Rational Selection of Strawberries for CNC Milling

• 1
  Key takeaways
• 2
  Which strawberry fits which job
• 3
  What a strawberry actually is in a rational selection of strawberries
• 4
  Tooth count, chip load and the feed you can actually run
• 5
  Helix angle and flute form: where the cutter pushes the part
• 6
  Coating and substrate: what to buy for your material mix
• 7
  Machine, workholding and coolant: the constraints nobody lists
• 8
  How to judge a supplier on this kind of cutter
• 9
  Six steps to pick a strawberry without guessing

In shop language a strawberry is a coarse-pitch shell mill or shank cutter with wide, open flutes. The name comes from the shape of the tooth form, not from any fruit. It is a roughing tool. Its job is to pull a large volume of material out of a pocket or off a face in the shortest number of passes.

The geometry is deliberately unbalanced. Uneven tooth spacing breaks the rhythm of the cut, so the cutter does not sing at one frequency. Wide flutes leave room for a thick chip. Fewer teeth mean each tooth bites deeper at the same feed per revolution, which is exactly what you want in soft material.

That same geometry is a liability in hard material. A deep bite on 4140 or 17-4PH puts a bending load on the edge that a fine-pitch cutter would spread across more teeth. Rational selection of strawberries starts with admitting the tool has a narrow comfortable range.

So the first question is not which brand. It is whether the feature in front of you is a roughing feature or a finishing feature. If the drawing calls for Ra 0.8 μm and a ±0.005 mm bore, the strawberry is the wrong tool for the last pass, no matter how good it is at the first one.

Chip load per tooth is the number that ties the cutter to the program. Take the feed rate, divide by spindle speed and tooth count, and you get the thickness of the chip each edge removes. On a 4-tooth cutter at 4,000 rpm and 1,200 mm/min, that is 0.075 mm per tooth.

For aluminium, a chip load of 0.05–0.15 mm per tooth is normal on a coarse cutter. For stainless, stay nearer 0.03–0.08 mm. For 4140 and harder, 0.02–0.05 mm keeps the edge alive. These are starting windows, not limits; the machine rigidity decides how far you can push.

Fewer teeth also means more chip room per flute. That matters more than the tooth count itself. If the flute fills up, the cutter rubs, heat climbs, and the edge fails within minutes. A 3-tooth cutter in a 60 mm deep pocket without through-spindle coolant will fail sooner than a 5-tooth cutter with good evacuation.

There is a practical ceiling too. On a 40-taper machine, a Ø63 mm coarse shell cutter running 3 mm axial depth at 70 percent radial engagement is already near the spindle's comfortable load. Adding teeth raises the feed but also the torque. Check spindle load on the first part, not the tenth.

One more check: the arbor. A shell cutter on a worn arbor with 0.03 mm runout will cut oversize and chatter regardless of tooth count. Indicate the arbor before the tool goes in the spindle.

Helix angle decides the direction of the cutting force. A 45° helix pulls the chip up and out of the cut and pushes the workpiece down against the table. That is helpful on a thin plate in a vise, and unhelpful on a tall, unsupported wall.

Low helix, 20–30°, sends more force sideways into the part. It suits cast iron and short-chipping bronze, where the chips break small and drop away. It also suits rigid setups where part deflection is not the limiting factor.

Flute form matters as much as the angle. A polished, uncoated flute in aluminium reduces built-up edge. A sharp, positive rake on stainless limits work hardening at the cut line. A heavy-core cutter trades chip room for stiffness, which is the right trade on a long reach.

Watch the corner geometry. A small corner radius, 0.4–0.8 mm, holds up far better than a sharp corner on a roughing cutter. A sharp corner chips first. If the drawing allows it, put a radius on the tool and save a deburring operation later.

Variable pitch is worth the extra cost on deep pockets. The uneven spacing spreads the harmonic load and lets you raise the depth of cut without the whole setup ringing.

Uncoated, polished carbide is still the best choice for aluminium and most plastics. Aluminium tends to weld to a coated surface, and a built-up edge changes the effective geometry of the cutter within a few minutes. Keep the surface polished and the rake positive.

TiAlN and AlTiN are the standard choices for steel and stainless. The aluminum in the coating forms a thin oxide layer at cutting temperature, which slows diffusion wear on the flank. On 304 and 316, AlTiN also helps limit the work-hardened layer that makes the next pass harder than the last.

For titanium, Ti-6Al-4V and similar alloys, keep the cutting speed low and the coolant flowing. Titanium conducts heat poorly, so the edge takes the temperature. A coated cutter with a sharp edge and a conservative feed beats a coated cutter run fast.

Coatings are not a substitute for the right substrate. A tough, fine-grain carbide grade survives interrupted cuts in 4140 that a hard, wear-resistant grade would chip. Ask the tool supplier for the grade, not just the coating name.

If your shop runs aluminium in the morning and stainless in the afternoon, buy two cutters. One cutter asked to do both will do neither well.

A coarse cutter is a high-load tool. On a small machine with a 20-taper spindle, a Ø50 mm coarse shell cutter at full width is asking for chatter and a poor surface. Scale the cutter to the spindle. A Ø32 mm head on a small machine often removes more material per hour than a Ø63 mm head that has to be run gently.

Workholding sets the ceiling on depth of cut. A part held in a single vise with 60 mm hanging out will deflect before the cutter does. Support the part from below or add a jack, then increase the axial depth.

Coolant choice is not a detail. Through-spindle coolant clears chips from deep pockets and keeps the edge at temperature. Flood coolant is fine for open face milling. Air blast works on aluminium where thermal shock is not a concern and you want the chips out, not wet.

On stainless, do not let the cutter dwell. A short pause in the cut work-hardens the surface and the next tooth meets a harder skin. Keep the feed steady and the entry and exit arcs smooth.

For a long reach, reduce the radial engagement and raise the axial depth. That is the opposite of what many operators do, and it is usually why a long cutter chatters.

Ask the supplier which grade and coating they recommend for your specific material and feature. A vague answer, or one that names a coating but not a grade, is a warning sign. The grade is what survives the cut.

Ask how they verify the tool after it arrives. Runout at the arbor, corner condition under magnification, and a first-article cut on your material tell you more than a catalog page. A supplier who will not cut a test part is selling you a number, not a tool.

Check the process around the tool. If the shop runs a 5-axis cell with 16 simultaneous 5-axis centers and a 100% inspection routine, cutter selection is part of a controlled process. If the tool is chosen by whoever is at the machine that morning, results will vary from lot to lot.

Certifications are a useful filter, not a proof. ISO 9001:2015 covers quality management, IATF 16949:2016 covers automotive work, ISO 13485:2016 covers medical devices, and ISO 27001:2022 covers information security for your drawings. Ask which one applies to your program and why.

Finally, ask about the finishing plan. A supplier who roughs with a coarse cutter and finishes with a separate, finer tool will hold Ra 0.8–1.6 μm more reliably than one who tries to finish with the same cutter. That single question tells you how the shop thinks.