Sheet metal

                              

New Stakesy's Colours are White Base with Burnt Orange Side Castings.
Bench top 610mm (24”) slip roll that allow rolling up to 0.8mm in mild steel or 1.3mm in aluminium down to a minimum diameter of 38mm. They can be set to slip on one side to produce cones which is great for making exhausts and inlet trumpets or for model makers. Bench mountable, they also have wire grooves of 6, 7.5 & 9.5mm to roll rod.

For those who want a little extra capacity over the 320mm (12") slip roll, our 24"  slip roll fit the bill. Features quick release top roller and bushings found on larger slip rolls. A nice unit.

• Fully Ground Roll
• Gear drive
• Cast iron frame
• Slip rolling for cones

Model	STA113
Max Rolling Width	610mm (24")
Max Capacity Steel	0.8mm
Max Capacity Aluminium	1.3mm
Min Rolling Diameter	38mm
Gross / Net Weight	40 / 37kg

Sheet Metal

Here you go ... this will take a lot of photos to try and explain how it is built.
The roller is wide enough to accept a 36 inch wide piece of sheet metal.
The rollers are made out of 2-1/2 inch diameter steel tube with 1/2 inch wall thickness.
I turned steel bushings to fit in the ends of the rollers. These bushings were then pressed into the ends of the rollers.
A 3/4 inch diameter rod was then pushed thru the rollers and welded to the steel bushings on the gear end.

First, overall photos of the roller.
( front )

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( right end, with the operating crank )

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( rear )

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( left end )

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The round silver handle on the lower right, controls the movement of the back roller in and out.
The knob on the far left, moves that end of the back roller in and out independently so you can roll a cone shape.
The lever down in the center, moves the top roller up and down for griping the sheet metal ( the top roller is in the down position now ).

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This is the rear showing the arm on the left side for the back roller and the linkage for adjusting this side for making a cone shape.

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This is showing the arm on the right side for the back roller.
This also shows the brackets and linkage for the center lever that operates the top roller.
This linkage rotates a shaft that runs across the rear to move levers on both ends for the top roller.

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This is the handle for the back roller.
It rotates a shaft that runs across the front to move the arms on both ends of the back roller ( the back roller is in the open position here ).

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Here the back roller is in the closed position.

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Here are two views of the adjuster for the left end of the back roller.
This lever and adjuster rotates with the lever on the right side for moving the back roller evenly at what ever setting this adjuster is set at.

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This is two views of the lever for the top roller.
First in the open position and then in the closed position.
The lever goes "over center" just a little to hold the top roller closed.

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Left end, showing both the back roller and the top roller in the open position.
There are springs on the levers for the top roller that hold the top roller up when it is open.
There are adjusting nuts on the control rods on both ends to tighten or loosen the top roller for gripping different thickness of sheet metal.

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Showing the top roller closed and the back roller still open.

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Both the top roller and the back roller are closed.

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The bracket holding the bearing for the top roller is split into two sections on this side.
There is a "thumb screw" on the front of the top half of this bracket. This unscrews so the top of this bracket can be lifted up.
This allows this end of the top roller to be lifted up off the bearing bracket.
If you roll a piece of metal completely around the roller, then this is how you get it off the machine.

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View from the front with the top roller held up with a block of wood.

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Right end showing the chain and gears to operate the top and bottom rollers.
The gear on the left is on the pivot pin for the top roller. The crank handle connects to this gear.
The chain from this gear goes to the top roller.
This gear is meshed with the gear on the bottom roller at all times.
when you turn the crank, it rotates the top roller thru the chain and rotates the bottom roller in the opposite direction to grip the sheet metal an roll it thru.
This shows the top roller in the open position.

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this shows the top roller in the closed position.

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The gear on the bottom roller has a cover made out of the end of a tin can.

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The cover for the gear and chain for the top roller is attached to the top roller bracket and moves up and down with it.

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Here is the front of the cover.

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There are two adjusting screws on the right legs that screw down to the floor to hold the roller in place.

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When I first built this, it was made to accept a piece of sheet metal that was 6 foot wide.
Even with these rollers being 1/2 inch thick, the force needed to roll a 6 foot section was so great that the rollers would flex in the center.
So I cut it down to 3 foot and it works great.
This is shows the joints from shortening the roller frame.
I just cut the rollers in half to shorted them.

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Sheet Metal

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.

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".