The last post left off at the point where I had finished drilling all the 1/8 inch pilot holes in the chassis and my next task was to use my newly arrived step-bits to further enlarge the holes to the sizes indicated on the drill-chart. I decided to start with the larger of the step-bits, to enlarge the 9-pin tube socket holes. These needed to be 7/8 inch in diameter, so I took note of the appropriate markings on the step-bit. They weren’t exactly very clear as they were offset from what I knew to be the correct one (I measured with vernier calipers). Once I knew the depth at which I had to drill, I set the depth-stop on my drill. I did not want to accidentally make the holes too wide by going one step too much.
Drilling with a step bit is a noisy and violent act. Even with the chassis clamped down to the bed, and with lubricant, it was a disturbing affair. After referring to those in the know (the Hoffman Amplifiers forum) it would appear that I was either drilling to fast (rotational speed of the bit) or feeding too quickly (pulling down too hard on the press). With hindsight I was doing both, and probably just due to impatience. So, I committed two terrible sins here…I rushed the job, and didn’t really listen to advice. Nice move, Simon.
The upshot was that I had some clean up to do. As you can see from some of the close-ups, there was an amazing amount of burring on the underside of the holes. I used my nifty Dremel mototool, with a cylindrical grinder bit, to get rid of the burring. While holding it almost like a pen/pencil I was able to ‘scrub’ the burrs away quite easily and quickly.
The next step is to finish cutting out the square hole for the AC power socket. I want to ensure that I measure a real one first before I start cutting into the chassis. To do this I need to procure one, so I’ve ordered some actual parts (potentiometers, fuse holder, light holder, tube sockets, switches etc). Once done cutting I can “offer up” parts and make sure the hole sizes are correct. This will also allow me to start amassing all the various fasteners that I’m going to need. Ace Hardware, which is 2 minutes walk from my house, will be seeing lots of me this weekend.
The amp is really starting to take shape: The drill-chart implies there’s lots of complexity, but when you actually think of which parts go where it’s much much simpler. With the holes all drilled I can clearly see where everything goes.
Pilot holes: I drilled too many pilot holes. Have a look around the tube-socket locations. It wasn’t until I was sourcing the sockets themselves that I realised nobody actually makes a socket that requires 4 holes in the chassis. The drill-chart said 4, so I did 4. It is my fault for not checking, so that makes me an idiot.
Step-bits: I wasn’t impressed. Next time I will have to do two things 1) Feed slower 2) Slower drill rotation. This really wont be possible on my drill-press as it only goe as slow as 650 rpm. A formula posited by a Hoffman forum member goes as follows:
I use an empirical formula Rpm = 200 / Dia, so for 1/2″ => 200 / .5 = 400 rpm for HSS bits in standard steel. Better bits such as carbide = 800 rpm, lower quality = 200 rpm. Aluminum alloys: Rpm = 800 / Dia, 1/2″ => 800 / .5 = 1600 rpm.
This would be 200/1.0625 = 188 rpm (for the 1 and 1/16th holes). Impossible on my drill. And in fact, I don’t know how I’d do this by hand with a hand operated electric drill. If I am going to do this more regularly I’ll either get chassis punches, or use aluminium chassis’ (or both). I’m betting that an aluminium chassis WILL be far easier with step-bits.