Technical Articles:
High
Speed in the Slots
High-speed techniques apply to milling slots on mold bases and
componenets
| The
Old Way |
5/8"
diameter roughing mill
Total
process time= 30 minutes
for six parts (not including roughing time) |
| The
New & Improved Way |
3/4" diameter Millstar carbide insert ball
mill with TiAIN coating Depth
of cut per pass= .05"
rpm=
5,000
Feed
rate= 60 ipm
Total
process time= 10 minutes for six
parts (no additional finishing reequired) |
| (See
Figure 2) |
The
process of milling a slot dates back as far as milling itself. It
used to be that mold makers would rough with an HSS or Cobalt roughing
mill and then finish with an end mill. A time-tested and effective
approach to be sure, but it was also time consuming and less than
efficient, especially considering how simple the operation is for
a CNC machine. Well, the times are changing. More and more, machinists
are embracing a high-speed technique to mill slots not just for
cavity and core but also for mold bases and runners, wire channels
and other mold components as well. This new high-speed approach
involves mold makers roughing with a carbide insert ball end mill,
another simple operation for a CNC but one that is considerably
faster and easier on the machine.
| Material |
4140 |
| Hardness |
18
HRc |
| Width
of slot |
3/4" |
| Depth
of slot |
2" |
| Length
of slot |
5" |
| Machine |
Conventional
CN with
5000 rpm maximum |
| (See
figure 1) |
| Special
note: The walls on both sides of the slot were only 2"
thick after machining and here was a 1" hole through the
middle of the part, creating an interupted cut |
Millstar
is a manufacturer of milling and profiling cutters. Here is one of
their own case reports showing dramatic process improvements utilizing
this high-speed milling technique.
In
this particular case, the bottom of the slot was left with the full
radius. The roughing and finishing were done in the same pass, which
is why the depth of cut amounted to only six percent of the diameter.
As such, the parts were completed in one-third the time with a much
better finish, eliminating the need for an additional finishing
tool. If a flat bottom is required, these slots can be finished
with a flat bottom insert.
 |
| Figure 1 |
 |
| Figure 2 |
This
example is just one of many cases in which roughing with a ball
mill produces better parts in considerably shorter cycle times.
An HSS or Cobalt tool cannot machine hardened materials, but utilizing
the carbide ball geometry and the Z-level machining technique allows
for slot machining in prehardened material up to 63 HRc. And deeper,
narrower slots can be machined with even better results by using
a carbide shank toolÐbecause of the rigidity difference between
carbide and HSS.
All
in all, the new high-speed process and the subtle improvement from
the old way to the new have produced remarkable results for mold
makers nationwide. Still new to the marketplace, it has a process
not yet widely utilized. But it has one that more and more machine
shops are adding to their repertoires to boost productivity and
the bottom line without adding additional expense.
Source:
Competitive Mold Maker, Volume 6, Number 1
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© 1999 Makino Inc. All rights reserved.
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