Hi All,
The machining of the valve. I knew this part, though it looked so simple on paper, was going to present me with a lot of machining difficulties. It had two internal features, a 7/64" through square hole and a flat bottom circular hole. The flat bottom hole I can deal with as, in the past, I have needed to made up a number of flat bottom drills which, can be a problem to produce in their own right if one does not have the right grinding equipment but fortunately I do so...
To have a chance with the 7/64" square hole, a new single lip square broach needed to be made. This size was smaller than any which had been made before and the start was not good as in the first go around I snapped off the pilot section of the broach while trying to machine in the undercut.
After that I reasoned why not try and machine a two piece broach as it would be much easier and simpler if it was done this way. And it worked! See the first photo. The undercut could now be simply formed through the use of a number 0 or 00 centre drill as seen in the second photo. The centre drill was drilled into the square end of the broach until the 60 degree centre hole just met the edge along the four sides of the square. No back relief was put on the square cutting end. The material was then hardened in oil but not tempered. The pilot piece was left soft as machined.
The broach also uses a head steady as seen in the second photo for reasons which will become clear. The third photo shows the square end test piece which I machined to exactly .1093"(7/64"). I use a spin index in the mill and sharp cutters to produce all of the square sections which I think is quite straightforward to do so no photos.
The setup for the actual broaching operation is seen in the next three photos. The bench arbor press may seem to be standard and nothing unusual but it has a number of features which make it rather unique together with all of the special fixtures made for it over the years so maybe I will describe all of this in a separate thread.
One not often seen feature is the ram has a reamed 5/8" dia. centre hole which is very useful when attempting a broaching operation like this as the broach head steady fits precisely in this hole and as a result one is able to precisely hold, locate and turn the broach to whatever position is needed quickly and accurately. I found out the hard way that small commercial multi tooth broaches are very delicate when pressing them and the slightest misalignment or arbor press ram slippage/wobble will result in a broken tool - hence the head steady. Also, not seen but the ram has been shimmed to eliminate any and all side play.
Another feature is the arbor press base had been drilled (square to the ram and how to set it up this way is another interesting operation) when I got it so special locating/holding fixtures could be used. One of these is being employed here in the broaching operation where there is a slot and centered hole in the fixture (which was designed for another entirely different operation) giving the broach clearance when it passes through the work.
The actual broaching technique needs a few words. It is best if the arbor press lever arm can be horizontal or near horizontal when broaching but things work out that this is rarely the case with these small cheap bench units (not being of the ratchet type) but clever people have figured out a modification to overcome this deficiency which involves a ram modification which I have not yet done.
Do not try and press through at one go. Carefully align the broach first in its required orientation and press slowly into the work ensuring that the pilot end of the broach slides down into the pilot hole in the workpiece without hanging up anywhere on any edge. Press firmly and release, pressing through the workpiece in small incremental stages. After the initial pressing, the pilot can be removed and the broach used without it as it will stay aligned and the open end of the broach allows for the displaced metal to more easily move into. When through gently tap out the broach with a non metallic small hammer as without any back relief the workpiece and broach are now firmly set together. As I have said before this type of broach has its limitations one being the metal thickness which limits it to a maximum of 1/8" thick.
The result is what is seen in the last two photos, a perfect square hole. As an aside, the pilot stem (being so slender in this case) did not survive but it is quite easily remade.
