Hub drilling

[Jmk89]

One of the things that I have always had difficulty with is setting up propeller blades to be drilled for fitting to the wire hub on duration rubber models (e.g. CdHs).

All the magazine articles talk about setting the pitch using a pitch angle and then say in a blasé way of marking a position on the blade or a dowel attached to the blade and drilling it. I have always had difficulty in ensuring that the hole was made in exactly the right position and in the right direction (so the pitch often wasn't what I had intended and, in the case of two bladed props, the pitch on one blade was often different from that on the other).

Of course if you are making lots of examples of the same props/hub then you can make a dedicated jig to ensure that this is easily repeatable. However, making something that was more adjustable to cope with other circumstances had always defeated me.

However, I have now printed up a couple of pieces on the 3D printer, which enables the prop to be held at its end at the hub end (particularly if I use a 6 mm stub dowel which is often very useful as it avoids drilling through the prop blade itself and removes the need for reinforcing the blade in that area. They also enable the blade to be set at the correct pitch angle. The photographs I think are relatively self-explanatory, at least of the pitch triangle (in this case it is 25°).  Please note extra photos to follow)

What I am more proud of is the hub holder. As you can see both it and the pitch angle are provided with holes that can be pinned to a building board are using pin clamps, so that this is a very versatile setup. The hub holder has a hole to hold the hub dowel set at about the height of the middle of the 25° triangle so that it is a relatively easy exercise to get the centre of the blade on the 25° angle at the same height as the hub.  The point at which the 25° pitch is set is simply marked with a pair of pencil marks on the edge of the blade at the appropriate point and the hub is then pinned at the appropriate distance. The dowel is twisted in the hole until everything is set up properly, then the screw at the top of the hub holder (the end of which has been filed to a point so that it holds the dowel firmly) is screwed in to tight finger pressure. Then the hole through the dowel can be drilled using the 2mm hole at the near end of the hub block. The blade and the hub block can be taken off the building board and put in a vice to keep everything steady while you drill. I have found that it is much easier to use our hand-operated egg beater drill to make these holes than a Dremel or similar or even a standard handyman electric drill. Somehow rather the hand drilled holes turn out much neater.

Once again, if you would like me to share the STL files of either or both of these elements, send me a PM with your email address and I'll send them to you.

[Jmk89]

Pictures of devices in use