In addition to 'C spanner', they are also known as 'hook spanner', so that might give more options when searching.

They are sized according to the OD of the nut so measure what you have. There is a table of sizes on this eBay listing, which might help:

https://www.ebay.co.uk/itm/194318571071

If there is an 'official' standard for them, it is DIN1810 (DIN 1810 specifies many 'Forms'. The one with the square prong is Form C).

https://www.ganternorm.com/en/products/3.2-Mounting-positioning-levelling-with-screws-clamping-and-supporting-elements/Nuts/DIN-1810-C-Spanner-Steel

If you have an ER collet chuck, try the spanner from that. It is the same type of spanner so it might fit.

Colin

This is the one I made perhaps 30 years ago.

Works well.

As I recall things I cut a 1/4" thick ring from the end of a piece of steel bar and cut it in half.

The pin was positioned by eye with reference to the collet block "nut" and welded in place before a bit of light filing to clean up to a

I cut the half ring down to what looked like a suitable length of arc and welded that to a short offcut of 1/4" plate. Bandsaw, file and linisher to finish off a bit smart with no sharp edges.

Objectively a bit more time and trouble on radiusing the edges with a bit more spit'n polish would have been nicer but made no difference to how well it works.

The taper on the handle makes it much more pleasant to use. Could have done a simple rectangular bar or even a rod but such tend to be unpleasant to hold. Sufficiently so as to make positioning harder. Learned a long time ago that tapered handles, large end out, are well worth the effort.

Clive

Jason!!!

In principle possible but trying to keep the thing well enough engaged to approach sensible tightness is likely to be a blood sport. A frustrating blood sport.

Its going flying at the first opportunity when you lean on it.

How do I know!

I tried, once, using the little tommy bar out of the horizontal – vertical milling table mounting 5C carrier. Tommy bar works OK on the H-V units because you only have to adjust the nut close enough for the tightening lever to work so its just turning a loose nut with no real leverage needed.

Used a small brass drift for final tightening on the collet block and worked around the workpiece not ending up where I'd ideally have liked.

C spanner construction started about 4 milliseconds after I'd cleared the machines!

These days I'd have made the spanner before finding drawer space for the block.

Its taken a decade or three for the penny to drop but I've finally come to realise its always best to take the extra time to make functional, easy and safe to use tooling / adjusters rather than steam in with the crudest thing that can be cursed into working. Takes 4 or 5 times longer to make but on balance you come out ahead after a few uses because the job goes so much quicker. Especially if accurate setting is required. Never forget your hands are likely to be slightly oily so grip less positive.

Hafta accept loosing on a few when it turns out the nice tool is for a job that pretty much doesn't repeat.

Still ahead overall.

Hate wiping oily blood drips off machines

Clive

ARC euro list ER collet wrenches which engage with 4 slots in the collet nut,the one that I use is painted green and shown on ARCS web catalogue,these grip better and less liable to slip plus the handle,they are strong and take fair amount of abuse,lot better than C spanners,

Colin

Great. I'm glad it's worked out well for you.

Only sorry I no longer appear to have the drawings which would have made life a little easier for you. Pretty sure the original computer files didn't make a jump between machines and/or program upgrades in the intervening years.

Back then I didn't keep hard copies after making the item figuring I could always print again if need be.

Something I discovered to be a very bad idea about 20 years ago when having to recreate some moderately complex stuff with grossly inadequate information due to no longer being able to read, let alone print, the original files.

Clive