Die & Mould Making - CNC - Computer Numerical Control

40
CHOICE OF HOLDING TOOLS
One of the main criteria
when choosing both
holding and cutting
tools is to have as
small run-out as possible.
The smaller the
run-out is, the more
even the workload
will be on each insert
in a milling cutter. (Zero run-out would of
course theoretically give the best tool life
and the best surface texture and finish).
In HSM applications the size of run-out is
specifically crucial. The TIR (Total Indicator
Readout) should be maximum 10
microns at the cutting edge. A good rule of
thumb is: ”For each 10 microns in added
run out - 50% less tool life!
Balancing adds some steps to the process
and typically involves:
• Measuring the unbalance of a tool/
toolholder assembly.
• Reducing the unbalance by altering the
tool, machining it to remove mass or by
moving counterweights on toolholders.
• Often the procedure has to be repeated,
involving checking the tool again, refining
the previous adjustments until the
balance target is achieved.
Tool balancing leaves several sources of
process instability untouched. One of these
is error in the fit between toolholder and
spindle interface. The reason is that there
is often a measurable play in this clamp,
and there may also be a chip or dirt inside
the taper. The taper will not likely line up
the same way every time. The presence of
any such contamination would create
unbalance even if the tool, toolholder, and
spindle were perfect in every other way.
To balance tools is an additional cost to
the machining process and it should be
analysed in each case if cost reduction
gained by balancing is viable. Sometimes,
however, there is no alternative to get the
required quality.