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Tool steel - Beginners guide ?

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Dave Whipp08/12/2019 22:41:35
23 forum posts

Hi All -

I have a 4 jaw chuck (Polish made) that came along with my second hand lathe purchase, however I do not have the correct chuck key for it. From measurement it's a 7mm square key that is required.

Now I do have an imperial chuck key that is a sloppy fit and will sort of do the job but the diameter above the square section is quite large and makes removing the jaws pretty awkward, so I really need a longer and slimmer shank with a 7mm square end. Looking online I could not find a suitably shaped chuck key for sale, so I machined one up from some mild steel bar I had kicking around.

However on first use of tightening the jaws down on a part the square section started to twist, so obviously some stronger material is required.

So my very basic understanding of tool steel (please correct me if I'm wrong) is that it is soft enough to be easily machinable and then by heat treating it becomes much tougher.

However I see there are many different grades available, I'm not sure which would be most suitable for my job - or maybe some other type of steel that does not need heat treatment would be better ?

For information, my only heat source is a bullfinch brazing torch that's running on Propanol (burns a tad hotter than regular propane so I understand). -But I don't have any Oxy, it's Propanol only.

Any pointers or advice gratefully received.

Cheers - Dave.

Vidar08/12/2019 23:15:15
57 forum posts

Not sure if you want a project or an actual lathe key chuck, but searching "Lathe Chuck Key 7mm" found quite a few suppliers online. Here is one in the UK. **LINK**

duncan webster09/12/2019 00:00:22
2736 forum posts
40 photos

I made my missing chuck key out of a big allen screw. Tough stuff but machined OK, fairly slow and kept pouring coolant on. Nowadays I suppose you'd use a carbide cutter

Emgee09/12/2019 00:04:10
1652 forum posts
224 photos

Hi Dave

You could try making a key from EN16T, tough enough without hardening.


Dave Whipp09/12/2019 00:18:46
23 forum posts
Posted by Vidar on 08/12/2019 23:15:15:

Not sure if you want a project or an actual lathe key chuck, but searching "Lathe Chuck Key 7mm" found quite a few suppliers online. Here is one in the UK. **LINK**

Thanks - That's almost identical to the one I already have. Problem is, if you want to remove the chuck jaw for cleaning or just to turn it the other way round, it fouls on the round section of the chuck key as it comes out of the chuck body because it is too large a diameter close to the square key end. You have to partially remove the key and angle it over to one side to allow you to remove the jaw.

I suppose I could buy one of those in the link, remove the T bar, then turn it down to a smaller diameter providing the metal is machinable, or possibly grind it down.

However I thought that making a custom one from scratch might be a quick little side project, and if it requires tool steel then I might learn something along the way. However I don't want to end up spending hours and hours on it.




Edited By Dave Whipp on 09/12/2019 00:20:52

Dave Whipp09/12/2019 00:19:24
23 forum posts
Posted by Emgee on 09/12/2019 00:04:10:

Hi Dave

You could try making a key from EN16T, tough enough without hardening.


OK, thanks, will look into EN16T.


Pete Rimmer09/12/2019 00:29:36
774 forum posts
50 photos
Posted by duncan webster on 09/12/2019 00:00:22:

I made my missing chuck key out of a big allen screw. Tough stuff but machined OK, fairly slow and kept pouring coolant on. Nowadays I suppose you'd use a carbide cutter

A decent socket cap screw would work as you say. I often make tools from 12.9 socket caps. I once made a brake lever pivot for an old Triumph TR6 from one, milled the square in the end and used the milled square as a makeshift broach to cut a new square in the actuating arm slightly bigger than the original. Worked great.

A 10mm shank socket cap is almost perfect diameter for a 7mm square to be milled, the across-corners dimension being 9.9mm

not done it yet09/12/2019 07:10:40
4893 forum posts
20 photos

Here’s approximately what I documented on another forum recently. Works perfectly well and easily selected from my other chuck keys. I may well add a couple of steel balls to at least one other chuck key, possibly all of them (might need to colour the others differently for ease of identification).

I’ve got several chuck keys - and often pick up the wrong one, first off!. My 160mm TOS chucks use different keys and I thought it about time to make a decent one for the independent chuck.

I decided to make one a little different as a novelty (and to investigate if it is easier to use).

A grade 8.8 5/8” stainless bolt was the basis. I left the head on for no other reason than it could come in handy should it ever be needed on another job. I can always turn it down later if I feel it needs to be ‘smoother’.

Threads were turned off the bolt and a square milled on the end (using the Raglans, of course). Drilled about 9mm hole. A piece of half inch ‘tough’ steel rod (an un-needed socket extension bar) was turned to fit the hole and a small ‘dimple’ hole drilled to secure with a grub screw through the bolt head.

A couple of 1” brass balls were drilled to fit the handle ends and simply loctited in place. Added because it will increase the angular inertia and aid hand positioning. Brass? Only because they were the only material available from my local bearing supplier at the time. A wide handle because the angular momentum is increased and I can always easily shorten them if necessary

It currently weighs about 450g and is about 250mm across the handle.


Michael Gilligan09/12/2019 08:04:47
16202 forum posts
706 photos

Interesting design, ndiy

Too big for anything I would need, but worth a look ... so I clicked on the image

It seems that the Raglan forum uses an uncommon definition of the word ‘Guest’

... Shades of ‘Catch 22’


not done it yet09/12/2019 09:10:50
4893 forum posts
20 photos

Simple. They don’t exist, as such!

