Hi Toms Wheels:
John.k has a point about the amount of material you're hoping to remove with this setup, both with each cut and the total material removal when you are grinding from the solid.
However, thread grinding
from the solid is a perfectly viable process, just not with a wimpy setup and most importantly not with a weenie little wheel.
When you look at a dedicated thread grinder you will see that they run large diameter wheels, in big rigid spindles, tilted at the helix angle of the thread, and deficiencies in these three factors are why the process is not working for you.
A big wheel means that every grain at the periphery of the wheel shares the work with many others, which obviously is not true with a small wheel.
This primarily is why the corner of your wheel is degrading so fast.
That and you can't conveniently re-dress the wheel on the fly and you (presumably) don't have flood coolant.
Also you do not have the benefit of the research thread grinder makers have undertaken to find the best combination of bond, grain size and wheel hardness to give the best balance of free, cool cutting and wheel shape retention.
All of these things matter immensely as you're finding out.
So given the limitations of your gear, you need to circumvent the worst of it.
If you are willing to accept a hard wheel you can preserve the shape better.
To get a hard wheel to cut without burning you need better cooling and you can't load it up by hogging with it.
So roughing the thread with a single point threading tool, dressing the wheel often (rig something up so you don't have to tear down your setup for each dress) find a way to flood cool it without trashing the lathe: all will help you.
Grab an "L" hardness wheel that's as coarse as you can get away with, and maybe snip out bits of the periphery to break up the cut (it'll run cooler that way).
That will also help you.
Tilt the toolpost grinder to the helix angle as Alex recommends in post #2.
These are all tricks you can try to help you overcome the fact that you're not running a 36" diameter wheel on a 50 horsepower wheelhead using a 15000 lb machine with a built in dresser.
If you do all those things you can grind decent threads...they won't be even remotely as good as what a thread grinder can make, but they will be good enough for many things.
My personal experience was with threaded mold cores...I've ground dozens in my career as a moldmaker and they were done with a surface grinder running a 7 1/4" wheel.
They had to have good fidelity of thread form and a great finish but did not need to be particularly accurate by the criteria that normally apply to ground threads.
I and about a bazillion other toolbreakers were able to routinely achieve threads that were plenty good enough for those kinds of applications, but they weren't micrometer spindle or leadscrew grade threads by any stretch.
So it's by no means impossible so long as you are realistic in your expectations.
Cheers
Marcus
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