A little bit about the terminology I was taught "back in the day": a door frame is rebated to take the door and generally pretty thick (45mm or thicker) and is almost always installed on exterior doors, although some buildings have them on interior doors, too, and you most often don't fix architraves on them; a door casing is thinner (generally 32mm for FD30 fire casings), is rebated to take the door and mostly has architraves fixed to the outside edges; a lining is similar to a casing (i.e. 32mm thick and for interior use) but has no rebates and the rebate is formed
after the door is hung by affixing a stop lath ("banging strip" in the Midlands, I believe) to the face.
It would help if you learned to use the correct terminology. The lock face plate is the piece of the lock
recessed flush in the edge of the door - the bit of ironmongery in the jamb of the door frame/lining/casing is generally referred to as the
keep or
receiver. And the significant bit about it is that you
cannot use a mortise or marking gauge to mark it out. If nothing else, if you think about it, the architraves will mess up any attempt to mark out from the edge of the lining or casing (in the case of interior doors) whilst the rebate will nobble you on door frames and rebated casings because the keep is very often only 3 to 5mm off the edge of the rebate. So, wrong tool!
It is now that you need to have made a good job of putting that lock in absolutely central to the door. Close the door so that you can see where the sprung latch will end up being and put two pencil marks across the rebate using a combi square. If the lock mechanisl has a mortise mark the top and bottom of that in the same way. Offer the keep up and mark two pencil lines for the top and bottom extents of the keep. You should now have either 4 or 6 lines running across the rebate.
Next measure the door thickness, which is maybe 44mm (for an FD30) and do the same for the rebate it is going into (door frames and rebated casings only). The rebate
should be about 1mm deeper than the thickness of the door. This is to allow for 1mm thickness of paint finish. If it isn't 1mm greater than the thickness of the door you have a problem qwhich will need to be dealt with a bit of adjustment (see below). As I said the lock should have been fixed in the middle of the door, so if you transfer the centre line of the lock drillings (i.e. the centre of the door) onto a combination square (directly transfer from the centre line marked on the door,
never by setting to the numbers on the scale as this is a sure fire way to introduce errors). Yo can also mark the top and bottom screw hole positions as you do need to avoid these when cutting the recesses. That is then used to mark the centre line of the keep in the door frame/lining/casing. You should now have a set of markings which looks like this in the rebate:
View attachment 256919
Now measure the width of the keep. This is often different to the width of the lock face plate. Add half the width of the keep to the measurement on your combi square, so if that measurement read as "23mm" and your keep (main body part) was 28mm wide, you'd add 14mm to the setting on the combi square, giving 37mm. Put a pencil line down the rebate. Do the same for the outside edge of the keep body, but
subtract the hald width from the centyre line measurement (23mm - 14mm = 9mm) and again make a pencil mark. Measure the widths of the pockets at the rear of the keep and repeat the exercise for them:
View attachment 256921
Next mark the top and bottom extents of the pockets on the rebate. The area in pink is the pocket you will need to recess to the thickness of the keep:
View attachment 256922
Then you need to mark out the position of the little bent protection tab if the keep has that, together with its' thickness. This is shown in yellow here:
View attachment 256923
What I tend to do then is drill out the waste with an appropriately-sized Forstner bit (in a tool kit you rarely need more then a couple TBH) then finish with a chisel, or in some cases I just take it out with a chisel (obviously you need to sever the fibres first, so your chisels need to be razor sharp - blunt doesn't hack it and will leave you with a result that ends up looking like you carved it out with a spoon):
View attachment 256929
Note: the lower one is shown drilled, the upper one is shown drilled and fully chiselled. This
shouldn't be done using either an auger bit or a spade bit because on very deep keeps you coiuld end up coming out the back of the casing/lining. I've found some nice bits of Amazon with built-in depth stops which are ideal for the job.
Having chopped out the recesses the shallow recess for the keep is taken out with a sharp chisel - yet again you absolutely need to knife the lines neatly before using the chisel:
View attachment 256933
You can now offer in your keep and make any adjustments necessary to get it to sit flush, such as cha,mfering the leading edge of the tab recess, radiusing the corners wher the tab meets the main plate, etc.
Obviously this doesn't cover more awkward features such as radius-ended keeps, keeps with plastic box inserts (for the recesses), etc which add further layers of complexity, but I wouldn't expect you to even look at doing those.
As you can see there is almost as much work putting the keep in properly and neatly as there is in putting the lock in in the first case. A properly fitted keep will close with a nice "click" and you shouldn't be able to rattle the door once it has been painted
Nota bene: Pencil marks should be as faint as possible, made with a very sharp H or 2H pencil (but certainly
NEVER one of those fugly*, useless carpenter's pencils which should be reserved for rough carpentry, not accurate stuff like this) and should always be removed afterwards (a damp cloth generally does the trick).
* - Composite word, I'm sure most people reading this will get my drift