Before I piled into drilling holes in the actual steel that will make the frame I thought it was best to do a dry run with some of the off-cuts that I have. I’m really glad I decided to do this as it has shown me that I’ve got some accuracy issues that I need to address before I start drilling holes everywhere.
I sawed off a spare bracket (started with a hacksaw and quickly progressed to the jig saw) and drilled four holes at 6.5mm. My initial test was supposed to be determining hole spacing and whether 6mm bolts would be strong enough. The holes were 10mm in from each side and 10mm and 50mm from the open end of the angle.
Once I’d drilled the hole I placed the angle on the piece of steel I’d be bolting too – in this case a scrap piece of bed frame – and transfer punched the hole using the 6.5mm drill bit I drilled the holes with. I realize that a drill bit shouldn’t be used as a transfer punch but it was the closet I had. The mark it left wasn’t great but I thought it would be enough.
I drilled holes for tapping in the box section at 5mm and eyeballed them with the angle. Oh dear, the alignment was terrible. Nothing moved while I was drilling so I concluded the problem must be where I placed the drill bit above the punch mark. With a 5mm drill bit it was difficult to see exactly where the drill would start and my punch mark wasn’t very good anyway. I’m fairly sure that this is what caused the almost 1mm alignment problem – to be fair it was probably 0.5mm on both holes but in such a way as the errors summed.
I re-drilled the angle to 7mm and tried again. The bolts wouldn’t quite go in so in frustration I drilled at 8mm (I couldn’t find a 7.5mm bit). At 8mm that gives me a full 1mm all around the bolt which is nice from an adjustability point of view but not exactly great from an accuracy point of view.
Although the experiment hadn’t gone entirely to plan there was no point in stopping now so I sawed down some bolts (my only M6 bolts were quite long) and bolted the pieces together as shown in the image. I did the nuts up what I would consider to be quite tight for an M6 but not so tight I feel there’s a risk of stripping the thread.
I deliberately left a slight lip between the upright portion of the angle and the end of the box section as I wanted see if I would make the joint slip. I rested just the end of the box on a piece of wood and then used the angle as if it were a pogo stick. Much to my delight I could detect no movement in the angle, the slight lip was exactly the same as it was before. I then disassembled the joint and examined all the parts which were all unharmed.
Although this piece passed the stress test with flying colours I wasn’t happy with the bolt spacing so I’ve done another dry run with the bolts 12.5mm in from the edges and 12.5mm and 45mm up from the open end. this gives a much more pleasing bolt pattern and means there’s a little more meat to take off should I need to expand a bolt hole.
This second run also suffered from alignment issues (although not quite so badly) so I’ve bought myself some proper transfer punches and will be following Tubalcains instructions here and here.
P.S. the paint for the steel has arrived so I’ll be painting things for the next few days I think.