Drilling then Tapping in Drill Press

[Anonymous]

Posted by Iain Marshall 1 on 11/12/2020 09:50:03:

Might it have been better to drill a smaller hole?

It won't make any practical difference. As said the bolt is within tolerance. The OD of the internal thread is set by the tap, not by the initial hole. Taps are also made to varying tolerance standards and are normally ground to the upper limit of the tolerance to allow for wear.

Incidentally your tapping drill is unnecessarily small. For M6 I drill between 5.2mm and 5.4mm depending upon material and application.

Andrew

[Iain MarshallParticipant @iainmarshall1]

Thanks for replies.

I suppose I noticed it because it was a fairly well long bolt into a 5mm or so bar.

Attaching the bar to something solid and bolting it down would likely be fine. Threads seemed well engaged.

Iain

[bernard towersParticipant @bernardtowers37738]

Sorry Andrew but if you are using a 5.3 mm drill for m6 tapping I would hate to see your percentage of engagement figures!

[Anonymous]

Posted by bernard towers on 11/12/2020 11:23:33:

Sorry Andrew but if you are using a 5.3 mm drill for m6 tapping I would hate to see your percentage of engagement figures!

The theoretical value is 65%, athough taps tend to extrude the material slightly, so possibly a bit more in reality. I have never stripped a thread, but have broken bolts when tightening.

Long ago I did some experiments for a work project involving M4 threads in 6082 and high tensile bolts, (grade 12.9). I drilled and tapped a series of holes starting with 50% engagement and working up. The M4 bolt broke in the hole drilled for 50% engagement without the thread stripping.

So for a coarse thread like M6 it's pointless drilling and tapping for high percentage thread engagement.

Andrew

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