The first thing you need to do is to determine the actual starting location of a bend produced by the Bending Die you installed
in the bender. This can vary between die sets and must be checked for every die set purchased. In the below example we are
using 1 1/2" O.D. tubing and a Bending Die with a Center Line Radius of 6 1/2".
Here's the procedure:
A) Place a piece of tubing (app. 2 1/2' long) into the
bender so that exactly 12" extends out from the
edge of the die to the end of the tubing when
the tubing is fully seated in the Bending Die's
groove. Place a little bending pressure on the
tube so as to seat the tubing in the Bending
Die. Not enough to start bending the tubing just
enough to seat it in the groove.
NOTE: If you
lay a small length of tubing in the groove of a
Bending Die you will notice the tubing does not
seat to the bottom of the groove. The Bending
Dies are deliberately machined this way so
that during the bending operation a side force
is developed in the tubing. This helps to reduce
flat spotting and wrinkles.
B) Using a Black Magic Marker mark a line on
the tubing precisely at the edge of the die. See
figure 43.
C) Bend the tube to an exact 90 degrees. Use a
carpenters square to check the angle. You will
have to over bend the tube a little to account for
springback. How much to over bend will come
with practice. If you over bend the tube a little don't worry. Because cold worked steel has memory, you can place the
tube in a vise or anything else that will retain it, and simply unbend it. Obviously this only works for small amounts of
over bend. If the tubing is under bent, it will be necessary to put it back into the bender.
D) With the tube bent correctly to 90 degrees locate the actual start of the bend. To do this, measure from the end of the
tube to the far end of the 90 degree bend. In the example in figure 44 this came out at 20 1/4". Subtract 6 1/2" for the
centerline radius (CLR) of the Bending Die, another 3/4" for the radius of the tubing not seated in the die, and 1/8" for
springback. (Substitute the CLR and tube radius to match your die set). The 1/8" figure for springback is an approxi-
mation, not an exact figure. However it is usually very close to the real thing and may be used without worry to determine
the actual starting location of the bend. So:
20 1/4" - 6 1/2" - 3/4" - 1/8" = 12 7/8"
Now subtract from the 12 7/8" the original 12" we had
marked earlier and you find that the bend will actually
start 7/8" in from the edge of the bending die. Now we
know for example, if we want 40" from the end of the
tubing to the start of the bend, we must subtract 7/8"
from 40" and set the tubing 39 1/8" from the edge of
the Bending Die.
Another ex ample, you want 36" from the bottom to the
top of a rollbar. Tube size is 1 3/4" and you have an
actual bend start 1/2" inside of the Bending Die's edge.
The CLR of the Bending Die is 7 1/2". So: 36" - 1/2"
(Actual Bend start) - 7 1/2" (CLR of die) - 7/8" (Half
of the tubing diameter) - 1/8" (Spring back) = 27". Set
the tube 27" from the edge of the Bending Die and
make the bend.
Bending Method #2 - Using Math Formulas
- 15-
Figure 43
Figure 44
Angled groove
closest to the
forming die
Straight
groove
Actual start
of bend
6 1/2" CLR
7/8"
20 1/4"
12"
Mark
