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Thread: How do I make tapered tubing?
02-26-2007, 10:44 PM #1
I have a need for a piece of tubing which tapers from 1 1/4 inch OD to 1 inch OD over 16 inches in length. Material would be DOM seamless steel tube, wall thickness would be 0.090. I need six pieces. Normally this would be made by rotary swaging, but generally swaging dies are only 12 inches long so it would take two sets of dies at roughly $2,500 per set. Too pricey for six pieces, probably they wouldn't touch such a small quantity anyway. I thought about using heavy wall tubing and turning the taper on the ID and OD but gosh, a 3/4 inch boring bar at 16 inches would chatter so bad I don't think it would work. Any good ideas?
02-26-2007, 11:38 PM #2
On "How its made" or "How do they do it" lately, I saw a little blurb on how they formed the taper next to the bell of a trombone. They began with a rather crude welded piece of brass, I suppose perhaps it was conical, and they had a nice tapered mandrel to go inside. They had some sort of a drawbench set up, with a heavy lead washer as a 'die'. They pulled the mandrel and the brass through the lead washer. As it was pulled, the lead washer deformed but still had enough pressure to force the brass down tight onto the mandrel.
It might be a lot of trial and error to determine the proper lead alloy that has the correct amount of flow.
Of course, the lead washer was single use. It was also very heavy to begin with....maybe an inch thick or so.
02-26-2007, 11:43 PM #3
Make a long tapered mandrel and Heat ?? Blacksmith style.
02-27-2007, 12:00 AM #4
If you stretch the metal wall at big end will go down to about .070
May have to start thick, stretch it with mandrel and heat, then turn OD to reestablish wall.
02-27-2007, 12:04 AM #5
I think I'd go about it by figuring out how to jam a tapered mandrel into the tubing under large pressure (or with the tube red-hot).
Of course the heat should also aid in retracting the mandrel too.
02-27-2007, 12:13 AM #6
spin it, that is how baseball bats are made. how are metal power poles made?
02-27-2007, 12:14 AM #7
If the metal went down to 0.070 at the large end I could live with that. I tried jaming a tapered mandrel in but no joy. I used a 40 ton H frame press which wanted to crumple the bottom end until the whole mess started to bend. Also tried the process with heat but I couldn't get the tube to uniform heat with a torch. Maybe with a tube furnace but gosh. I'd have to mount the tube furnace vertical in the press, seems like potential disaster. It takes incredible force to expand metal this way. I'm more hopeful for a machining process at this point.
02-27-2007, 12:17 AM #8
Maybe make a tapered boring bar. Does the wall thickness have to be even along the whole tube?
02-27-2007, 12:26 AM #9
The tapered boring bar idea isn't too bad. It would have to go from 1 inch down to 3/4 inch, so would probably still chatter. I imagine some sort of bushing that supports the bar inside the tube but don't really see how this works with a tapered ID. As far as spinning I don't know. How exactly? Three rollers that would surround the tube and somehow crush it as it spins? I think this would take unbelievable force. I can roll machinery around on 0.090 tube with out crushing it. Spinning bowls out of 0.040 annealed aluminum is one thing, this is different.
02-27-2007, 12:54 AM #10
Ahh, a small problem. Must pull the mandrel into the tube. I'd heat the tube at least to a dull red, weedburner torch pointed into a semi-enclosure of firebrick.
Automotive camshafts are made (assembled) by fixturing lubes upon a steel tube. A die-ball is pulled thru the tube, upsetting metal all the way thru and expanding it enough to lock it to the lobes evermore.
02-27-2007, 01:18 AM #11
If it was easy, brass musical instruments would be cheap!
The way any tapered tube on a brass musical instrument is made is to start with a piece of flat stock cut on a taper, use a mandrel to shape the tube then weld or braze the joint and flatten it to match the thickness of the rest of the tube. If you look carefully at the bell of a tuba or low brass instrument, the joint appears like a line of interlocking 1/8" rectangles with a filler metal in the very small gaps. Even very old brass bells have this appearance so the technique has been in use over 150 years.
02-27-2007, 01:40 AM #12
Would there be some way to hold the boring bar between centers (to avoid chatter), and somehow slowly spin the tube while carried along on the carriage?? I'm sure Rube Goldberg could come up with something ...
02-27-2007, 01:41 AM #13
Just a crazy idea but could you build up metal spray over a tapered mandrel?
02-27-2007, 01:51 AM #14
I would think a gun barrel manufacturer would be able to do this cheaply and have all the tool on hand to make it. You could sub it out and make 15% profit?
02-27-2007, 02:11 AM #15
Could wire EDM do this?
02-27-2007, 02:11 AM #16
A decent blacksmith could get you close without much sweat, and you could clean it up from there.
02-27-2007, 02:16 AM #17
Here are a few ideas. Make a die by boreing a piece of steel with the corect taper for the outside of the finished tube. In this case 1 1/4" to 1" 16" long. Cap the ends of a piece of 1" tube and slide it into the bored hole. A little black powder inside the tube should do the trick. Or the tube could be heated and air pressure applied. How about pushing a progressive series of short tapered mandrels into the tube until filled? That way you are not trying to expand the whole 16" in one shot. This could be done inside the above mentioned die if straightness became a problem. Or, Bore all of the short tapered mandreles, to maybe 3/4", and use a piece of 3/4" bar inside as a guide. Maybe fill the tube with water, put it in the die and freeze and thaw it a few times.
02-27-2007, 02:21 AM #18
Could wire EDM do this?
4 axis could , BUT 16", the only wire I ever saw that could do 16"s , was a machine that had been ste.....ched , and the 4 axis was no longer able to respond because of distorted geometry ...
I met the guy who stretched it .....very impresive work , even though we don't like each-other a bit !!! gotta give it when it's due !
P.S. surely there is factory WIRE @ 16" somewhere , No?
02-27-2007, 06:50 AM #19
Can't you just use 1 1/4" dom tube, 17" long... slice a wedge out of it with a small angle grinder (superthin disc)...squeeze together (prefereably over a mandrel, of aluminum) and weld a new seam. Grind to desired finish and chop ends true.
A lot of work I know as I've done similar, but not compared to anything else mentioned. And a lot easier than doing it from sheet metal with rollers.
Better alternative: Go find 'em someplace. Baseball bats, furniture legs, barbecue stands, lampposts... I dunno. Take your calipers and go on a scavenger hunt. They must be out there someplace.
PS: Sure they gotta be solid? Who says, and who will know the difference?
02-27-2007, 10:35 AM #20
What about making a die and forming the taper in two 180 degree segments and welding them together. Someone else may be able to add about using a copper backing bar for full penetration, but without having a protrusion.