Having reached the point of getting the major components to turn, the next step is to strip the engine down, and complete the block, and then paint, before erecting the entire assembly and tackling the valve gear.
Looking at the block I realised just how many additional holes and bits and pieces we still needed to be done, so a list of all the holes to be drilled, and components to be completed looked a little daunting, but a couple of good days in the workshop resulted in some good progress.
Most of this was a combination of simple turning jobs for the value gear stuffing blocks, and then some co-ordinate drilling – this latter activity is perhaps worthy of comment.
In order to position holes in things one can either set about traditional marking-out, with scriber, dividers, height gauge etc. OR resort to some technology in the shape of a DRO (digital read-out). I made the DRO for the milling machine, using an open-source design by Shumatech. (which I can highly recommend – see http://groups.yahoo.com/group/ShumaTech/ for a really good discussion group on this project).
This device includes a number of functions like finding mid points between two edges and automated flange drilling functions (that put a number of holes on a pitch-circle diameter). These provide a sort of manual CNC function, whereby the DRO tells you the offset to the hole (or sets a zero there) and then you twiddle the hand-wheels on the mill until you are located in the right place – a very accurate (half a thou) and error-free process.
HOWEVER, in order to use these functions you have to be able to find the reference edges of the work-piece so you can zero the DRO at the “origin” of your component.
I have tried many approaches (feeler gauges, Mk1. eyeball, bits of fag paper, and the often recommended “wobblers”). None of these provided great results – the wobbler was a particular source of angst, when having scrapped a couple of jobs I realised that the .25″ ball (on which the tool relies) was actually .21″ !!!! Cheap tools 😦
So in the end I decided to try to find a better approach, and settled on an electrical approach. I made an insulated tipped edge finder which I could mount in the Milling Cutter chuck. This has a small bearing attached to it, with a hole for a multi-meter probe (the bearing allowing the probe to remain stationary while the tool is driven at the slowest speed of the mill). The other side of the multimeter is attached to the frame of the mill, and then running slowly you approach the work piece by careful adjustment of the hand-wheels, while listening for the continuity alarm on the meter to go off. (at this point you have “found” the edge!)
By running the mill as you measure (and working off two opposing edges and using the mid-point finder function) you zero out any lack of concentricity in the edge finder. I had tried to make the edge-finder probe true (grinding it on the Quorn Grinder), but the necessity of having a plastic collar to provide the insulation means it always finished a bit off.
The results with this are very good – you can locate an edge to within about 2 thou, and a brake valve I made in three parts with 5, 6ba bolts on a 1″PCD assembled perfectly.
Back to the job
So as discussed two or three days of turning and drilling allowed me to virtually complete the cylinder block with all the stuffing boxes and valve covers in place! Here are some pics!