Assemble the slider blocks onto the spindle mount and fit two columns with the top
plate also fitted. Bring the slider blocks up to the top plate and using a 5 mm drill,
drill through the hole already in the top plate to mark the position for the leadscrew.
Due to the nature of the assembly you will probably need to do this with a hand held
drill. Dismantle and drill the 5 mm hole fully through the slider block. Follow this
by drilling the 12 mm diameter hole 10 mm deep, make sure you do this on the correct
side. This hole is a generous clearance for the leadscrew so any wander on the 5
mm hole is unlikely to be a problem. The 5 mm hole in the top plate can now be opened
up to 10 mm as per the drawing.
Mount the block in the four jaw chuck with the 5 mm hole visible and adjust to run
true using the centre finder as above and enlarge hole to 8.1 mm (thread core dia.
plus 0.1 mm). However, this hole must run true so drill say 7.8 mm diameter and bore
to 8.1 mm. The hole size is important as explained above regarding square threads
so make a plug gauge for checking. Also, bore the entrance to 10.1 mm by 3 mm deep,
again this diameter is important. This will indicate when the thread cutting tool
has reached the threads outside diameter.
Cut the square thread using the already finished leadscrew to check size, Photograph
12. With the hole starting at 8.1 mm diameter and the cutter needing to project from
the boring bar by a minimum of 1 mm, then the maximum boring bar size will be 7 mm
and preferably a little less. Because of this keep its projection to a minimum to
avoid it flexing.
Finally (maybe) make slots as per the drawing to provide the necessary adjustment,
Photograph 13, though my photographs show that I made the slots prior to cutting
the internal square thread. When using a slitting saw, do remember to reduce the
speed in keeping with its diameter. If this is not done, then whilst the saw will
start to cut very well it will rapidly overheat, as there is so little metal at the
saw's tooth to absorb the heat generated. Remember, running your 80 mm diameter saw
at 600 rpm is equivalent to running a 10 mm endmill at 4800 rpm, something that you
would never do. I include this advice from bitter experience.
Above, I added “maybe”, as it is possible that at this point you may run into a minor
problem. Due to internal stresses in the cast iron the size of the bored hole may
change when the slot is cut. If the hole increases in size then there will be no
problem as the final adjustment provided in the design will cope with this. If though
the slot closes, reducing the hole size, then assembly will be a little difficult
but more important the block may not slide on the column as easily as would be liked.