Zan09/12/2019 09:32:21
178 forum posts
12 photos

I used silver steel when I made mine, hardened then tempered to blue, ( spring temper) been using it for years

If it’s good enough for a screwdriver, it’s ok for a Chuck key

Edited By Zan on 09/12/2019 09:33:55

Dave Whipp09/12/2019 11:47:32
23 forum posts
Posted by not done it yet on 09/12/2019 07:10:40:



That's nice. And it's the sort of design I need with the shank being quite narrow.

However I thought stainless wasn't particularly tough, which is why stainless fasteners are not recomended for safety critical stressed applications on cars. I also thought most stainless bolts are graded as A2 or A4, but isn't 8.8 a regular mild steel bolt if I'm not mistaken ?

12.9 HT bolt sounds more like it though - and I might have one of those kicking around in my odds & sods drawer.... Will have a look later.

So general opinion is that making a chuck key from tool steel is a bit overkill then ?

Thanks - Dave.

Andrew Johnston09/12/2019 12:07:35
5635 forum posts
652 photos
Posted by Dave Whipp on 09/12/2019 11:47:32:

So general opinion is that making a chuck key from tool steel is a bit overkill then ?

It's worse than that; tool steels are the wrong material. They have properties you don't need.

I'd agree with Emgee, use a medium carbon alloy steel like EN16T or EN24T.

The designators A2 and A4 for stainless steel fasteners refer to the grade of stainless steel; A2 is basically 304 and A4 is 316. For steel bolts the grading is based on mechanical properties. The common 8.8 grade is rather more than mild steel, or should be from a reputable supplier. The material should be a medium carbon steel quenched and tempered.


Vidar09/12/2019 12:35:30
57 forum posts

(Stainless steel are generally tougher than carbon steels. That is not the same property as them being stronger though. As for strong there is a wide range of stainless with typical A2 and A4 is on the weaker end of the scale. The opposite end of the scale has stainless which is stronger than 12.9 with around 1500Mpa stainless vs about 1100Mpa for 12.9).

Practically speaking here though A2 or A4 stainless are generally weaker than hardened steels.

Tool steels are made with a wide range of properties catering to specific purposes in machining or manufacturing - ie steels for the tools that are used to make other stuff. (Including making hand tools). Some might be great for your purpose, others might have optimized properties for say high temperature, toughness or long term abrasion instead).

I think a 12.9 bolt or a hex key made into the required square shape will be more than up to the task.

(As a sidenote 17-4PH stainless steel is very friendly to harden, and would be up there with 12.9 in strength. It can be hard to find though).

Edited By Vidar on 09/12/2019 12:38:59

SillyOldDuffer09/12/2019 13:27:11
6192 forum posts
1345 photos

Whilst mild steel isn't ideal for chuck keys because it's a little soft and bendy, I would have thought it would be 'good enough' strength-wise. Is the chuck particularly stiff? The scroll might need cleaning because it's full of swarf.

Or are you an enthusiastic tightener? I've known a few chaps who habitually overdid it! One couldn't stop himself stripping RS232 fixings, and a friend is convinced it's essential to finish off tightening nuts by hammering on the spanner.


Andrew Johnston09/12/2019 13:38:04
5635 forum posts
652 photos
Posted by SillyOldDuffer on 09/12/2019 13:27:11:

One couldn't stop himself stripping RS232 fixings

That's unfair! The RS-232 4-40UNC screwlocks are useless. After fitting one gives it an extra little nip for security and the brass fitting snaps. No warning, no nothing! I always buy a spare pack when I'm fitting them.


not done it yet09/12/2019 14:21:27
4893 forum posts
20 photos

I’ve no idea on what the bolt is/was. It had likely been knocking around the garage for years and wasn’t showing any signs of rust. Hard to machine, as in tough, so good enough for a quick knock-up for a chuck key. The handle was even an even harder material to turn down. I’m not like SoD, I use whatever is to hand, that will do the job.

Brian Oldford09/12/2019 14:40:48
684 forum posts
18 photos

Wouldn't it be possible to turn down the shank of the one suggested earlier **LINK** ?

duncan webster09/12/2019 15:53:46
2736 forum posts
40 photos

The 8.8 grade refers to the UTS being at least 800 MPa and the yield being 80% of this ie 640 MPa. For imperialists that is 52 tsi and 41 tsi. This is not quite as strong as EN16 after it has been heat treated, but allen screws are often 12.9, ie 78/62 tsi. My key made from an 8.8 bolt has lasted for many years

Dave Whipp09/12/2019 17:33:56
23 forum posts

Well I do have some M10 12.9 bolts kicking around but they're not long enough so I have ordered some longer ones on ebay. I have also ordered a short piece of EN16T - thanks all. I'll have a bash at making a fresh key with whatever turns up in the post first.

As for my previous home made key made from 100% pure scrapbinium steel - I cut a 6.9mm square peg on one end and the T bar across the top is approx 6" long and fixed in the centre. I tightened it down with one hand and I could feel it start to twist as I nipped it up. Is that overtightening ?

I have resorted to using a 1/4" ratchet & extension bar in the meantime but it's a sloppy fit and it's a PITA to use if you are trying to centre a piece in the 4 jaw.

So thanks all for the advice - especially about the bolts & Allen screws - I'd never really considered repurposing bolts before but it makes sense.


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