Monday, November 28, 2011

Window Latch Aluminum Link

The Aluminum Link is made from .040" aluminum.  In order to form the shape needed it will be made from 5052-H32 aluminum.  To form the channel shape and the curved end I made a hardwood form block.  The first step was to locate and drill the 2 holes for the pivot pins.  The holes won't be drilled in the sides of the link until each side is bent.  The sloped end was then cut for shaping the bend in the top of the link.  A 3/32" radius was then filed on the 2 corners where the bends are formed.
Except for dead soft aluminum there is always some spring back when you make a bend.  As a result the form block needs to be tapered so the bend will spring back to 90 degrees.  The simple way to figure out how much to overbend the part is to just take a sample piece and bend it 90 degrees. The sample can be used to set the angle on the sander to taper the form block and check the finished block to make sure the angle is correct.

With the form block ready a template was made for the part.  The pivot hole will not be punched in the blanks.  It helped me to understand the part.  The small hole is for attaching the knob.  On the original part this was done with a 3/32" rivet.  I made the hole 1/8" so I could use it as an alignment hole to position the blank more easily for bending. And Locating The Pivot Holes.
A 1/8" hole was drilled in the edge of the wood form block where the rivet will later be installed in the part to attach the knob.  A hardwood backing block was made from 1/4" stock with a hole in it for a 1/8" steel rivet to serve as a dowel pin.  The hole was counter bored to allow the rivet head to be flush with the edge of the wood when everything is clamped in the vise for bending.

Don't hammer directly on the metal.  Use a wooden block to hammer on so the forces are evenly spread and the aluminum doesn't get dented.  With the relief angle on the form block the bend springs back to a nice 90 degrees.  I love it when a plan works.
Using the rivet hole to locate the part on the form block the 2 pivot holes on the first side can be center punched with a 3/16" transfer punch.  The 3/16" pivot holes can now be punched with the Whitney punch.
Two 3/16" bolts and the rivet are were used to hold the part in position for the second bend.

With both bends made the transfer punch was used again to center punch the pivot holes on the second side through the form block.  This assures the holes are positioned at the correct spacing and square so the linkage will move freely.
There is no way to get the punch into the marks for the hole on the second side so they need to be drilled.  I find regular twist drills hard to control accurately.  I also find it's easy for a twist drill to act like a reamer and open up the first hole in the thin aluminum by rubbing on the side of the drill.  The small pilot drill on a center drill along with it's smooth sides make it perfect for drilling through the first hole into the center punch on the inside.  You just have to leave enough of the drill sticking out of the chuck to drill through the second side.  I also used a thin 3/16" washer to protect the aluminum from the spinning chuck.
We now have a well formed part with all the holes properly located.  The next step is to form the end down to a nice curve.
In order to form this end down it is necessary to shrink this end of the channel.  The trick is to form the metal inward to form a wave in the middle, anneal the wave and then smash it down flat without widening out this end of the link.  This is why I chose 5052, it's easy to anneal the area needed without annealing the whole part.  First I ground off 2 washers to use to hold the metal tight to the form block while giving room to hammer the corner to the middle with the rawhide mallet.  The rippled area was then annealed by coating it with soot from the acetylene torch and warming it with a neutral flame to heat off the soot just in the rippled area.
To keep the sides from moving out the link was put back on the form block and the sides protected with fender washers.  Then the ripple was hammered down.  A harder form block would have made this work out better so I made a small steel block to just planish the area a little.

The finished part should work just fine.  The next part to make is the steel link.

Saturday, November 19, 2011

Window Latch Door Brackets (Cessna 0411595)

These brackets are the 2 aluminum brackets riveted and screwed to the inside of the door.  They are made of .040" aluminum and are badly worn.  Because they are so small and so worn, I've decided to replace them with steel parts made from .040" 4130 steel.
 These are easy parts to make by cutting out a blank bending it and punching the holes.  I would like them to line up with the existing holes and have the pivot hole square to help the latch work easily.  The first step is to make the blanks.  I prefer to make a template for every part from galvanized steel.  It allows me to work out any issues without wasting expensive aircraft materials.  If you are making more than one part it saves layout time.  Also holes can be transferred from the template to the part so they all come out within a tolerance which make them interchangeable (remember Henry Ford).
 The 2 mounting holes are 7/8" apart.  Once the template is made you can check it against the holes in the door to make sure it's right.  If you're not familiar with matched hole construction go read the articles in Sport Aviation on building the Thorp T-18 (Early 1960s).  Matched hole construction is one of the best kept secrets of sheet metal work.  Today we can accomplish this with a CNC punch press and a computer, assuming you have a couple hundred thousand dollars to spend on a press.  The rest of us just use a Whitney Punch (#5 Jr. Hand Punch) with an extra set of punches.  You need to take one set of punches and carefully grind off the centering nib.
By positioning the template on the metal for the part you then carefully slip the punch without the nib into the hole in your template squeeze and punch a hole in your part.  As I get each hole punched I insert a cleco clamp to help keep the template from shifting around.  You will note I am not adding the pivot hole at this time.  I'm good but not good enough to get them in the right spot after forming so we'll add them later.
 Once the holes are punched just draw the outline with a soft pencil or a scribe.  If you use a scribe just be careful not to scratch somewhere that will end up on the finished part.  We don't want any cracks forming on our stray scratches.  I prefer to punch all the holes I can before cutting out parts.  If I mess up punching hole I don't have to throw away my labor cutting out the part, just some metal.  I made some extra parts just-in-case my clever plan for forming them, etc. doesn't work out well and I think I'll use these latches for my next homebuilt project.
 The parts are cut out and de-burred, ready to form.
 To form the parts I cut a piece of 1/4" x 1" mild steel.  Next I transferred the hole locations to it, from the template,  using the 1/8" punch with the nib on it as a Center/Transfer punch.  In this case there was nothing to do but carefully drill the holes using these center punch marks.  I then counter drilled the back side of the form piece so I could use some 1/8" steel rivets as locater pins for bending the parts.  A 3/32" radius was ground and filed on the corner of the form block and the rivets were epoxied into the holes to make using the tool easier.
 With the epoxy dried the parts just need to be slipped over the pins and the whole thing clamped in the vise for forming.

