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Mcgyver
01-12-2012, 04:18 PM
Anybody know, or able to point me to the calculation for wire bending and spring back? ie for a given dia of mild steel wire bent through so many degrees, how much smaller should the die be that the desired finish size to account for springback

lots of material out there on sheetmetal, but couldn't find it for wire

thanks

DR
01-12-2012, 05:19 PM
I've never seen anything like that. I always use 7 degree overbend as a starter.

In my experience even the CNC press brakes with bend calculators need a test bend to zero in on the amount of overbend. There are so many variables, condition of metal, speed of bend, etc, etc.

First time I really got involved in thus stuff was with my press brake. We kept sneaking up on the bend angle maybe 5 degrees at a time until we were right on. Set the stops at this point.

Bent the first "good" part and it was way, way off. The reason was, our test bend was done slowly with a series of incremental bends, the good part was done to the stops, but in a continuous stroke.

We've bent a lot of heavy wire on my Di-Acro's. You learn real fast that the bending motion has to be consistent from part to part to maintain part tolerance.

Mcgyver
01-12-2012, 05:24 PM
I've never seen anything like that. I.


hmmmm... I suppose I could turn my die to nominal and then via experiment start turning it down more until its producing the desired size

thanks

becksmachine
01-12-2012, 06:05 PM
Why not use the figures for a corresponding thickness of sheet metal?

Dave

RussZHC
01-12-2012, 06:31 PM
http://bbs.homeshopmachinist.net/showthread.php?t=9993

that thread deals more with winding springs, so not exactly (guessing) what you are after...there does not appear to be a formula (per se) as a lot of discussion in that thread and elsewhere appears to focus on trial and error...found a bit of detail on some of the chain maille sites and can understand how for that process springback becomes of vital importance (gist seems to be too many variables to accurately track or plug into a formula)...I know there are some formulas when dealing with tubing (as in roll cage) but not sure if it would apply (and may only be available if the overall bending program software is purchased)...or if you could say change one of the variables to make it a solid and then shrink it to a wire size (the software may just balk without a wall thickness e.g.)

oldtiffie
01-12-2012, 06:44 PM
http://www.google.com.au/#sclient=psy-ab&hl=en&site=&source=hp&q=wire+bending+techniques&rlz=1W1IRFC_enAU360&pbx=1&oq=wire+bending&aq=3&aqi=g4&aql=&gs_sm=c&gs_upl=2333l5069l0l9847l12l10l0l2l2l0l402l3079l3-8.1l11l0&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=f87e263762c49db9&biw=1280&bih=542

http://www.google.com.au/#hl=en&cp=8&gs_id=w&xhr=t&q=wire+bending&pf=p&sclient=psy-ab&site=&source=hp&rlz=1W1IRFC_enAU360&pbx=1&oq=wire+ben&aq=0&aqi=g4&aql=&gs_sm=&gs_upl=&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=f87e263762c49db9&biw=1280&bih=542

Mcgyver
01-12-2012, 08:03 PM
Why not use the figures for a corresponding thickness of sheet metal?

Dave

that's a good point, I had a mental block because of the different in the depth of what's being bent, which I suppose doesn't really matter....thanks

japcas
01-12-2012, 08:22 PM
I think I misunderstood what you were wanting. I thought you were looking for a formula to calculate mandrel size to produce a certain size diameter spring. Marv Klotz has a utility for that so I'll share it just in case someone else can use it. I think Kozo Hiaroka wrote it and Marv Klotz coded it to make it easier to use. It's called Mandrel.Zip, and it is about 1/3 of the way down the page. Here is the link.

http://www.myvirtualnetwork.com/mklotz/

Mcgyver
01-12-2012, 08:38 PM
I think I misunderstood what you were wanting. I thought you were looking for a formula to calculate mandrel size to produce a certain size diameter spring. Marv Klotz has a utility for that so I'll share it just in case someone else can use it. I think Kozo Hiaroka wrote it and Marv Klotz coded it to make it easier to use. It's called Mandrel.Zip, and it is about 1/3 of the way down the page. Here is the link.

http://www.myvirtualnetwork.com/mklotz/

thanks, I've got the Kozo articles, my application is I want to bend some mild steel wire into 6" arcs and was hoping to engineer it to minimize the futzing around, you turn the die to size and works, without trial and error on the dia springback. That calculation is in a way what i was getting at, but I was thinking it might be quite different for spring wire than mild steel so thought there might be a more generic calculation....Kozo's original work was graphs that for music wire that i suppose Marv has codified

Don Young
01-12-2012, 09:57 PM
One thing I learned about this is that the tension or friction on the wire makes a difference in the springback. I would think the amount of springback with any particular setup would be relatively constant so one test setup should allow a pretty close final arrangement.

