Worked Project: Pocketed Mounting Plate

A 100×80×12 mm aluminum plate with a 40×30×4 mm pocket and four Ø6 through holes. This project ties together ramp entry, layer-by-layer roughing, corner geometry, and drill-tip breakthrough calculation.

Concept

The workpiece is an already-faced 100×80×12 mm aluminum plate. G54 work zero is at the lower-left corner on the finished top surface: X0–100, Y0–80, Z0 at top, Z−12 at bottom. This program does not face mill — that is a prior operation. We machine only the pocket and four holes.

The pocket boundary is X30–70, Y25–55 (40×30 mm), depth Z−4, internal corner radius R5. We use a Ø10 flat end mill (tool radius 5 mm) and program the tool center directly (G40 throughout — no cutter compensation). The tool center boundary is inset by one radius: X35–65, Y30–50. The four Ø6 through holes are at (10,10), (90,10), (90,70), (10,70).

Why It Matters

Several things go wrong if you only cut the pocket boundary: (1) the interior material is never removed, leaving an island; (2) plunging straight to Z−4 on a flat end mill is bad because the center of a flat end mill does not chip-load well — you need a ramp; (3) a through-hole is not done when the drill reaches Z−12 — the conical drill tip must also break through; (4) corner radius R5 means the Ø10 cutter naturally forms R5 corners without extra offset math.

How It Works

Setup and Tooling

Setup assumptions (classroom geometry, not a production fixture design):

• Fixture top does not exceed Z8. Clamping jaws are at X≤4 and X≥96, Y32–48.
• Support under the plate contacts the bottom at Z−12. Around each hole, a clearance channel of at least 5 mm radius runs from Z−12 to Z−18.
• Z50 is the cross-travel height between operations in this classroom model.

Pocket Roughing: Two Layers

We take two axial cuts: Z−2 and Z−4. Entry is a 30 mm long ramp that drops 1 mm per segment (slope 1/30 ≈ 1.9°). This requires a center-cutting end mill capable of slotting at this ramp angle — a classroom assumption, not a manufacturer endorsement.

Layer 1 (Z−2): rapid to (35,40,Z2), feed to Z0, ramp to (65,40,Z−1), reverse ramp to (35,40,Z−2). Then run the 17-point raster at F200.

Layer 2 (Z−4): G00 Z5, reposition to (35,40), feed down to Z−2 (already cleared), ramp to (65,40,Z−3), reverse ramp to (35,40,Z−4). Repeat the same 17-point raster.

The 17-point center path (stepover 4 mm, tool diameter 10 mm → 6 mm overlap between rows):

Corner geometry: The tool center stays on a rectangle [35,65]×[30,50]. The Ø10 tool sweeps a circle of radius 5 around each center point. The union traces the target rounded rectangle with R5 corners. The four center corners are the centers of the four R5 arcs — do not also round the center path corners or you will produce R10.

Coverage check: Stepover 4 mm is less than the 10 mm tool diameter, giving 6 mm overlap between adjacent rows. The four wall-cleanup passes ensure side edges and corners are fully swept. Target pocket area = 40×30 − (4−π)×5² ≈ 1178.5 mm². At 4 mm depth, theoretical volume ≈ 4714 mm³.

Drilling: Drill-Tip Breakthrough

A Ø6 drill with 118° point angle has a conical tip height:

h = (6/2) / tan(118°/2) = 3 / tan(59°) ≈ 1.8026 mm

Full-diameter breakthrough requires the drill tip to reach Z ≤ −(12 + 1.8026) = −13.8026. We choose a teaching target of Z−14.5, giving 0.697 mm extra breakthrough. The clearance channel below runs to Z−18, leaving 3.5 mm geometric gap. Drilling uses G98 (retract to initial Z50 between holes).

Example: Complete Program

O0028 (POCKETED PLATE PROJECT)
G21 G17 G90 G94 G40 G80 G49
G54
(T02 - 10mm FLAT END MILL)
T02 M06
S2000 M03
G43 H02 Z50. M08
(RAMP ENTRY LAYER 1 - Z-2)
G00 X35. Y40.
G00 Z2.
G01 Z0 F200.
G01 X65. Z-1. F60.
G01 X35. Z-2. F60.
G01 F200.
(17-POINT RASTER LAYER 1)
G01 Y30.
G01 X65.
G01 Y34.
G01 X35.
G01 Y38.
G01 X65.
G01 Y42.
G01 X35.
G01 Y46.
G01 X65.
G01 Y50.
G01 X35.
G01 Y30.
G01 X65.
G01 Y50.
G01 X35.
G01 Y30.
G00 Z5.
G00 X35. Y40.
G01 Z-2. F200.
(RAMP ENTRY LAYER 2 - Z-4)
G01 X65. Z-3. F60.
G01 X35. Z-4. F60.
G01 F200.
(17-POINT RASTER LAYER 2)
G01 Y30.
G01 X65.
G01 Y34.
G01 X35.
G01 Y38.
G01 X65.
G01 Y42.
G01 X35.
G01 Y46.
G01 X65.
G01 Y50.
G01 X35.
G01 Y30.
G01 X65.
G01 Y50.
G01 X35.
G01 Y30.
G00 Z50. M09
(T03 - 6mm DRILL, 118 DEG, PECK DRILL G83)
T03 M06
S1500 M03
G90 G00 G43 H03 Z50. M08
G00 X10. Y10.
G98 G83 X10. Y10. R2. Z-14.5 Q3. F80.
X90. Y10.
X90. Y70.
X10. Y70.
G80
G00 Z50. M09
M05
G91 G28 Z0
M30

Common Mistakes

• Only cutting the pocket boundary — leaves material in the center. You must raster through the interior.
• Plunging straight to depth — a flat end mill center has zero cutting speed; ramp or drill a start hole first.
• Adding G41/G42 on top of center-programmed coordinates — double offset. Since we program tool center directly, stay in G40.
• Forgetting the drill tip — Z−12 does not break through a 12 mm plate with an 118° drill; use Z−14.5.
• Not changing H offset after tool change — T03 must use H03, not still H02.
• Rounding pocket corners on the center path — the R5 corner comes from the tool radius, not from arcing the center path.

Practice

1. Pocket depth target is 4.00 ± 0.05 mm. You measure 3.92 mm. Is it acceptable?

2. For a Ø8 drill with 118° point, what is tip height h? On a 10 mm plate, what Z target gives 0.5 mm breakthrough?

3. The tool center boundary is X35–65, Y30–50. What is the actual pocket corner radius, and why?