Large Propeller Issues

[Swede]

A bit of background - I've got a large-ish radial engine that requires a propeller maybe 22" to 28" in diameter:

http://www.homemodelenginemachinist.com/index.php?topic=14182.0

Finding a prop this large isn't difficult with giant-scale R/C being as popular as it is, but I've always loved the look of variable-pitch propellers... with the round prop root, concealed piston, etc. Years ago, when this engine was made, there was a company called Syn-Tech in Canada who made cool ground-variable props with the correct "look." I bought a 2-bladed hub:

The ground pitch adjustment uses an insert in the hub that bears on pins in the prop root. The prop blades are hard maple. To make the prop work, I had to discard the adjustment mechanism, and machine a new insert. This was OK, because I could still adjust the pitch with a protractor. The keyed round interior of this mechanism is my own fabrication. It's worked well for a long time.

But the overall diameter was too small! Syn-Tech went out of business. Now, there's Moki props and more importantly a nice guy who has revived the line of large props. Blades and hubs are now available. They are expensive, but the quality is very high.

http://www.soloprops.com/

I bought a used 3-blade Moki hub, which is identical to the Syn-tech line. I needed to populate the hub, so I ordered 4 ea. 24" blades from the solo prop guy. They are very nice, but I have a problem... the diameter at the root is exact, but the machined retaining groove in the prop hub has changed profile! It's gone from square, to grooved... Old prop on the left, new on the right.

The fancy new blades no longer fit either hub. I have a couple of options:

1) I can machine the propeller blade root. I have done almost NO wood turning in my life, and I am a bit hesitant.
2) I can buy a new 3 or 4 blade hub (cha-ching! $250)
3) I can modify my own hub(s) for the new, rounded retaining flange. This would be relatively easy for the 2-blade hub, much trickier for the 3-blade hub.

Option 1 seems the most obvious. I have 4 blades (background, first pic), and I'd rather not scrap any, but I could live with one lost to buffoonery. I plan on using Min-wax wood hardener afterwards, or perhaps some thin epoxy painted in to harden and strengthen the root. These props will spin nowhere near max RPM.

Any suggestions on how to execute all of this would be appreciated. I also am planning a new stainless steel inner hub for the 3-bladed system. When done, it should look cool.

 

[maverick]

I understand not wanting to turn the groove on a lathe and have the prop hanging out there. How about gripping the
OD at the hub end in a collet in a spin indexer and mill the groove with 1 pass for every 1 or 2 index divisions.

 

[Captain Jerry]

Hard maple machines very nicely. It can be rotated by hand at very, very slow speed to eliminate balance problems. The tool should be very sharp with a lot of top rake. If the round bottom grove is to the proper depth, grind the tools as for a right shoulder cut and for a left shoulder cut. This will let the tool shear the grain of the wood. Try to avoid plunge cuts except with a narrow cut-off tool since this type of cut tends to lift and splinter the grain. Try it with a scrap of wood before committing to the real thing but I think you will find it very easy. SHARP TOOL, SLOW SPEED, TURNING not PLUNGING!

Good Luck

Jerry

 

[Swede]

Maverick and Jerry, thanks, that is what I wanted to hear, because I probably would have taken a parting tool to square up the groove, and in my mind, I could see the wood grain tearing terribly. So I'll go at it with tiny HSS left & right tools. The depth of the round groove is correct, fortunately.

The root of the blades is exactly 24.00mm, very nice tolerances. I don't have a 24mm 5C collet, so I'll either have to pick one up, or I can bore out an emergency collet, maybe in brass or nylon, for the task.

I think there's plenty of "meat" so the collet can get a good grip.

 

[Swede]

I've made some progress on this thing... kind of a sub-project, I guess you could call it. Here it goes...

My engine has a stepped prop shaft that is 0.500" and 0.375" in diameter. If I had it to do over again, I'd have tapered it, but this is what I have to live with, so I needed to modify the current 3-blade hub to fit this shaft. As I mentioned, the ground-adjustable variable pitch mechanism has to go to make room for the adapter.