J Tiers
01-12-2012, 11:05 PM
Is not the "springback" due to the yield strength?

When you bend any material, you stress it past the yield point. At some point in the bending process, the stress drops below the yield point (after some yielding) and the rest of the bend corresponding to that is all elastic.

If so, then the amount of overbend would be related to the radius etc by the yield strength. With some modification due to work hardening, presumably.

The overbend would be what is required to just reach the yield point... any bend past that would result in some yielding until the residual stress is below the "current" yield strength. And any bending past the yield strength point would result in a "set" to the total angle minus the angle of bend that results in a stress just at the yield point.

Mcgyver
01-12-2012, 11:21 PM
Been reading and learning...spring back is caused by residual tensions. Its dependent on the ratio of radius and thickness, R/t. These form a curve for different materials with spring back varying by the R/t ratio. Spring back is closest to zero at the minimum radius it can be bent and goes up as R/t goes up.

Not accustom to bending large radius of small dia stuff, I didn't appreciate how much sprint back there'd be. Grabbed a 6" disk and piece of 1/8 gas welding rod, clamp things to the mill table and got an arc that with a 4.5" radius!

I also read about tension to reduce spring back, apparently heat is also used to reduce it

always something new to learn

Rich Carlstedt
01-13-2012, 12:17 AM
Both J Tiers and Mcgyver are correct.
The size, yield strength and radius all interplay to give you springback.
We had spring back gauges that were protractors laying flat with a .5 radius arbor , and the steel ( we were using) was clamped and precisely bent around the mandrel to 90 degrees, and then released and the springback measured.
Indeed, we encountered coils of steel that varied .
Your best bet is to set a hardness for your raw material, and then test it as per the above.
Then you can plan your dies around that.
I doubt you will find a formula. I believe it is all testing based
Rich

I am not home, or I would take a pic of the protractor layout...believe I have one laying around

DR
01-13-2012, 06:45 AM
.................................................. ...........................................

Not accustom to bending large radius of small dia stuff, I didn't appreciate how much sprint back there'd be. Grabbed a 6" disk and piece of 1/8 gas welding rod, clamp things to the mill table and got an arc that with a 4.5" radius!

I also read about tension to reduce spring back, apparently heat is also used to reduce it

.................................................. ...



Okay, now I understand the situation better. My previous response was more aimed at simple 90 degree bends, etc.

Using a bender similar to the Di-Acro,

http://www.diacro.com/benders/hand-operated-benders.php

the spring back can be minimized by varying the nose pressure against the center mandrel (6" in your case).

To almost totally eliminate springback use a very high pressure of the bending nose against the material. In other words, the clearance between the bending nose and the mandrel is less than the wire diameter. This will result in "coining" the wire. The coining will leave noticeable flats on the wire,both inside of the bend radius and outside, usually not a problem.

With the coining I'd start with a mandrel a bit less than 6".

No matter how much you "engineer" the setup adjustments will still have to be made for different lots of material, but it's simple to change the nose clearance to accommodate.

We had a large job where we needed to bend 5/16" diameter mild steel into a closed eye with a 3/4" ID. Customer wanted no more than +.005" on the eye diameter. That material ended up with flats from coining of over 1/8" wide. We were able to easily hold size, the center mandrel had to be hear treated though.

J Tiers
01-13-2012, 08:29 AM
the spring back can be minimized by varying the nose pressure against the center mandrel (6" in your case).

To almost totally eliminate springback use a very high pressure of the bending nose against the material. In other words, the clearance between the bending nose and the mandrel is less than the wire diameter. This will result in "coining" the wire. The coining will leave noticeable flats on the wire,both inside of the bend radius and outside, usually not a problem.



That forces nearly all the material cross-section to yield, and leaves almost no less-than-yield stress areas, so there is almost no elastic bending left, and little springback.