 Okay, we'll go into match drilling of 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 the 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 true positions because they can be off a little bit and 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 are cases in which, see, in the first one there, the holes match, but the countersunk is not in line. So you have head bending where the arrow is up there. Over here we even use the wrong countersunk 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 but 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 up and drilled right and the other one drilled at a slight angle, then you're in real trouble. Now knife edges in a countersunk hole, knife edges are stress risers and are to be avoided. In fact, the airspace industry makes a big issue over this that 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 develop cracks really 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 countersink 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 flush surface up here and that's in the case 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 flush surface. Now as I understand it, this is still allowed on small airplanes. They still allow dimpling. Yeah, Mario, you fly. So this will make you feel better. They allow dimpled holes on small aircraft but they don't allow them on the big ones because of the fact that where you deform the metal like that, there's danger of developing cracks. So the major aircraft manufacturers prohibit that. Now dowel pins. They're a very important thing and have an important function but sometimes people want to use them in ways they shouldn't be used. They're close tolerance pins which are used to align mating components and that's really their major function. They're usually mounted in one of the pieces with a slight interference bit. Then the mating piece has a close tolerance hole to slip over it and you get good alignment of the pieces and then you bolt them together or however however you want to fasten them together and you analyze the bolts for the total load and you don't use the dowel pins and bolts together to calculate the load because one of them is interference bit 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 to carry out the load you can 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 fit 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 interference bit the bolt 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 shear 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 that you can take a punch to the backside and and knock the thing out or use a vetted pin with a groove or a flat edge for blind installations just to make sure you can 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 serrations on the edge of it that keeps it from rotating. Now shear allowables for dowel pins are usually determined by the manufacturer's test program because a lot of the times the irregularity of the cross section means it's difficult for you to calculate the cross sectional area that you have so it's easier to use the manufacturer's values for it. So some of the common types of dowel pins that we have here is a plain solid one and pebble grained even. This one is one of the ones that we Fred Yeris's people made for me with some sort of a new system that they had that gave it kind of a rough surface. Then we go to the drilled and tapped dowel pin with vents. 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 rod. Here's a grooved dowel pin and this one actually this this groove is on it to give it a little bit of compressibility so this the grooved end would be slightly larger and 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's just at the 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 that you don't pull any vacuum when you put the thing in the blind hole. Now here here's the 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 nut 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'll pull it. So those those are pretty good if you if you've got a place where you can use them that way. Now a rolled pins or sometimes called spring pins are actually made by rolling a piece of thin alloy steel or stainless steel to a given diameter with a chamfer on each end of it so you can take a hammer and drive it in the hole. It's then heat treated to 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 is not really rolled up it's just a cylindrical piece of tubing with a slot cut in it and you can use that also as a spring pin so there's one of each showing on the next page here. Here is the roll pin which is wound up if you look at that one I believe you see it better there that it's actually overlapped rolls of material so that it'll 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. 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 and our next section when we come back will be on rivets.