I'm currently trying to design a timber box sash window.
I've had a look at both "traditional" designs from books, and more modern double-glazed window designs, and most have the same general arrangement:
There are two sashes in the frame, the top/outer sash and the bottom/inner sash. The outer frame is built as a box in the factory. On site, the frame is fitted into the brickwork (often behind a reveal to mask the large sections needed to house the balance weights).
Next the outer sash is placed into the frame, the parting bead (which separates the two sashes, acting as a runner, and also holding the outer sash in place against the frame rebate) is hammered into a groove in the frame, then pinned into place with glazing pins or similar.
Then the bottom sash is placed into the frame, and the staff bead on the inside is pinned in over the top to hold this sash in and also act as a runner face.
The sashes then lock together in the closed position by a catch at the meeting rails.
What gets me is that the parting bead is what's keeping the bottom sash from being pulled out to the outside of the frame, but it's also visible/accessible from the outside. What's to stop somebody either chiselling off or levering away the parting bead, then yanking the bottom sash out of the frame towards them?
I've found a product that claims to address this here but the end result is that the parting bead is fixed into place via screws which are still accessible from the outside of the frame. They call it "secure" because the screwdriver bit needed is a "Torx" bit (star key), but in my experience any standard multi-bit set has a number of these in these days, so they're not exactly exotic and this doesn't really seem to solve anything.
How can I change the fixing method of the parting bead or alter the design in such a way that it's secure?
Assume that the frame is fitted with the most secure glazing available - I'm not interested in debates about the glazing as this is largely a solved problem.