AVT 206 A&P - P2 - Developing Sheet Metal Flats - The Math Behind the Bends

if you have ever tried bending a piece of sheet metal around a radius on a bending break you know that it's not simple and one of the things that happens is it doesn't come out correctly and most of us hate to be wrong we need to know about said setback Bend allowance Bend tangent lines and sight lines this is going to be a presentation where I'm going to show you where all of those things come from now before I begin I want to tell you that in theory the theoretical world exactly agrees with the real world but in the real world the theoretical world usually isn't the same as the real world and unfortunately most of the time the difference between what it should theoretically be and what it really is is bad for us we don't like that unfortunately or perhaps fortunately when we get the sheet metal some of those differences can be bad for us the some can be good for us in theory and in the real world things aren't always the same in theory the measurement to parts that we're going to form on the break are made to nice simple sizes but in the real world things become quite complex in theory there are sharp angled ends that make it simple to locate the corners but in the real world we have to go around a corner because a sharp angle would bend the aluminum or crack the aluminum so our theoretical math is nice and simple but our real world map is complex because of the radius and the theoretical things that we've talked about are called mould point dimensions and they're frequently what we get on our blueprints but in the real world the real things that happen are what we're going to be dealing with in there we're going to be giving them an end event at these mold tangent lines and we have to calculate where they're going to so what happen I have to do with bending medal I want you to notice the corners on pac-man you notice how abrupt they are you're like our theoretical metal bin or something moving one way and then suddenly BAM we move the other way but you know that that's very theoretical and as a real-world corners don't happen that way corners are well they're round and it takes some time to go around the corner here you can see what we're kind of still in smoke see don't see the looking from the top watch the pass the cars take it's not a gruff it goes around a radius we're going to look at a lot of the same things as we Bend our sheet metal around the radius so I'm going to use that analogy of the roadway and you're going to make a bed we're going to hang a right right at the first right so the mole point is the theoretical location of the term here's our turn and in our theoretical world we go right up there and then suddenly return but in that gives us a mole point which is right over here and that mole point is the theoretical point where we make the turn in the real world though we know we begin the turn before we get to the turn and we end the turn after we get to the turn and that leads us to the setback the setback is that distance that we begin before we got to the turn and that we ended after we got to the turn so that's our setback luckily it's the same number for both cases the next thing we need to look at is the bend radius and if you follow the set back line back to where they intersect you'll find that there's a point and that point is where we're going to measure our Bend radius from our Bend radius is the distance from that point to the inside of the bend was not least it's going to take us some material to sweep around the corner and the amount of material that's required to sweep around the corner is going to be called our Bend allowance now here's where that bit about the theoretical and the practical comes in in the practical world we find that when we go around the corner we actually cut a little bit off the corner and we get a head and that means that the bend allowance is shorter than the theoretical amount of material required to get around the curve so we can actually bend our piece of metal and it's likely deemed a little metal in the bed that's going to be an important thing to check for later on when we're checking to make sure we're correct all right so how do we calculate these well let's start with mold point if we talk about calculating mold points we're talking about the wrong thing because we don't calculate mold points we read the mold point off our blueprints or perhaps we take the mold point and we read it on a tape measure when we're measuring how big our parts should be we measure it on our aircraft so that one was easy let's move on to the next one setback okay for this lecture I'm going to assume a ninety degree angle and we can convert this to other angles fairly easily but let's just stick with the simple right now here's our theoretical mold point then and you'll notice there's our mold point nice square corner but in reality we know we have to bend it around a radius and I've drawn this circle here and of the radius of course is half the distance across the circle now once we know the big radius you can look up here and you can see that the amount of setback which is up here is going to be equal to the Bend radius plus the material thickness that gives us the setback when you see that we can write that down in a formula where our setback equals our Bend radius