How was this machined?

Dick Streff · Jan 15, 2007

I recently got a band saw with a broken trunnion. I know how I'm going to go about repairing it but it got me thinking while I was looking it over how the heck did they machine it in the first place.

Here are some photos of the piece in question:

Side view
Opposite side view.

A semi-circular groove isn't any trick, but I can't wrap my head around how they would have machined those ways inside that 2 3/4" space. I'd be surprised if they had made any one off machinery just for the purpose. And by looking at the bottoms of the semi-circular grooves it doesn't to appear to have been done by any endmill type cutter. Is there some kind of broaching process for cuts like this?

Just curious,
Dick

MitsTech · Jan 15, 2007

Actually it looks a lot like it was cut with a straight flute T-slot type cutter.

A vertical mill with a vertical rotary table, a slotting cutter, and you're good to go!

agrip · Jan 15, 2007

A face plate on a lathe, equipped with several angle plates that have position stops for the bolt face.

Now you can turn 6 or 8 at once.

Using a boring bar with an off set trepan rig, that is rigged for cuting both in the carriage advance or retro direction.

Cuts both slots in one setup.

The chatter marks in the slots are a giveaway.

Hth Ag

gbent · Jan 15, 2007

I'll vote for a cutter on a big horizontal mill arbor. Take a big side milling cutter, and relieve the inside. Run about 5 rpm, but lots of teeth. Load/unload is longer than cutting time.

Mark John · Jan 15, 2007

Looks like a skim job. The slot was cast and they just machined for flatness. Looks like a large radis in the corner.

My experience is t-slot cutter and rotary table.

JRIowa · Jan 15, 2007

T-slot or milling cutters! No way!

Probably would have been done on a horizontal mill with a special cutter. The cutter would have to be cup shaped on each side or like a fly cutter with tools sticking out both sides. You need to put both grooves in on the same set-up to keep them parallel. On a horizontal, you could put 2 parts on the table and be machining one while changing the other.

agrip also hit it. That could have been done on a lathe. Probably a turret lathe. All that you would need is a special tool holder, an angle driver, and you could cut both grooves in the same set-up.

You might be able to do them on a jig-mill with a special bar.

Most depends on the quantity that they made. Nobody is going to invest a lot in fixtures or tooling for 500 parts/year. Special machine - probably not.

JR

tools · Jan 15, 2007

I agree the slots were probably cast. Easy to clean them up with a specially made fly cutter and form tool.

The tool would govern the radius, once that is established, the rest isn't very important.

Tools

stephen thomas · Jan 15, 2007

I've studied a few and am pretty sure it was done on a lathe. Some kind of setup as has already been mentioned with a faceplate and angle irons to locate the part, then with "the biggest dia honking boring bar possible". The bar would have to have a cross slide on the nose to adjust radius, with the tool projecting forwards and backwards so the grooves could be machined with alternate motions of the carriage.

When it comes to woodworking equipment, a lot of the old shops seemed to have done nearly everything possible on lathes & shapers.

smt

Damien W · Jan 15, 2007

How about a vertical borer. I can't see any sort of milling cutter being successful. I can't see both slots being machined at the same time. The base and sides must be datums for a fixture, the job is rotated to present each side of the casting to a heavy single point offset tool with suitable clearance for the radius.
What about those machining marks? They do suggest chatter?

Pete in NC · Jan 15, 2007

Wouldn't a lathe leave cutting marks in the other axis in the slots?
whilst a T slot mill in a hurry would leave the marks shown?

fen2art · Jan 15, 2007

Pete

T-slot mill would not leave square sides shown on piece.

Phil

Damien W · Jan 15, 2007

A flycutter or boring and facing head in a vertical mill with the part fitted to a rotary table also sounds reasonable but for higher metal removal rate I am still opting for a vertical borer. Either way it is still a single point tool.

In the picture of the side view there are what appear to be 2 ground pads near the base. These could be datums for fixturing.

Racer Al · Jan 15, 2007

Clearly, they used one of those handy flexible end mills.

Just kidding, of course.

As mentioned above, the giveaway to the OEM method is the chatter marks in the bottom of the groove. It cut all three sides of the groove at once with a big, flat-faced single-point cutter, probably ganged up on a big faceplate.

