I see that there is an adjustable throat, but is there also an adjustable piece behind the blade?
No, that is the bottom of the tiltable frog.
(Photo below was taken earlier in the build, when the sole was only surface ground, before scraping it flat)
In order to support the iron/blade the whole way down, the frog also has to extend as far as possible.
Yes, it supports the iron (the blade).
How have you found it to work?
It took a while to work into being "conversant" with using it. Often the inclination does not need to be much more than 50 deg. Though at times more is better.
Have you established at what point the chipbreaker stops being helpful?
In an adjustable throat plane, the chip breaker is always merely an alternative option.
Neither facility can be optimized if both are to work together.
So practically, it is an either-or option.
If the throat is close enough to control the chip, the chipbreaker/cap iron cannot be because it would protrude almost through the throat & completely block it. When the throat is primary chip control, the cap iron has to be back from the cutting edge a material amount - in this case perhaps 1/16", certainly at least 1/32" or it would prevent the throat from closing.
When the chipbreaker is primary, the throat needs to be more open so there is space for the chipbreaker to approach within desired chip thickness of the cutting edge. Say perhaps .005" or less if it can be managed. (Piece of typing paper being aprox .0035" thick as an eyeball reference)
So, wood being weird, sometimes a close chip breaker and a somewhat open throat works, sometimes the opposite.
Hand scrapers and scraper planes work without throats or with very wide throats and no chipbreakers.
That depends how the concept chipbreaker is perceived.
A burred scraper as is typical, does force the forming chip back on itself quite abruptly.
So does a sharp chisel (no burr) used as a scraper, at a (back) angle in the same direction as the tool is being moved while scraping.
Have you tried different bevel angles as well? So many options to explore.
Being bevel-down,. there is no advantage to a cutting bevel other than one with adequate cutting clearance at the most acute angle of inclination.
You are correct that there are so many options to explore - a good reason why i was inclined to try to construct a "plane type" platform in which any of the parameters can be varied and compared. & it is always good to hear questions that i might not have thought of.
smt