Blunting and rounding a Diamond Shape Leading edge

[Mike Griffin]

I was wondering if anyone had ever come up with a sanding tool/form for blunting and rounding off a square leading edge when put into fish mouth ribs in the diamond shape.  I have always just done it the old fashioned way by using a strip of sandpaper and "shoeshine" it.  A mold with sandpaper lining seems like it would work better.  Anyone have a better method?

Mike

[FLOYD CARTER]

I use a "sanding bar" about 30" long and 3" wide.  Attach sandpaper with 3-M spray adhesive.  The sanding bar makes a straight edge.  You have to "eyeball" the radius, but that isn't hard.  You are way ahead by starting with 3/8" or even 1/2" square leading edge before  planking.

[Dan McEntee]

  I carve it to rough shape with a razor plane, then use a sanding bar like Floyd mentions. To fine tune it, make a contour sanding block from scraps of plywood. Take two rectangular pieces of 1/8 or 3/32" plywood and transfer the airfoil leading edge to each one and cut out. Make sure they match. Assemble theses to more ply or hard balsa to form the basic tool. Then lay in some 1/64" ply into the airfoil shape to give you a surface to attach sand paper to. when you have gotten things close to finish with the T-bar, finish up with the contouring tool. Then try to keep track of the contouring tool until you build another wing so you won't have to make another one!
  Type at you later,
  Dan McEntee

[pat king]

In my kits I machine a radius on one edge of the square stock to reduce the sanding necessary to finish the leading edge. I used to machine them with a router mounted in a router table. I now use a wood shaper.

Pat

[Dave Hull]

Mike,
You may be thinking of the contour sanding bar from Great Planes.

     http://www.greatplanes.com/accys/gpmr6190.php

This is an extruded aluminum bar that you can attach your self-adhesive sanding roll material to. This paper is quite durable if just used on balsa. I don't actually have one of these, so I can't comment on the larger radius on the tool as far as suitability for a particular stunter's wing.

I do have a homemade version, probably made from solid birch, that I cut a cove into. If you have a table saw you can do this with a skew cut--as deep as you'd like to go. The shape approximates an ellipse, but is wider by the thickness of the blade. With standard router bits you can cut an internal round. Sand smooth. A little polyurethane would make the paper stick better.

If you are patient and have a good eye, and can restrain the wing without warping it (to avoid sanding the warp into the structure) then just a flat bar works fine as Floyd suggests.

There are also some contour detail sanders out there for furniture making. These are much shorter, but would work fine if you work evenly along the full length. Here's what Rockler has, which would set you back ten hard-earned Simoleans, less one cent, plus shipping of course.

     https://www.rockler.com/rockler-angled-contour-sanding-grips

You could make a custom one of these, say 6" long. Just make up a sample wing section, then pour urethane molding compound or similar into a form-box around it. Cure. Pop loose. Attach paper. Go to work.

Lots of choices. Find one that meets your expectations that you can live with....

The Divot

[Greg McCoy]

I used a router with an MDF fixture.

[Mike Griffin]

Thanks to everyone for their input.

Mike

[Dan McEntee]

I used a router with an MDF fixture.

     I would be interested in a more detailed thread on how you do this. Might open some doors for some ideas in other areas, such as airfoiled shaped leading edge, or replicating the milled leading edge stock like the old kits had.. I don't have any wood working tools other than a band saw and a 4 inch power plane, but would invest in the rtight tools to make neat and usable stuff.
   Type at you later,
  Dan McEntee

[Dave Hull]

You can get custom-ground bits that would match any LE shape you want. These aren't cheap, but since balsa is neither very hard or very abrasive, they might last a long time. But they have to be very sharp to cut balsa. With a router, I found that if a feather of wood pulled out of the stock and wrapped over the cutting edge, you were kind of done. Any comments or experience Greg has along these lines would be good info.

Not sure if wetting the wood would help, haven't tried it, but would be a worthwhile experiment. Might even stick the wood in the freezer before cutting?

Recall that an awful lot of guys buy a heavy duty plunge router so they can table mount it upside down to mimic a shaper. An actual shaper is a bigger, more powerful and much more expensive machine--which would be total overkill for any modeling task. But the control of having a router table would be very useful. A shaper has a spindle, and the cutters stack onto it. Router bits are generally one piece deals.

Gang-cut LE stock such as is in the Brodak 1/2A kits could be cut by a shaper or horizontal mill or equivalent. Single separate cut LE pieces could be cut by router, though. The setup Greg shows seems logical for a diamond rib notch. For a flat-front rib you'd need more sophisticated setup and cutters. A little curious if Greg gets much (any?) chatter from the wood in his setup. Or if he uses any double-stick tape?

