Material Machinability Ratings for Deep Hole Drilling — Complete Reference

Machinability Ratings by Material

Baseline: AISI 1212 free-machining steel = 100%. Higher percentage = easier to machine (higher speeds, longer tool life). Lower percentage = more difficult (reduced speeds, shorter tool life).

Carbon Steels (ISO P — Steel)

Grade	Condition	Rating	Hardness (HB)	ISO 513 Subgroup	Notes
12L14	Leaded	170%	—	P01–P05	Lead addition acts as chip breaker and lubricant
1215	Hot rolled	136%	—	P01–P05	High sulfur free-machining
1117	Cold drawn	100%	—	P05–P10	General purpose free-machining
1018	Cold drawn	78%	126	P10–P15	Most common low-carbon steel
1020	Annealed	72%	111	P10–P15	Low carbon, slightly gummy
1045	As-rolled	57%	163–179	P15–P20	Most common medium-carbon steel for shafts
1045	Q&T	45%	250–300	P20–P25	Prehardened condition
1095	Annealed	42%	190	P20–P25	High carbon, abrasive carbides

Alloy Steels (ISO P — Steel)

Grade	Condition	Rating	Hardness (HB)	ISO 513 Subgroup	Notes
4140	Annealed	66%	197	P15–P20	Standard Cr-Mo alloy
4140	Q&T (prehardened)	45%	269–302	P20–P25	28–32 HRC — common for shafts
4140	Q&T (high strength)	35%	350	P25–P30	Reduced speeds required
4340	Annealed	57%	217	P15–P20	High-strength Ni-Cr-Mo
4340	Q&T (aircraft grade)	38%	340–430	P25–P30	36–45 HRC — difficult
8620	Annealed	66%	190	P15–P20	Carburizing grade
300M	Annealed	40%	—	P25–P30	Aerospace, very high strength
4130	Normalized	65%	187	P15–P20	Commonly used for aircraft structures

Stainless Steels (ISO M — Stainless Steel)

Grade	Condition	Rating	Hardness (HB)	ISO 513 Subgroup	Notes
416	Free-machining	85–110%	—	M10–M15	High sulfur, best stainless for machining
303	Free-machining	78%	160	M10–M15	Lead/sulfur added
304	Annealed	45%	135–175	M15–M20	Work-hardens severely — use sharp tools
316	Annealed	36–45%	135–175	M20–M25	Tougher than 304, lower speeds
316L	Annealed	38%	135	M20–M25	Lower carbon variant
420	Annealed	45%	190	M15–M20	Martensitic, can be heat treated
17-4 PH	H900 aged	38%	350–430	M25–M30	Precipitation-hardened
Duplex 2205	Annealed	30–35%	250	M25–M35	Very difficult — high cutting forces
Super duplex 2507	Annealed	25–28%	270	M30–M40	Highest strength stainless, most difficult

Cast Iron (ISO K — Cast Iron)

Grade	Condition	Rating	Hardness (HB)	ISO 513 Subgroup	Notes
Gray iron Class 20	As-cast	110%	150–200	K10–K15	Excellent machinability, graphite lubricates
Gray iron Class 30	As-cast	95%	187–241	K10–K15	Most common engineering gray iron
Gray iron Class 40	As-cast	80%	180–260	K15–K20	Higher strength, more abrasive
Ductile iron 60-40-18	Annealed	85%	130–180	K15–K20	Ferritic, good machinability
Ductile iron 80-55-06	As-cast	65%	170–230	K20–K25	Pearlitic, more abrasive
Ductile iron 120-90-02	Q&T	40%	248–352	K25–K30	High strength, difficult
CGI (compacted graphite)	As-cast	60–70%	170–230	K20–K30	Intermediate between gray and ductile
Malleable iron	Annealed	120%	130–160	K10–K15	Easiest cast iron family

Aluminum and Non-Ferrous (ISO N — Non-Ferrous)

Grade	Condition	Rating	Hardness (HB)	ISO 513 Subgroup	Notes
C360 brass	Free-cutting	300%	80–100	N05–N10	Easiest common metal, sharp tools
C932 bearing bronze	As-cast	85%	60–80	N10–N15	Leaded bronze
C510 phosphor bronze	Hard	80%	149–202	N10–N15	Work-hardens
C954 aluminum bronze	As-cast	60%	160–210	N15–N20	Treat like tough steel
6061-T6	T6	200–270%	95–100	N05–N10	Most common structural aluminum
7075-T6	T6	140–240%	150–160	N05–N10	High-strength aluminum
2024-T4	T4	180%	120	N05–N10	Aircraft aluminum
Copper C110	Annealed	70%	35–80	N10–N15	Gummy chips — use polished carbide
Magnesium AZ31	—	500%+	—	N05	Extremely easy (fire hazard — no water coolant)

Titanium and Superalloys (ISO S — Heat-Resistant Alloys)

Grade	Condition	Rating	Hardness (HB)	ISO 513 Subgroup	Notes
Ti-6Al-4V	Annealed	22%	310–340	S15–S20	Low thermal conductivity — heat at cutting edge
Ti-6Al-4V	Aged	18%	370–400	S20–S25	Higher strength, even harder to machine
Ti-6Al-2Sn-4Zr-2Mo	Annealed	20%	330	S15–S20	High-temperature titanium
Inconel 718	Aged	12–15%	370–470	S25–S30	Work-hardens severely
Inconel 718	Solution treated	15%	330–360	S20–S25	Slightly easier than aged
Inconel 625	Annealed	15%	235–270	S20–S25	Similar difficulty to 718
Hastelloy X	Annealed	12%	235–270	S25–S30	Very difficult
Waspaloy	Aged	10%	—	S30–S40	Most difficult common alloy
Rene 41	Aged	10%	—	S30–S40	Similar to Waspaloy

