Fabricating Precision Sheet Metal:
This is a specialty of mine so this section will be fairly extensive. You may be thinking that a Preceptor Pup is a rag and tube airplane and the sheet metal work makes up a very small portion of the airplane but when you take that innocent piece of sheet metal and try to put a bend in it, things can get complicated in a hurry. I will try to keep my explanations as simple as I can but because the process can be complicated this will take some effort for me to explain completely and for you to follow along if you have limited experience with sheet metal. Don't be to discouraged though, you can do anything you put your mind to and this isn't brain surgery.
We should start our discussion by taking a look at the state of the art in aircraft sheet metal manufacturing and work our way toward the information and techniques you can use in your home shop. I will supply the math you need and I also have an Excel spreadsheet available so your computer can do some of the hard stuff.
So what is the ultimate process available for the design and manufacturing of your dream plane? Lets start with an unlimited budget. Throw in a large team of experienced aerospace engineers, the latest greatest CAD/CAM/CAE software, every piece of manufacturing equipment you can imagine, shake it all up and what do you get? Well it probably won't be state of the art. My experience with the local aircraft manufacturers suggests that they're still making sheet metal airplanes the same way they did in the forties for the most part. The technological advances in computers and manufacturing equipment have definitely been put to good use, but you will have to look somewhere else for the leading edge.
So what does it take? Lets start by creating a complete, accurate 3D solid model of the entire airplane with the best software you can find. Lets make sure to model every rivet location, every wire, hose and radio. Make sure to finish the computer model before you start making parts because if you don't you will make a lot of them again. I know you're anxious to get some parts in your hands but it will go faster in the end if you finish the details on your model first.
Ok, you have a model, it has been approved by everyone important, now what? For the sheet metal parts, we are going to need to determine the manufacturing processes necessary to fabricate them. The majority of the parts can probably be brake formed, roll formed or hydro formed and these are the processes I will concentrate on. Parts that get draw formed, stretched or formed with a drop hammer are beyond the scope of what you would normally find on most homebuilt aircraft so they won't be dealt with here.
Sheet metal normally comes in flat sheets, so we need flat patterns of all the parts to cut out of those sheets. We will be discussing how to develop accurate flat patterns. After the flat patterns are made, we'll need some tooling to form the parts. We will create 3D computer models of our form blocks directly off the models of the parts for complete accuracy. Be sure to allow the proper spring back for the condition the material is in at the time of forming. Next we'll cut the form blocks on our CNC mill and cut our flat patterns on our CNC router, then form the parts. Parts fabricated this way can be held consistently to tolerances plus or minus .005" on the webs and flanges. You didn't forget to pre-drill for all your fastener locations did you? OOPS! This is where the major aircraft manufacturers drop the ball. With the accuracy of the manufacturing equipment available today and the power available in the computer on your desk, there shouldn't be any excuse for being unable to pre-drill all the holes should there? It sure would be nice to be able to gather all your parts, cleco everything together and start riveting without having to rely on some massive assembly fixture to locate all your parts, wouldn't it?
Wait a minute. Doesn't Van's Aircraft supply most of their parts for the RV-7, RV-8 and RV-9 pre-drilled just as we have been talking about? I'm sure there are others doing the same thing. What's wrong with everyone else? We'll try to determine that later but for now we will get into the nuts and bolts of developing flat patterns.
Lets start with some definitions.
FLAT PATTERN - The outline definition of the part in it's flat state before forming. This can be a physical drawing or template or a computer model. Lets go ahead and include all the fastener locations.
OUTSIDE MOLD LINE or OML - The intersection of two outside surfaces of a part extended past the bend radius. This intersection is not actually something that can be measured on a part directly unless the bend angle is 90 degrees. Most drawing dimensions for formed sheet parts are to the outside mold line.
INSIDE MOLD LINE or IML - This is the same as the OML except using 2 inside surfaces instead. This is also the form block line.
BEND TANGENT LINE - The line created where the radius of the bend meets (is tangent to) either flange. There are 2 bend tangent lines for each bend.
CENTERLINE OF BEND - The line midway between the two bend tangent lines. This is useful for brake forming bends on a press brake.
FLANGE LENGTH - The distance measured from the edge of the flange to the outside mold line.
NEUTRAL AXIS - The arc through the bend between the bend tangent lines where the metal neither stretches nor compresses.
T - The sheet metal thickness.
R - The bend radius. This is always the inside radius, not the outside.
a - The angle of the bend in degrees.