One of the issues with any shaped parts is how straight they are afterwards. Most of the stock is going to bow. You may be able to straighten it, or may be soft enough that you just pull it into line during assembly. Heavier wood is going to be easier to cut, but I would guess more likely to bow. Hobby shops seem to have ancient LE stock that is so bowed that it never sold. It does keep the rack filled up though, just as it has the last twenty or so years....

The Divot

[pat king]

The router fixture shown will work OK for a small number of different sizes of leading edge. The back-up/router spacer board will change for every different square stock dimension. I use router bits in the shaper. I have more than 30 different leading edges I produce. They vary from 1/4" square stock to 1" square stock. They have corner radii from 1/16" to 1".  I use a 1/2" square carrier piece with a 1/4" corner notch for the 1/4" square stock. When I design a wing I set the radius about .010" beyond the actual airfoil shape at the closest point. Some balsa comes curved, some curves as it sits in the shop, some curves after one corner is machined, some starts straight and stays straight. If the leading edge is curved it can easily be straightened by wetting it with Ammonia water, strapping it to a straight piece of extruded Aluminum angle and allowing it to dry thoroughly. For any extended amount of cutting use Carbide edged router bits. Wood is abrasive, it will quickly dull carbon steel or high speed cutters if they are run at wood cutting surface speeds. If run on a mill at Aluminum cutting surface speeds they will last for a longer time.

Pat

[FLOYD CARTER]

In the past, I've attempted to shape balsa using a high speed router, or a table saw.  In all cases, the heat generated by high speed cutters tend to warp the wood due to heating.  Slow and easy is the ticket.

[Ken Culbertson]

Creating sanding tools is something that I never did in the past.  Now I have had one of them for about everything you can sand.  I use a band saw to cut a 3" block mold and line it with sandpaper. (put a little taper at the edges to keep from sanding in groves.   If you are duplicating an airfoil you can wrap the sandpaper around the LE then mold some wet 1/16 around that.  When it is dry lightly soak the curve with some thin CA.  Don't plan on needing to do shaping any more though.  Molding!!

Ken

[Peter in Fairfax, VA]

FYI, at least check, the Great Planes Easy Touch Sander was made of "unobtainium."  I sold some from an estate on eBay and they were very, very popular.  Here is an auction running right now:

[Matt Brown]

Nothing in my modeling history has been more satisfying than shaping LEs and other parts with my David razor plane! I’ve had several other hobby grade razor but none compared to my David plane.


Matt

[Ken Culbertson]

Nothing in my modeling history has been more satisfying than shaping LEs and other parts with my David razor plane! I’ve had several other hobby grade razor but none compared to my David plane.


Matt

A right of passage...
Ken

[Howard Rush]

I did lots of these with a sanding block.  It's pretty easy. 

Lately I've been using the router-mounted-under-the-saw-table trick.  My most recent adventure was roughing a 1.6"-radius LE for a couple pointy-LE stabs using a 1.5"-radius router bit.  I made a box that mounts on the rip fence against which to slide the stab past the router bit.  The bit is a monster and vibrates so much that the router recedes into the table. 

[pat king]

The picture shows the leading edges and splices I make at this time.

Pat

[Joe M]

pat I cant read the sizes in your attachment. Can you supply .625 radius on .75 square stock?  30 inch in length?
 Joe

[Mike Griffin]

pat I cant read the sizes in your attachment. Can you supply .625 radius on .75 square stock?  30 inch in length?
 Joe

Joe did you try tapping on the picture and blowing it up?  The resolution remains good if you do and you might be able to read it that way.

Mike

[pat king]

pat I cant read the sizes in your attachment. Can you supply .625 radius on .75 square stock?  30 inch in length?
 Joe

Joe,
I have the following radius cutters: 1/16, 3/32, 1/8, 5/32, 3/16, 1/4, 5/16, 3/8, 7/16, 1/2, 9/16, and 3/4. I have a 5/8 R. cutter on order.
I can put a radius on any length up to 48" as I am now set-up.  I could do 6 or 7' pieces if I rotate the machine and clear a path for the stock. I don't think it is worth messing with pieces longer than 4', besides I don't think balsa sticks longer than 4' are available.

Pat

[Joe M]

Thanks Pat. since I need 5/8 i will hand shape the set i need.
 Joe

[pat king]

Joe,
I'll have the 5/8 cutter in 6 to 8 days.

Pat