What to Do When the Chips Are Down

Deep-cavity milling requires proper chip creation, evacuation

Milling deep pockets

The main challenges in milling deep pockets are chip creation and evacuation, especially as you get deeper.

When the chips are down -- way down, deep inside a pocket -- it is imperative to get them out of the work zone quickly and easily so they do not get recut.

The second challenge in deep pocketing is reducing vibration. The enemy in any type of cutting, vibration will worsen as the pocket deepens.

Another challenge that occurs as the pocket gets deeper is that the part’s side walls can act as barriers, and toolholders and adapters can even collide with the part. Steps can be taken, however, to ensure that these features common in die/mold work are created productively.

The first steps are efficient chip creation and evacuation.

Chip Control

“The main challenges in milling deep pockets are chip creation and evacuation, especially as you get deeper,” explained Tom Hagan, milling product manager for Iscar Tools Canada.

The first of these challenges, efficient chip creation can be achieved by using the correct tool for the machining process and material; the second challenge, evacuation, can be solved by using high-pressure coolant (HPC).

“In the past five years, grades have gotten much better and changes in tool geometry have taken place that help break the chip better,” said Hagan. “Iscar, as an example, created the MillShred P290 line, which is suitable for machining very deep cavities because the chips can be very easily removed from the machining area.”

This happens because the tool has serrated milling inserts with internal coolant channels. This design reduces cutting forces, reduces chatter, and makes it productive, even with high overhangs, according to Hagan. The inserts reduce the size of the chips by essentially shredding them.

Reducing chip size reduces cutting pressure so higher productivity can be attained by increasing cutting parameters, specifically speed and DOC.

Using tools with deep chip gullets also is important in deep-pocket applications, said Hagan.

Chipping insert serrated edge

Inserts with a serrated edge break the chips into smaller, more manageable pieces.

Using HPC

The application of high-pressure coolant is a necessity in deep milling operations because you simply must evacuate the chip before it can be recut.

“If you start recutting chips, that will cause the insert to break,” said Hagan. “If you use just flood coolant or through-coolant that is not high-pressure, the chips will just be sitting at the bottom of the cut.”

This is especially problematic for mold and die work if the part is set up on a magnetic tombstone fixture. The chips at the bottom of the pocket need that high-pressure coolant to get them out of the work zone. In extreme-depth situations, an air blast can even be employed, along with high-pressure coolant, to ensure that chips leave the zone.

“When high-pressure coolant is applied, the chip also can be broken easier. This becomes even more important in materials like 300 series stainless steels, which are very gummy,” said Hagan.

The material being cut will affect the tool’s ability to evacuate chips. Selecting the appropriate insert grade and geometry is the first step in achieving acceptable tool life and chip evacuation. Typically, short-chipping materials such as cast iron aren’t much of a problem in terms of chip evacuation. Stainless steels and heat-resistant superalloys (HRSA) are more problematic because of the long chips that are created during the cutting process.

A lot can be learned from the chips produced.

Chip appearance varies based on several factors, but workpiece material is the main driver. The two most important details to watch for are chip color and chip size. The sizes of chips produced when cutting cast iron are vastly different from those made when cutting steel. And, depending on the material you are machining, there are certain colors to look for as chips are created.

Three characteristics of the machining process to keep an eye on are:

  1. The chips.
  2. Machine vibration.
  3. Load meter.

Abnormalities in any of these could point to an improper tool, holder, or cutting parameters.

Eliminate Chatter

The second step in deep-pocket milling is reducing the vibration caused by the machining process.

vibration-dampening toolholders

Vibration-dampening toolholders allow machinists to run deep-pocketing operations faster.

Vibration reduces tool life, creates poor surface finishes, and in extreme cases can lead to insert failure. Dampened tools should be considered, especially in long-reach applications.

Reducing chatter during cavity milling operations will improve productivity because it increases metal removal rates, creates finer surface finishes in fewer steps, and reduces scrap production.

“The benefits of reducing vibration are less wear on inserts, better surface finishes, and less stress placed on the machining center, especially the spindle,” explained Tim Aydt, product manager for Seco Tools. “Proper fixturing, workholding, and machine tool maintenance also all work together to reduce vibration.”

A vibration-dampening tool reduces chatter. Most of these tools create the effect by having inserts with different pitch angles. Tools can be purchased with both variable pitch and different helical angles, which are suitable for machining high-temperature superalloys to reduce chatter.

“If you have too much vibration in your process, you will break inserts and you also will get terrible surface finish on the wall,” said Aydt.

Vibration-dampening holders also should be employed to get the best results.

Vibration-damping mill holders increase productivity, improve surface finish, and lengthen both tool and spindle life.

“Seco’s Steadyline™ vibration-damping system improves the dynamic rigidity of the milling tool,” said Aydt. “This allows higher cutting data to be used with the best overall stability.”

Large, complex workpieces and deep cavities demand long-overhang tools. These applications are becoming increasingly common, especially in die/mold work. A typical long-overhang milling operation can be performed faster using vibration-dampening holders, and tools will last longer.

Benefits of running a vibration-dampening holding system include:

  • Machine overhangs up to 5xD with the highest possible cutting data.
  • Better surface finish.
  • Better tool and machine life.

Machine tool and setup rigidity play big roles in any machining process, but they become even more critical in a long-overhang situation. The more variables there are that add vibration to the milling process, the more difficult it will be to eliminate them.

There is no single cause of vibration, and that means there is no single cure. Vibration creates uneven wear on cutting tools and thereby shortens tool life.

Four main alterations can be tried if vibration becomes a problem during deep-cavity milling. They are:

  1. Use a dampened adapter.
  2. Use a larger adapter to create more stability.
  3. Use the smallest possible milling cutter.
  4. Reduce the radial and axial cutting depths.

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