Roll Forming
Basic Roll Design
BASIC ROLL-FORM TOOLING DESIGN:
PROCESS OVERVIEW, INSTRUCTION AND TIPS
1.0
INTRODUCTION
This paper is intended to provide “basic”
Roll Forming
Roll Tooling
design instruction and tips for the beginning or novice
designer. In my experience, the ideas expressed in this
report can be applied to many situations but I will stop
short of claiming that these are the only solutions. Many
roll designers have different approaches to both design and
development (testing of tooling), and amazingly enough,
there is more than one way to successfully form a profile.
However, there are common design practices that must be
understood and applied to the design process, coupled with a
certain degree of “magic” (gained through previous
experiences).
In other words,
a mixture of science and art is still very prevalent today
in this industry. For a
Roll Tooling
design, considering all the variables and
applying “theory” whenever possible is vital in producing a
good running product, with minimal start-up problems.
Because the roll forming process has a seemingly limitless
capacity to produce complex profiles with just as many
variables, a high percentage of designs are one-of-a-kind.
This is why theory can only go so far. Past experiences,
correctly applied, are still the key ingredients to the
design and development of
these applications.
More commonly
formed, or basic shapes, tend to have fewer variables and
limitations, which in turn have higher levels of results
which are both theoretical and repeatable. With this in
mind, we will only scratch the surface in this “basic”
design guide. The basics are essential for the beginning
designer.
2.0
DESIGN PROCESS OVERVIEW
For each new tooling requirement, the
designer should use the following steps:
- Develop a cross-sectional
drawing (with part, material and mill specifications /
tolerances).
- Calculate the Estimated
Strip Width.
- Produce a Bend Progression
or “Flower”.
- Layout and design the
Roll Tooling
around the flower.
- Incorporate fixtures,
guides, side rolls and straightening devices where
applicable.
After the cross-section has been finalized,
and before the rolls can be designed, the proper number of
passes and rolling mill must be determined. At the same
time, other questions need to be answered such as:
- Steel type and grade.
- Pre-notching, pre-cut
lengths, multiple gages.
- Combination sets.
- Secondary processes, which
can limit roll design options.
3.0
CALCULATING THE ESTIMATED STRIP WIDTH
Probably the most common problem associated with designing
Roll Tooling
is predicting how a forming bend will react during the
rolling process. Using the proper “K” factor or “Bend
Allowance” is the main item the designer must consider when
calculating a strip width. Various factors contribute to the
bend allowance such as, material type and yield strength,
profile characteristics such as large radii or 180-degree
bends and roll design techniques.
When calculating strip widths and designing
rolls, the maximum thickness within the gage range is
generally used. This is done to eliminate interference
between the male and female rolls when the material is
passing through. Unfortunately, many companies purchase
their raw materials to the low side of the thickness range
to net more footage per ton, which leads to a lesser quality
cross-section (poor dimensional and bend characteristics).
The use of restricted tolerance material will greatly
improve part consistency but is rarely done due to increased
costs for
raw materials.
FIG #1:
C-CHANNEL CROSS-SECTION WITH MATERIAL AND MILL SPECS
First, consider
the material to be formed. Using steels with yield strengths
of 30-55 KSI with forming angles of around 90-degrees and a
1t – 5t inside bend radius, a “k” factor between 35-40% of
the material thickness is most common. Higher yield
materials between 60-85 KSI (with a low elongation
percentage), a factor between 40-55% may be necessary. For
bend angles over 120-degrees and inside bend radius less
than 1t, the “k” factor should be around 50%. Again, these
examples are only a general rule of thumb for typical roll
formed bends.
Note that the
strip widths should always be considered an estimate, and it
is important not to order large quantities of raw material
prior to proving the tooling. There are instances where the
width will need to be modified during testing.
4.0
FLOWER DEVELOPMENT
After
the estimated strip width is calculated, the flower can now
be developed by using the “arc” and “straight” lengths. This
basic C-channel will be the example given. On typical
C-channels, the return leg (bend #2) is the first forming to
be done. I like to form this leg to approximately
70-80 degrees in three passes depending on
the length of the leg. This angle is important because if
formed all the way to its finished angle of 90-degrees,
access to the inside corner of bend #1 will be limited in
the later passes. By keeping bend #2 open 10 to 20 degrees,
one additional pass will have contact with bend #1. The
final 10 to 20 degrees of forming would take place after the
last pass that contacts bend #1.
