121.3.2 - Manual Machining

Speeds & Feeds

SFM & Speed/Feed Variables

Spindle Speed (RPM)

RPM: Revolutions per Minute

SFM: Surface Feet per Minute (varies depending on machinability of material)

Feed Rate (IPM):

IPM: Inches per Minute

IPT: Inches per Tooth (Flute or Cutting Edges)

CPT: Chip per Tooth (Thick ness of Chip)

.002": Generally good for finishing

.004-.005": Generally good for Roughing

Stepdown (DOC) & Stepover (WOC)

Depth of Cut (DOC) & Stepdown Guidelines for Manual Milling and Turning

Depth of cut (DOC) and stepdown selection are crucial for manual milling and turning, affecting tool life, surface finish, machine load, and overall efficiency.

Manual Milling: Depth of Cut (Stepdown) Guidelines

DOC, also known as stepdown in milling, is the vertical depth per pass.

DOC Calculation Formula for Milling

For end mills and face mills, the recommended DOC can be calculated as:

DOC=D × R

Where:

DOC = Axial depth of cut (stepdown) in inches or mm
D = Cutter diameter in inches or mm
R = DOC ratio (decimal form, typically 0.05 to 1)

For example:

¾-inch end mill (D = 0.75 in) with 40% engagement (R = 0.4):

DOC = 0.75 x .4 = 0.3 in.

- → Set DOC to 0.3 inches for medium roughing.

Stepdown for Face Milling

For face milling, DOC should be limited based on cutter diameter:

DOC= ⅓D

🔹 Example: If using a 2-inch face mill, the max DOC should be ⅓ × 2 = 0.66 inches per pass.

Best Practices for Milling DOC Selection

(A) Heavy Roughing (Max Material Removal)

✔️ DOC = 50–100% of tool diameter
✔️ Use lower feed rates to avoid excessive tool wear.
✔️ Use climb milling if backlash is controlled.
✔️ Use rigid setups to prevent vibration.

(B) Medium Roughing (Balanced Cutting)

✔️ DOC = 30–50% of tool diameter
✔️ Use moderate feed rates for efficiency.
✔️ Control chip load to prevent overheating.

(C) Light Cutting (Minimizing Tool Pressure)

✔️ DOC = 10–30% of tool diameter
✔️ Increase feed rate slightly to maintain chip thickness.
✔️ Use conventional milling if backlash is a concern.

(D) Finishing (High Surface Quality)

✔️ DOC = 5–15% of tool diameter
✔️ Use high RPM and light cuts for a smooth finish.
✔️ Multiple passes may be required for best results.

Manual Turning: Depth of Cut (DOC) Guidelines

In lathe turning, DOC is the radial depth of cut per pass.

DOC Calculation Formula for Lathe Turning

DOC = (D × P)/2

Where:

DOC = Radial depth of cut (DOC) in inches or mm
D = Workpiece diameter in inches or mm
P = DOC percentage (decimal form, typically 0.03 to 0.1)

For example:

2-inch diameter workpiece with 5% DOC (P = 0.05):

DOC = (2 × 0.05)/2

→ Set DOC to 0.05 inches for roughing.

Best Practices for Lathe DOC Selection

(A) Rough Turning

✔️ DOC = 0.050 – 0.250 inches
✔️ Use low RPM & high feed rates for best efficiency.
✔️ Ensure tool rigidity to prevent chatter.

(B) Finishing Passes

✔️ DOC = 0.005 – 0.030 inches
✔️ Use high RPM & low feed rate for best surface finish.
✔️ Use sharp tools to minimize tool pressure.

(C) Special Considerations

✔️For Hard Materials: Use lighter DOC to avoid tool wear.

✔️For Thin Parts: Use lower DOC to prevent deflection.

✔️For Interrupted Cuts: Reduce DOC to avoid tool impact.

Final Takeaways

Best DOC (Stepdown) Practices

✅ For Manual Milling:
✔️ Roughing: DOC = 30–50% of cutter diameter.
✔️ Finishing: DOC = 5–15% of cutter diameter.
✔️ Face Milling: DOC = ⅓D of cutter.

✅ For Manual Lathe Turning:
✔️ Heavy Roughing: DOC = 0.050–0.250 inches.
✔️ Medium Roughing: DOC = 0.030–0.100 inches.
✔️ Finishing: DOC = 0.005–0.030 inches.

Bottom-Up Machining Strategy

A bottom-up milling strategy involves starting the cut at the lowest point (bottom) and working upward rather than conventional top-down machining. This approach is commonly used in pocketing, slotting, or deep cavity milling to manage tool deflection, heat dissipation, and chip evacuation.

Benefits of a Bottom-Up Strategy:

✅ Better Heat Management – Chips are cleared more easily, reducing heat buildup.
✅ Improved Tool Life – Less tool deflection and breakage due to reduced stress.
✅ Superior Surface Finish – Reduces vibration and chatter for better accuracy.
✅ More Effective Chip Evacuation – Gravity helps remove chips from the cut zone.

Maximum Flute Engagement

Maximum flute engagement refers to the axial depth of cut (Stepdown, DOC) or radial depth of cut (Stepover, WOC) where the cutting edges (flutes) of a tool are fully engaged with the material without exceeding the tool’s capabilities. Proper flute engagement is crucial for tool life, surface finish, and preventing chatter or excessive tool wear.

Key Factors Influencing Maximum Flute Engagement:

Tool Material & Coating – Carbide tools can handle higher engagement than HSS tools.
Workpiece Material – Harder materials require less engagement per pass.
Machine Rigidity – A more rigid setup allows for higher flute engagement.
Chip Evacuation – Poor evacuation can cause tool clogging, reducing effective engagement.
Cooling/Lubrication – Proper coolant helps prevent excessive heat buildup.