The hub comes as two clamshell halves with a 0.687" bore. I needed to "sleeve" this and also provide for positive keying between engine, adapter, and hub components. I had to first turn down and modify exterior portions of the hub. To do this, I made use of an expanding mandrel. These are super-handy if you don't mind taking light cuts.

I wanted to reduce that large diameter, and also face it down on both halves somewhat. Piece of cake...

Next, the insert itself was turned from a 1.500" steel bar.

 

[Swede]

The piece was sawed off using a bandsaw and faced to create a flange that was 0.125" thick. In my opinion, it's easier to do that, rather than parting 1.5" of steel.

The adapter had to be bored so as to follow the stepped-diameter prop shaft. This meant a small portion at the front needed 3/8" and the bulk of it bored to 1/2". A tiny carbide-shanked ISCAR tool did the honors.

Towards the end of a precision boring, it is important to take light passes so as to remove any spring in the tool. This little boring tool can really shave, makes some nice, fine cuts.

You can use telescoping bore gauges, or if you want more confidence, a set of plug gauges comes in really handy for this sort of thing.

One of the harder parts was cutting the keyway, as the prop shaft has a woodruff key. Because the bore of the adapter is stepped, the keyway broach has to go through both diameters, and thus the key cut in the 3/8" portion was especially deep. It went off without a hitch, and test fitted to the engine, everything looked good so far.

 

[Swede]

Next, the steel adapter had to be drilled and countersunk for attachment to the rear 1/2 of the hub clamshell assembly. It's getting tight in there, and I wanted everything to be exceptionally secure, so I created a 28mm bolt circle arrangement on the adapter, centered it in the mill, and used generated X,Y coordinates to spot all the holes.

To secure this, I am probably going to use 6-32 x 1/8" FHCS with a TORX head, as these need to be very snug indeed, and if one of them backs out while the engine is running, it'll run into the front cover of the engine, and do tremendous damage! Not good.

The rear half of the hub needed matching holes, except drilled and tapped, obviously. I got to use one of my more enjoyable tools, one of those centering indicators. In this case, it is an import, and does a great job. Setting up takes longer than the actual centering, which is easily good to 0.0005" or better. When care is taken, coordinate-generated bolt circles work very well. I would not rely on a blued layout or popped surface for this sort of thing, although I know it can be done. DRO's make life sooo easy.

 

[Swede]

Getting close... the hub halves, and the adapter:

The prop blades were turned for the correct hub interface, and the maple DID turn very nicely using a 5C collet bored to 24mm. I'm not sure why I was so chicken, it was simple. But the turning exposed some bare wood that'll need painting. So right now, the blade roots have been painted, and I am going to add the correct decal, and give it a coat or two of clear to keep everything intact and hopefully it'll shrug off oil and gas. We'll see.

When it's completely done, I'll get a pic or two of the installation. I'm looking forward to running it with this new prop, and the condenser added to the spark curcuit. The added mass of the 3-bladed prop should improve idle a bit, and slow the engine down, which was turning too fast with the old setup.

 

[maverick]

That 3 blade prop is gonna look soooo nice on the radial.

Regards,
Mike

 

[Omnimill]

Yes thats certainly looking nice. Would like to see a movie of it running when it's done.

Vic.

 

[Swede]

I'm very tempted to turn the roots of this hub to a rounded profile, like this 4-bladed hub:

It'd be relatively easy by turning a 24mm mandrel in the shape of a propeller blade root and mounting the hub to it... turning it with a left-hand tool, from left to right. I think it'd look better.

The new decals went on nicely. I plan on shooting the blades with some enamel clear-coat to give them a bit of protection. And I do plan on getting a video up. Perhaps someone with a You-tube account, and the knowledge to do it, can take my raw Sony video file and submit it? I really know nothing about current video technology.

Thanks!