Forming a bend
that does not have a roll holding the inside corner is
called “air”, “blind” or “ box” forming. When this occurs,
forming the remaining angle from bend #2 helps to reduce
distortion in the radius of bend #1.
FIG #2:
EXAMPLES OF AIR FORMING AND FLOWER WITH ROLL CONTACT
If the bend corner of #2 has a pre-punched
slot or hole, it would be better to finish the bend while #1
is flat (0-degrees forming), because any forming in the
later passes would increase distortion to and around the
notch. However, I would still hold the return leg as shown
above after contact with the inside corner is no longer
possible. This will prevent bend #1 from lifting while
minimizing bend # 2 distortion.
FIG #3:
3D VIEW OF NOTCHED BEND
The chart below
describes where the forming takes place. The angles shown
are how much forming is being done at each pass.
FIG #4:
FLOWER BASED ON CHART WITH CALCULATIONS
PASS
|
BEND 1
|
BEND 2
|
1
|
0
|
28
|
2
|
0
|
23
|
3
|
0 |
19 |
4
|
12
|
0
|
5
|
16
|
0
|
| 6 |
17
|
0
|
7
|
20
|
0
|
8
|
13
|
12
|
9
|
9
|
11
|
10
|
6
|
3
|
*SEE OPTIONAL (12) PASS NOTE.
The following
layout describes how to determine the theoretical radii for
the bends. The radii that are calculated are considered a
“reducing” or “constant length” type. Which means that in
every forming pass the entire “arc length” (.4285”) is being
used within the radius. This is why the earlier passes have
larger radii when not completely formed to the finished
angle of 90 degrees. There are other forming methods used to
for bends such as; “constant radius”, “arc-in” & “arc-out”.
These methods have certain advantages and disadvantages, but
for the most part the “constant length” method is most
widely used. In many cases I like to calculate all the
passes using this method to develop the progression and then
sharpen the radii in the early passes slightly to help the
section track properly.
Over-forming a bend beyond the finished angle
is necessary to account for material spring-back. In this
example section where the material is .060” CRS with a yield
strength of 35 KSI, a 2 or 3 degree over-form is required to
overcome the spring-back effect. Materials that have high
elongation rates (Aluminum for example) may only need or 1
degree while high strength and high yield materials may
require 5 to 15 degrees of over-form. NOTE: The amount of
over-form can vary based on numerous cross-sectional
characteristics such as section depth, size of the
inside radius, thinner material running
through tooling designed for thicker and the amount of “air”
forming involved. Try to avoid over-forming more than one
bend at the same pass because the operator may need to
control one more than the other using roll pressure. In the
chart above, bend #2 is over-formed at pass #9 then allowed
to spring back to 90-degrees at pass #10. Bend #1 is
over-formed in pass #10.
Care should be
taken to the limit the amount of lateral and vertical
movement the material is subjected to from pass to pass,
especially lateral movement. Note how this flower
progression is forming more at first and less towards the
end. The material flows more laterally once the forming
angle approach 90-degrees. This increases load against the
forming rolls and creates many problems like marking or
scuffing, end-flare, twist with asymmetrical profiles and
camber or bow.
In pass #10, we
are over-bending #1. There is no “finish pass”. This allows
the mill operator more control of the finished forming angle
of bend 1 and at the same time, minimizes end-flare as
opposed to a more traditional overform/finish pass.
FIG #5:
OVERFORM / FINISH PASS EXAMPLE
When developing the flower the designer must
consider the correct number of passes. This ten-pass example
should be okay if running non-notched, post-cut mild steel.
If the section is either notched, pre-cut or high strength
material, more passes will be necessary to achieve desired
results. Additional passes allow the material to flow
through the mill with less strain. This is a must,
especially for pre-cut blanks. The pre-cut lengths must
self-feed through every pass without damaging the lead edge
of the strip. A good rule of thumb is to try to design all
roll
passes to self-feed even when running coil stock. The mill
operator should not have to feed a new coil and “pry” the
section into the next pass.
*NOTE:
If end-flare is a concern and a “finish pass” is desired,
consider adding two more passes to this ten pass example.
Pass #11 would unfold Bend #1 to approximately 86 degrees
and pass #12 would re-form Bend #1 to the finished angle of
90 degrees. The use of this unfolding / re-forming method
would improve angle and flare control.