K=tan(a/2)
KT - The distance between the OML to the IML projected to the plane of one of the surfaces. KT=[tan(a/2)]T or KT=K*T
BA - Bend allowance or the length of the neutral axis. BA=a(.01743R + .0078T)
BT - The distance from the OML to the bend tangent line. BT=K(T + R)
D - Setback. The amount of material saved by going through a radius instead of a square corner on a bend. D=2BT - BA.
Lets go through the math for a simple bend. Say we need a part with a 90 degree bend with two 1" leg lengths. We will be using .032 material with a .125 bend radius. The part needs to be 4" long. Lets start by listing what we know.
T=.032
R=.125
a=90
Now we'll plug the numbers in our formulas.
K=tan(a/2)
K=tan(90/2)
K=tan(45)
K=1
BA=a(.01743R + .0078T)
BA=90*(.01743 * .125 + .0078 * .032)
BA=90*(0.00217875 + 0.0002496)
BA=90 * 0.00242835
BA=0.2185515
BT=K(T + R)
BT=1 * (.032 + .125)
BT=1 * .157
BT=.157
D=2KT - BA
D=2 * .157 - .2185515
D=.314 - .2185515
D=0.0954485
Not too difficult is it? It's just a little tedious. There are sheet metal books available that have charts made up with the numbers calculated for each of the common material thickness and bend radius combinations. The charts are calculated for every 2 degrees of bend or so. I have created an Excel spreadsheet that calculates this for you so you don't have to worry about misplacing a little book. It is better than a book because you can plug in the exact angle you are bending. If you want you can make up your own book by printing this spreadsheet for each angle you need to bend. You can download it here. Sheet Metal Forming Table
All right, now we can layout our flat pattern. First draw a line 4 inches long to represent one edge of our part (#1). Now draw another 4-inch long line parallel to the first 1 inch away to represent the outside mold line (#2). From there we draw another line that is offset our D distance of 0.0954485" towards the first line from the second (#3). D is the amount of material "saved" by going through a radius. From this line measure over 1" to the second edge (#4). What is represented by the four lines we have drawn is the two edges on the outside and two outside mold lines separated by the distance D. If there was no bend radius and you could bend a perfectly square corner, the flat pattern layout would measure 2 inches wide for the two 1" legs by 4 inches long. Because you can't bend a square corner and our bend radius is .125", our flat pattern should measure 1.9045515" x 4". Normally 3 decimal places is more than accurate enough for our purposes so our flat pattern would measure 1.905" x 4.000".
This is a specialty of mine so this section will be fairly extensive. You may be thinking that a Preceptor Pup is a rag and tube airplane and the sheet metal work makes up a very small portion of the airplane but when you take that innocent piece of sheet metal and try to put a bend in it, things can get complicated in a hurry. I will try to keep my explanations as simple as I can but because the process can be complicated this will take some effort for me to explain completely and for you to follow along if you have limited experience with sheet metal. Don't be to discouraged though, you can do anything you put your mind to and this isn't brain surgery.
We should start our discussion by taking a look at the state of the art in aircraft sheet metal manufacturing and work our way toward the information and techniques you can use in your home shop. I will supply the math you need and I also have an Excel spreadsheet available so your computer can do some of the hard stuff.
So what is the ultimate process available for the design and manufacturing of your dream plane? Lets start with an unlimited budget. Throw in a large team of experienced aerospace engineers, the latest greatest CAD/CAM/CAE software, every piece of manufacturing equipment you can imagine, shake it all up and what do you get? Well it probably won't be state of the art. My experience with the local aircraft manufacturers suggests that they're still making sheet metal airplanes the same way they did in the forties for the most part. The technological advances in computers and manufacturing equipment have definitely been put to good use, but you will have to look somewhere else for the leading edge.
So what does it take? Lets start by creating a complete, accurate 3D solid model of the entire airplane with the best software you can find. Lets make sure to model every rivet location, every wire, hose and radio. Make sure to finish the computer model before you start making parts because if you don't you will make a lot of them again. I know you're anxious to get some parts in your hands but it will go faster in the end if you finish the details on your model first.
Ok, you have a model, it has been approved by everyone important, now what? For the sheet metal parts, we are going to need to determine the manufacturing processes necessary to fabricate them. The majority of the parts can probably be brake formed, roll formed or hydro formed and these are the processes I will concentrate on. Parts that get draw formed, stretched or formed with a drop hammer are beyond the scope of what you would normally find on most homebuilt aircraft so they won't be dealt with here.
