The Dragonfly

Last Update-03/13/2010 01:41 AM         

Page 2 - Casting and Machining                                       51mm.  Machined set         


The lightening holes that are cut into the ribs and spar need to be flanged for added strength.  I have seen some people make the flanging punches from wood as well as steel and aluminum and all seem to work ok.  I have decided to build two patterns for the punches and have them poured from T356 aluminum at a local foundry I use to build patterns for.  The foundry only charges about 3 bucks a pound for poured aluminum.  6061-T6 plate for the same project would run me at least 150 bucks.  So, Ill build the patterns.



The first thing to do in building a pattern is to make a layout.  We have to draw the layout a bit bigger taking into account the shrinkage of the metal as it cools.  I will build a pattern for the male and a second for the female.  The foundry can make many copies from the same pattern.   A round pattern like this needs only the side view laid out to be able to build it.



Utilizing a set of dividers, I measure the distance from center to the outer edge of the punch and add about an 1/8".  This will give me some extra material for turning it on the lathe.





Next, I draw a circle on a piece of 1/8" MDF.  I'm creating what they call a sector or segment.  I divide the circle into six equal parts and cut out the sector.





Next I lay out 12 sectors on a piece of 3/4" pine.  It will take 6 to make each course for my pattern and each pattern will have 2 courses.  (Usually a patternmaker No No. You should always use at least three course to avoid warpage but I think Ill be ok on this one) I'll need another twelve to make the second pattern.




Here you see 12 sectors drawn onto the face of the board.





The sectors are rough cut out of the board to within a 1/16" of the line.





Now, sand till the line just disappears.  You may have noticed an X written on the tops of all the sectors.  This is to aid in identifying the top and bottom of the piece.  You see, the sander can never be set exactly to 90 degrees.  We set the sander for about 2 degrees and then  when we glue the sectors together, we simply flip flop the pieces and they butt perfectly together.  Neat trick ay?




I guess I should explain why we segment the circle like this.  We are trying to achieve the "longest chord of arc" across each segment at the outer edge.  We want as little of the end grain on the outer edge as possible to help in turning a nice smooth surface with the lathe.  You'll see as we progress through this project.

After "sizing" the exposed end grain with a bit of glue and allowing it to dry, we glue the pieces together flip flopping them to give us a nice little semi circle.



The completed semi circle.





Use a good quality wood glue for a project like this.





I forgot to take pics of the full course or layer being glued up but here we see the two course glues face to face to give us enough thickness to be able to face it down to 30 MM thick as required by the pattern.  All dimensions will have the shrinkage factor added to it while building the pattern.




Thought I'd throw in a pic of the US Navy Patternmaker manual.  A lot of good stuff in there.







I have cut a false faceplate to attach to my cast iron faceplate on my lathe.  The pattern stock will then mount to the false faceplate once I face the surface of the false faceplate smooth.  This helps keep my bit off the cast iron by spacing the pattern stock off the iron faceplate.




Here the false plate has been mounted and ready for facing.





Facing the false faceplate. 





Now we mount the pattern stock we made to the false face plate using screws through the back of the false face plate.





Screwing the stock to the plate.





Here I take the thickness of the stock plus a bit for machining allowance I forgot to add to the layout,,,,,





and transfer it to the stock mounted in the lathe.





Using a surface gauge, I scribe the thickness all around the pattern stock by rotating the faceplate.





Now,  face that bad boy to the correct thickness.  Stop just when the line disappears.





The stock faced down to thickness.  Those segments start looking pretty after awhile.





With the dividers, I pick up the outer diameter on the layout.  Notice that the bottom of the pattern layout is wider than the top.  I added 3 degrees of "draft" to the pattern to it will pull out of the sand during ram up in the sand.





Its easy to find the center point of the face.  Simply run the bit all the way to the middle and then hold a pencil in the point darkening the spot.  The use the dividers to scribe the outer diameter onto the pattern stock then darken in with a pencil.  Patternmakers mark everything with knife cuts or scribe lines and then darken them in.  Then you can sand till the line just disappears for the exact dimension.



Now pick up the draft angle from the layout with the sliding T bevel. We will use this to set the compound slide on the lathe. 




Setting the draft angle on the slide.  Now we can turn the outer diameter with the draft included.





Turning the outer diameter.  This lathe is so small you have to get creative on how you set up your tool holder.  I WANT MY NAVY PATTERNMAKER LATHE BACK !






I sort of screwed up by making my segments solid for the female die.  I have never tried this before, but I needed some way to cut out the center of the beast to form the opening so I enlisted the trusty circle cutter.  No need to pay for more aluminum than you need right?  Plus, it will make the machining faster.




Well, the trusty circle cutter wasn't long enough so I had to cut a bit out of the center for the snout of the circle cutter to go into.





See, like this.  The center popped right out when I hit bottom.





Finished pattern.  Just needs some sanding.  I forgot to get pics turning the center hole, sorry. 





Nice and sanded still on its mount.





Just wanted to give you an idea of how large it is.  Man is it nice and smooth.





This is why many people collect old foundry patterns.  They are generally very neat looking when complete.  Just look at those segments.  Now we need some lacquer.




WOW, what a before and after.  Now I just need to turn the right one.  That will be the punch portion. 





Tonight I turned out the punch and I'm done!  A little Deft, some 320 grit paper followed by some 000 steel wool and we are ready to take these bad boys to the foundry. 




Most patterns are built to form the majority of the shape of an object with enough machining to get it to match up to whatever it needs to.  The patterns I have made here are more for billet stock to give me the correct size of material needed to machine the punch and die.  More to come after casting.


On to the Casting