The Hinge for the Main Mast

I have been making the parts of the hinge for the main mast. These are three 316 grade stainless steel plates, one aluminium packing piece and a couple of pins. Machining stainless steel can be a problem because it easily work hardens. Any rubbing, rather than cutting, quickly raises the temperature causing hardening. I had the stainless supplier cut the plates to shape for me on a CNC plasma cutting machine. This left me with the job of accurately drilling a number of different sized holes through the plates. Fortunately I have a small CNC milling machine to do this job. Here is a photo showing the aluminium packing piece being cut; the white surface is powder coating on the scrap aluminium being reused.

Here are the two identical plates for the upper part of the hinge having the holes cut. Note the clamps and bolts through the first couple of holes to keep the plates together and stop them moving relative to one another. Note the coolant stream to prevent both cutter and material heating up and also to carry away the chips.

Here are the completed parts, upper hinge plates (4mm thick) on the left and right, lower hinge plate (6mm thick) left of the steel rule and the white packing piece (6.35mm thick).

 

This next photo shows the parts assembled. The notch in the corner of the lower hinge plate hits against the pin which passes through the two upper plates to provide a positive stop when the mast is vertical. The packing plate (white) is slightly thicker than the lower hinge plate to ensure that the hinge doesn't bind.

The next two photos shows the hinge clamped to the top of the lower part of the mast its up and down positions. In the up position there is a locking pin (just below the lower bolt in the upper hinge), this will probably only be used when the mast is pushed up until the forestay is tightened and when the forestay is released to lower the mast.

The lower part of the mast is still square at this stage so that the lower hinge plate can be used as a template to drill the five dowel holes. Having the mast square will also help with cutting the slot in the mast for the hinge plate. Doing either of these jobs on a circular mast would be difficult to get right and the position of the hinge plate in the mast is critical.

The plate will be glued into the mast with epoxy after roughening the surface of the steel. Hardwood dowels will reinforce the glue joint. For some reason that I can't explain I have put 5 dowels through the lower plate and four through the upper plate! Maybe I decided that the hinge pin which passes through the upper part of the mast provided some additional support.

It's now a day later and I have fitted the lower hinge plate into the lower part of the mast. Here it is in place in the boat. The mast still needs to be shaped (square to circle) and I am pleased that I drilled the holes and cut the slot while it was still square. I will do the same with the upper part of the mast.