Machinist (metal milling) troubleshooting
Chatter / vibration marks
Cause: Lack of rigidity in tool, holder, workpiece or machine, or an aggressive toolpath (Harvey). Doubling the stick-out gives about 8x the deflection; doubling the diameter gives 16x less (Harvey).
Fix: Reduce speeds, feeds and axial/radial depth; hold the shank deeper and use the shortest tool that reaches; check spindle runout; re-fixture; use variable-pitch or variable-helix tools; climb mill (Harvey).
When this fix doesn't apply: Don't cut chip load so low that the tool rubs: Harvey notes too small a chip load itself causes chatter.
Sources: Harvey Performance: End Mill and Milling Troubleshooting Guide · Harvey Performance: 3 Steps to Shutting up Tool Chatter · Harvey Performance: Optimized Machining With Miniature End Mills · Harvey Performance: chip load too large / too small (Chip Thickness articles)
Tool breakage
Cause: Too large a chip load packs chips into the flutes and breaks the tool; on small tools, deflection force exceeds the strength of the carbide (Harvey).
Fix: Use the manufacturer's chip load for the diameter, cut deeper passes with reduced chip load (-25% at 2xD, -50% at 3xD, Amana), maximize shank grip and minimize stick-out (Harvey).
When this fix doesn't apply: If the tool chips rather than snaps, check chatter first (Harvey lists the same fixes).
Open calculator with this change
Sources: Harvey Performance: chip load too large / too small (Chip Thickness articles) · Harvey Performance: Optimized Machining With Miniature End Mills · Amana Tool: Plastic O-Flute speed chart v5 (depth-of-cut rule) · Harvey Performance: End Mill and Milling Troubleshooting Guide
Built-up edge in aluminum
Cause: Gummy aluminum welds to the edge, usually because speed or feed is too low and the chip isn't hot or thick enough to slide off (Harvey).
Fix: Use sharp, polished, material-specific geometry; raise SFM toward the manufacturer's range (Harvey: 800-1500 SFM for 6061/7075 carbide); aim coolant at the cutting edge; use a low-aluminum-affinity coating such as TiB2, not AlTiN (Harvey).
When this fix doesn't apply: On an 8,000-10,000 RPM spindle with small tools you may not reach the SFM; the metal calculator flags this as spindle-limited.
Sources: Harvey Performance: Causes & Effects of Built-Up Edge · Harvey Tool: Tool Coatings (TiB2 -C8) · Harvey Tool: General Machining Guidelines
Work hardening in stainless
Cause: Heat goes into the workpiece instead of the chips, and a rubbing (too thin) chip hardens the surface (Harvey).
Fix: Use the manufacturer's speeds and feeds with a constant feed, climb mill, use flood coolant and suitable coatings (Harvey). Harvey's general range: 100-350 SFM, chip load from .0005" (1/8") to .006" (1").
When this fix doesn't apply: Conventional milling is only for high-toughness or hardened materials (Harvey).
Sources: Harvey Performance: Work Hardening and When It Should Scare You · Harvey Performance: Slaying Stainless Steel · Harvey Tool: General Machining Guidelines · Harvey Performance: chip load too large / too small (Chip Thickness articles)
End mills wear out early
Cause: Harvey: surface speed above the tool's rating, or feeds and speeds that are too light (rubbing). Also abrasive or hard material without a suitable coating, the wrong helix angle, and re-cutting chips.
Fix: Lower RPM if surface speed is too high, and set feeds and speeds from the manufacturer's chart rather than running light. Clear chips with depth of cut and coolant/air so they aren't re-cut. Choose the coating and helix the manufacturer recommends for the material (Harvey).
When this fix doesn't apply: If the edge is chipped rather than evenly worn, see 'Edge chipping'. Flank wear that grows slowly and evenly is normal tool life.
Sources: Harvey Performance: End Mill and Milling Troubleshooting Guide · Harvey Tool: Tool Coatings (TiB2 -C8)
Cutting edges chip
Cause: Harvey: feed rate that's too aggressive, both during the cut and on entry, and an aggressive depth of cut. Poor rigidity and spindle runout add to it.
Fix: Reduce overall feed and entry feed. Cut axial and/or radial depth. Improve rigidity with a better holder, more shank in the holder or a shorter tool, and better workholding. Check spindle runout (Harvey).
When this fix doesn't apply: If wear is even and slow rather than chipped, see 'End mills wear out early'.
Sources: Harvey Performance: End Mill and Milling Troubleshooting Guide
Chips pack in the flutes
Cause: Harvey: feeds and speeds beyond the tool's capacity, flute gullets too small for the chips being made, and not enough coolant volume or pressure to clear them. Packed chips are a common cause of breakage.
Fix: Bring feeds and speeds back to the manufacturer's chart. Use fewer flutes, which give deeper valleys that clear chips more easily. Increase coolant or air volume and pressure, and aim the nozzle closer to the cut (Harvey).
When this fix doesn't apply: In deep router slots in wood, chips trapped by a downcut bit are a different problem. Onsrud's fix is to route chip channels into a dedicated spoilboard below the cut path.
Sources: Harvey Performance: End Mill and Milling Troubleshooting Guide · LMT Onsrud: Fixturing and Routing of Plastics with CNC
Heavy burrs on milled edges
Cause: Harvey: wrong feeds and speeds, dull cutting edges, or the wrong helix angle.
Fix: Check feeds and speeds against the manufacturer's chart and consider reducing them. Use climb milling, change to the correct helix angle, and replace or regrind dull tools (Harvey).
When this fix doesn't apply: Some burr is normal at the tool's exit edge. If the burr appears only there, a chamfer or deburr pass is cheaper than retuning feeds.
Sources: Harvey Performance: End Mill and Milling Troubleshooting Guide
Parts are out of tolerance (walls or pockets off size)
Cause: Harvey: aggressive depths of cut and poor tool or machine rigidity, which let the tool deflect.
Fix: Reduce axial and/or radial depth of cut. Inspect and repair the machine, tool, holder and fixturing. A tool with more flutes (a stiffer core) can help (Harvey). A light, consistent finish pass also reduces deflection.
When this fix doesn't apply: If the size is off by the same amount everywhere, check the tool diameter or offset in the CAM or control before changing cut parameters.
Sources: Harvey Performance: End Mill and Milling Troubleshooting Guide