Makin’ holes in stuff

Making holes in wood and metal is a big part of the DIY tube amp building hobby, but practical construction strategies aren’t something that get a lot of attention on forums or websites. We (myself included) probably spend 90% of the time in thought experiments and circuit analysis and 5% of the time on fabrication (the last 5% is chasing math and rounding errors).

I usually build enclosures from raw materials: 3/4″ hardwood and 1/8″ aluminum. While this is by no means the only way to do things, here are some of the tips and tools I’ve accumulated for my style of construction. The focus here is on making holes (especially in metal); for tips on making a simple wooden box, see this page.

Drill Press

drill press

You do not need a drill press, but it makes many things easier. A drill press is more stable than a handheld drill and easier to setup for repeatable depth or consistent spacing. Drill presses are more powerful than handheld drills and have more settings for speed, both useful features when using different types of bits and materials.

Limited throat depth is a disadvantage of the drill press. A press advertised as “10 inch” swing or throat depth can drill to the center of a 10 inch piece of material. This means the distance between the chuck and the vertical support is 5″. In my experience, 10 inches is the minimum size press that will be practical with tube amp top plates. Even better if you can fit a 12 inch or larger in your budget and work-space.

Drill presses work best on flat stock. While this isn’t necessarily a disadvantage, it is something you need to plan for while building enclosures. Drill first, then glue and assemble!

In most cases a press is slower to setup for cuts than a hand drill. I still have a good quality battery powered handheld drill when I just need a quick hole for mounting bolts/screws, when placement isn’t critical, or when I don’t have clearance to use a press.

Drill Bits

drill bit pilot point

You don’t need anything super fancy or expensive for drilling in wood and aluminum, but you should invest in a decent set of bits. Regular twist bits with a pilot point have worked great for me in both presses and hand drills. The narrower end on a pilot point also helps with hole placement when cutting to precise locations on the press. I suspect that brad point bits would not hold up well to lots of aluminum/metal drilling and I’ve broken a lot of cheaper standard point steel bits. A bit set with a good coating on it will definitely stay sharp longer.

Bi-metal Hole Saws

holesaw

For cutting out socket holes and holes for mounting large capacitors, I use bi-metal hole saws. I’ve tried punches, but I like working with 1/8″ aluminum and I haven’t found a punch that works for this thicker material. If you’re working in thinner steel, a good set of Greenlee punches may be your best friend.

I cut octal holes with a 1 1/4″ hole saw and 9-pin holes with a 3/4″ (sometimes it’s necessary to enlarge this to 7/8″ depending on the socket and tube). Both of these sizes will work with a handheld drill, but larger sizes for motor run caps really beg for a drill press.

I’ve been very impressed specifically with the Milwaukee Hole Dozer series. They’re easy to find at the big box home improvement stores, the arbor can be swapped between saws, and they’re easy to clean out shavings and stuck plugs. I’ve drilled a lot of octal socket holes without serious dulling of the saw.

Unibit

unibit

Unibit is your BFF for drilling out grommet holes to run transformer wiring. It is also very handy for a quick 9-pin socket hole. I’ve found the 7/8″ size to be perfect for most tube amp needs (anything bigger than this is a hole saw job). Tip: use painters tape to mark your depth on the bit so that you don’t overdrill to the next size larger than intended.

The unitbit has a tendency to grab the stock you’re drilling into. Whether using this with a handheld drill or a press, be sure to super clamp your stock and avoid spinning top plates of death.

Unibits bits aren’t cheap, but they’ll last forever if you get a good quality one.

Countersink Bit

countersink

This one isn’t the most used bit in my toolbox, but I do think it adds a nice touch for top plates. I use these to put the head of mounting screws level with the top plate and to clean up any holes drilled for chassis air flow. These work on wood and aluminum.

Forstner Bit

forstner

The forstner bit is your hole saw equivalent for wood work. A forstner bit allows you to drill circular holes in wood, removing the material to a specific depth. Most jacks and pots do not have much of a bushing on them. If you want to mount jacks and controls in the wooden portions of your chassis, you’ll probably need something like this to reduce the wood to a manageable thickness:

IMG_20171022_125533053.jpg

I use forstners to remove wood from both the inside and the outside of my enclosures, depending on where I want to locate the recess. A good forstner bit leaves a very clean edge so I use them wherever possible in wood. Don’t try these in aluminum.

Jigs

Because I incorporate solid state parts on heatsinks in many of my builds, I drill a lot of holes to vent the chassis:

A thin and flat ruler taped to a straight fence makes a really simple and useful jig for making sure the hole spacing and placement is consistent on the drill press. Pictured below:

IMG_20180902_141052248.jpg

  • First, layout the lines along which you’d like to drill the holes in your plate with a square and a permanent marker (remember to use the mirror image if marking on the underside of the top plate).
  • Second, mark your material along the edge using a square. Exact location is somewhat arbitrary.
  • Now, line up the material and bit to cut the initial hole at your desired starting location (but don’t drill yet).
  • Adjust the fence to line the mark on the material’s edge up with any whole inch mark on the fence/ruler (you see the marked line at the 10″ spot on the ruler in the photo). The material should be flush against the fence at this point.
  • Finally, clamp the fence/ruler combo down securely and recheck that you are drilling the first hole where you want and that the mark on the material’s edge is lined up with a convenient mark on the ruler.
  • Now you can drill the first hole and move the material along the fence to keep the rest of the holes in a perfectly straight line. Using the mark on the edge and the ruler, you’re able to very precisely control the distance between holes.

 

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