plus our material thickness so that's a nice simple formula to remember if you take the material thickness and the bend radius and put them together you know how far you need to begin before you reach the corner and how far you in after the corner all right so continuing along the Bend radius how do we get our bed radius well again we don't calculate our Venn radius we actually select the Bend radius and how do we know how to select a vendor a dias well they may tell us when they tell us to make the part what our bed radius needs to be but they may not tell us and this means they don't tell us we need selected on our own and there are three things that we need to consider ourselves when we go to select is on our own number one we need to consider we concern ourselves with the fact that it needs to fit so if I get too big of a bend radius it may not fit in the position it needs to be number two I need to have the proper radius diets to make that particular corner and number three I cannot exceed the minimum say then I can't bend it tighter this is a minimum amount that I can do over here you can see that been radius going on we'll come back to this diagram in a little bit so we select our bed radius here I've got a minimum safe Bend chart and on this minimum safe bed chart we can start looking details up we are going to be working with 6061 alloy t6 and so we select this row across here and then we need to select the column and on this case and in this example we selected 30 mm and when we go to right where they come together we find that there is a 1/2 T to one and a half T allowance for our minimum stay fit what does that mean a half of the thickness - one and a half times the thickness that's how much I can go so I have to multiply by 0.32 and I find exactly where that's going to come in this is our minimum safe bed and that is point zero one six two point zero four eight is our minimum state dent and in this in that range if we get down into the really lower parts in between there then we have to be careful not to crack and inspect our work as long as we're above point zero four eight we're good to make that then now our particular bed is going to be made at point one two five and because of that that's way bigger than the minimum state then and we're good all right the last thing we need to talk about is the bend allowance and the bend allowance is the amount of material we use in the real bed now this one is definitely going to be a calculation for us so calculating our bin allowance and again this is for a ninety degree then here's our same picture up here and now we need to see what the bend allowance is going to be the only thing I've done different about this particular picture though is that little dotted line right and and that little dotted line right in the middle tells us about the what we call the neutral axis as we begin bending this metal metal an inside here shrinks in metal on the outside expands or gets stretched but metal in the middle doesn't and so our radius across from here here is our material thickness plus 1/2 I'm sorry 1/2 of our material thickness plus our Bend radius working with that 1/2 a material thickness plus our Bend radius that tells us what the radius of the neutral circle is going to be so if I want to find out what the circumference of the neutral circle is I need 2 pi times the radius or 2 times pi times my bed radius plus 1/2 of my material thickness to get me what that whole circle is well I don't need the whole circumference how much is it recovers do I need I need 1 out of 4 or 1/4 so I'm going to simply take that 2 pi and I'm going to stick up for underneath it and I get 2 pi over 4 times the quantity then radius plus 1/2 the material thickness and that gives me my formula for what I'm going to do but a lot of us don't like all that that fancy way of expressing it in terms of math so what we're going to do is we're going to simplify and by the way I'm going to give you a little warning here when I may give you the simplified version I'm not actually moving exactly 1/2 across here I'm giving point four five five because that turns out to be a little more accurate for our aluminum's and different materials are different but our formula turns out to be Bend allowance equals point one one point five seven times the bend radius plus point seven zero 2 times the material thickness and that will tell us how much material is going to take to go around the bend so we've covered how to do it all but we don't really do it until we do it so as I did all right putting it all together in theory here I have a problem and this happens to be the problem I assign my students on project number two where they have a four and a half inch white bar with three quarter inch deep flanges so that's our mole point to mention score and a half by three quarters that's the theoretical part we need to build the real part so I can use the theoretical part and I could lay out four and a half inches for knit I'm sorry three-quarter four and a half three quarters and I would know that six inches would pull this spot keep that area of that number in mind while we keep working by the way 24 inches long is how wide they have to work again this is our simplified version using both points but in real life we don't get to use the simplified version we have to consider the bend