Saturday, May 11, 2013

Teflon Fabric for Rub Strip Arrived

 The Teflon fabric is here.  I bought 1 yard (37 1/2" wide).  I have a nice straight edge for cutting.  It's an aluminum extrusion which is easy to hold, doesn't bend and has rubber on the bottom so it wont easily slide while cutting.  It happens to be 3" wide, just what we need for these strips of fabric.  It must have been meant to be.

I like my disc cutter.  It doesn't wander as easily as a knife and it doesn't ruin the cutting board surface as fast.  They make various discs for it.  I mostly use the straight and pinking discs.
Very quick and easy.

 To make it easy to sew the slippery strips I ironed a fold into them.  You need just enough heat to hold the crease while sewing.

 The first stitch is about 1/4" from the edge to hold the ends together once the felt is inserted.  I used some Nomex thread because it is designed to take the heat.  I used red so it would show in these photos.
 To insert the felt I sharpened the end of a piece of welding rod with the belt sander and bent the end.  Think long Bodkin.  You slide it in the fabric pouch, hook it in the felt, and pull the felt through.  Very simple.

Ready to sew the second stitches.

 You need a presser foot for sewing welting.  I wanted to use my good machine for this but never bought that foot.  I used the foot with our cheap Singer which appears to be for sewing zippers, but I got them sewn.

Just like new.  Ready to cut to length and staple to the baffles.

Tuesday, May 7, 2013

Cutting Felt Strips

My first bright idea was to make a guide to hold the felt straight while I cut off a 7/16" wide strip with a razor blade in a block to hold it square.  Felt this thick is just too hard to cut with a razor blade.  I managed to cut off some strips but then cutting the 1/2" x 7/16" strip to 7/16" sq. just did not work at all.
OK dump that idea.
My next idea was to use the band saw to cut off strips.  I had once heard of cutting foam rubber with a blade turned upside down so the teeth don't tear the foam, or in this case shred the felt.  When cutting vinyl house siding with a circular saw you put the blade in backward.

I decided to do more than that.  I wanted to put a pointed edge on the blade more like a serrated knife.  To do it I tipped up the table on the belt sander and used it to guide the blade along the belt to sharpen each side of the blade.  I used a worn out metal cutting blade for this.

To install the blade upside down you just turn it inside out.  I used some 1/2" aluminum angle to make a guide to hold the felt while sawing it.  I thought it would be easier to cut a straight line if the felt was held down snug as well as along a fence.  The angle is used to hold the felt down.

Because I want strips 7/16" wide the angle also made convenient guide for the width of the cut.  The angle is 1/16" thick so having the blade just touch the edge of the angle gets a 7/16" wide strip.

I had to hold the felt snugly in the guide but it worked as expected.  You mostly hold it in the guide and pull it through from behind the blade.  The curved ends at the front of the guide are to prevent the felt from snagging, it worked.

To control the thickness of the felt during the cut I positioned the angle differently for the first and second cut.  For the first cut the sheet of felt is 1/2" thick so the angle was positioned with the bottom of the flange 1/2" above the bottom of the wood.

For the second cut the strip is now 1/2" wide but only 7/16" high so the angle for this cut is positioned 7/16" form the bottom of the wood.

For the second cut I needed a guide on the free side of the strip to hold the felt compressed in both directions.  A full fence would have gotten in the way of pulling the strip through the blade.  Instead I used piece of 1x4 in the area of the blade.

It worked great.  The cut is still slightly fuzzy.  I need a sharper blade.

The strip on the right is the first cut and the one on the left is the finished strip.

Now I need to sew up the fabric piece which hold the strip in place.

Thursday, May 2, 2013

Cowl Baffle Seal Felt Arrived

The felt arrived from The Felt Store.  The smallest pieces they sell are 1 ft. x 6 ft. (the width of the roll).  It appears the SAE F-15 felt is the best choice.  The F-26 doesn't seem to spring back as well.  It also seems to be made from coarser wool fibers.
Now I need to rig up a way to cut nice even strips.  The Cessna strips are 7/16" square but they only sell the felt in 1/8" increments so I bought it 1/2" thick.  I'll need to cut the felt in both directions.
I think I need some 1/2" angle to make an edge to hold the felt against while I make the first 7/16" cut from the sheet and then another piece of angle to make a channel to hold that strip while I cut off 1/16".  I think I could use a single edge razor blade with a holder to run along the angle.  Sounds like a trip to Lowe's.

Wednesday, May 1, 2013

Engine Baffle Seals ( Rub Strips P/N 0450234 )

 Last summer I started working on the seals for the engine baffles.  I ordered some SAE F-3 felt only to have it arrive and realize it was way to stiff to use for this.  So let's start over.   The Parts Manual calls these Rub Strips.  This is probably a better name for them since they rub on the cowl as the engine shakes in the cowling.  They're on the back baffle and the ones on each side along the cylinder heads.  The side seals are long gone but the ones on the top of the back baffle are still there.
 This section of seal has been wired on with some safety wire.  This is not how the factory did it.
 The seal is made of a strip of soft felt in a rubberized piece of fabric.  The seal is assembled into a "P" shape with the loop of the P setting on the flange of the baffle.  The tail of the P is stapled to the wall of the baffle.  At first I thought the felt was 3/8" square.  After more careful measuring I believe it is 7/16" square. The loop for the felt is 1 3/4" from the stitching around to the stitching, 7/8" if you lay it folded flat.

 The staples as I noted earlier will be replaced with 0.041" safety wire staples.
 To figure out what felt I should have ordered I needed to determine the density of the felt and the pressure required to compress it 10%.  The other specs are beyond my abilities to determine, and probably won't matter if no one sells felt with all the different percentages of wool in the SAE specs.
My first effort was to determine the compression.  I washed the felt in MEK to remove years of oil.
To compress the felt I'm making a channel to stack 2 pieces and hold them in place while compressing them.  I'm using some 1" aluminum angle for the walls of the channel.  I've clamped the first wall to a piece of scrap steel.  It's overkill for stiffness but it was free and it actually worked well.
 On the second wall I inscribed a line .078" from the top.  The wall is 1" tall. The 2 felt pieces total 7/8" tall and I'm using a piece of 1/8" aluminum for a plate to apply pressure to the felt.  When the top of the plate reaches the line I've got 10% compression.  OK it's not perfect but I'm just trying to decide which felt to buy from a list of available types.

I used the compression plate as a spacer to position the second wall to give me a 7/16" gap.

 The one strip was long enough to use for the 2 pieces.

The test fixture looks like it should work as planned.

 I set the test end of the fixture on the scale and the found a balance point along the steel which gave me a starting weight close to zero to give me the full range of the scale for the test.

 I marked a center point on the on the compression plate and pushed down with a pencil until I could see the line on the wall.  To take the readings I just photographed the scale, the joys of digital cameras.  After several readings I averaged them to get a compression of 1.5 psi at 10% compression.  Based on that the felt choices would be SAE F-12, F-13 & F-15 at 3 psi or F-26 at 1 psi.
My sample piece was small and weighed only 10 grams.  which converts to 11 lb/ sq. yard (1" thick).  The first 3 felts are spec'ed at 8.48 and F-26 at 7.2 so I need a better scale to do more with that.

In the end I ordered a piece of F-15 and a piece of F-26 from The Felt Store.  Yes there is such a place.  We'll see which one works best when they arrive.