{short description of image}Strap Fitting
{short description of image} {short description of image} {short description of image}











      About Me
Strap Fitting
Have you ever come across a fitting like one of these on a set of plans and wondered what the designer was smoking when he drew that? Is it possible to make something like this in my home shop? I have personally read a lot of books and magazine articles on homebuilt aircraft construction and have never come across anything for making these. I have four similar to these to make for my N3 Pup and until recently I didn't have any ideas how to do it. Then it happened. I went and bid a job that needed them and I won the job. Time to come up with something fast. I called Stan Blankenship on the phone for advice. Stan has been involved in the aircraft industry longer than I have been alive and has more knowledge about making aircraft parts than anyone I know. He tells me "Get a pin the right size and just wrap it around". Thanks Stan. I could have thought of that myself. Then I remembered a tool I purchased for bending music wire for a landing gear on a radio control airplane. It was just a small plate with 2 pins pressed in. It had a round handle with a hole to pivot on one of the pins in the plate and a pin pressed through the handle. Thread the wire through the pins, crank the handle around and you can make pretty little bends all day long. Thanks again Stan, really!

Ok, we have a direction to go but how do we implement it? The two fittings pictured above are very similar. The parts are made of .063" 4130-N. They both get welded to ¾" tubing. One is wrapped all the way around with two flanges .375" apart. The other is only wrapped half way with two parts welded to the ¾" tube opposite each other to form two .375" gaps. The distance from the center line of the .250" hole in the strap to the center line of the ¾" tube is the same for both parts. I also had a close tolerance for the .375" gap because a spherical rod end gets bolted between the flanges and after welding, the flanges are not going to flex much. We need an accurate flat pattern and precision forming to make a quality part.

The techniques I am going to describe can be easily adapted for the home shop. It is very helpful to have a CAD system of some sort for the flat pattern calculations and to plot a full-scale outline of the profile. If you have the math background the calculations can be performed with a scientific calculator. You could also manually lay the profile out full size and through a process of trial and error, come up with a flat pattern by measuring and guessing. Making scrap is very expensive for me, so I prefer to invest the time up front to make accurate parts.

Bend allowance tables don't work well for these parts because there is a .375" radius for the large bend around the ¾" tube going directly into a .125" radius bend for the flange. You have to know the number of degrees the bend is going through to use the bend allowance tables. If you are willing to wade through the math necessary to figure the angle of the arcs, go for it. I prefer to use a CAD system. It's just like a large pocket calculator that does most of the work for you.
{short description of image}
The first thing we need to draw is the profile of the part. I usually start with a full 3-D solid model of the part I am going to make. The picture above is a screen shot of the solid model I had on my computer. On my screen the background is normally black and the part image is much higher quality, but I wanted them to print without using all your toner or take 3 days to download.
{short description of image}
The next thing required for this part is to split the solid in half as shown above. This is a very easy thing to do in a solid modeler but if you are doing a 2-D wire frame drawing, just draw the profile of the part through the center to start with. Now for the beauty part of your CAD system. Offset the profile .4466 * T (material thickness) from the inside of each bend. This is a close approximation of the neutral axis of the bend and is represented by the green line above. You can't really see it well in the image above but the ends of the green arcs do not line up (click the image for a larger version). This doesn't matter though because all we need to know is the length of the green entities to find the overall length of our flat pattern. Most CAD systems have a function to analyze the length of any entity. Use it for each one and add them all together. You now have the overall length of your flat pattern.
{short description of image}
The next step is to lay out the flat pattern. If you are only making one part, you can lay it out directly on your material. I do it in my CAD system again because I cut the flat pattern on a CNC mill and I need a model for programming. Cut out your parts to get ready for forming. You can cut the radius on the end and drill the two holes after forming if you want. If you can cut the flat pattern accurately and bend the two .125" bends accurately, it is easier to drill and cut the radius first.
{short description of image}
Time to form the part. The two .125" bends need to be located very accurately on the flat pattern in order for the .375" gap to come out right. If you can imagine, miss-locating the bends by a small amount will make a large change in the gap. You should also concentrate on making both bends the same. I prefer braking parts on a press brake. Parts tend to slip in a leaf brake and the bends will be miss-located. You could also form the part in a vise but it is very hard to locate the bend correctly. In a press brake there is a backstop to locate the bend accurately and consistently. Because the part has a radius on the end, I clamped a 1-2-3 block to the female die to square the part to as you can see above.
{short description of image}
I plotted out the profile full scale to check my parts against. Use some scrap material to check your setup before forming your good parts. If the scrap has square corners, it is very easy to check against the plotted profile because the square corner will be down against the plot. You don't have to eyeball straight down and guess at the length of the flange if the corners are square.
{short description of image}
Here are eleven parts ready for final forming. See how consistently the press brake forms the parts.
{short description of image}
This is a model of the form tool for forming the .375" radius. The green pin is what determines the size of the radius. If you have ever formed any sheet metal you know that you have to over-form the material because it springs back. The radius of the bend springs back as well as the angle of the bend. The larger the bend radius, the more the bend radius springback becomes a factor for making accurate parts. I have springback information in my Fabricating Precision Sheet Metal Aircraft Parts article. For this .375" radius I made the forming pin .700" diameter. The red pin is pressed into the blue block. It holds the material in place for forming. The cyan pin is pressed in the handle and does the forming when you rotate the handle.
{short description of image}
Here is one of the parts on the tool. The distance between the .700" pin and the .250" pin is .063", the material thickness. The farther the small pin is from the large pin, the farther the .375" formed radius will start from the .125" bend. When forming the part, do not wrap the part completely from the first side. It will not form the part completely to the second .125" bend if you do. Start the .375" form on the first side, and then flip the part to finish the form from the second side.
{short description of image}
This is the finished product setting above the full sized plot. The other strap was formed with the same setup, it was just formed farther around. This technique can be adapted to other parts as well. I hope you find it useful.

Home | Fuselage | Wing| Empennage | Engine | Fittings | Shop| About Me

This document maintained by paul@vermilyea.com.
Material Copyright ©; 2003 Paul Vermilyea