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Groove or slot milling

Groove millingSlot milling

Groove or slot milling is an operation in which side and face milling is often preferred to end milling.

  • Slots or grooves can be short or long, closed or open, straight or non-straight, deep or shallow, wide or narrow
  • Tool selection is normally determined by the width and depth of the groove and, to some extent, length
  • Available machine type and frequency of operation determine whether an end mill, long-edge cutter or side and face milling cutter should be used
  • Side and face cutters offer the most efficient method for milling large volumes of long, deep grooves, particularly when horizontal milling machines are used. The growth in vertical milling machines and machining centers, however, means that end mills and long-edge cutters are also frequently used in a variety of groove milling operations

Comparison of cutter concepts

Side and face milling

+ Open slots
+ Deep slots
+ Adjustable widths/tolerances
+ Gang milling
+ Cutting off
+ Large product range for different widths/depths
– Closed slots
– Linear grooving only
– Chip evacuation

End milling

+ Closed slots
+ Shallow slots
+ Non-linear slots
+ Versatility (additional methods):
  • Trochoidal slot milling for difficult materials (hard steels, HRSA, etc.)
  • Plunge milling as a problem solver for long tool overhangs
  • Additional semi-finishing/finishing operations can be added easily
  • An endmill can be used for operations other than slot milling
– Deep slots
– High forces
– Vibration sensitive if deflected

Side and face milling

Side and face milling cutters can more efficiently handle long, deep, open slots, and provide the best stability and productivity for this type of milling. They can also be built into a “gang”, to machine more than one surface in the same plane at the same time.

How to apply

  • Choose cutter size, pitch and position so that at least one edge is in the cut at all times
  • Check chip thickness to achieve the optimum feed per tooth
  • In demanding milling, check the requirements for power and torque. Stiff arbors and overhang are very important in applications in which arbors have a free end
  • Fixture and arbor support must be strong to handle up-milling cutting forces


  • First Choice method
  • Use a firm stop in the direction of tangential cutting forces to prevent them from forcing the workpiece down against the table. The feed direction corresponds with the cutting forces, which means that rigidity and eliminating backlash are also important, since the cutter has a tendency to climb


  • Alternative in applications where problems arise due to insufficient rigidity, or when working on exotic materials
  • Solves problems generated by weak setups and chip jamming in deeper grooves


  • Good complement for weak setups and when available power and torque are low
  • Position the flywheel as close to the tool as possible
  • Strengthening the workpiece mounting is always a good investment

Milling open slots using side and face milling cutters

Calculating feed per tooth

A critical factor in peripheral milling using side and face milling cutters is achieving a suitable feed per tooth, fz. Insufficient values cause serious disadvantages, so extra care should always be taken during calculation.

The feed per tooth, fz, should be decreased for deeper slots and increased for shallower ones, in order to maintain the recommended maximum chip thickness. For example, when full slotting with geometry M30, the starting value for maximum chip thickness should be 0.12 mm (0.005 inch).

Note: Because two inserts work together to cut the full slot width, feed is calculated using half the number of inserts zn.

ae / Dcap (%)​ fz (mm/tooth)​ hex (mm)
(0.006 inch)
(0.005 inch)​
(0.008 inch)
(0.005 inch)
(0.011 inch)
(0.005 inch)

Depth of cut

For deeper slots, a special cutter can be ordered. If deeper slots are to be machined, feed per tooth should be decreased. If the slot is shallower, increase feed.

Note: The depth of a slot can be limited by the diameter of the arbor boss, the deformation strength of the driving keys and the capacity of the chip pockets.

Flywheel – on horizontal machines

Only a few teeth are engaged at any one time in side and face milling operations, which can generate heavy torsional vibrations due to intermittent machining. This is detrimental to the machining result and to productivity.

  • Employing a flywheel is often a good solution for reducing these vibrations. Problems caused by insufficient power, torque and stability in the machine are often solved by the correct use of flywheels
  • The need for a flywheel is greater in a small machine with low power, or in a machine with greater wear, than in a larger, more stable and powerful machine
  • Position the flywheel as close to the tool as possible.
  • Using a flywheel results in smoother machining, which in turn leads to a reduction in noise and vibration, and longer tool life
  • In addition to up-milling, a flywheel can be fitted to the arbor on which the milling cutter is set up
  • In order to further improve stability when side and face milling, use the largest possible flywheel that the application permits
  • Combining a number of round carbon steel discs, each with a center hole and key groove to fit the arbor, remains the best method for constructing a flywheel

Gang milling using cutters mounted in a staggered pattern

Cutters that have bore mounting with 2 keyways can be arranged in a staggered pattern for milling more than one slot at the same time. Displacing the cutters in relation to each other assists in avoiding vibration. This also reduces the need for flywheels.

Milling of narrow and shallow slots and grooves

Versatile cutters have multiple edge inserts that are available in shapes to fit most types of small grooves. Common applications include the machining of internal circlip and seal ring grooves and of small straight or circular external grooves, particularly on components that cannot be rotated.

