Wrinkles are caused by the tubing not being fully supported on the compression side (inside of bend). While anything you can do to better support the tube (shaping a nice groove in the die, filling with sand or cerro bend) will help, it's just plain hard to bend larger diameters of thin wall tubing without wrinkles.
In 4130, anything with a diameter:wall thickness ratio over 20 is getting into tough territory. I would plan on wrinkling a few tubes before getting things right. I wouldn't attempt anything with a ratio over 30 without a very fancy mandrel bender.
Professional tubing benders are expensive because they control so many aspects of the bending process at once (tubing support, clamping, smooth, steady bending action, accurate & consistent die alignment). If you're doing very many bends, though, they are worth every penny. Here's a checklist of things to try when bending tubing:
1. Anything you can do to make your bending jig more rigid. 4130 is very strong. If the jig breaks or shifts, the bend won't be even and will be more likely to wrinkle.
2. Support the tubing fully and evenly (nice groove in bending die, no gaps between clamps/braces and die, no sudden or sharp edges touching tube).
3. Ensure the tubing is well clamped and doesn't slide or slip. This is an important secret. Tubing doesn't want to bend evenly. It wants to crumple, focussing all the bending at one point. If it can't slip, it's harder for all the bending to happen in one place (one end or other of the tubing has to slide to do this). Imagine holding a firm tension on the tube as you bend it.
4. Fill with sand (preferably tamped wet sand; you'll have to do this 1 or 2 inches at a time). I've never been able to find low temperature metal alloy bending material (cerro bend?), but it sounds like good stuff.
5. Arrange enough leverage to enable you to bend slowly and steadily. Repeated lunging and grunting tends to crumple rather than bend.
6. Large radius bends are a lot easier to do without wrinkling than small radius bends. Figure anything with a bend diameter:tubing diameter ratio under 8 or so will be difficult.
--Pat Franz
TerraCycle Inc.
503-231-9798 (USA)
Minnesota Human Powered Vehicle Assn.
Subject: Re: Lowracer drawing/Large Tube Joint Design
Warren's photo and Felix's drawing show large diameter tubes that appear to have been butt welded at an angle. Given the difficulty of getting large diameter, thin wall tube bent this makes sense. However, unless you have inserted internal diaphragms to prevent the resulting elliptical joint from distorting, this could lead to failure.
I learned about this from Peter Ross the British builder of Trice trikes and Ross Recumbents including the Festinal Low Racer. He was answering a question about bending large, thinwall tubes.
Peter wrote;
Bending large diameter tubes with thin wall thickness is a very specialised job, and in the United Kingdom there are not many companies that will do it.
The company must have a machine which draws an internal mandrel down the tube at the exact point where it is being bent.
For many years we could not find anyone to do it, and our Trice was built using straight tube with an angle. This is OK but it does not look as good. If you do it with straight tube you MUST put a diaphragm inside the joint, otherwise the tube will 'lozenge' at the joint when it bends and this will rapidly lead to fatigue failure.
A diaphragm can be made of very thin sheet, and is just an elliptical shaped piece of metal which fills the tube at the joint (like a wall across the tube.)
I tack with gas weld, and take the tube to my local TIG welder who makes sure his weld penetrates the tube and creates a sound joint. TIG welding also looks neater than most gas welding.
You have also to ensure that the steel you are using has properties which allow welding without creating cracks.
I asked him;
Is the diaphragm slightly larger or smaller than the tube?
Peter replied;
The diaphragm is very slightly larger to allow metal to be melted back to the original diameter. You could make it the same size and use more filler rod when you weld. The important thing is to ensure that all three components are fused together.
With a pro welder, the larger diaphragm would certainly save time. Given my limited welding skill, I'd opt for a smaller one and filler rod to ensure penetration.
BTW the tubes Peter was joining this way were 50mm diameter and 1mm (0.040 inches) wall. My guess is that they were Reynolds Chrome Manganese instead of the U.S. 4130 Chrome Molybdenum.
Whether or not you can get away without such a diaphragm would depend on the angle that the tubes meet at, and the thickness/diameter ratio.
From: Marten Gerritsen Factors influencing the formation of wrinkles are the tube thickness to diameter ratio and the curve of the bend. Tight bends in thin-wall tubing (< 20:1) are therefore more difficult to obtain than bends with large radii. I've seen mild tubing being bent over a flat circular form (for Dutch roadster ladybikes), this is basically the same process as used in bending forkblades. For higher quality tubing I suppose it is more usual to bend the tube between three rollers. For tight bends use a mandrell bender.
Freebending with heat (fill the tube with -dry-blasting shot, weld the ends shut and use the biggest torch you can find) is OK for exhausts, but I wouldn't recommend it for bicycle frames.
WWW: HTTP://tubus.nl