Stephenson’s Link (2)

by John

The next step in making the Stephenson’s link reversing mechanism, is to make the yokes for the links.  See the previous blog. I decided to drill and tap the BA10 holes, while the bar stock was still rectangular, for ease and precision of clamping the pieces.

BA, in case you are not familiar, stands for British Association.  BA threads were standardised in 1884, using imperial measurements (fractions of an inch), but to metric specifications.  All very confusing.  BA threads are rarely used these days.  Model engineers, instrument makers, and restorers of ancient British cars and motor cycles being the exceptions.  Builders of model engines often use them, because the bolt heads and nuts are nicely scaled for the models.

BA10 bolts are only 1.7mm diameter.  If a BA10 nut falls on the floor, it is gone forever.  I can barely see the thread of a BA10 bolt.  I shudder to think of using the even smaller BA12’s.

The tapping drill is only 1.4mm diameter.  A bit thicker than a human hair, (OK, many times thicker), but very delicate.  And the holes needed to be at least 5mm deep.  I do not possess a drill press capable of drilling such fine holes.  Any run out of the chuck, or excessive pressure would just destroy the drill bit.  Also, on working out the feeds and speeds of the drilling, it was apparent that the optimal drilling rpm’s would be 12,000 .  Twelve thousand.

So, once again, CNC to the fore.

I reattached the high speed head to my CNC mill, worked out the XYZ co-ordinates, and did a practice run on some scrap.  No problems!   Worked like a charm. 12000 rpm and feed 100mm/min.   Then to the actual job.   Centre drilled all pieces.  Then using the Pro-stop by Edge Technology  vice stop, repositioned the work pieces and deep drilled them using the 1.4mm twist drill, using CNC peck drilling at 1.4mm intervals.

Then to tap the holes.  The BA10 tap seems even more delicate than the 1.4mm drill.

I attempted to hand tap the holes, in the belief that holding the tapping handle in one hand, and the work piece in the other, would be the most sensitive system.  But these hands, which once performed microsurgery, were not up to the job.  It was inevitable that the tap would break in the job, so I abandoned the method before disaster struck.

The work piece was repositioned on the mill, again using the vice stop, and I used the CNC positioning to centre the tap fairly precisely squarely above the holes.  I made a spring loaded point to apply light pressure to the tap, and to keep it centered. (see photo below).  24 holes and about 2 hours later the threading was completed.   No breakages!

Tapping the BA10 thread.  Note the Protech vice stop, and the spring loaded centering tool.

Tapping the BA10 thread. Note the Prostop vice stop, and the spring loaded centering tool.

BA10 bolts OK.  Now to shape the yokes.

BA10 bolts OK. Now to shape the yokes.  (seen in the background.)


I had made a video of the CNC drilling, but the broadband downloading speeds here are so slow, that you will just have to imagine the excitement of the drilling.

OK, the video finally uploaded.  It is pretty crappy.  To see it, click on the link which follows.