It is a good idea to use a magnifier to look at items like this. With regard to drill size I tend to use charts as a guide only and usually increase the size for small taps which are prone to snap. I often conduct a quick test on a scrap of the same metal to see what size will then tap easily. For non critical applications it is often surprising how large the tapping drill can be and still provide a decent thread.

Doug

I rarely use serial taps having gone over to Spiral Flute and Spiral Point, so much easier even at M2.

Here's a picture of some M8 taps:

The top tap is M8 taper, next one down is M8 bottoming. I don't bother buying 2nd taps as I never use them. The third tap from the top is M8 spiral flute – should be obvious why! The tap at the bottom is M8 spiral point (known in the US as a gun tap).

The spiral flute tap is intended for blind holes, it usually taps within a couple of thread pitches of the bottom of the hole. The spiral flutes push the swarf upwards and out of the hole. In ductile materials you get three continuous ribbons of swarf exiting the hole.

The spiral point tap is intended for through holes. The cutting edge is angled such that it pushes the swarf down and away from the tap, and out of the bottom of the hole.

Although spiral flute and spiral point taps are intended for machine tapping they work perfectly well when used by hand. They have better swarf clearance so may be slightly less prone to breakage than hand taps. However, you still need to take care over the size of the tapping hole or they will jam and break. Generally I buy spiral flute taps as one tap then does everything.

I don't know if ME suppliers deal with spiral flute and spiral point taps, but any commercial/industrial tool merchant will stock them. In the UK I use Drill Service for mail order.

Andrew

The sprials don't need to be reversed to break the chip but if your problem is really down to not aligning the tap with the hole they could easily make things worse.

It's also worth bearing in mind that a poor maximum length for thread engagement between the screw and the tapped hole is 2 diameters. The usual rule is if more than 1 1/2 cut the screw down. The deeper the tapped hole is the more likely alignment and other problems will break the tap.

John

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It's interesting to look at the usual rule for tapping drills on metric just subtract the pitch.

If that is used m2 would be drilled 1.6mm. Above says the core diameter is 1.567mm. So a 1.6 drill give a bit over a thou clearance. Sounds great before both the drill and the tap are measured.. Drills usually under size, taps over plus brass effects – the drill tends to cut a hole much closer to it's size, The other aspect is those sharp corners usually shown in images like this. The actual tools are very unlikely to have them. Clearing any rads makes a big difference to the effort needed to tap a hole. It's extremely noticeable on BS threads.

I reckon all of the taps and dies I used during training must have been H6 and ground thread maybe even 5.

John

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If I was having problems with M2 Brian I would probably use a No50 drill in brass but would also measure the tap.

John

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3 .You can use a deburring tool, twist a countersink bit in by hand, twist say a 6mm drill bit by hand all will take the burr off. Or if using spotting drills then use them to chamfer the edge before drilling and tapping the hole like these M2 tapped holes

Spotting drills are very stiff and if positioned slightly off centre will chamfer eccentricly, on a small hole this will show up visually. If you use the spotting drill first then it creates a lead in that the more flexible twist drill will follow concentrically. It is similar to using a centre drill in a lathe to start a hole on the spindle axis.

Martin

A shallow cut with the clearance drill before tapping will ensure the tap does not throw up a burr.

Not in my experience, a quality carbide spotting drill leaves no detectable raised burr.

Andrew

Not if it's kept sharp!

Russell