The Build: Chassis fabrication to power-on

[SoulFetish]

This picks up on a conversation which started in Uneumann's thread "Help with chassis" found here:

http://music-electronics-forum.com/t43010/

I wanted to post some of the updates as I progress through a build and share some of the things I've learned (and am learning) -– what has paid off and things I would do differently. I had posted some images I'd taken of different stages of the chassis and left off talking about the different techniques for cutting the square hole needed for an IEC. I came up with a way of creating the mounting hole using a drill and file which worked out way quicker than I had anticipated. This works very well for those who don't have access to a punch or mill. This is the result:



To Keep the chassis clean and simplify the physical layout, I opted to use some insulated turret terminals with a threaded base. This required tapping some holes in the chassis. With the proper tools (and a little patience to make sure to locate them and wait for them), this ended up being a painless task.
Here are a couple of pictures of the terminals and tapping 2-56 threads:


-


Up to this point the all of the holes were falling withing really close tolerances to the drawing I had created. All of the planning was really paying off... until I got impatient. I made a DUMB-ass mistake.
I'm have to work in the morning, so I'm crashing. But, I'll tell you about it later and ask for some advice on the best way to deal with it.

[MarkusBass]

There are three important information missing here:
- where did you buy insulated turret terminals with a threaded base?
- what was the DUMB-ass mistake (we all want to learn)?
- why didn't you use such terminals?

They would make your life much easier.

Mark

[SoulFetish]

There are three important information missing here:
- where did you buy insulated turret terminals with a threaded base?
- what was the DUMB-ass mistake (we all want to learn)?
- why didn't you use such terminals?
[ATTACH=CONFIG]41484[/ATTACH]
They would make your life much easier.

Mark

Mark, those are 3 great questions! I'll answer them in the order you asked them:
1. I have two different types, but I think I scored those on eBay. I have some others that are swage mount which I am thinking of selling as it is easier for me to tap the chassis, rather than swage mount them. I don't have the tooling to properly press fit them. Unfortunately, eBay is probably the best way to acquire them as buying them through a distributor is prohibitively expensive in my experience.

2. The mistake: I was getting frustrated that progress was going slower than I had hoped. I made the decision to use M3 fasteners over 4-40, or 5-40 because I think it is a superior thread for small fasteners. This can be a challenge because here in the States, the standard is.... well.., Standard (or Imperial), and M3 is Metric. In any case, clearance holes for an M3 fastener range from around 3.2mm(close fit) - 3.6mm(free fit). I had opted to look for a 3.3mm-3.5mm stub drill made of TiN (Titanium Nitride) coated HSS (High Speed Steel). This material was chosen for drilling aluminum for a few reasons, but I'll indulge anyone who wants to know why later in an effort to shorten the answer. I knew it was going to be another several days until the drill was going to arrive, and I already cleaned the shop (twice). I was getting impatient one night and I though "I have an 1/8 inch drill I can use. It's a tight fit, but I can make it work". So I said "f_ck it" and decided to make some holes. While marking the chassis for the board standoffs, I marked a standoff hole that wasn't supposed to be there (it was to feed some wires through the board). I only caught the mistake after I drilled the hole. So, now there is an 1/8th inch hole in my chassis which isn't supposed to be there, and it REALLY burns my ass. The problem wasn't that I used an 1/8th inch drill, the problem was my state of mind and the motivations behind my decision making which made it much more likely for mistakes like this to happen. It would have been wise to wait for the tools and materials which were chosen for thoughtful reasons, over making impatient decisions trying to "get away with it".

3. I could go with "easier" and use those types of terminals. But, I think for this application the terminals I chose are more mechanically rugged, make much more efficient use of space, are marginally more difficult to install, and present a more elegant appearance. I've used that style terminal before and if I needed to make several electrical connections in the area, I probably would opt for something like that.

Oh, so does anyone have any suggestions on how to fix or plug the 1/8" hole in my aluminum chassis??

[mozz]

Rivet.

[SoulFetish]

Rivet.

Moz, you suggestion sounded good at the time... But, now it looks weird with a rivet sticking out of the outside of my chassis.

[The Dude]

It wouldn't bother me, but I understand OCD. If it really bothers you, you could have it welded closed and file/sand/polish it back smooth. A lot of work for a tiny hole. I don't think they make an 1/8" box fan.

[SoulFetish]

no, the rivet was a great idea! It's not really sticking out of the chassis (I'm not that soft). It's flush against the outside and hidden inside under the turret board inside.

[SoulFetish]

If it really bothers you, you could have it welded closed and file/sand/polish it back smooth. A lot of work for a tiny hole.

I'm not going to lie, that thought crossed my mind.

I had done all the other chassis work except for the standoff holes, and after I realized I screwed up, I considered scrapping the chassis and starting again. But, I backed away from the ledge when I realized that's the kind of crazy which compels someone to build a plateau sculpture out of mashed potatoes and heading for the hillside.
I was pretty sure I'd bounce back just fine.
But, Moz, your friggin' rivet solution... so simple, yet so brilliant.

