# Is there a formula for calculating springback on mandrel bending 3003?

• posted

We are building a mandrel to bend a 3" rad(6" dia.) on a 14" long piece of 1/8"

3003. Problem we are encountering is the amount of springback opens the piece back up to 7 1/4" dia. Even when held in place and heated to relieve tension the part still has more spring than we would have thought. This doesn't happen when we spin it but in this particualr case we need to mandrel bend the part. Any help would be greatly appreciated. We are also planning to bend other rads so trying to build the right diameter mandrel first time is essential.
• posted

I don't know a general formula and also don't know of a source of appropriate constants if you had a formula.

Here are some lines from Marvin Klotz's MANDREL.C mandrel size calculator program, which has wt = 0 for music wire and wt = 1 for phosphorus bronze:

/* MANDREL.C M. W. Klotz 2/05 Computing mandrel size for spring winding. Based on Kozo Hiraoka's article in "Home Shop Machinist", July/August 1987, pg. 30. */ ... int wt; //wire type index dbl c0[]={0.980364,0.012436}; //constant coefficient dbl c1[]={-0.012436,-0.11018}; //first order coefficient dbl id; //spring internal diameter (in) dbl ds; //average spring diameter (in) dbl dw; //wire diameter (in) dbl fact; //empirical factor (nd) dbl dm; //mandrel diameter (in) ... ds=id+dw; //spring average diameter fact=c0[wt]+c1[wt]*ds/dw; //empirical factor dm=fact*ds-dw; //mandrel diameter

You could set up a handful of different mandrel sizes, eg {3, 4, 5, 6, 7} inches, bend a piece around each, and from the resulting measurements see if the model applies and what the constants should be. The bend you mentioned, with a 6" diameter mandrel giving a 7.25" diameter piece, gives a data point with dm=6, dw=.125, and ds = 7.25 + dw (if the 7.25" was an inside measurement).

I assume you are bending a bar or plate instead of a wire but don't know if the model needs to be adjusted because of that.

• posted

Thanks. We are already making smaller mandrels to find what works. Only problem with this is it is cost prohibitive when we have to pay a machine shop to do so. However, if we find some ratio that is common for our part we will then be able to get other change parts much closer on the first attempt. Kindest regards, Randy.

• posted

Just remember with smaller mandrels , the springback is less. As long as y ou are using the same rod , you should be able to plot the amount of spring back for three mandrels and be able to predict the springback for other siz ed mandrels. Couldn't you use some stock sized pipe or tubing to see how m uch springback you get at various diameters to get real close to the right size of mandrel needed. How precision do you need the finished product?

Dan

• posted

-- Thanks Dan, but we are bending 1/8" sheet. Piece is 14x18 x 1/8. The centerline of the bend will be at the halfway point of the 18" side. We will plot as suggested as there are several different pieces (diameters) we will eventually need to make. Using some off the shelf pipe is an excellent suggestion. The shop should have plenty of end cuts laying around. Randy.

• posted

You also should be able to use say 1 inch wide strips of 3003 to get how much springback there is. I see no reason to use a wide sheet.

Dan

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