Drilling small holes

[NealebParticipant @nealeb]

To clarify the title – I'm talking 1.5mm in 3mm steel (laser-cut loco frames of unspecified composition). So, nothing exotic. I can manage down to 2mm with care, but not only do the 1.5mm drills keep breaking, but they tend to leave the tip in the work which creates further problems. I've tried high (2K rpm) and low (500 rpm) using my vertical mill. I put a drop of oil on the centre-dot.

A complicating factor is that I "marked-out" the work using my CNC router with a small spot drill. The spindle is too fast for drilling but I thought I could get away with making small pits with the tip of the spot drill. However, I do wonder if the problem is that the spot drill has left the steel work-hardened in the mark which takes the point off my HSS drill which then snaps when I apply more pressure. I'm using Irwin branded drills, of which I fortunately(?) bought a box some time ago. I have also started trying to reform the broken drills using a Wishbone sharpener but haven't had a chance to try these yet.

Thoughts and suggestions warmly invited! I had thought that after 45-odd years playing about in the workshop, I wouldn't be tripped up by such a simple problem…

[NealebParticipant @nealeb]

[JourneymanParticipant @journeyman]

Too slow! Drill speed should be in the order of 3500 to 4000 rpm. No pressure on the drill just peck very gently. If you have the CNC router I would be inclined to give it a go provided you can set the Z downfeed very fine and peck away at the holes.

John

[HopperParticipant @hopper]

Irwin drills are usually pretty good, so I don't think that's your problem.

You could have already identified your problem – work hardening from over fast spotting drill. Depends on what rpm your router was running at? Possibly, use a similar spotting drill to remove the hardened metal at lower rpm in the same drill press you then drill the 1.5mm hole? Or the very tip of a good centre drill.

Another thing to check is how true is your drill chuck running? I had problems with small drills breaking and found that small drills held in my cheap Chinese drill chuck were running well over 10 thou out of true, checked by dial indicator on the shank. A better quality drill chuck solved the problem. Haven't broken a drill since.

Edited By Hopper on 21/04/2018 10:34:59

[Anonymous]

It's nothing to do with work hardening; I doubt the steel is anything other than low carbon. It's all to do with spotting to start with. In my experience it's a waste of time when using "small" drills. Preumably you're moving the work between machines between spotting and drilling? How are you lining up on the spot drilled location?

If I was drilling said holes I'd do it like this:

1. Use a 4-facet drill – I use Dormer

2. Just drill straight off, don't try spotting or centre punching first

3. Use a squirt of WD40 or very thin oil to lubricate

4. For this size drill I'd be running my Bridgeport at about 2000rpm – could be faster but it gets noisy!

5. Let the drill cut and be positive with the feed – at 2000rpm, two flutes and 1.5thou per rev per edge that's 6 inches a minute

6. For 3mm depth you probably don't need to peck, but one might be used if one is on the cautious side

If yoiu're careful when starting the drill you can get away with a 2-facet drill. Shown here are 1.2mm holes drilled 8mm deep in tungsten alloy on the Bridgeport with no spotting (tungsten carbide drill):

Andrew

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