Advice on drilling holes in chassis

[dude]

I never drilled a tube socket hole in a chassis, better to ask first then make a mistake.

I'm doing a conversion and need to relocate four tube sockets, I have a step drill. 

When using a step drill is it necessary to first drill a small hole before using the step drill?

And most important if the tube socket being drilled has another small hole, not anywhere near the center of the hole being drilled but off to the side, will the step drill bit drill the hole "round" without being "pulled" off center by the small hole in it's path?

I'm drilling a 6V6 socket hole and there is a 3/8" hole off to the side but in the outer path of the 6V6 hole to be drilled.  I'm very limited on where I can drill sockets, every things tight.

thanks,
al

[stratele52]

I never drilled a tube socket hole in a chassis, better to ask first then make a mistake.

I'm doing a conversion and need to relocate four tube sockets, I have a step drill. 

When using a step drill is it necessary to first drill a small hole before using the step drill?
Yes , use a punch fist and a small 1/8  bit.


And most important if the tube socket being drilled has another small hole, not anywhere near the center of the hole being drilled but off to the side, will the step drill bit drill the hole "round" without being "pulled" off center by the small hole in it's path?


Won't be easy to keep the step drill in center . You may need to fix chassis with clamp and use press drill .

I'm drilling a 6V6 socket hole and there is a 3/8" hole off to the side but in the outer path of the 6V6 hole to be drilled.  I'm very limited on where I can drill sockets, every things tight.

thanks,
al

[dude]

Thanks

I have a drill press and can clamp the chassis tight that way things should stay pretty much alined.

al

[eleventeen]

What's critical is if you are drilling aluminum vs steel! A drill bit in steel almost *always" wanders some. That is even if it is in a drill press; unless you chuck the bit "up" in the chuck and end up with a very short length of drill below the jaws of the chuck.


DEFINITELY recommend a pilot hole whether alum or steel. Absolutely. Give yourself every advantage you can. Yes, there is a drill bit looking tip on a step bit but they tend to be....not so good.


What I would do: "rehearse" a practice hole in a piece of scrap, if you can. Do a few of them until you feel comfortable.


STOP drilling the hole you drill in your real chassis short of its final size. With a knife or a file, get rid of the burr both on the top and the bottom that the step drill will create.


See if you can stop with the step drill just a tad short of final size and finish off with a rat tail file. If the flange of the socket is to be mounted on top of the chassis, you can be a little more aggressive with your drilling, as it will be covered. But if the socket mounts from underneath, try to finish with a rat tail file. It helps to have a BIG rat tail file. Most people have little (less than 1/4" in diameter one) but a larger one will make your life easier.


You shouldn't have a problem with an intersecting hole, if it's not too large. Especially w/your drill press. But...it is not necessarily the easiest thing to clamp a chassis without bending it, either.


Again, give yourself every advantage you can: Drill a pilot hole, file off the burrs before you reach the final size of your hole(s). 

[sluckey]

I center punch and drill a pilot hole for any hole I'm gonna put in a chassis. One exception. If I'm using a brad point bit in aluminum I do not drill a pilot hole. Still use a center punch to mark location though.

I like to drill to the exact size I need, even when using a step bit. Hold the chassis firmly. Use clamps if needed. Use slow speed and steady pressure. Not too aggressive.

For deburring, I simply turn the chassis over and use the next size of the step bit to slowly remove the burr. I usually use a hand drill at this point because you only need to spin the bit very slowly and only a few revolutions. The intention is not to drill all the way thru. Just remove the burr. I have done this just holding the bit in my hand  (like using a hand reamer) but it's much easier and faster if you can get a variable speed drill to fit.

If I'm drilling a hole for an additional tube socket in a existing amp, my method will vary depending on the particular chassis. My main focus is put the hole where I want it without allowing the drill bit to grab and spin the chassis, destroying good stuff.

In your case drilling a 1" diameter hole overlapping an offset 3/8" hole, use a drill press, go slow, lubricate if steel, and have the chassis firmly clamped to the table. Remove all glass before drilling.

[dude]

Thanks for the great advice.

I'll post how it went.

al

[kagliostro]

I don't know how much the 3/8" can interfere with the new hole
it may be that it is worthwhile to put a small piece of plywood (or aluminium sheet) fastened under the hole 3/8 " (which later will be removed)
before you start making the new hole
K

[dude]

Very good idea kagliostro

thanks,
al

[PRR]

> another small hole, not anywhere near the center of the hole being drilled but off to the side

That may be a Problem.

I had to drill a 2" hole in the side of a fusebox which was peppered with 3/4" knock-outs. I knew that if I did manage the big hole, there would be very little left for my fitting to grab on to. So I bought summa Doug's lovely glass-board stock to go on both sides. Then ripped a chunk of oak (flooring). Stacked the whole pile up with heavy clamps.

Then I used a HOLE-SAW. This doesn't try to rip-out the whole hole, it cuts a circle. It would ignore your off-center off-edge stray hole. It didn't much mind a handful of knock-outs. There's a bit of a wrist-break when the bit comes through the steel, though going next into glass-board it wasn't too bad.

I ended up with a 2" hole with three part-3/4" 'bites' around the rim. Also two pieces of glass-board with perfect 2" holes. I sandwiched the mess (using Doug's lovely little screws/nuts to hold alignment), and my fitting went right in just like UL had approved it.

You can get hole-saws about the size of an Octal. Not exactly, so plan your sockets and your filing.

You want METAL-cutting hole-saw. The ones sold for wood will barely hack Aluminum, and would quickly die on steel.

You want a HIGH torque drill, excellent bracing/clamping, and a strong grip on the motor. A 2" in steel is brutal, but even a 7/8" in Aluminum can get bound-up jammed.

[jjasilli]

Thanks

I have a drill press and can clamp the chassis tight that way things should stay pretty much alined.

al

Whereas that sounds good in theory, in practice, unless everything is truly square, clamping the workpiece often results in off-center drilling.  Typically on a drill press, nothing is truly square including the chassis or workpiece itself.  Another alternative is this machinist's "trick": keep the workpiece (chassis) loose, but blocked from spinning.  The drill bit will self-center each hole, by pulling the workpiece into alignment.  Start with a layout marked on masking tape or the like; or directly on the chassis.  Scribe fine, accurately placed cross-hatch marks.  Use a sharp center punch to locate the intersection mark by feel, and tap it.  Let a fine drill bit "find" each punch mark, and pull it into alignment for drilling.  Repeat with progressively larger drill bits, or a step bit.  Se:  http://www.micro-machine-shop.com/drill_presses_chucks.htm

[G._Hoffman]

Greenlee hole punches are the thing, but you have to be expecting to do a lot of holes to justify the expense.  A 3/4" punch for 9-pin sockets is $50+, and that's just for the round ones!  The square hole punches are just stupidly expensive.  They do a nice job, though!  You can find cheaper punches, but I've heard bad things about the quality.  Still, you drill a 3/8" hole for the bolt, don't worry about deburring it,  and you end up with a perfect hole. 


Gabriel