In Part 1 of this article, we covered the theory of Cyborg CNC, and saw a tool that implemented that theory to produce cutting templates that can be used to carve any (uncambered) NACA Series 4 airfoil. In Part 2 we're going to demonstrate how to use the templates to cut a centreboard for a dinghy I'm building...
The 'old school' method for constructing an airfoil is nicely captured in the document Rudder Blades and Centerboards. The majority of the details continue to be valid under Cyborg CNC, with one change to the process: the shaping of the material blank into an airfoil (the bit that scares most would-be makers). This is the bit that Cyborg CNC makes 'fool-proof'.
First up, it is useful to describe what the templates obtained via the NACAPlot application actually do. The 'SVG Data (Long)' template file is used to produce (via a laser-cutting service like Ponoko) two pairs of templates: a left- and right-handed copy of the router template, plus a left- and right-handed copy of the drilling guide. The former are rounded in shape, and the latter are rectangular.
The drilling guides are used at several points in the fabrication process. Firstly, they are used to accurately locate the holes for the screws that will attach the router templates to the blank (the router templates need to be perfectly square to each other, and this is hard to achieve with a curved template). They can also be used to 'true up' your blank (if necessary) on your router table before attaching the router templates (the drilling guides will hold the blank at a fixed height above the cutting head). After shaping, the drilling guides can be reattached to the cut object to facilitate 'square on' drilling (also hard to do on a curved object).
The router templates are the bits that do the bulk of the work. They ensure that the blank can not be cut too deeply, no matter the angle or direction of the pass over the cutting head.
One optional extra that I can recommend you squeeze onto your Ponoko laser-cutting file is a jig for accurately setting the cutting head depth (not provided via the NACAPlot application). This simply consists of a sufficiently wide rectangle with a notch cut out of one side that matches the 'Router bit depth' specified on the NACAPlot application screen (also captured within the raw SVG file, which can be opened in any text editor). See 'Step 01.' in the table below for an example of this jig.
One final point I should mention is that your router needs to be mounted in a table of sufficient size to handle the object you're cutting. In my case a 1.1m [43"] -long centreboard needed a table at least 2.2m [7.2'] long. I handled this by setting my router into my 2.4m [8'] workbench (as can be seen in the photos, below).
This section details the Cyborg CNC method of carving your material blank. It assumes you have constructed your blank as per the Rudder Blades and Centerboards document. I can recommend making your blank a little oversize, especially across the chord, to ensure a good result.
This process requires you to work over an unguarded router head. Follow all appropriate safety procedures, especially securing any loose clothing or hair.
I can recommend including a router bit depth jig on your Ponoko laser-cutting file. The depth of the notch needs to match the 'Router bit depth' specified on the NACAPlot application screen (also captured within the raw SVG file, which can be opened in any text editor).
Attach the drilling guides to either end of your blank. Do this on a flat surface to ensure that the guides are square to each other. Ensure the 'front' of each guide (the 'fat' end of the airfoil) is aligned to the same edge of your blank. Ensure that the airfoil outline engraved on each drilling guide is completely covered by your blank.
After setting the router bit height using the jig, I set about 'truing up' the centreboard stock by running the blank over the router. This effectively planed the bits that were too proud back to the required thickness.
Next the router templates are attached to either end of the blank using the same holes that held the drilling guides in place.
Now we run the blank repeatedly over the router head, tipping the blank forwards and backwards as required to ensure the full depth is cut. The templates ensure that it is impossible to cut too deep. Handle the blank+templates by the templates only and ensure you don't press down on the blank itself, as you may bow it down onto the router head.
One thing to notice in these photos is that the leading edge of my blank was slightly bowed in and was not oversized enough to create a perfectly round edge. Don't make the same mistake - make your blank a little oversize, especially across the chord.
Cutting the leading and trailing edges requires you to roll the blank up and over whilst continuing to pass it over the router head. Keep making passes at all angles of tilt to ensure the best results.
Routing all done. Drilling guide reattached.
Next up I traced the final shape onto the cut blank and located the various holes that needed to be drilled. By laying boards across the drilling templates I was able to use a portable drill-press to drill perpendicular holes through the centreboard. The large hole visible in the photo on the right will hold a lead 'puck' to ensure the final centreboard has a sufficiently high density to keep it under the water.
The centreboard shape is then cut out and sanded. The transition from airfoil to stock is pretty rough at this point and will need to be built up and formed using HD-thickened epoxy. I used flexible plumbing pipe pressed into the thickened epoxy to create a nice fillet at this transition point. Some solid sanding completes this step.
I probably should have taken a photo of the cross-section before cutting out the centreboard, but this pair of photos, taken from slightly different perspectives, give you an idea of how good the final shape is...
I made the lead puck casting mold by using the round wood cut-out. First, I part-filled a cake tin with regular plaster-of-paris, and cast the shape of a round item that was a good few inches wider than the wood cut-out. Once set, I part-filled the cavity with 'high temp' plaster and cast the shape of the wood cut-out itself. This outer+inner layer plaster approach meant I consumed less of the expensive, 'high temp' plaster, but still ended up with a robust mold. I then melted about 3kg [6.6 lbs] of lead* in a stainless steel saucepan set over a camping gas stove, helped on its way with a blow torch. I made sure that I did this outside on level concrete to ensure the impact of fumes and any accidents was minimised. Before starting to melt your lead, make sure you have a clear plan of what you are going to do and have all your tools to hand. I can also recommend 'cooking' your plaster mold in the oven at a medium-low temperature for at least half-an-hour before you use it to ensure any water vapour lurking in the plaster is driven off (molten lead + water vapour = possible explosion). Pour the molten lead into the still hot mold and leave to cool. Slightly over-filling the mold helps mitigate any shrinkage. If the resulting puck is too thick, a 3lb lump hammer on a smooth, solid surface can sort that out. HD- thickened epoxy is used to cement the puck into the centreboard. A Surform makes quick work of shaping the puck area after the epoxy is cured.
*Lead melts at 330°C [620°F] and has a density of 11.34 g/cm³. My puck has a volume of: π x 5cm x 5cm x 3cm = 236cm³ = 2.7kg [6 lbs].
An optional extra at this stage involves slicing off the bitter end of the centreboard and then reattaching it with epoxy glue. This creates a waterproof barrier between the bulk of the centreboard and the bit that is likely to get abraded and subsequently waterlogged (see Rudder Blades and Centerboards for more). I forewent this step as I plan to attach a sacrificial strip of aluminium around the tip of the finished centreboard (which I'm hoping will take the brunt of any abrasion). To finish off, I wrapped the trailing edge of the centreboard in fibreglass, epoxied the whole shebang a couple of times and finished with a couple coats of marine paint. All done!
Did this save you time, effort and/or money? Consider making a donation, commensurate with the benefit you've received, to support further development of resources and tools. Donors are encouraged to offer enhancement ideas in the comments area on the PayPal Donations screen...