Boom Gallows Poles | Stanchion Fastenings

How to hold the boom gallows support tubes securely in their stanchion support fittings had been on my mind for a very long while.

Each stanchion support is equipped with a pair of grub screws which are tightened onto the tubes when fitted, like this.

The grub screws themselves are M6 in A4 stainless steel with a conical tip, like this.

I assumed that they would be secure enough when tightened, but an engineering-minded visitor pointed out to me that the poles could easily slip loose because the points of the grub screws do not bite into the tubes to hold them in place. A friction only grip would not suffice. We needed dimples in the poles to seat the grub screw tips.

OK, understood. So how to do this?

It was clearly necessary to drill a small dimple in each tube to act as a recess for the screws.

So firstly I used a permanent marker to highlight the screw holes on each tube, like this.

Then I tightened each grub screw until its sharp point bit into the tube, showing where the dimple needed to be. Like this, with the tube removed.

So far, so good. Now I needed to drill the dimples.

Previous experience drilling stainless steel showed me that a drill bit would wander on the rounded surface of the tube unless it was guided by a small indentation as it cuts.

So, a centre punch was required.

Some research quickly revealed that an ordinary metal worker's punch would not suffice - it would not be hard enough to penetrate stainless steel.

We needed a special punch. It transpired that Starret make such a punch - their number 117 E was what was required.

Here it is.

And here are the hammer and punch ready for action.

Nylon vice clamps were deployed in the drill vice to hold each tube while it was centre punched, like this.

That worked well, and drilling the dimples was then straightforward enough. Here we are using the pillar drill to do so.

I used a 3mm drill, because I didn't want the dimple to be so big that the screw would not grip tightly.

And here are the resultant dimples.

A test fit worked well. Here are the grub screws locking the tube firmly in place.

Ans lastly here is the boom gallows, securely held on its support poles.

Another job complete!