Machining Allowance Guide: How Much Stock to Leave for Each Operation
One of the most common questions in CNC machining is: “How much stock should I leave for the next operation?” Leave too much and you waste cycle time. Leave too little and you risk missing the work-hardened layer, getting poor surface finish, or even failing to clean up the previous tool marks. This reference provides specific machining allowance values for turning, milling, drilling, and grinding across all common operations.
Why Machining Allowances Matter
Machining allowance (also called “stock allowance” or “cleanup stock”) is the intentional amount of extra material left on a workpiece during one operation to be removed by a subsequent operation. Correct allowances:
- Ensure the finishing pass cuts below any work-hardened surface layer
- Remove tool marks, scale, and surface defects from the previous operation
- Account for thermal distortion and clamping stress relief between operations
- Provide consistent cutting conditions for predictable tool life and surface finish
Turning Allowances: Diameter Stock to Leave
Values below are diameter stock (radial stock × 2). For example, “1.0 mm on diameter” means 0.5 mm radial stock per side.
| From Operation | To Operation | Part Diameter (mm) | Allowance on Diameter (mm) | Allowance on Length (mm) | Notes |
|---|---|---|---|---|---|
| Rough Turning | Semi-Finish Turning | <50 | 1.0–1.5 | 0.3–0.5 | Standard carbon/alloy steel |
| Rough Turning | Semi-Finish Turning | 50–150 | 1.5–2.5 | 0.5–0.8 | Larger parts need more stock for distortion |
| Rough Turning | Semi-Finish Turning | >150 | 2.5–4.0 | 0.8–1.5 | Heavy parts — allow for thermal effects |
| Semi-Finish Turning | Finish Turning | <50 | 0.3–0.5 | 0.1–0.3 | Must exceed work-hardened layer depth |
| Semi-Finish Turning | Finish Turning | 50–150 | 0.5–0.8 | 0.2–0.5 | Standard finishing allowance |
| Semi-Finish Turning | Finish Turning | >150 | 0.8–1.2 | 0.3–0.5 | Larger diameter = more stock for rigidity |
| Finish Turning | Grinding | Any | 0.15–0.30 | 0.05–0.15 | Grinding stock must be uniform |
| Rough Turning | Grinding (skip finish turn) | Any | 0.5–0.8 | 0.2–0.4 | Only if surface quality allows |
Milling Allowances: Stock to Leave on Faces and Walls
| From Operation | To Operation | Feature | Stock Allowance (mm) | Notes |
|---|---|---|---|---|
| Rough Face Milling | Finish Face Milling | Flat face / floor | 0.3–0.8 | Depends on surface flatness from roughing |
| Rough End Milling | Finish End Milling | Vertical walls | 0.2–0.5 | Radial stock on walls |
| Rough End Milling | Finish End Milling | Pocket floors | 0.3–0.6 | Axial stock on floor |
| Rough Milling | Semi-Finish Milling | 3D contours | 0.5–1.0 | Step-down stock for 3D surfaces |
| Semi-Finish Milling | Finish Milling | 3D contours | 0.1–0.3 | Light stock for ball nose finishing |
| Rough Milling | Grinding (flat surfaces) | Ground faces | 0.2–0.5 | Uniform stock for grinding wheel |
Allowances by Material Type
Different materials require different allowances due to varying work-hardening tendencies, scale thickness, and thermal distortion characteristics:
| Material | ISO | Work-Hardening Tendency | Min. Finish Allowance (mm) | Recommended Finish Allowance (mm) | Why |
|---|---|---|---|---|---|
| Low Carbon Steel | P10 | Low | 0.2 | 0.3–0.5 | Minimal work-hardening, standard stock |
| Medium Carbon Steel | P20–P30 | Low–Medium | 0.3 | 0.4–0.6 | Slight work-hardening on surface |
| Alloy Steel (4140) | P30–P40 | Medium | 0.3 | 0.5–0.8 | Harder surface layer after roughing |
| Austenitic Stainless (304, 316) | M10–M30 | Very High | 0.5 | 0.6–1.0 | Severe work-hardening — must cut deep enough |
| Duplex Stainless (2205) | M30–M40 | High | 0.5 | 0.6–1.0 | High strength + work-hardening |
