I have and use a bench grinder, and then use stones to finish edges, but when I use
my friend's tool grinder it is great to set angles on the table and miter guide and just grind a
perfect edge and angle. REALLY nice to make up a handful of thread cutters or touch
up the existing cutters. Kind of reminds me of working on my table saw.
One thing that seems to help me is using "too large" cutter blanks. Certainly when making
tiny boring bars of solid HSS, you are absolutely correct to use smaller blanks, but for other
cutters, some of my engineering knowledge comes into my mind. The stiffness of a cantilever
beam (cutting tool) is proportional to the CUBE of the height of the tool, so a tool twice as
tall is eight times as stiff. A 1/2 by 1/2 cutting tool is 3 times as stiff as a 3/8x3/8 cutting
tool, and is 16 times as stiff as a 1/4x1/4 cutting tool. The sixteen X comes from eight times due
to the height change and another factor of two due to width change.
Also, the overhang of the cutting tool needs to be as short as is possible while clearing features.
The deflection of a cantilever beam with a tip loading is proportional to the CUBE of the overhang
distance.
Examples: a 1/2 x 1/2 cutting tool extending 1" from the tool holder, with a 1000 lb load
at the tip due to cutting forces (arbitrary load) will deflect 0.0021". A 3/8 x 3/8 cutter
set up the same way, with same load will deflect 0.00675", basically triple deflection. A
5/16 x 5/16 cutting tool set up the same way will deflect 0.01398", twice the 3/8 cutter and
seven times the 1/2 inch cutter.
If you cut the overhang distance from 1" to 3/8", the deflections drop by a factor of 19
to 0.00011" for 1/2 cutter, 0.00036" for 3/8" cutter, and 0.00072" for 5/16" cutter.
The key take-home is that small increases in tool cutter height make very big reductions
in deflection (relates to chatter, and smoothness of cut, plus accuracy of cut), and small
reductions in tool overhang also make very large reductions in tip deflection - cube functions.
Tool cross section width is directly proportional, assuming the cross section height stays the same.
A 1/2 " tall cutter that is 1/2" wide, if changed to 1" wide is twice as stiff. If that is
rotated 90 degrees to 1" tall by 1/2" wide it is eight times as stiff as 1/2 x 1/2.
Use a "too big" cutter and the shortest practical overhang past the tool holder.
Simple engineering truths that help when selecting cutter sizes and setting the cutter in
the tool holder.
Bill