Tool Steels (ISO H — Hard Materials / P — Steel)

Grade	Condition	Rating	Hardness (HB/HRC)	ISO 513 Subgroup	Notes
P20	Pre-hardened	55–65%	270–300 (28–32 HRC)	P15–P20	Best mold steel for machining
H13	Annealed	50–60%	210–235 (20 HRC)	P15–P20	Pre-machine before heat treat
H13	Hardened	20–25%	430–560 (45–55 HRC)	H15–H25	AlCrN coating recommended
A2	Annealed	55–65%	220–235	P15–P20	Moderate machinability
A2	Hardened (57–62 HRC)	15%	600–700	H25–H30	Very poor, use AlCrN
D2	Annealed	40%	220–240	P20–P25	High carbide content, abrasive
D2	Hardened (58–64 HRC)	12%	600–750	H30–H40	Extremely difficult
O1	Annealed	85%	190–200	P10–P15	Easiest tool steel to machine
S7	Annealed	75%	220–230	P15–P20	Good machinability
S7	Hardened (48–58 HRC)	18%	460–600	H25–H30	Very difficult

ISO 513 Classification Notes

ISO 513 classifies workpiece materials into six groups, each with a subgroup number (01–50) indicating the required balance of hardness and toughness:

Group	Color	Material Types	Deep Hole Drilling Notes
P	Blue	Steel and alloy steel	Most common; P15–P25 for general gun drilling
M	Yellow	Stainless steel	M15–M25 for austenitic; M25–M35 for duplex
K	Red	Cast iron	K10–K20 for gray iron; K20–K30 for ductile
N	Green	Non-ferrous	N05–N10 for aluminum/brass; N10–N15 for bronze
S	Orange	High-temp alloys / titanium	S15–S25 for titanium; S25–S35 for Inconel
H	Grey	Hardened steel (>45 HRC)	H15–H25 for 45–55 HRC; H25–H40 for >55 HRC

Lower subgroup number (01–15) = higher wear resistance, higher speeds, finishing operations. Higher subgroup number (25–50) = higher toughness, lower speeds, roughing operations.

Drilling Speed Adjustment Factor

To estimate the cutting speed for a material relative to 1045 carbon steel (57% baseline, typical Vc = 70 m/min for carbide gun drilling):

Rating	Speed Factor vs 1045	Example Vc (carbide gun drill, m/min)
300% (C360 brass)	× 3.5	245
200% (6061-T6 Al)	× 2.5	175
100% (1117 steel)	× 1.75	123
57% (1045 baseline)	× 1.0	70
45% (304 SS)	× 0.8	56
30% (2205 duplex)	× 0.55	39
22% (Ti-6Al-4V)	× 0.4	28
12% (Inconel 718)	× 0.25	18

Formula: Estimated Vc = (Material rating ÷ 57) × 70 m/min. Always verify with manufacturer data and adjust based on observed tool wear and chip formation.

Cost Impact of Low Machinability in Deep Hole Drilling

Rating	Relative Tool Wear	Relative Cycle Time	Relative Cost per Hole	Example
200%+ (Aluminum)	0.3×	0.3×	0.3×	6061-T6
57% (1045 baseline)	1.0×	1.0×	1.0×	1045 steel
45% (304 SS)	1.5×	1.3×	1.5–2.0×	304 stainless
30% (2205 duplex)	2.5×	1.5×	2.5–3.5×	Duplex stainless
22% (Ti-6Al-4V)	3.0×	2.5×	3.0–5.0×	Titanium
12% (Inconel 718)	5.0×	4.0×	5.0–8.0×	Inconel 718

Key takeaway: A material with a machinability rating of 22% (titanium) will cost approximately 3–5× more per hole than 1045 steel at equivalent geometry, due to reduced cutting speeds, shorter tool life, and increased inspection frequency.

Factors That Reduce Machinability

Factor	Effect on Rating	Common Materials
Work hardening	Reduces 25–50%	Stainless steel, duplex, Inconel
Abrasive carbides	Reduces 30–60%	D2 tool steel, Inconel, CGI
Low thermal conductivity	Reduces 40–60%	Titanium, Inconel, stainless
High ductility (gummy chips)	Reduces 20–40%	Copper, low-carbon steel
High hardness (> 350 HB)	Reduces 40–70%	Hardened steel, 2507 super duplex
Casting scale / surface defects	Reduces 10–20% at entry	Cast iron, cast stainless

Using Machinability Ratings for Parameter Selection

Machinability ratings provide a starting point for estimating cutting parameters when manufacturer data is not available:

Identify the material’s machinability rating from the tables above
Find the base cutting speed for 1045 steel for the specific operation (gun drilling, BTA, or conventional)
Multiply by the speed factor from the adjustment table
Reduce further by L/D correction factors (typically 10–30% for holes exceeding 10×D)
Adjust feed rate proportionally to maintain chip load — lower machinability generally requires both reduced speed and reduced feed
Verify with a test cut and adjust based on chip form and tool wear

For detailed cutting parameters by method and diameter, see the Speeds and Feeds Reference for Deep Hole Drilling. For material-specific parameter guides, see the 4140/4340 Alloy Steel Guide and the Deep Hole Drilling Troubleshooting Guide.

Sources: ISCAR Die & Molds Guide; PMPA Craftsman; MSCI machinability ratings; SawbladeUniversity; EOXS; Stanford Advanced Materials; Engineers Edge; Outokumpu machining guidelines; Admiral Metals; Aobo Steel.