A hardwood block works fine to bend the parts over the form block.  By using a narrow piece of steel for the form block the part can be slightly over bent to get the angle exactly 90 degrees.

Left and right parts are just a matter of reversing the part when forming the bend.

The next step is to locate the pivot hole in one of the parts.  I used a scrap of some 1-1/2" x 1/16" aluminum angle from Lowe's for this fixture.  First a line was drawn on the inside of the fixture.  The top rivet hole and the pivot hole are on the same line.  The top hole was transferred to the fixture so it was on the line and then the bottom hole, keeping the bent part clamped tightly to the side of the fixture.  The 3/16" pivot hole was punched in the fixture.  By clecoing the parts to the fixture the pivot hole can be transferred so each parts is within a tolerance that they are all interchangeable.

Then on the outside of the fixture a line was drawn through one of the mounting holes and the finished part clecoed in place.  The last part was clamped in place an the mounting hole lined up and center punched on the fixture with the 1/8" punch.  With that part clamped and clecoed to the first hole the second mounting hole can be marked and punched.

Now the part with the pivot hole and the mating part without can be clecoed to the fixture and the 3/16" pivot hole transferred to the second part.  The parts now fit the holes in the door and the pivot holes are squarely aligned so everything will move freely.  I have enough parts to do both doors, they are interchangeable and I have some spares for my next project, as well as the tools to make more.

Owner Produced Parts are really a lot of fun and easier to make than to write about.

Thursday, November 17, 2011

Window Latch Parts (Cessna 0411186)

Since I have no latches for the windows I was lucky to find one for sale from Rick Jeorgensen in California.  This one is worn and missing the knob for opening and closing but it should work fine as a pattern to make new ones.  The latch is a simple folding mechanism which when folded holds the window closed and when unfolded holds the window open.
Here are the parts disassembled.  The key parts are (L - R) the extruded aluminum bracket which is riveted to the window frame
an aluminum link, a steel link, and the simple bent brackets which attach to the door.  There's enough wear that the hole locations are not obvious.  Things like metal thickness are easy with Verniers, etc., but for some of the other dimensions needed I find photos and scans of the parts easier to use.

By using these images in TurboCAD it's much easier to reverse engineer things like hole locations and the outside shape of complex parts.  The drawing at the right shows the scanned images of the key parts and the assembly drawing made to check that the parts will open and fold up correctly.  It helps to keep in mind that these parts were probably designed using a tee-square, triangles and a scale in 1/32" increments even though drawings were dimensioned in thousandths of an inch.  To scale the images I measure some easy dimensions and adjust the scale of the image until those dimensions are correct.  For the 2 links I measured the inside edge to edge distance between the 2 holes, easily done with a vernier caliper.

Time to go make some parts.

Sunday, November 13, 2011

Door Handle Stops (Cessna 0411615)

The inside door handles work in the reverse of what you would expect from opening car doors. To open the door you push the handle forward. It has to do with how the handle connects to the latch. There is a long thin strip of aluminum which is pulled forward to operate the latch. Because the handle is located just below the window the link from the handle to the strip makes the handle work in reverse. This could have been solved by placing the handle about 3 inches lower on the door but it would have put the handle in a very awkward location.

Because of all this a stop is needed to prevent buckling the long aluminum strip if someone pulls on the handle instead of pushing. The parts manual shows the stop (20) but mine were both missing. From looking at the picture in the parts manual the stop has 2 tabs which fit into 2 of the 4 holes in the inner bearing plate for the handle. The outer bearing plate is identical but without the holes for the stop. This stop looks weak, even more so if it were made from aluminum, which is probably why they were both removed. A change to the design was needed to make a stop which would work.
The material could be no thicker than 0.040" so a piece of .040" 4130 steel was used to make the stops. The original stop appears to be loaded in bending, which make it weak, so the new stop tab was designed to be loaded in shear with a welded corner for extra stiffness (see pictures below).

The process for making the stops was straight forward.  A paper pattern was made to check the fit.  The parts were laid out on the steel and as much hole punching, drilling and cutting done while the parts were still held on the sheet of steel.  The tabs were filed close to finished size but the notch for the stop was not cut to shape until all the bending was done.

Once the tabs were all bent they were filed to a snug fit in the holes of the bearing plate.
The Stop Tab was then welded.  The notch could then be cut and filed to a good fit so the handle would stop with the latch in the fully closed position.

The pictures below show the stop with the handle in the latched position and then in the unlatched (pushed forward) position. The cotter pins have not been bent for these pictures because that can only be done once all the parts are in the doors.

The left door with the latch and handle reassembled.  Next is the window latch, new glass in the window and then seals for the window and the door.  I don't plan to install the interior fabric panels until the plane is flying or I win the lottery.