Sheet metal normally comes in flat sheets, so we need flat patterns of all the parts to cut out of those sheets. We will be discussing how to develop accurate flat patterns. After the flat patterns are made, we'll need some tooling to form the parts. We will create 3D computer models of our form blocks directly off the models of the parts for complete accuracy. Be sure to allow the proper spring back for the condition the material is in at the time of forming. Next we'll cut the form blocks on our CNC mill and cut our flat patterns on our CNC router, then form the parts. Parts fabricated this way can be held consistently to tolerances plus or minus .005" on the webs and flanges. You didn't forget to pre-drill for all your fastener locations did you? OOPS! This is where the major aircraft manufacturers drop the ball. With the accuracy of the manufacturing equipment available today and the power available in the computer on your desk, there shouldn't be any excuse for being unable to pre-drill all the holes should there? It sure would be nice to be able to gather all your parts, cleco everything together and start riveting without having to rely on some massive assembly fixture to locate all your parts, wouldn't it?
Wait a minute. Doesn't Van's Aircraft supply most of their parts for the RV-7, RV-8 and RV-9 pre-drilled just as we have been talking about? I'm sure there are others doing the same thing. What's wrong with everyone else? We'll try to determine that later but for now we will get into the nuts and bolts of developing flat patterns.
Lets start with some definitions.
 |
| Air Craft Sheet Metal |
FLAT PATTERN - The outline definition of the part in it's flat state before forming. This can be a physical drawing or template or a computer model. Lets go ahead and include all the fastener locations.
OUTSIDE MOLD LINE or OML - The intersection of two outside surfaces of a part extended past the bend radius. This intersection is not actually something that can be measured on a part directly unless the bend angle is 90 degrees. Most drawing dimensions for formed sheet parts are to the outside mold line.
INSIDE MOLD LINE or IML - This is the same as the OML except using 2 inside surfaces instead. This is also the form block line.
BEND TANGENT LINE - The line created where the radius of the bend meets (is tangent to) either flange. There are 2 bend tangent lines for each bend.
CENTERLINE OF BEND - The line midway between the two bend tangent lines. This is useful for brake forming bends on a press brake.
FLANGE LENGTH - The distance measured from the edge of the flange to the outside mold line.
NEUTRAL AXIS - The arc through the bend between the bend tangent lines where the metal neither stretches nor compresses.
T - The sheet metal thickness.
R - The bend radius. This is always the inside radius, not the outside.
a - The angle of the bend in degrees.
K=tan(a/2)
KT - The distance between the OML to the IML projected to the plane of one of the surfaces. KT=[tan(a/2)]T or KT=K*T
BA - Bend allowance or the length of the neutral axis. BA=a(.01743R + .0078T)
BT - The distance from the OML to the bend tangent line. BT=K(T + R)
D - Setback. The amount of material saved by going through a radius instead of a square corner on a bend. D=2BT - BA.
Lets go through the math for a simple bend. Say we need a part with a 90 degree bend with two 1" leg lengths. We will be using .032 material with a .125 bend radius. The part needs to be 4" long. Lets start by listing what we know.
T=.032
R=.125
a=90
Now we'll plug the numbers in our formulas.
K=tan(a/2)
K=tan(90/2)
K=tan(45)
K=1
BA=a(.01743R + .0078T)
BA=90*(.01743 * .125 + .0078 * .032)
BA=90*(0.00217875 + 0.0002496)
BA=90 * 0.00242835
BA=0.2185515
BT=K(T + R)
BT=1 * (.032 + .125)
BT=1 * .157
BT=.157
D=2KT - BA
D=2 * .157 - .2185515
D=.314 - .2185515
D=0.0954485
Not too difficult is it? It's just a little tedious. There are sheet metal books available that have charts made up with the numbers calculated for each of the common material thickness and bend radius combinations. The charts are calculated for every 2 degrees of bend or so. I have created an Excel spreadsheet that calculates this for you so you don't have to worry about misplacing a little book. It is better than a book because you can plug in the exact angle you are bending. If you want you can make up your own book by printing this spreadsheet for each angle you need to bend. You can download it here. Sheet Metal Forming Table
All right, now we can layout our flat pattern. First draw a line 4 inches long to represent one edge of our part (#1). Now draw another 4-inch long line parallel to the first 1 inch away to represent the outside mold line (#2). From there we draw another line that is offset our D distance of 0.0954485" towards the first line from the second (#3). D is the amount of material "saved" by going through a radius. From this line measure over 1" to the second edge (#4). What is represented by the four lines we have drawn is the two edges on the outside and two outside mold lines separated by the distance D. If there was no bend radius and you could bend a perfectly square corner, the flat pattern layout would measure 2 inches wide for the two 1" legs by 4 inches long. Because you can't bend a square corner and our bend radius is .125", our flat pattern should measure 1.9045515" x 4". Normally 3 decimal places is more than accurate enough for our purposes so our flat pattern would measure 1.905" x 4.000".
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