Building The Mixer Frame - in 100 taps

The basic frame of the Mark three consists of a number of one metre wide sub-frames attached to a pair of wooden cheeks.  Each sub-frame consists of a par of end plates with standard extrusions bolted between them. The bolts fit into tapped holes at the ends of the extrusions. The standard extrusions you buy are already tapped so assembly is simply a matter of bolting together the end plates and extrusions. The one metre lengths provided by Schroff are intended for people who want to cut them to non-standard lengths and then tap the ends themselves. This means the standard one metre lengths are supplied untapped. Since I had purchased ten front and ten back rails, each needing four taps each and ten centre rails each needing two taps each this meant I was looking at cutting 100 taps. Regular readers of my blog will know that mechanics is not my favourite occupation so the prospect of doing 100 M4 taps by hand was a bit daunting. I don't even have a set of taps or the right diameter drills and the last time a tapped a hole was during my apprenticeship in 1969. I needed someone else to do this for me.

Fortunately I found a small engineering company, North Norfolk Engineering, less than half a mile from my home. Bernie, the owner, was very helpful and soon had a simple jig set up to tap my extrusions which were completed in a few days. I now know what it costs to tap these holes so I have asked Schroff via their UK distributor to quote for supplying one metre extrusions ready tapped. It is interesting to note that Holger told me that Fischer tapped his one metre extrusions for free.

The next step was to assemble a one metre sub rack just to make sure the taps worked, so I built a 3U by one metre sub rack. I attached a 2 slot mother board to the back and plugged in one of the mix amps from the EZTubeMixer project:

Next we need to attach the end plates to the wooden profiles in the correct positions and use them as a template to drill further holes on the wood. The end plates are attached to the wooden checks using insert nuts which look like this:

To use these, you drill a 7mm hole in the wood and screw the insert nut in using an M6 Allen key. The insert nut is 13mm deep and I am using 18mm thick wood so the nut can be screwed in so it is flush with the surface of the wood. This allows the end plates to be fitted snugly against the wooden end cheeks. Once the end plates are fixed to the wooden cheeks, you need to drill holes in the wood everywhere there will be a bolt attaching an extrusion. The really nice thing about having the end plates already fixed to the wood is that the end plates can be used as a template. No measuring required, just drill  a pilot hole through each existing hole in the end plate with a 4mm drill. Then you remove the end plates and counter bore the holes will a 10mm drill which makes the holes wide enough to accommodate the heads of the bolts. After that it is just a matter of assembling each sub-rack and then bolting its end plates to the wooden cheeks. This is the result:

I didn't have enough insert nuts to fit all the sub racks; I have some on order so I'll update the picture in the next post. However, you can see the main sloping 9U section. The upper 6U is for the channel module and you can see I have fitted an EZTubeMixer module to check it plugs in OK to a 2 module motherboard. Beneath the channel module is a 3U space into which the routing module will plug. At the back you can just see the rear facing 3U unit (the one pictured earlier). When I get some more insert nuts the 4U fader nose can be fitted to the front and the 6U rear panel section to the back.