[JackOfDiamonds]

I've built a lot of frames and have some nice equipment to make life easier. However it seems like the final frontier that I'm missing is the ability to make good smooth tight bends in chromoly.

I have SwageLok benders for 1/2 and 3/8 tubing which works down to 0.035 pretty well, but I can't bend anything with a thinner diameter / thickness ratio.

I did some research and it seems like this is possible only with a special bender with an internal mandrel. This is probably not something I could make or buy for a small shop.

So, I just don't make bends. If I can't do it with a very gradual bend, then I have to cut it off and braze an angle joint. But I'm starting to get annoyed at the limitations. I want to be able to make uni-crown forks and swoopy brake bridges and beach cruisers with step-through top-tubes and other things like that.

Are there any companies out there that will do CNC bending for small batches?

 

[Little_twin]

Bike fab supply can do custom bends for you in most common diameters. You can also buy Cobra frames tubing bender which is designed specifically for thin wall bike tubing.


Sent from my iPhone using Tapatalk

 

[GrayJay]

2-year old thread on frame tube bending that is worth reviewing;
https://forums.mtbr.com/frame-building/how-bend-1-375-x-0-035-tube-1076551.html#post13665904

Have you tried using tightly supportive wooden forms and ice or sand internal support?

 

[Eric Malcolm]

A few years ago, a hobby builder who built a cruiser style MTB using the name : Trailmaker. He posted good detail on tube bending and it would be worth your while studying his posts.
His cruiser was a rather awesome, 26" fat bike.

Eric

 

[mickuk]

I've tried ice, sand and home made foundry sand which is sand + bentonite clay (cat litter made into powder with the cheapest possible Wallmart blender). Ice didn't seem to work at all. The others were slightly better than an empty tube but nothing like proper mandrel bending.

 

[Halekai]

I've built a lot of frames and have some nice equipment to make life easier. However it seems like the final frontier that I'm missing is the ability to make good smooth tight bends in chromoly.

I have SwageLok benders for 1/2 and 3/8 tubing which works down to 0.035 pretty well, but I can't bend anything with a thinner diameter / thickness ratio.

I did some research and it seems like this is possible only with a special bender with an internal mandrel. This is probably not something I could make or buy for a small shop.

So, I just don't make bends. If I can't do it with a very gradual bend, then I have to cut it off and braze an angle joint. But I'm starting to get annoyed at the limitations. I want to be able to make uni-crown forks and swoopy brake bridges and beach cruisers with step-through top-tubes and other things like that.

Are there any companies out there that will do CNC bending for small batches?

I am picking up what you are laying down. There are a lot of well known builders out there who have mediocre bends in their builds. artifacts, kinks, flat spots assymetry etc.

I have put a ton of effort, time and expense into learning how to bend thin wall tube without packing or heating etc.

I can bend 1/2" x 0.028 at a 2.25" Radius (truss fork work, wishbone seat stays, bridges)
...3/8" x 0.028 at 1.25" Radius (Seat stays wishbones and fork truss work and racks) Ive never seen anyone else pull that off. I made a custom bender that cradles the tubing on exit to eliminate deforming.
...7/8" x 0.035 at a 6" radius (chainstays, fork blades and H-Bars)
1" x 0.035 I can work into a perfect circle that is 22" across and with a perfectly round crossection.

I may be able to get tighter on all of these but they work for what I am doing and dies can be expensive. I have over $1k in dies that proved to be too ambitious and that Ill never use.

I have a JD squared model 32 bender that I use for the smaller tubes up to 7/8"

For 1"-2" I us a Harbor Freight roller bender with some custom shop made dies and other custom dies made by SWAG Offroad. You can do perfect bent seat tubes as well as smooth arc'ed top tubes etc. The drawback of the HF bender is that there can be some "wasted tubing" because there is an entry and exit on each end of your tube that does not see the roller pressure. Even this can be minimized with some mods to your bender. I have also used this bender with large diameter dies to bend and form smaller diameter tubing. To flatten the profile of chainstays for tire clearance. I made one unicrown fork a long time ago with this method.

I also have a Diacro #2 that I plan on using for 3/4" tighter unicrown bends when time permits. With mods, you can use JD-2 dies on your Diacro bender. The Diacro is a superior tool. I plan to use it more in the future but dies are expensive. I am planning to send Diacro a cache of tubing of all sizes and wall thickness and they agreed to see what they can get away with. Ill post that info when I get it compiled.

There is so much going on with bending thin walled tubing...entry and exit profiles, follow blocks, slippage, lubrication, speed of bend, too much for this entry but If you want to speak to me directly feel free to PM me...

If you want to see some of the bends Ive done on real machines...check out my Instagram... @actualsurfer

Cheers and good luck.