These milling methods were originally developed for roughing and semi-roughing of difficult materials, like hard steels, ISO H, heat resistant super alloys (HRSA), and ISO S, but can also be used in other materials, especially in vibration sensitive applications.
The techniques are based on a small radial depth of cut, ae, which:
- Generates a low radial cutting force that places less demand on stability and enables a large depth of cut, ap.
- Means that only one tooth is in cut at a time, which minimizes vibration tendency.
- Reduces the heat in the cutting zone due to the short contact time, making it possible to use higher cutting speeds.
- Generates a small chip thickness, hex, but a high feed, fz.
It can be divided into:
- trochoidal milling – primarily used for machining slots.
- slicing – usually used for semi-roughing of corners.
Both these slicing methods have proven to be very secure and productive methods.
Choice of tools
- The most commonly used tool for slicing operations is the CoroMill Plura.
- CoroMill 316, CoroMill 490 or CoroMill 390 are alternatives when the depth of cut is lower.
- The slicing technique can also be used with long edge cutters that combine small ae with large ap.
How to apply
Slicing uses a higher cutting speed, vc, and an axial cut, ap, but with only small radial engagements, ae, and feed per tooth, fz. This is possible due to:
- Thin chip thickness
- Small arc of engagement
- Lower cutting force/deflection
- Reduced temperature at cutting zone
- Deeper axial cuts
- Higher speeds
An excellent method for slotting when vibration is a problem; it is also suitable for rough milling of confined cavities, pockets and grooves.
Trochoidal milling can be defined as circular milling that includes simultaneous forward movements. The cutter removes repeated "slices“ of material in a sequence of continuous spiral tool paths in its radial direction.
It requires specialized programming and machine tool capabilities.
The tool is programmed with a roll entry into and exit from cut, with the radial pitch, w, kept low, which means that:
- The controlled arc of engagement generates low cutting forces, which enable high axial depths of cut.
- The whole cutting edge length is utilized, ensuring that the heat and wear are uniform and spread out, leading to longer tool life than traditional slot milling.
- Due to the short arc of engagement, multi-edge tools are used, which enable high table feeds with secure tool life.
- The maximum radial cut, ae, should not exceed 20% of the cutter diameter.
ap ≤ 2 x Dc
ae = small
vf = high
vc = up to 10 times that of conventional methods
For groove widths less than 2 x Dc
The tool is programmed on a continuous spiral path that feeds in the radial direction to form a groove or a profile. The feed is constant, with a continuously varying radial cut. 50% of the time the tool is out of cut.
1) The radial cut is constantly changing and, at the greatest immersion, it is higher than the programmed stepover, w.
2) It is important to keep the cutter diameter to a slot width ratio below 70%,
and the radial pitch, w, below 10% of Dc.
3) The feed is constant; however, the tool center feed, vf, varies from the periphery feed, vfm. When the feed is programmed based on the tool center, then the peripheral feed must be calculated.
- Max. cutter dia Dc = 70% slot width
- Stepover w = max. 10% Dc
- Radial cut max. ae = 20% Dc
- Axial cut ap = up to 2 x Dc
- Start feed per tooth fz = .004 inch
Calculate programmed feed vf
Machining cases using trochoidal milling
1 – Narrow groove – Inconel 718 (44HRC)
Trochoidal milling provides a far more secure process, when compared to traditional slotting or plunging, with increased tool life and reduced tooling costs, as a .315 inch (8 mm) tool replaces a .472 inch (12 mm) tool.
For grooves wider than 2 x Dc
A continuous spiral path, such as those programmed for the narrow groove where 50% of the time is spent with the tool out of the cut, can be optimized as the groove, becomes wider:
1. Roll into cut – programmed radius (radm) = 50% of Dc.
2. G1 with ae = .004 x Dc.
3. Roll out of cut – programmed radius (radm) = 50% of Dc.
4. Rapid movement to next start position.
5. Repeat cycle.
- Radial depth
– CoroMill Plura ae = 10% Dc
– CoroMill 390/490 ae = 20% Dc
- Axial cut ap = up to 2 x Dc
- Start feed per tooth fz = .004 mm
- Radius feed radfv = 0.5 x G1
2 – Wide groove – Scallop
Number of slots/component 8
Width 1.772 inch
Depth .630 inch
Thickness .157 inch
Tool 1 – CoroMill 390 – Ø .630 inch
Tool 2 – CoroMill Plura – Ø .472 inch
a) Stainless steel – 316
b) HRSA – Inconel 718 (44 HRC)
CoroMill® 390 vs CoroMill® Plura
- Stainless steel – CoroMill 390 offers the fastest time – 140% faster than CoroMill Plura.
In stainless steel, the CoroMill 390 performed without material "clogging" or jamming in the flutes, which allowed for a faster radial cut, ae, and higher feed per tooth, fz, than the CoroMill Plura.
- HRSA – CoroMill Plura was 120% faster than CoroMill 390.
In the harder HRSA, the extra teeth and high helix of the CoroMill Plura produced a much smoother operation.
Slicing – corner milling
Slicing is a semi-roughing technique used in corner milling where the larger tool used in the previous operation could not reach.
Unlike trochoidal milling, no roll into or from cut is required, as the radial cut builds from zero to a maximum in the middle, and then drops back to zero again.
Multiple passes successively remove material, ensuring consistent low radial immersion/engagement angle and low cutting forces.
Feed rate reduction in corners:
- As with all radius contouring, when programming with a tool center feed, vf, the feed rate needs to be reduced relative to the tool periphery feed, vfm, to maintain a constant feed per tooth.
- Depth of cut can become too great to be able to run at same high feed as with straight line cutting, depending upon cutter diameter to corner radius relationship.
- However, the ratio between programmed cutter path diameter, Dvf, and hole diameter, Dm, is constantly increasing towards the finished corner radius; which means that the feed needs to continually decrease for each pass.
- Process becomes unstable and vibration occurs.
- A machine tool with good dynamic stability and tool center feed reduction control is essential for successful milling of internal corners.
Dvf and vf continually decreased for each pass
w = radial stepover
radm = component end radius
radw = component start radius
Typical values for a CoroMill Plura R216.24-xxx50-xxK xxP
- Maximum cutter diameter Dc = .069 x radm
- Radial stepover w = 10% Dc
- High axial cut ap = up to .079 x Dc
- Start feed per tooth fz = .004 inch
- Cutting speed – approx. 3-6 times the normal recommendation.
For the same start and end radii, the number of passes required will vary depending upon the corner angle.
For corners with angles less than 60˚, plunging using the CoroMill 390 or a plunge drill can be a good solution.
Angle of corner