Monday, July 2, 2007
A couple of weeks ago I posted an article...
...showing how to install a Permanent Magnet dynamo on the pulley-end of a VW engine. In the article I mentioned that VW crankcases are not identical. That came as quite a surprise for a number of folks who had somehow gotten the notion Volkswagen engines were pretty much the same. In the non-technical sense I suppose you could say they were a close match but in the same vein you could say a .30-‘06 cartridge was a pretty close match to an 8mm x 57 cartridge, which is to say they are visually similar. Functionally, the two cartridges are not interchangeable, which is also true for VW crankcases when you’re installing the coaxial dynamo.
Race-winning reliability comes from keeping things simple; eliminating anything not absolutely required, a philosophy that also applies to Flying Volkswagens. Once you have reduced a part to its bare essentials all that is left is to make the parts accurately, install them securely and ensure the fasteners can not come loose in normal use. Fortunately, doing all that isn’t as difficult as it may appear and the dynamo bracket serves as a nice example.
Creating a part with CAD allows you to make the usual prototyping mistakes on paper. So long as your printer is accurate enough you should be able to eliminate the task of laying out a part by simply printing an accurate pattern like the one shown in Figure 1. Also shown is the piece of scrap I’ll use to make the bracket.
The pattern is cut out (Fig. 2), the metal is given a spritz of spray-glue and the pattern is affixed to the metal (Fig. 3). The chore of fabricating the part is now reduced to simply cutting, drilling and filing. This also applies to the other parts that make up the dynamo’s mount, which I’ll eventually post in DeltaCAD format in the archives of the Chuggers Group.
After the part has been roughed out I center-punched the intersections (Fig. 4) and opened them up to about 11/32", since the fasteners are going to be M8-1.5 cap screws.
Figure 5 shows how the part does not fit on an aluminum crankcase from Brazil, one of two in the shop at the moment.
Nor will the bracket fit the machined curve on a German crankcase with its characteristic asymmetric curve as shown in Figure 6.
But the bracket is a nice fit on a recent Mexican crankcase shown in Figures 7 & 8, since I made the original bracket pattern to match this particular case. (Actually, it needs a tad more filing; note that the poor centering of the bolt hole.)
When satisfied with the fit of the part the central hole is opened up with a step-drill (Fig. 9) to accept a common pipe plug as shown in Figure 10, allowing the opening to be used as the oil filler port.
In a future post I’ll show how the bracket is aligned to the dynamo mount and the two are riveted together, although you probably won’t see this bracket again. Upon removing the pattern (dampen it with a wipe of lacquer thinner, peel it off then remove the spray-glue) I saw that I’d included a series of extra holes along one edge, making the part look a bit too tatty.
If this engine was to be fitted with a fuel pump the bracket would be made of mild steel .050" to .063" in thickness. The stock phenolic thermal insulator would then have that amount removed from its upper surface or the fuel pump’s push-rod would be extended by that amount (a common repair procedure for a worn push-rod).
At the moment there are eight engines in the shop, most using new Mexican crankcases but that number includes two aluminum crankcases and a few post-‘71 German cases, all of which have slight differences in the area where the dynamo’s mount is to be installed. Accommodating those differences is not difficult but if you’re not aware of the wide variation in VW crankcases it can come as a nasty surprise.
Although the part shown above is about as simple as it gets, novice builders have told me the most daunting aspect of building from scratch is the amount of time and degree of precision needed when laying-out the parts. Those road-blocks are removed when the builder has access to drawings they may print-out and use as patterns. This same method will be applied to all of the other metal fittings needed to for the Chugger's airframe and engine installation.
-2 July 2007