If I were to do a one-off job using my flimsy HSM equipment, I'd start by determining the diameter of the groove, then welding up a "T" shaped boring bar.

The length of the cross bar of the "T" will be a little longer than the diameter of the groove. One side of the "T" will be drilled to accept a 1/4" lathe bit & a perpendicular hole tapped for a hold-down bolt. The other side of the crossbar is there just for balance.

The lathe tool would need quite a bit of "side" clearance since it's working like an axial plunge cut.

The vertical leg of the "T" will fit your micrometer boring bar holder for your vertical mill. You might find that the boring bar holder won't hold the "T" bar tight enough, and I don't know any solutions for that off the top of my head.

I might be tempted to make the crossbar out of hefty stock to reduce flexing, but it's probably doomed to microscopically light cuts no matter how stiff you make the "T" bar. Since the cutter is waaaaay out there, it needs to run at a very slow spindle speed.

Oh, yeah, keep your fingers well away from the whirly things. I might even suggest setting up a temporary plexi/lexan shield because it would be *super* easy to forget how big the swept area of the cutter is.

Mike C. · Jan 15, 2007

I'm with the horizontal mill and cupped cutter. The slots could have cast in, but that's at least a triple roll mold and two or three piece pattern to manage to draw the pattern in green sand. A big horizontal mill, two side mills back to back, right and left hand. Both the parts on the table in a jig, traverse to the first. Power feed in to the stop, power feed rev to the stop. Rapid back to the middle. Next.

tools · Jan 15, 2007

That is what I was thinking Mike. However, even easier would be a large dia (2" or so) arbor. Mount a standard (easily as large as 1 to 1 1/2" dia) boring bar with the cutter at 90". The cutter would be the same size as the slot, relief ground all three sides, essentially a form tool.

The distance from the arbor to the cutter determines the radius, so easily adjustable. The overall length of the cutter must be narrower than the opening, but long enough and ground on both ends so you don't have to rejig the part to do both slots. Just like Mike said, y axis in for one, out for the other.

Would be cheap, rigid enough, and would likely leave those chatter marks. Could EASILY be shop fabricated by any one of us, doesn't need rotary tables or anything "fancy".

Maybe this is more easily described as swinging a boring bar like a flycutter from an arbor in a horiz mill. The arbor would have to have a provision in which to mount the boring bar. Perphaps some mild steel mounted in a large diameter end mill holder, the far end could ride in a bushing in the arbor support.

Tools

Tools

stephen thomas · Jan 15, 2007

Well, heck, then on a large lathe, add tailstock support to the arbor, mount the part on your tee-slot carriage, and run the carriage back & forth stop to stop.

smt

BadDog · Jan 15, 2007

I've got an "offset head" that could easily do the cleanup on a cast slot like that using a simple end mill and a vertical mill (fits my Bridgeport).

Set the part sideways on a horizontal rotab with axis at arc C/L, set up offset head, locate cutter, swing arc using rotab, flip for second side OR set the end mill on the other side of the offset head and cut 2 without breaking setup (to ensure coaxial arcs).

awake · Jan 15, 2007

You all are overlooking the obvious solution. SMT has already hinted at it:

When it comes to woodworking equipment, a lot of the old shops seemed to have done nearly everything possible on lathes & shapers.

Obviously, they used a rotary shaper -- like a regular shaper, except that it makes a curved stroke. Simple, eh?

tools · Jan 15, 2007

Rotary shaper, eh? Hmmm...

I don't know what the heck that is, BUT I WANT ONE!

Tools

FranH · Jan 15, 2007

In the days before CNC, there were circumferential broaches used in the manufacture of jet engine blades and vanes.

Certainly could easily be used here, but cost for a dedicated machine tool for one operation, one part is expensive.

How was this machined?

Aluminum

Stainless

Cast Iron

Diamond

Aluminum

Diamond

Hot Rolled

Diamond

Stainless

Aluminum

Cast Iron

Stainless

Stainless

Diamond

Hot Rolled

Diamond

Stainless

Titanium

Hot Rolled

Hot Rolled

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