Tool steel for making a die

[Lo-Fi]

Evening!

I have a small job on making a few spring dies. These are not for the tool room! They're intended for graunching down studs in a boiler firebox to accept the next size down of sacrificial nut to stop the end of the stud itself burning away.

I'm looking for some input on what type of tool steel to choose for the job. It needs to be tough above anything, doesn't need to hold a razor edge and doesn't need any special kind of heat tolerance as it'll be hand use only. It will need some fatigue resistance and spring as it's intended to form a "contracting die" to get from one size to the next. Heat treatable in the shop with a gas torch is what I'm hoping for, but I'm not averse to splashing out on an oven.

Pics of the original BR 11/16>5/8 tool I'm kinda copying:





The studs (stays) protrude about half an inch beyond the inner firebox plates. Any rotational forces applied in dieing them down to accept new nuts need to be kept low or you risk breaking the caulking which keeps them steam tight in the plates. The nuts eventually burn away and require replacement, hopefully having protected the stays, but it's not always 100%. You either have to break the corrosion off to accept the same size nut or go down to the next size if there's not enough meat left. You can hopefully see why special tools are needed to get to the point where you can run a traditional die down... They're a bit of a curiousity, being a Whitworth thread form but 11tpi regardless of size, and can be anything from 1/2" to over an inch. I'll be making a complete set or sets. I suspect the originals are just some low grade carbon steel, but I might as well start with something good as I have the choice.

Thanks for reading!

 

[brino]

Sorry @Lo-Fi , no answers here, but I am following along to learn something!
-brino

 

[benmychree]

A good starting point may be with 0-1, a basic tool steel, easily machined and heat treated. not familiar with "graunching" on this side of the pond ---

 

[Ulma Doctor]

cool project!

 

[wrat]

So it sounds like the studs are basically boiler plate (!) or almost nothing when it comes to hardness. They etch and erode well past any case-hardening and thermal cycle well beyond any ordinary heat treat.
It also sounds like you're not working on a hot boiler, or at least not full temp 750F+.
Regardless, seems like it can be *anything* that will take a decent hardening. It needn't be A6, Waspalloy, or any kind of heavy exotic. Just ordinary carbon steel or even a chrome moly like 4130-4340.

A good starting point may be with 0-1, a basic tool steel,

Yes, agree. Just about anything.
I'd go as far as to say you could use any SAE1XXX steel and carburize or case harden it for this duty.
IMO, the trick will be the heat treat. Hard, but tempered back. It doesn't look hammered-on, but looks like it takes a little torque.

 

[Lo-Fi]

Thanks chaps! Sorry taking ages to get back to the thread. Life, you know!

Thank you to all for the suggestions; I've ordered some O1 bars for the job.

Anyway... CAD happened:



The 2D gets dimensioned, of course, and working from that I produced a test piece in mild steel:



The lathe taper sttachment came in really handy



Threading 11tpi:



I've made this an 11/16 to 5/8, but only had a 5/8 on hand... So I had to get creative. Yes, that's a tap in the lathe tool holder, not the tailstock.... Every thread, regardless of size is 11tpi here, remember



Fresh out of the lathe:



And into the dividing head:



I've designed it so all the flutes can be cut with a 4mm carbide end mill with 8mm relief holes required. All offsets and angles are set up in multiples of 30 degrees, meaning very simple indexing operations on the dividing head using only the 18 hole plate.

Offset Y 4mm (2mm+2mm for cutter radius), cut through three times indexing 120 degrees each time
Offset Y -4mm, index 60 degrees, then 120 each time.
Zero Y, rotate 30 degrees to bisect the two sets of cuts 60 degrees apart, drill x 8mm3 relief holes.
Swap back to 4mm cutter, finish reliefs.



It was lovely having that all figured out beforehand in CAD and on my prints. Nailed it! I loooove dividing head work.

I had to check how the old style nut fit. It's not to my design which supports the fingers far better, but I couldn't help but try the thread fit. It actually works!



Side by side:



I've ordered some tool steel for the body and 50mm hex bar for the nuts and have formed a cunning plan for cam machining the fingers to get the nut to squeeze the leading edges more than trailing, which was an inherent weakness of the original design. I've improved the cutting geometry, but sacrficed the ability to get the leading edge into the work when squeezed if I don't do something. Finishing work will be needed in the tool grinder once the fingers are hardened to bring up a nice edge on the internal cutting edges, of course.

As it's only a prototype, I didn't bother to cross drill the driving holes at the base. I'm thinking I might shorten it and machine a hex on the end, which would save some expensive tool steel stock. All in all, the prototype was well worth it even if it's useless for cutting anything - I learned a lot.

Stay tuned for more!