Hi Andy,
start with a length of box tube 38cm long ( yes this is over long)
make sure both ends are cut at 90 degree straight cut....
from the right hand end mark a scribe line 290mm along to the left,
then
scribe this line all around the box tube
then
scribe another line 69.5mm further to the left
then cut a 26degree angle at the end of the steel that you started the first measurement from,
once thats done,
turn the box tube 1 flat over away from you,
the scribe a 14degree line
turn the tube over by 2 flats,
scribe another 14degree line,
then
turn the tube back 1 flat & join the 2 scribe lines,
this is the base compound mitre cut,
place into a vice & cut each scribed line, turn the tube in the vice to cut each line,
then
scribe out a 13degree angle on either side of the 290mm scribe following the diagram in the book ( sorry , i don,t have the book with me at mo, so can,t give the page number but its the cutting list diagram in rear of the book)
scribe the 13 degree on opposite sides of the tube then join these scribe lines together on the flat in between them,
place in a vice & cut these scribe lines, turning the tube to do the opposite side,
once that is cut, make sure the cut faces are clean then bend the tube so it closes the tube together,
then using the jig as per book scribe & cut the 14 degree cut at the top scribed line which is 69.5 mm from the "notch" you,ve just cut.
you will then lose the extra length from the 38cm( this just helps set up the measurements without running out of steel
)
The other FF part is just the same but in opposite hand.
sorry if this sounds long winded but its easier to do than explain in writing
hope this helps
by the way....make sure your jig for the front frame is spot on as per book spec.
cheers
andy