After seeing the need for absolute accuracy last week I decided to create a very accurate template for producing the bridging brackets.

I used a scribe, measured very carefully and centre punched the positions before drilling them.

I then drilled through this piece in to the blank template piece so that transfer of positions between template and work piece would be correct.

Then for further accuracy, I did not merely use the template to mark up position by pushing a texta through the holes. I clamped the template on to each blank piece using an "over the centre" jig and drilled slightly through the template holes in to the blank.

Then when I was drilling each piece on the drill press, the drill bit was locating accurately in to the slightly predrilled position.

Imagine my dismay, when after producing 20 brackets, I found that they were way out of position.

This mainly shows up when the brackets are rotated through 90 degrees and fixed back to back, as they would be either side of a purlin.

I went back to my template, checked the relative positions of the holes and found that I had made a mistake there.I think what happened, and this is a risk when you use an engineer's scribe, is that I mistook a scratch on the surface for one of my markings

In order to avoid wasting the output from a whole morning, I drilled some additional holes in the template to produce the error in the opposite direction, then produced another 20 brackets.

I then paired the first and second batches up so that when bolted back to back there is no resultant error.