After a little filing,squaring up and such, the strap castings were drilled through for 7BA tapping size in each shoulder, and split as per the plan using a fine slitting saw - gently!
Not forgetting to centre pop the mating halves to avoid confusion later!
The lower (squre) segments were tapped and the uppers (curved) drilled through clearance and the two halves bolted together with the (supplied) small hex headed 7 BA screws.
I appreciate this is all pretty basic stuff for many - but for beginners - its not so daunting.
Each prepared strap then needs boring out to the actual diameter and facing/thinning to the size of its respective eccentric, plus a tiny whisker.
I chose to use the four jaw, fitting up a back-stop to support the rear face of each strap.
In the SB article, the centre is found using a fixed pointer to get the casting hole "sort of where you want it" - if it works for you, then fine. I don't like that and would prefer to mark properly the centre point.
I find the quickest and easiest way is to fill the casting hole with candle wax :-
I just "caressed" the knob of wax with a very low propane torch (any heat source will do - even a hair drier!).
When the wax is nearly set, I gently rub the face along a flat plastic surface to finish it off.
Any mistakes can be easily rectified with a few dribbles from the candle!
The correct centre point from the plan can then be gently scribed and the point gently popped with the pointy bit of the scriber. (Just think classical Latin/Greek writings! - the result is quite resilient).
The strap can then be set up in the 4 jaw in the usual fashion using a wobbler in the centre point. (Just be a tad more gentle when setting it up). :-
The bore can now be machined - I use a Glanze boring bar in a home made holder. The wax comes away quite easily!
After boring to size, the face can be finished and bought to near size for thickness. :-
The strap is then turned around in the 4 jaw putting the machined face against the backstop, and the other face machined to size to fit the eccentric.
Here's the result - awaiting final finishing. :-
I always build - up as I go to ensure everything fits and works together, but leave the final surface (cosmetic finish) to the end of the build - just in case of accidents when handling!
The next job is the valve and pump rods.
The original plan calls for these to be simply made from flat bar stock - functional but looking a bit basic!
There are plans given - to a certain extent, one page is missing! - to fit alternative tapered, round rods in place of the originals. Fitting these to the eccentric strap - buckles looks a bit of a job - involving milling out a pair of steel square shaped forks to fit around the gun metal tongues on the straps. Need some thoughts on this.
