Milling machine overarm - solid or tube

[SteveM]

I have searched for an overarm for a horizontal mill on which to mount a bridgeport M head. My head came with a mount for a 3-1/4" bar, which I though odd at the time I got it, and then realized that 3-1/4" was a pretty standard size overarm, so they were targeting the horizontal market.

The overam on the mill is one that has the curved end cast integral with the arm, and I could use it by reversing it, but that means a loss of flexibility where I can place the mill because I will need enough room to get the overam and me around back to put it in. Also, it won't stick out enough for some jobs. It's also dang heavy with the curved section on the end.

I have looked online, but besides the material costing a lot, shipping is a bear.

We have a local steel yard that has a "bargain barn" and they had a piece of 3-1/4" steel tube with 1/2" wall, a bit more than 3' long, which should be more than enough. It's in the bargain barn where I think it's priced at 50 cents / lb. My estimate is that would be about $25 (even if I'm off on the price by a factor of two, it's still only $50).

Alternatively, they have a solid bar of alloy steel, 29-1/2 long, but it's not in the bargain barn. It's 29-1/2" long, which may be long enough. It's priced at $2/lb and works out to about $130.

An M head only has a 1/2hp motor and 1/2" collet, so it's not like I'm pushing a lot of metal.

Would the tube be enough or am I just inviting harmonic vibration?

Steve

[CaptonZap]

Most of the bending stresses borne buy a round member are in the outer 20 % of the diameter. Fill the tube with expanding concrete, (your building supply house should know what that is) and don't worry about it. The concrete will dampen some of the vibrations.

CZ

[RichD]

Steve, I believe the Mhead overarm on my 9x32 BP is a hollow casting. FWIW.

[Steggy]

We have a local steel yard that has a "bargain barn" and they had a piece of 3-1/4" steel tube with 1/2" wall, a bit more than 3' long, which should be more than enough. It's in the bargain barn where I think it's priced at 50 cents / lb. My estimate is that would be about $25 (even if I'm off on the price by a factor of two, it's still only $50).

While I believe the 3-1/4 inch tube has the needed rigidity, I question whether it will be straight enough for this application.

[earlgo]

Steve:
The overarm on my small ProductoMatic H-mill is 2.975 diameter, is 5/8 wall, and projects 15" with the 1.25dia mill arbor in place. It is 31" OAL. I don't use it much but it has never vibrated during use, but then again there is only 2HP driving the cutter(s).
I am glad it is hollow because I can run it back in nearly flush and with a 36" thru bolt, fasten a plate to the front of it to stick a magnetic indicator to. No other place is so convenient.

Producto Matic

I'd not hesitate to use the tube if it is straight enough as pointed out by our Pleistocene friend.
--earlgo

[Steggy]

I'd not hesitate to use the tube if it is straight enough as pointed out by our Pleistocene friend.
--earlgo

Ahem...I'm late Jurassic.

[earlgo]

You bet, Jurassic.
--earlgo

[SteveM]

While I believe the 3-1/4 inch tube has the needed rigidity, I question whether it will be straight enough for this application.

I was thinking of bringing a mic to check it, and if it is oversize, I could turn it on the lathe to ensure that it is reasonably straight.

A pair of Keith Fenner's rollers would come in handy for checking straightness.

As the head will be able to tilt in both directions, if it is not straight, I can still tram it. If I move it, I would have to tram it again anyway.

Steve:
The overarm on my small ProductoMatic H-mill is 2.975 diameter, is 5/8 wall, and projects 15" with the 1.25dia mill arbor in place. It is 31" OAL. I don't use it much but it has never vibrated during use, but then again there is only 2HP driving the cutter(s).

The difference between that any my application is that on yours, your overam supports the end of the arbor. On mine, I'm supporting the M-head.

Here's an M mounted to a K&T with the same adapter as mine:


Steve

[earlgo]

Steve:
You are right, I forgot the important detail.
With a few dimensions, weights, and material specs, one could calculate the deflection of the tube/bar in question.
However from your picture, it would seem that the overhang is way too short for any flexing to occur, but calculations could verify/disprove that.
As an aside, the fit on the overarm to the main body is astonishingly close on the Producto Matic. It is as if it were lapped in place.
--earlgo

[spro]

The over arms were either solid steel as in 1911 Hendy Universal #1 1/2, Brown & Sharpe # 2 Universal single ram and others. These were 3 1/2" OD . There were larger mills at all those times with larger solid rams but it got to a point there were dual rams of smaller diameter. Fast forward many years and there was tube but we don't know if it was steel or semi steel. Semi steel is like iron and it won't bend until it cracks. The mill drills used this hollow column for years. The interior was rough. The OD was about 4 1/2" but they weren't shipping solid columns across the ocean. These were cast hollow and finished at the outer surface. There were all many ways of decreasing the "flex" of the column but it wasn't the hollow column, it was the mounting to the table base. That and the heavy 2hp heads racked up and down contributed to some frustration.
This is a new thing with that weight ~ 120 lb. hanging off it but much of the end weight is bolstered by the duplex head adapter and the mill's support.

[rudd]

Having both horizontals and a vertical, I have to think that the cutting forces on an overarm holding a 3/4" EM are going to much much less than the forces imposed by a horizontal slabbing cutter or even a 6" dia. x 1/2"W horizontal mill cutter.