Building Masters For A 1/48 SEAWOLF Pump-Jet, Part-4

Work went pretty quickly yesterday -- assembled the blades to the rotor hub and

faired everything in. Here's the story, in all its gory details.

Today I'll hack out the cones that forms the forward portion where the pre-swirl
stators fit, and also the dunce-cap with its projecting post-swirl stator blades. This
weekend I'll do battle in the shed with my wood turning lath -- blasting out the big,
two-piece shroud ... chips will fly!

A simple blade assembly jig was used to position and index the six blades around the rotor hub. The hub sits on the face of the jig, its 3/16" shaft projecting into a hole about which the hub can spin. The jig is marked off with the six radiating lines from center and a single index mark placed on the base of the hub. Rotating the hub to match its index mark with a line insures exact equal distancing between blades as they are glued to the hub. Some of you will remember the rather complicated jib I used to index the many blades of the VANGUARD pump-jet --needed because there were so many blades.

The crutch (formed from Evercoat filler) was formed off of one blade that had been attached to the hub -- that initial blade aligned with the aid of the blade alignment jig discussed previously. Coating the bottom of the blade (the suction back) with mold release was insured that there would be no adhesion problems as I slathered on the filler between jig surface and bottom of the blade. Once the crutch is formed it becomes an alignment foundation that will insure perfect symmetry of blades as they are glued into the slots cut into the hub. Those slots are marked off with a blade that was cut to be a sight fit against the side of the hull. A pencil traced around this marking blade marked out where I would have to cut a recess to accept the base of each blade. Renshape 40 is easily routed out with a Moto-Tool equipped with a 1/16" drill bit. Once the blades were trimmed, the hub recessed cut out and the crutch formed, it took only ten minutes to secure the remaining five blades to the hub. I used CA adhesive filled and accelerated to a hard cure with liberal sprinklings of baking soda.

It's a good practice to build up the propeller/rotor with a diameter a bit oversize and to face off the blade tips on the milling machine. In support of that job I created a simple holding fixture that would permit me to rigidly mount the rotor upon its blade assembly jig atop the bed of the machine. The pin through the hub not only extended downwards through the blade assembly jib, but also down into a hole drilled atop another wooden plate bolted to the bed of the machine. The object is to set the work so that by hand rotating the rotor/blade assembly jig over the bed, the end-mill cutter could take off the required amount of material to achieve the desired rotor diameter.

It was vital that the blades of the rotor, as they passed through the cutter bit, be supported rigidly -- white metal in such thin section bends easily, and the CA adhesives used to secure the blades within the hub fails easily in the presence of shock loads (vibration). That's where the crutch came in for further use. With fingers out of injury range (just barely!) I pressed a blade down on the crutch, the fixture was rotated into the arc of the cutter, and the excess metal cut off the tip of the blade. For each cutting operation, the machine was stopped, the rotor lifted off the fixture, a fresh blade positioned over the crutch, and the cutting operation repeated. Damn, I'm good!

The raw rotor master with it's tips trimmed to correct diameter. At this point I applied fillets to the base of each blade, using only my finger as the radius tool. Once the filler had cured I worked it with files to get a uniform, smooth transition where the blades penetrated the hub. I then gave all blades a good scrubbing with #100 sandpaper to ready the metal for pickling.

To insure good, tight adhesion of the primer to follow I 'pickled' the white metal blades by soaking them in Ferric Chloride acid, followed by a thorough rinsing in fresh water, spiked with a little making soda to kill the acid. The acid has no effect on the filler or the small celled polyurethane foam which comprises the Renshape 40. The work was then dried and given the first coat of primer. The neutral gray color of the primer permits easy identification of pits and other flaws.

The pickling produced millions of microscopic pits onto the surface of the metal

blades -- insuring excellent mechanical 'tooth' that resulted in a very tight adhesion of the primer to the metals surface. Without pickling, most metals do not adhere at all well to primers and paints. Here I'm using my Paache H Model spray-brush to apply a thin coat of primer -- at this stage I only want the gray on to help me find flaws in the finish of the rotor. Later, heavy coats of primer will work to 'fill' tool marks and scratches, but not the first coat, it's just for looks. Use automotive products here -- the hobby shop putties, fillers, primers and paints are shit!

After the initial primer had cured, the master was examined under a light so positioned as to cast shadows on the blemishes observed on the surface of the master. The pits and holes were filled with Evercoat two-part filler. I prefer their, Glazing Putty, a very tight grained filler that works easily with file and sandpaper. Once the filler had cured I worked the master with files and sanding sticks, then repeated the pickling and priming operations. All these steps repeated until a smooth finish had been achieved. Only after the master was in this condition would I go on with the next operation: manufacture and installation of the ring which fits to the tip of the rotor blades. Stay tuned, sports fans.