First time sand bending tubing
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  1. #1
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    Default First time sand bending tubing

    I decided to try sand bending some DOM tubing as a learning exercise for a future project. The tubing is 5/8" diameter, 0.120" wall thickness. I needed an inside radius of about 1/4" (if possible).

    I made some wood plugs to cap the tubing, packed tight with dry sand, clamped the tubing in a vice against a piece of 1/2" diameter steel rod. I heated the tubing with an OA torch and bent away. The results were poor. I'm pretty sure I know why, but would like any feedback anyone can offer. First problem is that I was using a tinted face shield and working outside. I think I was supposed to heat to dull red. Being the the tubing is small, I got to orange pretty quickly (did not see dull red). Second problem is that with the tubing in the vice horizontally I think I heated the crap out of one side and did not heat the other side quick enough. The photos show significant distortion at one side of the outside of the bend and cracking at the other side.

    I would appreciate any general input of heating methods. BTW, I was using a rather large cutting nozzle (don't recall the size, but probably rated to cut 1/2" steel-I need to buy a rosebud). I was using what I think was a neutral flame (maybe even a bit carburizing? and not using extra oxygen.
    Attached Thumbnails Attached Thumbnails heat-bending-resized.jpg   heat-bending-1-resized.jpg  

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    I would start by welding caps on the end rather than wooden plugs. Also with the tight of a radius it will have to be done in several steps cutting it open and adding more sand each time as the volume of the tube is increasing as the outer radius thins. Many years ago I had to make hairpin bends with boiler tubing during an emergency repair and I can tell you it is far from a one shot deal.

    Also bend radius is generally measured from the center line, CLR. So in theory a 1" tube bent 180 degrees on a 1" CLR would be touching itself. Bending to a CLR that is tighter than 1.5 times the diameter can be a real project.

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    The sand contains voids which compress and spoil your bend. To cold-bend it, try a low-melting-point alloy and thread caps into the ends. To bend it hot freehand, don't heat the outside of the bend, heat the inside. But in neither case is that tight a radius possible without flattening the cross section.

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    If I really needed to do a bend like that, I wouldn't.

    What I would do is machine however many bend elements are needed from a appropriate steel using a rotary table and lollypop cutters, then have a competent welder TIG them into place on the cut tubes. At worst, there'd be two 90*s for every complete 180, so three welds per 180*

    From a stress and thermal effects standpoint, can't be worse than trying to heat-bend the tube itself...

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    I questioned whether I could get right of a bend. I was happy with the inside radius and thought that the outside would have been better with even heating. I did apply the heat to mostly the outside. I will try it again. but I am concerned that the bend is too tight.

    I had been thinking about making the bend with solid 5/8 rod and then welding that to the tubing. Since the tubing is only about two feet long I guess the simple approach would be to use solid rod for the entire piece. Not much difference in weight.

    Do you think that this bend with solid rod would work ? The material is low carbon steel (1020?).

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    Solid stock would make for a nice bend heated to dull red..

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    So this is a mechanical application, not a fluid or gas passage one? If that's the case, go solid. If weight is an issue, make a 180* solid end form (you have CNC milling capacity?) and drill relief holes in the two ends of the "U" to lighten and make thermal mass roughly the same as the tubes you'll be welding on.

    Now you have the actual geometry you wanted and almost the same weight as tube, with just two simple welds.

    I'm willing to bet that even with solid rod you'll get a flattening or other distortions making that tight a bend, so if looks are important I'd stay with a machined "U" that can be profiled with a ball endmill pretty easily with decent CAD and some sanding after cutting.

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    Inteerrestingah snippet from the Rooskis….
    Attached Thumbnails Attached Thumbnails russian-tubebending.jpg  

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    Not to rain on your parade but if you want nice clean bends I would suggest having a shop with the proper equipment make all your bends and then do the rest of the fab yourself.

    A while back I needed a smooth bend in .120 wall square tubing to fit into a trailer hitch. I got a quote and had that part done by a shop that specialized in tube bending. The cost was actually quite reasonable. The result is neat and strong and I was able to complete all the rest without the hassle and worry of improvised bending.

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    This is a mechanical application. Its for a future project that I am just playing around with so far. I want to build a close replica of an antique motorcycle. I have not decided yet whether it will be for display only or if it will be functioning, but very little actual use.

    I have a SB 9 inch lathe, OA torch, 120 volt mig, 220 amp ac stick, and 200 amp ac/dc tig. Any other fab work is by way of angle grinders, Sawsall, Portaband, etc.

    As a learning experience I'll probably try bending the solid rod. I have a scrap piece.

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    I tried bending solid 5/8" diameter rod today. The radius is a little bigger than my attempt with the tubing, mainly because I clamped the 1/2" diameter rod I was bending against a little too far into the vice and heated the rod a little further out. It bent without much distortion. There is a little dimple where the 1/2" rod was placed. I think I can get a tighter bend without much problem.

    I will probably try the tubing again just for the experience, but the solid rod seems like the way to go.
    Attached Thumbnails Attached Thumbnails bending-solid-rod-2-resized.jpg   bending-solid-rod-1-resized.jpg  

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    If you have a tube bender that will bend it, fill it with water and freeze it solid. You wont be able to heat it though.
    If you do sand and heat again, heat the inside of the bend so the material can compress.
    You really need to pack the sand in tight, use a rod that just fits in the tube and a hammer to pound sand. Also on the solid heat more on the inside, if you can.

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    I anneal the tube and fill it with Woods metal....works every time...Phil

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    When I need a bend close to the end of the finished piece. I get a piece of stock a bit too long, then make the bend as I can with stock to spare on both ends. Then cut off the excess length to finalize the part.

    For what you show, I would cold bend that in a vee block. over a bending anvil ( a piece of round bar) in a 20 ton hydraulic press.

    I did just that only a few days ago making a spacing bar for the tractor. ;-)

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    Quote Originally Posted by Rob F. View Post
    heat the inside of the bend so the material can compress.
    This is something that my dad taught me when I was a teenager. He explained that it made a lot stronger bend than if you heat the whole pipe or especially if you mostly heat just the outside and it stretches it thin.

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    Forget about bending DOM at that diameter and radius. ERW will do better but the best is to get your hands on some ASTM J524 or J525. Sometimes called hydraulic tubing those products are made for bending. You will be amazed at the difference. Pieces are often available on ebay, get one and try it out.

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    Big B, I have read that in the past, but did not recall it when I was heating.
    adh2000, that is good information to know.
    CalG, one of these days I will buy or build a hydraulic press. I have little space for one presently. I need to convert one bay of my garage from an office back to a garage to get more space.


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