Welcome to our lesson about bend
allowance you may be wondering what
exactly is bend allowance if you've
never worked with sheet metal before
well when a sheet is bent in a press
break the part of the sheet close to and
in contact with the punch elongates to
compensate for the given
Bend if you compare the length of this
part in my example here before and after
the bending you're going to find that
they're different as Engineers if we
don't compensate for this variation the
final product won't have accurate
dimensions of course this is more
critical for parts where you've got to
maintain a tighter allowance or
Precision if all the variables in my
diagram are already making you zone out
fear not this tutorial is by no means an
exhaustive discussion about Metal
bending Theory however I Do cover some
of the basic problems and principles
you'll have to deal with regularly when
working with sheet metal before we get
rolling I wanted to comment on something
there's not really a scientific method
or formula for determining a truly exact
calculation of the bend allowance and
that's because there's so many factors
at play during the production of your
sheet metal part for example actual
material thickness an infinite variety
of tooling conditions forming methods on
and on and on there are many variables
here and in reality many methods are
used to calculate band allowance trial
and error is probably the most popular
one Bend tables are another popular
method and we'll be learning about those
in this video Bend tables are usually
available from metal suppliers and
manufacturers as well as in engineering
textbooks some companies develop their
own bending tables based upon their own
standard
formulas let's get back to Solid Works
how exactly does solid works calculate
bend allowance solid works actually uses
two methods bend allowance and bend
deduction I'm going to explain what
these methods are and show you how
they're used in solid
works the band allowance method is based
on the formula that I've got up here in
my diagram the total length of the
flattened sheet is equal to the sum of
L1 that's the first length and L2 plus
the band
allowance the bend allowance region is
shown in Green in my diagram this is a
region where in theory all deformation
occurs during the bending
process generally the bend allowance is
going to be different for each
combination of material type material
thickness Bend radius Bend angle and
different Machining processes types
speeds and so on truly the list of
potential variables is very
extensive the value of the bend
allowance that comes from sheet metal
suppliers and manufacturers as well as
engineering textbooks is provided in
Bend tables and a Bend table looks
pretty much like what I've got on my
screen now this is an Excel
spreadsheet the B table approach is
probably the most accurate approach for
calculating bend allowance you can input
your data manually into a matrix of the
bend angle and bend radius if you're not
sure of the bend allowance value you can
run some tests you need a piece of the
exact same sheet metal you're using to
manufacture your part and then you'd
bend it using the exact processes that
you'll be using during your Machining
just take some measurements before and
after the bending and then based on this
information you can adjust the bend
allowances
needed another method that solid works
uses is the bend deduction method the
formula is as follows the flatten length
of the part that's LF equal D1 plus D2
minus the bend
deduction there's D1 and there's
D2 minus the bend
deduction as with bend allowance Bend
deduction comes from the same sources
tables and manual
testing as you can see it's easy to
imagine how these values are related to
each other based on what these formulas
indicate another method for calculating
bend allowance uses the K Factor where K
is the neutral axis offset the general
principle of this formula goes like this
the neutral axes are in my diagram here
in red they don't change during the
bending process during the bending
process the material inside the neutral
axis will compress and the material
outside the neutral axis will
stretch the neutral axis will be closer
to the inside bend the inside Bend is
indicated in blue in my
diagram the more the part
bends now the more the part bends the
closer the neutral axis will lie to the
inside of the
part the K Factor as we've seen here
equals t or the offset distance to the
neutral
axis we divide that by Big T that's the
thickness of the
material in this formula here the bend
allowance equals 2 * pi multiply by a
that's the
angle multiplied by the sum of R the
bend radius plus the K
Factor multiplied by T the thickness of
the
material and then you divide all of this
by
360 in theory the K Factor can be
anywhere between 0 and one but for
practical purposes it's more like 0.25
to 0.5 for example you'll find that hard
materials like steel have got a higher K
Factor such as
0.5 softer materials like copper or
brass are going to have a lower K Factor
closer to .25 and don't worry that's the
last formula we'll be walking through in
this lesson it might seem a little bit
confusing now but once you practice it a
little bit it'll become second
nature one last Point here let me take a
look at an example there's a wiped hem
on this part it's got a k factor of
something
like3 on the other hand a softer Bend or
for example the gradual bend that I've
got on the other side of this part has
got a higher K factor of about
0.5 and this concludes our lesson on
bend
allowance