Howw do I put a square peg into a very small round hole....

[Beanwood]

Ok so this is what I am trying to achieve

It’s viewed from the top.
Ignore the bottom’bar’ as that will be screwed or dowelled on.
I was expecting to need to cut from the face which adjoins the bottom bar to get the square corners. So from bottom bar to end of slot is 6mm. Hence 6mm deep cut wit rest of slot being 1.5 mm wide. It needs to be this orientation due to the shape of object I am piercing holes in.
Behind the punch face I can relieve with a larger bit. Does this make sense now?

 

[Kram]

I can reduce the thickness of the die to about 1mm I suppose, in an area local to the actual die hole

Perhaps if you are only using it on paper? I'd suggest it needs to be much thicker for anything else. Note that harder steels are typically more brittle and 1mm thick hard steel can be snapped between fingers.
5mm plus would be a good starting point.

In other news- how does one cut tool steel? Put it on my new automatic hacksaw and barely even a scratch!

Which grade do you have, there are many varieties. Annealed O1 ground flat stock is the most common for purchasing and can be cut nearly any way you like, I have hacksawed inch thick lumps in the past.. If it were hardened it would be very difficult with normal tooling, same with HSS bits.

Very hard stuff you can use CBN tools, lightly hardened is ok with carbide. Either way it can get expensive very quickly if you wear or break cutters.

 

[Screwdriver]

Ok so this is what I am trying to achieve
View attachment 457481
It’s viewed from the top.
Ignore the bottom’bar’ as that will be screwed or dowelled on.
I was expecting to need to cut from the face which adjoins the bottom bar to get the square corners. So from bottom bar to end of slot is 6mm. Hence 6mm deep cut wit rest of slot being 1.5 mm wide. It needs to be this orientation due to the shape of object I am piercing holes in.
Behind the punch face I can relieve with a larger bit. Does this make sense now?

Probably a bit easier to reorient your slot:

One part being just a flat straight surface, the other shown at the right with the milling tool above it. In that orientation, any flat bottom cutter less than 6mm diameter will give you the square corners. Read and digest the suggestion above from @Agroshield regards clearances.

 

[Beanwood]

Probably a bit easier to reorient your slot:

One part being just a flat straight surface, the other shown at the right with the milling tool above it. In that orientation, any flat bottom cutter less than 6mm diameter will give you the square corners. Read and digest the suggestion above from @Agroshield regards clearances.

Thanks - love the design - far better than my workshop sharpie on a bit of cardboard!

I really need to learn to think more laterally, and if you're going to help me design how to do this, give you the whole spec - not just bits of it.

I need the die plate to be 65mm long in total, with the cutout at 35mm. Ideally 23mm wide, to form a nice flat plate for the piece being punched. It can't be wider as there are 'downward' protruding edges on the puched item. It doesn't want to be a different size, as this design will hold it in the correct place for punching.
I need the slot to run across the width - i.e. 8.5mm from each edge - 6mm length.
The width of the slot will be 1.5mm. This can be cut into one face.
@Agroshield is spot on with his assessment of it being much easier to judge/adjust tolerances if using the design you have shown above. (I'm punching 0.2mm tin - so the 'gap' should be about 0.06mm if my studies are correct - so that cigarette paper is a good choice)
I also need to have a way of holding this on a fly press (So bolster or clamps)
I was thinking of cutting the way I described as I have some 12mm square tool steel (Lathe tool blanks). I could then cut to length, and Bob's your uncle!
To cut it as you all suggested above will mean I need to use a piece of Gauge plate - which I think will work ? (I have found a piece of unknown grade and temper lying around of just about the right width/length, but only 8mm thick, so joining with bolts could be a problem, I may need to go the silver steel dowels route.)

I've had a quick practice run in mild steel tonight, and apart from the mill alignment being slightly off - it's really easy so far

 

[Beanwood]

Yeuch

 

[Screwdriver]

Something like this:

 

[Beanwood]

Something like this:

That is pretty awesome But don’t you ever sleep

 

[Agroshield]

I also need to have a way of holding this on a fly press

With it being so small, you would not want any play in the ram of the press. You might have to design punch and die as one unit on guide rails/pins so the punch always enters central to the die. Then you do not need to hold it as such on the press, just make sure it is somewhere below the centre of the ram.

 

[Screwdriver]

That is pretty awesome But don’t you ever sleep

Nearly all the time but in a different time zone.

Alignment is an issue as @Agroshield suggests so you definitely want to keep the punch as short as possible. Depending on the size of the part, you might be able to incorporate a guide pin - a large sliding connection or perhaps a hinge to keep them together.

The obvious datum would be the split line which one or other part might be able to slide against to maintain alignment. Would look a bit like outriggers since they have to give clearance to the part. On my flypress, I rely on the Hunton die holder but there are also those big "blocks on springs" die press holders that have the punch and die mounted top and bottom. I might even have a spare if you need one.

 

[Beanwood]

Thanks - for your help on this so far chaps - love the design - far better than my workshop sharpie on a bit of cardboard!

We have pivoted a little - and will be putting the rectangular punch 'on hold' for a while.
Over the weekend I made another couple of round hole punches (Very quick and easy to do) which will enable us to keep moving forward until I have time to experiment some more.