keen on trying a slotted screw in it).

I made one of the Hemingway rotary broaches with a parallel shank which I hold in a drill chuck on the lathe. Works fine for squares or hexagons up to about 6mm. You have to use a lot of pressure to get it to cut so no chance of the morse taper coming out.

If you are going to try to broach 3/8" square holes you would need a hefty lathe. I think.

I made a rotary broach many years ago, it has a 3mt tanged taper. Holds it well. Don't forget that that the cutting tool will need will be pulled out of the work piece, especially so if the tool is not as sharp as it should be!

The largest hole I've broached is 10mm square in brass with a hardened silver steel tool, that took a lot of pressure! I've also done a 6mm hexagon in mild steel with a HSS tool, again considerable force need but an entirely satisfactory job resulted.

If I were to make the broaching head again I would make it so it would accept commercially available cutters, google "Somma". Heated treated silver steel is fine for cutting brass and aluminium, its a pain making tools from HSS.

The only downside I can see with rotary broaching is that the tools are unserviceable, that is to say you might get one resharpen then its grind the tool down to the next smaller size.

John

I would expect the ‘oomph’ needed might depends on several factors - material to be cut, design of the cutter, accuracy of the build, rigidity of the tailstock (if used on the lathe) and perhaps other minor factors.

Probably the most I would attempt, in metal, would be 10mm across the points. What would you call a hefty lathe?

The Hemingway design looks quite small - mine will be somewhat larger, simply because of the need to secure the MT blank to the broach body. I plan on using a 40mm blank arbor - so bearings, etc to suit, and 10mm cutters.

Thanks John. That site looks like it will be interesting to me.

I chose to use the MT option, because it would be easy for the mill and lathe spindle with a drawbar. Are you suggesting they cut sort of ‘undersize’? Perhaps I might need to make provision to fit hooks on the body so I can secure it to the tailstock? I intend making the cutters concave - they may not stay sharp as long but should be keener at the start.

I'm definitely no expert on this topic and can only refer to my experience as an amateur. My lathe a a very good condition Myford 254 and my cutters were made of accurately ground HSS. The Hemingway tool is very good for cutting squares in brass for my clockmaking and will also cope with 6mm hexagons in steel. I agree its small in size but even if I had a larger broach I doubt my lathe would be up to pushing it through the workpiece to be honest.....

Edited By lfoggy on 15/11/2019 20:31:14

That’s OK then. I have the predecessor to the 254 - a Raglan 5” in very good condition.

Were your cutters flat on the ends, or concave? The screw hex I need to make, for the grip-tru, is only about 3-4mm across the flats, so no problem for the lathe or mill - the hard bit will be making the tiny cutter accurately.

My cutters have flat ends and seem to work OK. I am fortunate to have a Deckel SO tool and cutter grinder clone. This tool can grind accurate squares and hexagons in HSS really quickly and easily. Happy to assist in grinding a cutter for you if you.

I have taken some pictures of the rotary broach I made. The bearings are a double row deep groove and a thrust bearing, the er25 collet works for different size cutters. The number etched on the side is the length the cutters must project to have their tips at the null point. I made cutters for square and hexagonal holes.

Here is my version of a rotary broach. To give an idea of size the taper is 3MT. I used a needle roller as the main support for the shaft, inner sleeve, if memory serves, is 1" diameter. location and thrust is taken by a ball race located with a circlip. Bearing choice was made by what was in the "junk" box. The shank of the cutter is 12mm diameter.

John