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Old 16th Mar 2021, 12:03 pm   #21
David G4EBT
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Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 4,567
Default Re: Enlarging an existing hole in a steel chassis.

Originally Posted by Nanozeugma View Post
Of the possibilities suggested, I rather like the idea of a suitable cone drill.
The chassis is mild steel, approx 1.3mm thick and there is adequate room to use a cone drill.

The problem I have is selecting a suitable specimen.
Reading users experiences of cheap ones tends to suggest that they blunt at anything mildly resistant. Prices range widely.

It would be ideal if someone could point me to **one that experience has proven is successful** pitted against thin(ish) mild steel.
I obviously don't want to spend an arm and a leg on something that I may, quite possibly, never have to use again.

I am, however, grateful to all those who have taken the time to volunteer ideas and experiences (in case you thought otherwise!)
I've found the cheap sets of three such as the one in the link I posted earlier are fine. They come under several brands names and are fairly generic. They're high speed steel, and some (as those are), are also titanium nitride plated (sometimes referred to as 'TiN' - not to be confused with 'tin'). Some, (such as those in the earlier link), have a hex shaft, so that when in the chuck and drilling larger diameter holes, it obviates the risk of the drill shaft spinning in the chuck.

I do quite a lot of metalwork and woodwork/woodturning, and the step drill set that I use as branded as Bergen' which market quite a wide range of budget priced but quality tools. I've had my set for maybe five years or so and have drilled countless holes in steel, acrylic, aluminium sheet and diecast. For front panels for my home-brew projects which I've posted about on the forum, (most recently the 'mini-mod), I invariably use scrap double sided PCB FR4 laminate, which quickly blunts normal twist drills. Sometimes I use 2mm clear acrylic as a protection on front panels too, and a step drill overcomes the risk of 'snatching' and cracking the sheet, which can happen with normal twist drills.

Before I started using step drills, if I wanted to drill say a 15mm diameter hole in any material, I'd do it in the time-honoured way of starting with a pilot hole, then progressively larger bits, and even with a table vice clamped on the pillar drill table, you get lots of jitter.

I don't have any sheet steel to hand to demonstrate the drilling capability, so I've drilled a couple of holes in 2mm aluminium to show the results. The smaller hole is 22mm - the larger holes is the maximum diameter - 32mm. In some respects, though aluminium is softer, it's more problematic to drill than steel, especially with twist drills, as the swarf 'picks up' on the flute, whereas with steel is tends not to. You'll see how cleanly the drill leaves the holes, with no burrs. It took under five minutes to drill those two holes.

I've attached some pictures of my step drills, which as I recall cost 10.00 five years ago. There's also a pic of a set of Bergen countersink bits which I bought last summer, still with the 10.00 sticker inside. (When conditions allow, I visit a large monthly 'autojumble' at Rufforth near York, which is where both Bergen sets came from). Cheap though they are, but in my experience, not 'cheap tat'. They're been fine and get lots of use. If they cease to do the job, no drams - I'll bin them and get another set.

Bergen seem to be also marketed un the 'U.S. Pro' brand. Might be worth a bit extra for the nice metal box:

At the other end of the scale, for those who are spending the firm's money - not there own, RS do one for 73.63 inc VAT:

The only hole I've drilled in steel recently with a step drill was to fit a BNC socket to the steel rear chassis apron of a Heathkit RF1U signal generator. No need to centre punch, no pilot hole needed, all done in a iffy. The only downside of step drills is that you really need to use them in a pillar drill rather than a hand-held power drill, but knowing the antics of some DIYers, I don't doubt that some will have attempted it and may have succeeded.

Hope that's helpful.

Every success with the project.

Pic 1: Bergen set of three step drills.
Pic 2: 22mm & 32mm holes drill cleanly in 2mm aluminium sheet.
Pic 3 & 4: Bergen C/S bits
Pic 5: Hole drilled with step drill in Heathkit RF1U steel chassis for BNC socket.
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