| Gray Cast Iron | K10–K20 | Low (brittle chips) | 0.2 | 0.3–0.5 | No work-hardening, but hard chill layer |
| Nodular Iron | K20–K30 | Low–Medium | 0.3 | 0.4–0.6 | Some ferrite work-hardening |
| Titanium Ti-6Al-4V | S10–S20 | Medium (alpha case) | 0.4 | 0.5–0.8 | Alpha case layer must be removed |
| Inconel 718 | S20–S30 | Very High | 0.5 | 0.6–1.0 | Extreme work-hardening, age-hardened surface |
| Aluminum 6061 | N10 | Very Low | 0.1 | 0.2–0.3 | No work-hardening, minimal stock needed |
Casting and Forging Stock Allowances
When machining from raw castings or forgings, the total stock to be removed is much larger:
| Source | Feature Size (mm) | Total Radial Stock (mm) | Total Axial Stock (mm) | Recommended Passes |
|---|---|---|---|---|
| Sand Casting (gray iron) | <100 | 3–5 | 3–5 | 2–3 rough + 1 finish |
| Sand Casting (gray iron) | 100–300 | 5–8 | 5–8 | 3–5 rough + 1 finish |
| Investment Casting | <50 | 0.5–1.5 | 0.5–1.5 | 1 rough + 1 finish |
| Die Casting (aluminum) | <100 | 0.3–0.8 | 0.3–0.8 | 1 finish pass |
| Closed-Die Forging (steel) | <100 | 2–4 | 2–5 | 2–3 rough + 1 finish |
| Closed-Die Forging (steel) | 100–300 | 4–7 | 4–8 | 3–5 rough + 1 finish |
| Open-Die Forging (steel) | >200 | 8–15 | 8–20 | Multiple rough + semi + finish |
Grinding Stock Allowances
| Grinding Type | From Previous Operation | Stock on Diameter (mm) | Stock on Face (mm) | Notes |
|---|---|---|---|---|
| Cylindrical Grinding (OD) | Finish Turning | 0.15–0.30 | — | Uniform stock critical for roundness |
| Cylindrical Grinding (OD) | Rough Turning | 0.50–0.80 | — | Rough-turned surface may need extra cleanup |
| Internal Grinding (ID) | Boring | 0.10–0.25 | — | Smaller stock due to wheel size |
| Surface Grinding | Milling | — | 0.20–0.50 | Depends on milling flatness |
| Surface Grinding | Blanchard/rotary grinding | — | 0.05–0.15 | Fine stock for final pass |
Drilling and Boring Allowances
| From Operation | To Operation | Hole Diameter Range | Allowance on Diameter (mm) | Notes |
|---|---|---|---|---|
| Drilling | Boring | 10–30 mm | 0.3–0.8 | Drill undersize for boring cleanup |
| Drilling | Boring | 30–80 mm | 0.8–1.5 | Larger holes need more boring stock |
| Boring (rough) | Boring (finish) | Any | 0.2–0.5 | Light stock for dimensional accuracy |
| Boring (finish) | Internal Grinding | Any | 0.10–0.25 | Minimal stock for grinding |
| Drilling | Reaming | 5–25 mm | 0.1–0.3 | Reamers need light, uniform stock |
Practical Tips for Setting Allowances
- Always leave more than the work-hardened layer depth. For austenitic stainless, this is typically 0.3–0.5 mm. For Inconel, 0.4–0.6 mm. If your finishing pass only takes 0.1 mm, you are rubbing on hardened material, not cutting.
- Uniform stock produces better results than variable stock. If roughing leaves uneven stock (e.g., 0.8 mm in one area and 0.2 mm in another), the finishing pass will deflect differently, causing dimensional errors.
- Account for thermal growth. Large parts machined in roughing will grow from heat. If you measure and finish immediately, the part may be out of tolerance when it cools. Allow cooling time between roughing and finishing.
- For hardened parts, leave grinding stock in the hard state. Do not finish-turn to final size before hardening — leave 0.3–0.5 mm for post-hardening grinding.
- Thin-walled parts need extra care. Clamping forces during roughing can distort thin walls. Leave 30–50% more stock than standard for finish machining after reclamping with lighter force.
Need help planning your machining sequence? The application engineers at hooguu.com can review your part drawing and recommend the optimal roughing-to-finishing strategy, including Korloy insert grades and chipbreakers for each operation step.
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