[mozz]

Well i'm glad it was tried. They make a aluminum rod that you melt with a propane torch, have never used them myself though.

How about a bias test point?

[SoulFetish]

Well i'm glad it was tried. They make a aluminum rod that you melt with a propane torch, have never used them myself though.

How about a bias test point?

Moz, I think the rivet idea was the winner. Plus, it was better than a "dummy" screw idea which i thought of doing as a last resort.

I'm getting ready to mount the transformers and some of the hardware, and start the wiring. The first connection I'll make is the earth bond. Then I was going to wire the heaters and mains connections. Any advice? Something I may not catch and is easy to miss?

[SoulFetish]

How about a bias test point?

Oh, my bias points will already be here:

[mozz]

How about one of these?

[Enzo]

Pop rivets have the hole, but other rivets are solid like the above example. DUmmy screw? If there are no other exposed rivets, but there are other exposed screws, then an extra matching screw head won;t look out of place.

I have to admit, I have mounted smallish Dale or Arcol aluminum power resistors to chassis before, and went to far as to make a template for the hole spacing. They have opposite corner mounting holes. Like pins 1 and 5 on a DIP8. Sure enough, after drilling my carefully spaced holes, I found out I needed to invert the pattern

[SoulFetish]

I have to admit, I have mounted smallish Dale or Arcol aluminum power resistors to chassis before, and went to far as to make a template for the hole spacing. They have opposite corner mounting holes. Like pins 1 and 5 on a DIP8. Sure enough, after drilling my carefully spaced holes, I found out I needed to invert the pattern

Enzo, I hear you. I have a graveyard of Hammond aluminum which serves as a memorial to those moments when I realized I drilled the wrong @#*!-ing hole(s).
But it was supposed to be different this time.. I even made sure to label my templates as "inside view" and "outside view" (I'd have been screwed if I didn't do that).



But I'm going to cut myself a little slack on the errant drill hole. I've been reading Bob Pease's book "Troubleshooting Analog Circuits" (great suggestion by the way), and from the very beginning he champions the wisdom of creating space in the building(testing) phase for changes which will need to be made due to issues which present themselves in "real world" application.

Here is a practical example of a couple of issues I ran into during the actual building phase:
Taking the time to score, center-punch, and drill pilot holes was making for quick and precise fabrication of the chassis. Initially, everything was going according to how I planned it and had drawn it out. (...Other than drilling an errant mounting hole which has been discussed ad-nauseam above I'm sure)
The first amp I built, I did from the ground up(which was the last amp I built) and essentially improvised the entire build. The amp went through so many iterations and rebuilds that the chassis now looks like it was dragged through the Normandy invasion. One of the areas I remember running into difficulty was lining up the mounting holes for the tube sockets. I though I would solve that problem this time by keeping to tight tolerances by precisely marking the center points, and drilling pilot holes. Here is the result.





Not too bad, I though. However, in actuality, doing the preparation like this (in the order in which I did it), proved to complicate the actual fabrication and make for more work in post production in ways I hadn't anticipated. I'll tell you about it in the next post because I think I'm going to run out of attachments in this one. To be continued....

[SoulFetish]

Shop maintenance and re-tooling

I've been meaning to circle back here and update some of my progress.
So much of the construction phase of this project has been research, parts procurement, waiting for materials, and acquiring the tooling I need to do the kind of work I set out to do. The right tools are clutch! What ever tools I've needed, I've tried to get the best quality tools I can afford. I have to be thrifty and resourceful with this because I have limited resources for the most part. If the tools I need aren't readily available or out of my budget, I try and come up with a way to build or make what ever I need. Here is an example:
I needed a drill press. There are many drill presses I would LOVE to get my hands on and use. But I needed something to start this amp and I found someone selling a Delta 11-980 for $65. This is about as base model as you can get. But, I bought it, did the chassis work with it, and realized that the chuck was in rough shape.
So, while I was waiting for some drills to come in and some other parts, I found an old Jacobs chuck with the same taper mount as my drill press. So I rebuild the Jacobs chuck and cleaned it up. Once the chuck was off, I couldn't help it and took the entire drill press apart and cleaned and rebuilt that. After reassembly, I set up a dial indicator to check the chuck and spindle alignment and it showed .003" of runout. That's pretty good for this machine.

I was up and running and ready to make the turret board. I wanted to make a good quality anvil to set the turrets into the board, so I made a 1/4" stainless steel anvil. It just so happened that I had so old RV4 pots with the long shafts which I wasn't going to use so I cut the shaft off to use for the anvil. After a couple of failed guesses at how to drill this out, I found a great technique for drilling a dead center hole in a small diameter rod. (I'll tell anyone who's interested, but it makes this a longer story). Anyways, Here are some pictures of the anvil and board assembly:





Dude, I was psyched. This thing lined up dead nuts.