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Material selection or additional process

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colin hamilton25/09/2021 19:36:41
43 forum posts
19 photos

So I'm just getting back into machining. The first thing I made was a chuck key for my 4 jaw. It isnt very big and the square end of the key is only 7mm across the flats. I forgot to leave additional length so the available material in the square end is reduced by the centre drill. I made it out of black mild steel from my local steel supplier. I used it today and the square end just twisted into a spiral.

Should I be using a specific grade of steel or do I need to do something to it after machining to make it suitable in this application?

David George 125/09/2021 20:33:41
1656 forum posts
497 photos

Hi Colin. The black mild steel is too soft for a chuck key. I would use EN 40B which is a tough material or silver steel which will need hardening and tempering. Also I would make it from a larger diamiter bad and just reduce the 7mm AF end to allow a larger handle diamiter without weakening the key.


Nigel Graham 225/09/2021 20:47:54
1706 forum posts
20 photos

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.

SillyOldDuffer25/09/2021 21:30:27
7549 forum posts
1680 photos

Like Nigel I'm surprised mild-steel twisted: over-tightening or something wrong with the chuck maybe?

As a pair of chuck keys used together are useful for centring a 4-jaw, I made some out of mild-steel for that purpose and they're OK.


I expected to have to remake the keys in Silver Steel but so far no need.


Frances IoM25/09/2021 21:59:01
1157 forum posts
28 photos
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.
colin hamilton25/09/2021 22:07:59
43 forum posts
19 photos

I did use larger diameter stock (25mm). Since I hand cut the square I'm wondering if I was a bit undersized and combined with the center drill just didnt leave enough stock. This is the before photo. I'll add one of the damage tomorrow.


Martin Kyte25/09/2021 22:17:35
2558 forum posts
45 photos

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

duncan webster25/09/2021 23:03:09
3508 forum posts
63 photos


Edited By duncan webster on 25/09/2021 23:03:48

Neil Lickfold26/09/2021 03:27:44
720 forum posts
127 photos

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.


not done it yet26/09/2021 07:01:49
6322 forum posts
20 photos

You haven’t provided details of the diameter of the centre drill used - that may have some bearing on the matter.

I usually use a high-tensile bolt (old cylinder head bolt?) or silver steel. Never heat treated one yet.🙂

Machining a flat and holding the tommy bar with a couple of fixings (slightly deeper head on the chuck key?) can make a tool which can easily be dismantled. Not as quick as welding, but likely a neater job.

Tony Pratt 126/09/2021 09:35:12
1692 forum posts
8 photos

T bar is way too long.


SillyOldDuffer26/09/2021 09:50:20
7549 forum posts
1680 photos
Posted by Tony Pratt 1 on 26/09/2021 09:35:12:

T bar is way too long.


Too short is bad too! My genius let me down badly with these:


I thought short, fat adjusters would make it easier to rapidly centre opposing jaws by twirling. Brilliant, until they foul the jaws:


Silly me. embarrassed


Dave Halford26/09/2021 10:35:59
1726 forum posts
19 photos

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.

colin hamilton01/10/2021 08:51:13
43 forum posts
19 photos

So I'm going to have another go at making the chuck key this weekend. I've got myself a length of EN40b and EN8 and was going to make one from each (for practise). I'm using HSS tooling. Is there anything specific that I need to consider now I'm using these specific grades? Also as I dont have a mill will I still be able to file the square end?

Howard Lewis01/10/2021 12:55:35
5298 forum posts
13 photos

Why does it need a centre drilling in the end?


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!


colin hamilton01/10/2021 19:49:47
43 forum posts
19 photos

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!


Now that is good advice 😀😀😀😃

colin hamilton03/10/2021 19:51:50
43 forum posts
19 photos

Thanks for the input. Mk 2 in EN8 - much better!!



Edited By colin hamilton on 03/10/2021 19:52:14

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