How to Machine Bumps CNC: 7 Proven Setup Steps

• 1
  What matters before you cut
• 2
  What counts as a bump and what the machine must control
• 3
  Pre-machining preparation: floor, datum, and stock
• 4
  Tool selection for bump machining
• 5
  Programming the toolpath so bump height repeats
• 6
  Quality control: measuring bump height and spacing
• 7
  Step-by-step process for machining bumps
• 8
  Bump machining by material

A bump is any small raised feature left standing above a milled floor. It can be a hemispherical contact point, a flat-topped pad, a short rib, or a row of dimples used for press-fit alignment. The geometry changes, but the controlled variables do not: bump height above the floor, top size, spacing, and edge quality.

Height is the variable that breaks first. On a 3-axis mill, bump height equals the distance between the finished floor Z and the bump top Z. That means the floor must be cut to a known number. If you leave 0.1 mm of stock on the floor and cut the bump at nominal depth, every bump is 0.1 mm low.

Spacing is a machine and CAM issue. Small pitch distances, often 0.5–3 mm, push the tool into a corner where chip evacuation and tool runout matter more than spindle speed. Runout of 0.01 mm on a Ø1 mm tool changes the effective cutting diameter and shifts the wall position.

• 1
  Flat padConstant top, easy to measure with a drop gauge.
• 2
  DomeRequires 3D surfacing or a form tool. Height is harder to verify.
• 3
  RibLong and thin. Watch for deflection along the rib axis.

Start with a clean datum. The bump top and the surrounding floor must be called out from the same Z zero. If the drawing dimensions bumps from a top face, set that face as your work offset and verify it with a probe or an indicator before cutting.

Rough the pocket or floor area first and leave 0.2–0.3 mm of stock for finishing. Skim the floor with a face mill or a large flat end mill. A floor that is flat within 0.01 mm gives you a reliable base for height measurement.

Check stock consistency on the raw part. Castings and forgings can vary by 0.3 mm or more. If the bump height is measured from a machined floor, that variation is removed. If it is measured from an as-cast surface, it is not.

• 1
  One Z zeroBump top and floor share the same offset.
• 2
  Floor firstFinish the floor before the bumps so height has a reference.
• 3
  Leave stock0.2–0.3 mm on the floor, 0.1 mm on the bump walls.

For bumps larger than 1 mm in top width, a 2-flute or 3-flute carbide end mill with a corner radius of 0.1–0.2 mm holds up well. The corner radius spreads the load and reduces chipping on the bump edge. For bumps under 0.5 mm, you are in micro-tool territory, where a Ø0.5 mm or smaller tool is common.

Tool length is the single biggest lever on accuracy. A Ø1 mm tool hanging 15 mm out of the holder will deflect far more than the same tool at 6 mm. Measure your reach from the holder face to the tip and keep it as short as the geometry allows.

Coatings matter on abrasive materials. AlTiN or AlCrN coatings extend life in stainless and tool steel. On aluminum, an uncoated or DLC-coated tool avoids built-up edge. For PEEK and other plastics, a sharp uncoated tool with high rake cuts cleaner than a coated one.

• 1
  Ø0.5–1 mmMicro end mill, high spindle speed, light chipload.
• 2
  Ø1–3 mm2 or 3 flutes, corner radius, AlTiN for steel.
• 3
  Form toolUse when the dome shape repeats across many parts.

Cut the floor and the bump top in the same operation when possible. If the floor is finished with a Ø16 mm face mill and the bump top with a Ø2 mm end mill, any Z offset error between the two tools shows up directly in bump height. Keep the tools in the same setup and measure both with the same tool setter.

Use a constant-Z finishing pass for flat-topped bumps. Contour-parallel toolpaths work for domes but leave scallops that change the effective top height. For a flat pad, a single Z-level finish pass with a 0.02–0.05 mm stepover gives a clean top.

Leave the bump wall stock light. A 0.1 mm radial finish pass on the bump perimeter reduces side load and keeps the wall vertical. If you cut the full wall in one pass with a long micro tool, the wall will taper and the top will be undersized.

• 1
  Same setupFloor and bump top cut without re-fixturing.
• 2
  Constant ZFlat pads get one Z-level finish pass.
• 3
  Light wall pass0.1 mm radial stock, 0.02 mm stepover on top.

Bump height is a differential measurement. You need the floor Z and the bump top Z from the same part, in the same orientation. A drop gauge, a height gauge on a surface plate, or a vision system all work. Pick one method and use it for every part in the run.

Spacing is checked with an optical comparator or a vision system when pitch is under 2 mm. Calipers are not reliable at that scale because the jaw pressure deforms small bumps. For domes, measure the height at the apex, not at the edge.

Document the first article with the actual numbers, not just pass or fail. If bump height drifts across a 100-part run, the data shows whether the cause is tool wear, thermal growth, or fixture movement. That is the difference between fixing the process and re-cutting parts.

• 1
  First articleRecord actual height and pitch, not just pass or fail.
• 2
  In-processCheck every 10–20 parts on long runs.
• 3
  Final100% visual for edge damage, sample for dimensions.