Drilling holes for mods in chassis

[leydenjar]

Sometimes I need to make a new hole in an already functional amp for an extra potentiometer, tube socket, switch, e.t.c. I often find getting a drill bit to sit right where I want it to is the difficult part when starting the hole.

Would those of you with experience with this situation please tell me how you handle this?

Thanks,
leydenjar

[J M Fahey]

Well, you need to center punch it first.

If the hole will be large , it may pay to drill first a small guide hole, with a 2 or 2.5 mm drill, to help guide the larger one.
*This* is what a guy made when trying to enlarge noval to octal chassis holes, with a hand held cup saw, without using a center guide hole first

So now you know

[Wittgenstein]

It's always nice when you can use a greenlee chassis punch.

[Chuck H]

Yupper. Center punch and then, if the hole will be large, a pilot hole. Still not 100% with aluminum if you're working by hand. The softness of the material can gouge to one side when the bit grabs and throw you off. A press is ideal. I only have a little portable thing that won't do a chassis. Sometimes I do need to scrape away a small amount of plastic from the face plate holes to get things stuffed. Nothing so far off that you can detect it with the naked eye though. So, yeah, punch, drill, drill again for larger holes. You really only need a dent from the punch to keep a smaller bit in place. Back the chassis with wood before smacking. I have lot's of scraps in different thicknesses that I can stack and use to back the chassis for hammer blows.

[pdf64]

I find a set of step drills Buy Conecuts and Step Drills Step drill 4-12mm X 1mm RS T54000412 online from RS for next day delivery. massively helpful for this type of job, much better than large drill bits. I still tend to drill a pilot hole first to save wear on the step drill tip but a dot punch is the basic essential. I have a 'automatic' spring loaded dot punch, which does away with the need for a hammer; just press on it and at a pre-set weighting, it will 'fire'. It works better on aluminium than steel. Buy Centre Punches Sets Small automatic centre punch,120mm L Eclipse E171RS online from RS for next day delivery.
Pete

[Tom Phillips]

All of the above is good information. I offer the following additional info.

There are also other considerations than the size of the drill bit. I recently purchased a drill doctor so that I could sharpen my old bits. It has been interesting to learn how to optimize the drill bit point for the material that you are drilling. For example a 118˚tip angle for most wood and soft metal applications vs. a 135˚tip angle for harder material such as stainless steel. The bit tip can also be configured as a “slit point” which is self centering and also performs better on a curved surface. Just like many other things these optimizations are not always required but once you know about them you can get spoiled.

When I was reading user reviews about the drill doctor I found several people who just said that they buy new bits when their old ones get dull because they felt that they would never recover their investment in the sharpening tool. However, when it’s late at night and you need a sharp drill it’s really nice to be able to dress a drill bit tip on the spot and to the exact configuration that is optimum for the task at hand. Skilled machinists can sharpen a bit with a grinding wheel but I never had any success with that.

Hope you find this information useful or at least interesting.

Regards,
Tom

[tedmich]

this unit is a godsend in SS sheet metal work
M35 cobalt step drill 3/8" shank 9 hole 1/4" to 3/4" | eBay
$13 delivered , US supplier, high quality M35 Co HS steel and it's flat ground for easy resharpening, no "Doctor" needed!