5.0 ROLL LAYOUT
After the
flower has been completed, the designer needs to choose
drive diameters and check for maximum flange roll sizes and
possible interference with the rolling mill. “Step-up” or
increasing the pitch diameters of the rolls from pass to
pass is important to consider during the
design phase. In many situations “overfeeding” or “buckling”
between passes (particularly in the early stages) will
occur. This problem is caused when the material (especially
lighter gages) from a previous pass is driving faster than
the next. The amounts of “forming torque” and “surface
contact” on the rolls are the key factors as to why this
problem exists. However, after the profile has ample column
strength, step-up can be reduced or eliminated. With
pre-notched applications, caution should be taken not to
overly increase drive diameters because this may lead to
increased part lengths and / or distorted notches.
General guidelines for Step-up:
Pre-notched
materials (.015” to .030” gage):
Use approximately .010” diameter step-up per pass.
Thin gage /
non-pre-notched (.015 to .030” gage):
.050” diameter step-up passes 1-3, .030 diameter passes
4-up.
Our .060”
C-Channel example (non-notched):
.030” diameter step-up all passes.
Other solutions
to overfeeding can be idling key spindles on the mill or
idling key forming rolls, clearance angles on rolls that
appear to be driving too hard or reducing roll diameters in
a previous pass.
FIG #6:
ROLL LAYOUT WITH DRIVE AND FLANGE DIAMETERS
With the drive
diameters chosen the vertical centers are 7.060” and falls
between the 4” to 8” range. The maximum roll diameter shown
is based on the required “flange” needed for smooth material
transition from the previous pass. It is important to check
that this maximum diameter does not interfere with the mill
base or any other maximum diameter from an adjacent pass.
The location of the profile on the roll space, roll widths
and spacer lengths are also determined in this layout.
6.0 ROLL DESIGN
These days
almost all companies use some sort of computer aided design
software in their engineering departments. With this in
mind, specific calculations will not be covered; rather a
visual description is shown.
FIG #7: PASS LAYOUTS WITH DESCRIPTIONS
TYPES OF “TRAP” ROLLS:
7.0 ROLL DESIGN TIPS
When forming a
bend which has a 1t or less inside radius and when “box”
forming, a “coining” or “bead” pass is very effective in
setting the corner. By coining the material a slight
thinning will occur. However, the finished bend corner will
be crisp.
FIG #8: COINING
PASS
Another tip to
reduce material scuffing is to idle flange rolls so they are
not driven. The speed at which these idled rolls spin is
dictated by the section (at or near the highest point of the
leg) rather than the mills’ drive. Generally this method is
necessary when the formed legs of the profile are greater
than 70-degrees.
FIG #9: IDLED FLANGE CROSS-SECTION
8.0
TOOLING ACCESSORIES
Most
sections require a straightening device after forming which
removes unwanted twist, camber and bow. These straighteners
are normally a “block” type (made from Aluminum bronze or
nylon) but can also be done with rollers. The key to
straightening is to have the device fairly close to the last
pass (between the last two passes if pre-cut strips). The
length of the block should be long enough to achieve
approximately 1”-1-1/2” of working area (or land), while
incorporating a good lead-in and exit radius or taper. The
clearance around the profile should have about .015” per
side and more at the strip edges. Avoid trying to fully
contain the profile because if the device is adjusted it may
also affect the geometry of the cross-section.
FIG #10: STRAIGHTENER: BLOCK & ROLL
TYPE AND FIXTURE
Side Rolls,
or rolls which are mounted on a vertical axis and located
between passes is an excellent way to help a profile enter
the main roll passes. These are especially good for light
and heavy gage materials alike. Thinner gages tend to spring
while heavy gages have high lateral forces against the
flanges. In addition to smoothing out the forming transition
between passes, side rolls also reduce scuffing of the
vertical legs because they spin on the same axis that the
profile travels.
FIG
#11: TYPICAL SIDE STAND & CUSTOM DESIGN BETWEEN PASS SIDE
ROLL STAND
9.0 CONCLUSION
Although this C-channel example may not
represent your particular requirement, it does contain all
of the same steps that are needed to complete a roll design.
There is much more that could be added in this report
detailing other areas of a design. Again it is only meant to
give the beginning designer a guide and hopefully it will be
used as a reference to help you. Good luck!
Basic Roll-Form Tooling Design:
Process overview, instructions & design
tips
Written by:
Chuck Summerhill
Roll-Kraft Inc.
8901 Tyler Blvd.
Mentor, Oh 44060
Toll-free: 888-953-9400
Fax: 440-205-3110
e-mail:
csummerhill@roll-kraft.com
SolidWorks drawings by:
Chaz Rau
Roll-Kraft Inc.
Email:
crau@roll-kraft.com
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