Internal grooving

  • A smooth entrance should be programmed when using circular milling.
  • Consider the relationship between the cutter diameter and the hole diameter, Dc/Dw. The smaller the relationship, the larger the engagement will be.

End milling of slots

End milling is selected for shorter, shallower slots, especially closed grooves and pockets, and for milling keyways. End mills are the only tools that can mill closed slots that are:

  • Straight, curved, or angled
  • Wider than tool diameter, designated pockets

Heavier slotting operations are often performed using long-edge milling cutters.


Choice of tools

End milling and long-edge cutters

Solid carbide end mills Shoulder face end mills​ Long-edge cutters​ Exchangeable-head end mills
Machine / Spindle size​ISO 30, 40, 50ISO 40, 50​ISO 40, 50​​ISO 30, 40, 50
Stability requirement​High​MediumHigh​​Low
Roughing​Very good​Good​Very good​Acceptable
Finishing​Very good​Good​Acceptable​Very good
Cutting depth ap​Large​Medium​Large​Small
Versatility​Very good​Good​Acceptable​Very good
Productivity​Very good​Good​Very good​Good

How to apply

  • Use light-cutting end mills with a long, predictable tool life, mounted in high-performance chucks
  • Minimize the distance from the tool chuck to the cutting edge to achieve the shortest possible overhang
  • Consider feed per edge to produce satisfactory chip thickness. Use coarse pitch cutters to avoid thin chips, which can cause vibrations, bad surfaces and burr formation
  • Use the largest possible tool size to achieve the best diameter/length relationship for stability
  • Use down-milling as often as possible to achieve the most favorable cutting action
  • Make sure to evacuate chips out of the groove. Use compressed air to avoid chip congestion
  • Use Coromant Capto® coupling for best stability and support for the spindle

Grooving using end-milling cutters

Machining a groove or slot, often called full slotting, involves three machined faces:

  • Slots closed at both ends are pockets, requiring end mills that can work in the axial direction
  • Full slotting with an end mill is a demanding operation. The axial cutting depth should be generally reduced to around 70% of the edge length. Machine rigidity and chip evacuation should also be considered in determining the best method for the operation
  • End mills are sensitive to the effects of cutting forces. Deflection and vibration may be limiting factors, especially at high machining rates and with long overhangs

Keyway slotting

This operation requires some specific guidance, in addition to the general recommendations for milling of straight surfaces and grooving. A slot milled in a single step will not have a perfectly square form due to the direction of the cutting forces and the tendency of the tool to bend. The best accuracy and productivity will be achieved if the operation employs an undersized end mill, and is divided into two steps:

  1. Keyslot milling – roughing of full slot
  2. Side milling – finishing all around the slot, using up-milling to create true square corners

The radial depth of cut should be kept low in finishing operations to avoid deflection of the cutter, which is a major cause of bad surface finish and/or deviation from a true 90° shoulder.

Keyslot milling in two steps

Methods for opening up a closed slot or pocket in a solid blank

​In preparation for milling long, narrow, full-width slots, linear ramping is the most common method (after drilling) for opening up a pocket.

For shallow slots, peck milling can also be an alternative. Circular ramping is used for milling wider slots and pockets.


Comparison of three different methods

Conventional slot milling

+ Conventional 3-axis machines can be used
+ High removal rates under stable conditions
+ Simple programming
+ Wide choice of tools
– Generates high radial cutting forces
– Vibration sensitive
– Deep slots require repeated passes

Trochoidal milling

+ Generates low radial cutting forces – less vibration sensitivity
+ Minimal deflection when milling deep slots
+ A productive method for:
  • machining hard steels and HRSA (ISO H and S)
  • vibration-sensitive applications
+ The cutter diameter should be maximum 70% of the slot width
+ Good chip evacuation
+ Low heat generation
– More programming is required

Plunge milling

​+ A problem solver in vibration-sensitive applications:
  • with long tool overhangs
  • in deep slotting
  • with weak machines or setups
– Low productivity under stable conditions
– Requires a rest milling/finishing operation
– End cutting might obstruct chip evacuation
– Limited choice of tools

Rough slotting with long-edge milling cutter

  • Cutters with large metal removal capacities are generally used for rough machining
  • Shorter versions may produce slots up to a depth equal to the diameter in stable and powerful milling machines
  • Use stable ISO 50 spindles, as these cutters are more likely to accommodate considerable radial forces
  • Check power and torque requirements, as these are often limiting factors for optimum results
  • Consider the optimal pitch for each type of operation

Longer designs are primarily
intended for edging operations.
Pitch​L ​​M ​​H 
​Application area​Long setupUniversal​Short setup​
​Shoulder milling​Deep ap/ae​Medium ap/ae​Moderate ap/ae
​Slot milling​Moderate ap/ae​Limited​__
​vc ft/min
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