How well do the Magnetic face 5c/whirly chucks work?

I've cut a couple of fairly large (~3.25") x 3" long Punches that I will need to make sometime next month. Typically I just wire EDM punches, but in this case I will likely need to make a few Provisionals to adjust the final size as needed.

I was thinking that instead of wire EDMing them I would buy one of those small diameter mag chucks and mount them to one of my whirlies (simco microgrind or harig duo-form)

But how well do they work? is all that extra weight hanging off the end of a 5c shank too much to get consistent results?

And if they suck is there a better option to use on a manual surface grinder?

They do not have a lot of holding power.
Add up it's length and another 3 inches and you are out there a ways.
Is the back end of the punch square to within microns or is there a holding fixture/block involved?
5C may run nice at 1 to 3 inches out. Pass that and there will be problems. All of which one can comp in the grind or remount the chuck until true and then indicate the part mounting and fiddle with it.
On just normal 5C we program the grinder to go into the LVDT and check the part load for runout/position before the cycle starts.
This is expensive in time used but is nice when the machine says "I''m not going to run this part,... try again".
You do not mention tolerance or true to it's mounting surfaces in use.
A whole lot of people do like these things. I have one. It just did not work for me as I had hoped. I keep it because you never know and it has paid for itself.
Your mileage may very so yes/no is impossible. The sag from the weight is constant so that easy enough.
Also that weight preloads all the things in your spin fixture to help them repeat better.
Is the grind work being done up top or on the side. How aggressive?

Maybe something like this also works? Suburban Tool Faceplate & V-Block - CM-5C-FVB - Penn Tool Co., Inc

Bob.
(I'll bet you hoped for a simple answer here. Waiting for our many other members here for real world stuff or even just concepts)

I had a friend who did ID and OD grinding on a face magnet. He had V blocks and block-in devices to hold work.
I never saw him doing the grinding. My part of it was designing and making holding arbors, stub base mandrels, and odd shape V blocks for his work. Much if not most was one-ups and few-ups tickle grinding after heat treat.
I would guess that a two or three-finger push might be all many of his set-ups could bear.
Most grinding needs a two-finger strength hold or it will not grind.

Bob and Buck, I appreciate the input.

Based on what I am reading here they are a no go for my purposes.

I think I will instead look at making a solid 5c arbor to mount them to, that way the stickout length can be kept to a minimum there won't be risk of the part moving during roughing passes. it should also allow me to grind the entire length in one shot so long as the arbor is smaller in diameter than the punch.

The area at the base of the punch needs to be held fairly tightly +0.000 -0.0002/3 to fit in a retainer block. The portion of the punch that does the trimming isn't so important as there will be 0.002" per side breakage. It doesn't need to come off the arbor square to the base, it just needs to run concentric. I will just be sure to leave enough stock on the bottom to grind it into square after the fact.