I
okay we'll go into match drilling and
fastener holes this is something that I
touched on earlier which has to do with
making sure that the holes match whether
they're in their true position or not
and by having a pilot hole in one part
and then drilling all the way through
with the mating pieces clamped in
position this way you have a precision
hole even if the holes are not in their
troop positions because they can be off
a little bit and still still be close
enough and I have some examples of
mismatched countersunk holes in the next
figure now notice that this mismatch
usually causes head bending which is bad
news these these are cases in which see
in the first one there the holes match
the counter sunk is not in line so you
have head bending where the era is up
there the over here we even use the
wrong counter snug hole and because
there are two normal types 82 degree and
100 degree countersunk heads and so if
you use the wrong one you're in trouble
here we have the holes parallel but not
in line and here the holes weren't even
parallel so we're real trouble there on
bending on both of these so what you
have here is a case in which you need on
countersunk holes to use the same drill
fixture put all the holes in through
everything so that at least even if it's
a little bit off the 90-degree alignment
at least everything will match and
you're in better shape that way than you
are if you have one of them drilled
right and the other one drilled it a
slight angle then you're in real trouble
now knife edges in a countersunk whole
knife edges or stress risers and are to
be avoided in fact the aerospace
industry makes a big issue over this
thou shalt not do it so if we go on to
the next sheet it will show some
examples of this and I can just talk
from the examples there is a knife edge
right here and you see that edge can be
very jagged and developed cracks real
easy so therefore you're not supposed to
have that at all in a critical
application you're supposed to make sure
that you have enough thickness that you
can counter sink and still have a piece
left here to avoid that knife edge in
fact having the countersink be no more
than two-thirds of the thickness of the
sheet is one of the criteria that the
aircraft companies use then going to the
next page now here we have dimpled and
countersunk holes in this case we have
the countersink in the bottom sheet we
dimpled it just simply by hitting it
with a tool to make this one fit so we
could have a place surface up here and
that's in in the case where you where
the top sheet is too thin to countersink
in it then we're both of them are too
thin you can actually dimple both of
them and still have a flesh surface now
as I understand it this is still allowed
mr. murky on small airplanes they still
allow dimpling yeah Mario you put you
fly so so this will this make you feel
better they allow dimpled holes on small
aircraft but they don't allow them on
the big ones because the fact that where
you deform the metal like that there's
danger of developing cracks so so the
the major aircraft manufacturers
prohibit that now dowel pins there
they're a very important thing and have
an important function but sometimes
people want to use them in way
they shouldn't be used there are close
tolerance pins which are used on lane
mating components and that's really the
their major function they usually
mounted in what in one of the pieces
with a slight interference fit then the
meeting piece has a close tolerance hole
the slip over it and you get good
alignment of the pieces and then you
bold them together or ever ever how you
want to fasten together and you analyze
the bolts for the total shear load and
you don't use the dowel pins and bolts
together to calculate the load because
one of them is interference fit and the
other one isn't so therefore the dowel
pins would load up first so now if you
want to put enough dowel pins in the
carry all the load you could do that and
then just hold them together with the
intention with the bolts but you can't
use two different fasteners that have
different tolerances and say that both
of them are going to carry load equally
just like you don't use bolts and rivets
together because the rivets would fail
before the bolts pick up any load
because the rivets and her parents fit
the mold isn't so so that's that's the
way that they're supposed to be used now
you can design them to carry all the
cheer load although normally you don't
and now here's here's one of the things
you can run into with dowel pins if you
put them in blind holes they're kind of
hard to remove so particularly if it's a
solid pin so it's a lot better to have a
through-hole to put a dowel pin in so
you can take a punch to the back side
and knock the thing out or use a vetted
been with a groove or a flat edge for
blind installations is to make sure you
get the thing out and tapered dowel pins
are available and pins with external
serrations or ridges to prevent pin boat
rotation so you drive the thing in place
and it has the Russians on the edge of
it that keeps it and rotating now sure
allowables for dowel pins are usually
determined by the
manufacturers test program because a lot
of the times the irregularity of the
cross section means it is difficult for
you to calculate the cross sectional
area that you have so it's easier to use
the manufacturers values for it so some
of the common types of dowel pins that
we have here is a plain solid one and
pebble greened even this one is one of
the ones that we Fred year since people
made for me with some sort of a new
system that they had gave it kind of a
rough surface then we go to the drilled
and tapped dowel pin with vents these
are these even have little grooves
around them so that they will vent and
you can pull them out and then they have
a drilled and tapped hole so that you
can run a threaded rod in there of some
kind or screw and actually pull the
thing out with the ride here is a groove
dowel pin and this one actually this
this groove is on it to give it a little
bit of compressibility you so this the
grooved end would be slightly larger and
that then you can pound it in and the
groove will close up on some as you're
pounding it in which is because you
notice the groove doesn't go all the way
down it is just it to one end then this
is a vented dowel pin here and the
groove does go all the way down so that
you can get so she don't pull any vacuum
when you put the thing in the blind hole
now here's a tape that that I like and
we use some of these because you can
pull them this is the tapered dowel pin
with a jacking net so that one you can
slap it in the hole and then when you
get ready to take it out all you got to
do is tighten the nut up and that will
pull it
so those those are pretty pretty good if
you if you've got a place where you can
use them that way now roll pins are
sometimes called spring pins are
actually made by rolling a piece of thin
alloy steel or stainless steel to a
given diameter with a temper on each end
of it so you can take a hammer and drive
it in the hole it's then heat-treated a
real high hardness and the coiled cross
section on it decreases in diameter as
you're driving it so that you have an
interference fit now the slotted tubular
pin is one that it's not really rolled
up it's just a cylindrical piece of
tubing with a slight cut in it and you
can use that also SI spring pin so
there's one of each shown on the next
page here here is the roll pin which is
wound up if you look at that one I
believe you can see it better there that
it's actually overlapped rolls of
material so that it will develop more
load of course than the single slotted
tubular pin here these are used for
installing cranks and I know that I've
seen them used on bicycle cranks to hold
them together and they're easy to tap in
place and if they're in a through-hole
then you can take a punch and knock them
out when you get ready to take them out
the once again the load carrying
capabilities for these are usually
determined and tabulated by the pin
manufacturer because due to the
irregularity of the cross section it's
hard for you to calculate it directly
and that will conclude roll pins in our
next section when we come back we'll be
on Ribbit's