The spark plugs in the full-size engine are angled down and away from the head to avoid interference with the stock exhaust manifold. Since the same head is used on both sides of the engine, the plugs on each side will face in opposite directions. Even though I'll likely fabricate a set of headers, the model's plugs were set to slope 30 degrees down and 10.3 degrees front to back.
A crosswise mounted angle table supplied the 10.3 degrees, and a 60 degree shop-made angle plate provided the 30 degrees. That poorly thought-out plate was a drawn-out effort typical of what happens when I'm machining without a drawing.
The long side of the workpiece was easily indicated along the mill's x-axis, but the angle plate prevented alignment of its short side with the y-axis making it difficult to indicate. The spark plug seat depths are important because they set the plug depths inside the combustion chambers where there is little excess stock around the plug noses. In order to insure accuracy in such a complex setup, the drilling was done with the aid of tooling balls.
Normally one would incorporate the tooling ball into the machining fixture, and although the dowelled fixture plate had the needed accuracy it didn't have the real estate. Instead, the stem holes for the balls were drilled in the excess workpiece stock that will be milled away later. With the workpiece mounted flat in a vise, a hole was drilled/reamed adjacent to each spark plug location. A single tooling ball can be sufficient in some setups, and its location usually arbitrary. My thinking was that one ball per spark plug might reduce error build-up in my fairly complex setup. The same x,y,z incremental coordinates could then also be used to locate each hole which would reduce my chances of making a careless mistake.
But, what the heck is a tooling ball?
A tooling ball is simply a precision metal ball attached to a stemmed pedestal. Commercial tooling balls can be purchased in various sizes, or they can be shop-made. Their big advantage is that once they become a part of the workpiece or machining fixture, the ball's center can be indicated and used to locate features on the part regardless of the part's orientation. The ball becomes an integral part of the design, and difficult features can be dimensioned to it. The ball's height above its installed surface is fixed by its design. The use of a tooling ball in drilling the spark plug holes and seats is illustrated in the sequence of photos.
The first photo shows the particular tooling ball that was used. This one was from McMaster-Carr and is designed for a close slip fit in a .250" reamed hole.
The second photo is a CAD rendering of the workpiece showing where the holes for the tooling ball stems were drilled into the workpiece while it was clamped flat in a vise. The actual locations were arbitrary, but the trivial drilling setup allowed them to be precisely placed. Only one ball was actually used, and it was moved from hole to hole.
The third photo is a CAD rendering of the workpiece fixture'd in virtual space. The spark plug seat surfaces are parallel to the milling table and ready for drilling. The pre-calculated angles of the table and plate were verified in this model.
The next photo is the actual part set up in the mill and ready for drilling. Before proceeding, a number of sanity checks were made using a spindle microscope to verify the setup. The NW and SE corners of the workpiece were picked up, and all four tooling ball locations were indicated.
The fifth photo is a worksheet showing measurements recorded during the sanity checks. The worst case error in the x direction is .005" and in the y direction it is only .002". Not bad considering all the potential error sources in such a setup.
The next CAD drawing shows the x-y incremental moves to be made from each tooling ball to locate each drilling point.
The seventh drawing shows the seat depth referenced to the top of the tooling ball which includes a piece of .001" shim stock to protect the tooling ball from the end mill during touch-off.
The next photo shows a typical hole machining operation in progress. The seat was plunged with an end mill before spotting, drilling, reaming, and threading a .165" hole for a 10-40 VR-1 spark plug.
The last two photos show the final results. All eight holes ended up where intended and as can be seen there was little margin for error. The next steps are to mill away the workpiece material and finally reveal the heads. - Terry