tangent so our single line we're going to have to set back from our single lines our whole point lines to form two and tangent lines so I'm just going to go ahead and draw these in and now you can see me to then tangent lines where at each place where I had a single mold point line not still let them open one lines in there just for you can see up now I want you to notice that here for the width of this piece I need to pull back one setback from my mold point line and so I can actually calculate that I know my setback is quite 165 using my formula over here and so I can take my 3/4 inch that went over here and subtract one setback from it and I can find out what value that's going to be in this case point five eight five 3/4 minus one setback 0.5 85 now I come over to here and in my metal flat I need you to see that there is one setback for next line and one step back from this line and so when I put the two of them together I have to subtract two setbacks 4.5 minus 0.185 minus 0.185 gives me four point one seven zero for my Center flat and of course my bottom plat is just the same as my top slab because the math is identical so now I know what the three flats are but I don't know what the two ends are yet so I still don't know what my total on the material is now I have to go in and calculate my Bend allowance and my bindle Rama's formula was one point five seven times my Bend radius plus point seven zero two times my material thickness and I do that and I find this 0.224 and my 0.224 den goes in as the amount of material uses on each of these bins now I can add the five pieces together and when I add the five pieces together I find five point seven eight eight I've gone ahead and put this in on my diagram and that five point seven eight eight inches is how much I'm going to need to make this in real life remember my mole point dimension said it was going to be six so this is a little less than it was and I both like to mention and this is good reality check because if you cut the corner a little bit and you come up with a little more than you had before you're doing something wrong check your math make sure you're coming up with a little less The Sharper are maybe more you cut the corners the less material you're going to need all right now we've got the fitting the pieces laid out but we're still not ready to actually put it in the machine and bend it and that's because we need to talk about something called the sight line here I've got an eye and my eye is looking down at the material I need to put my bend tangent line right underneath the radius and my bending bridge but the problem is here's a done tangent lines another Bend tangent line out here the problem is I can't see where this Venn tangent line is because it's underneath there so I can only see one Bend radius away so what I'm going to do is I'm going to add a material are not a mark called a sight line and that sight line is going to be one Bend radius away from my bend tangent line and I'm going to see that from straight above now it doesn't matter which then tangent line I decide I want to measure my one band radius away but once I decided which one it's going to be I need to make sure that's the one that's directly underneath here so I like to add a little X Y measure away from my bed Hanjin line I put a little X next to the one it was so I don't forget which one I need to put underneath the jaw the bending call also let's talk about why we have to look from straight above when we go to do that then you need to know that if you look from straight above here we are you can see that one then radiates and everything lines up correctly but if I redraw this diagram and I'm not looking from straight above I want you to notice that the sight line isn't directly underneath anymore and now I get an error that is a problem we don't like errors all right so I had my sight line here our bend radius is an 8 so I take this same thing and I go ahead and I measure 1/8 an inch away and I'm measuring 1/8 of an inch away from the inner ones as I measure that 1/8 an inch away I could pull my line here shown in blue I can pull that line across and then I can go ahead and put that little X on each side to show me which one it is so I can stick this underneath the brake and I can measure that now the last thing I need to say about all of this is just like flying a tail dragger you need to practice this man-made sense while I talked about it but unless you've done it any times when you go to do this in real life or when you go to take a test and do this if you're a student it just doesn't seem to make a lot of sense so develop quite a few plaques spend your time doing this practicing it and then when it comes time to actually do it and realize it'll be a whole lot easier

Channel: 1donagin

Views: 364,851

Keywords: avt, 206, siuc, actech, sheet, metal, a&p, aluminum, bending, brake, setback, bend allowance, sight line, developed flat, developing, flats, bend, radius, bend radius, curve, radiused bend, calculating, math, demo, tutorial, lab, project, project 2, P2, Aviation, Technologies, University, spar, splice, airplane, plane, cessna, maintenance, manual, srm, ac-43.13-1B, iaw, i.a.w., bending brake, radiused caul, how to, diy, BTL, Bend tangent lines, tangent

Id: HemwD3NpKXk

Length: 15min 56sec (956 seconds)

Published: Sun Feb 05 2017