Hi Folks – this is a short video on drill bit selection.

I get questions usually in the form of complaints quite often regarding the drill bit selection.

Here is the large diameter SN Gent tube – as you can see the caliper clearly shows a 14.4mm diameter tube. In the drill press we have a 14.5mm drill bit.

The classic example is this - “On your website - you recommend a 14.5mm drill bit for a SN Gent and I purchased the 14.5mm drill bit from you, but when I come to using it, the brass tube will not fit into the hold – you’re a dud, you’re a liar, you sell crappy stuff.”

Well the problem is not usually the drill bit, it is the blank. And here is the proof.

After I have drilled a stable material – in this case a Acrylic pen blank, you can clearly see that the brass tube will slide in a out of the blank.

But now I will show you in the same take – what happens when you drill a timber pen blank that is not stable – the cause may be that the blank has a lot internal stress or the timber is still green (hasn’t had the time to dry), or your drill bit is blunt causing excess heat causing the blank to distort.

What actually happens is the timber blank starts to change shape after you have finished drilling the blank. The blank can cup, or bow or even worst, the blank can go oval shaped.

As you can see, it’s a very tight fit. The timber that I have drilled here is a piece of Fiddleback Blackwood that is still a little bit green. Even though I am using the same drill bit, the brass tube is just able to fit into the hole.

Here is another one that I drilled a couple of hours earlier, the brass tube will no longer fit into the hole.

So what has happened is this.... Timber being an unstable material – will want to find a new equilibrium after you have altered its equilibrium position. Just remember when I have drilled it, I have removed about 85% of its internal volume. It may bow or cup or twist – When it bows or cups, it reduces it internal clearance diameter – symptoms of this is being only able to push half of the tube into the blank. When it goes into oval shape, it reduces the clearance diameter which translates into not being even able to fit the brass tube into the pen blank.

The easiest fix to overcome this problem is to drill the hole then let it sit over night and come back and drill the hole again, this method ensures that the brass tube will fit into the hole. (You’re not impacting on the piece of Timber as much as you did the first time).

I hope you have found this video helpful and interesting. Now get off the computer and head back over to the workshop and make some saw dust!

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