You can't toughen black mild steel, but over-drilling the centre may well have weakened it. It should also be a close sliding fit in the socket, with the edges slightly chamfered.

Try again, leaving the end un-drilled. Bright-drawn mild-steel would be better, too.

Otherwise I would think you'd need one of the higher-tensile steels such as EN8 or EN 24 (or their moden equivalents), or perhaps a free-cutting stainless-steel.

Even so, I would expect a square as small as 7mm A/F to withstand the torque intended by the chuck's manufacturers: that for my Harrison L5's hefty great 4-jaw chuck is only 8mm A/F.

Did you make the whole key from small-size black steel? Usually they are cut from round bar of diameter typically a bit above the square's diagonal (which = square-root 2 x D = 1.414D). This would give 10mm stock diameter for 7mm A/F; but it can be bigger provided it is still below the inside diameter of the jaw thread as that emerges above the chuck body.

maybe the steel was the same ‘metal’ used in the set of Poundland allen keys bought out of necessity as no hardware shop was in locality and I needed one of the small ones to remove a grub screw – it twisted like a corkscrew and job had to wait for me to return with a proper set of keys.

That is an overly large T bar for the size of square. I would say therein lies the main problem. Half that length would suffice. That and the hollow in the square from your centre dilling.

regards Martin

Using a high tensile steel like 4340 or the similar high tensile will last a life time. When milling the flats, I suggest using the side of the cutter , as it creates a nice radius in the corner. If you use a #2 centre drill, but shorten the front of the centre drill so it only has 1mm or so parallel section before the 30 deg taper starts. Then you can use the centre support o=if need be when milling the end. Keeping the inside of the square drive of the chuck very clean will make the new pone last a very long time. I suggest that you make one with a T type handle for final tightening up, but be aware of over tightening on the jaws. The other I would suggest is 2 more but shorter in length and with about 30mm or so round handle for adjusting two opposite jaws at the same time for a quicker indication of parts. The shorter 2 can be made even from mild steel as there will not be a great deal of torque applied to these smaller ones.

Neil

T bar is way too long.

Tony

I would have thought you stood a good chance of damaging the jaws with that much leverage.

That said if a chuck can't hold without that much force being applied it's already ruined. I bought an old lathe with a bell mouthed 6" 4 jaw and a ground down oversized key.

Why does it need a centre drilling in the end?

Suggestions.

1 Measure the square socket in the chuck.

2 Carefully file the square on the end of the embryo key, offering up to the chuck, frequently.

3 To keep the flats the same length, make a collar and clamp it to the shank, to act as a length stop.

With a square of only 7 mm , the chuck would seem to be small, so a long tommy bar is not needed, and as you have found out, damaging! Probably 125 mm would quite sufficient, 150 mm max, in my book.

Using excess torque will only strain and wear the scroll and jaws needlessly. You are looking to grip a piece of relatively small bar, not moor the Queen Elizabeth!

Howard