Tool Steel Machining Allowances Chart
Standard machining allowances for hot-rolled tool steel square and flat bars: how much stock to remove from the top, bottom and edge surfaces of the as-supplied mill form, by specified bar thickness and width. These are the allowances to plan for when ordering tool steel bar and preparing it for heat treatment.
| Specified width mm | Specified width in. | Machining allowance (a) — top and bottom surfaces | Machining allowance (a) — edges | ||
|---|---|---|---|---|---|
| mm | in. | mm | in. | ||
| Specified thickness: under 12.7 mm (1/2 in.) | |||||
| 0-12.7 | 0-1/4 | 0.64 | 0.025 | 0.64 | 0.025 |
| >12.7-25.4 | >1/4-1 | 0.64 | 0.025 | 0.89 | 0.035 |
| >25.4-50.8 | >1-2 | 0.76 | 0.030 | 1.02 | 0.040 |
| >50.8-76.2 | >2-3 | 0.89 | 0.035 | 1.27 | 0.050 |
| >76.2-101.6 | >3-4 | 1.02 | 0.040 | 1.65 | 0.065 |
| >101.6-127.0 | >4-5 | 1.14 | 0.045 | 2.03 | 0.080 |
| >127.0-152.4 | >5-6 | 1.27 | 0.050 | 2.41 | 0.095 |
| >152.4-177.8 | >6-7 | 1.40 | 0.055 | 2.67 | 0.105 |
| >177.8-203.2 | >7-8 | 1.52 | 0.060 | 3.05 | 0.120 |
| >203.2-228.6 | >8-9 | 1.52 | 0.060 | 3.30 | 0.130 |
| >228.6-304.8 | >9-12 | 1.52 | 0.060 | 3.56 | 0.140 |
| Specified thickness: over 12.7 to 25.4 mm (over 1/4 to 1 in.) | |||||
| >12.7-25.4 | >1/4-1 | 1.14 | 0.045 | 1.14 | 0.045 |
| >25.4-50.8 | >1-2 | 1.14 | 0.045 | 1.27 | 0.050 |
| >50.8-76.2 | >2-3 | 1.27 | 0.050 | 1.52 | 0.060 |
| >76.2-101.6 | >3-4 | 1.40 | 0.055 | 1.90 | 0.075 |
| >101.6-127.0 | >4-5 | 1.52 | 0.060 | 2.41 | 0.095 |
| >127.0-152.4 | >5-6 | 1.65 | 0.065 | 2.92 | 0.115 |
| >152.4-177.8 | >6-7 | 1.78 | 0.070 | 3.30 | 0.130 |
| >177.8-203.2 | >7-8 | 1.90 | 0.075 | 3.81 | 0.150 |
| >203.2-228.6 | >8-9 | 1.90 | 0.075 | 3.94 | 0.155 |
| >228.6-304.8 | >9-12 | 1.90 | 0.075 | 3.94 | 0.155 |
| Specified thickness: over 25.4 to 50.8 mm (over 1 to 2 in.) | |||||
| >25.4-50.8 | >1-2 | 1.65 | 0.065 | 1.65 | 0.065 |
| >50.8-76.2 | >2-3 | 1.65 | 0.065 | 1.78 | 0.070 |
| >76.2-101.6 | >3-4 | 1.78 | 0.070 | 2.16 | 0.085 |
| >101.6-127.0 | >4-5 | 1.78 | 0.070 | 2.67 | 0.105 |
| >127.0-152.4 | >5-6 | 1.90 | 0.075 | 3.18 | 0.125 |
| >152.4-177.8 | >6-7 | 2.03 | 0.080 | 3.68 | 0.145 |
| >177.8-203.2 | >7-8 | 2.03 | 0.080 | 4.19 | 0.165 |
| >203.2-228.6 | >8-9 | 2.41 | 0.095 | 4.32 | 0.170 |
| >228.6-304.8 | >9-12 | 2.54 | 0.100 | 4.32 | 0.170 |
| Specified thickness: over 50.8 to 76.2 mm (over 2 to 3 in.) | |||||
| >50.8-76.2 | >2-3 | 2.16 | 0.085 | 2.16 | 0.085 |
| >76.2-101.6 | >3-4 | 2.16 | 0.085 | 2.54 | 0.100 |
| >101.6-127.0 | >4-5 | 2.16 | 0.085 | 3.05 | 0.120 |
| >127.0-152.4 | >5-6 | 2.16 | 0.085 | 3.43 | 0.135 |
| >152.4-177.8 | >6-7 | 2.29 | 0.090 | 3.94 | 0.155 |
| >177.8-203.2 | >7-8 | 2.54 | 0.100 | 4.32 | 0.170 |
| >203.2-228.6 | >8-9 | 2.54 | 0.100 | 4.83 | 0.190 |
| >228.6-304.8 | >9-12 | 2.54 | 0.100 | 4.83 | 0.190 |
| Specified thickness: over 76.2 to 101.6 mm (over 3 to 4 in.) | |||||
| >76.2-101.6 | >3-4 | 2.92 | 0.115 | 2.92 | 0.115 |
| >101.6-127.0 | >4-5 | 2.92 | 0.115 | 3.18 | 0.125 |
| >127.0-152.4 | >5-6 | 2.92 | 0.115 | 3.56 | 0.140 |
| >152.4-177.8 | >6-7 | 2.92 | 0.115 | 4.32 | 0.170 |
| >177.8-203.2 | >7-8 | 3.18 | 0.125 | 4.83 | 0.190 |
| >203.2-228.6 | >8-9 | 3.18 | 0.125 | 4.83 | 0.190 |
| >228.6-304.8 | >9-12 | 3.18 | 0.125 | 4.83 | 0.190 |
- (a) Minimum allowance per side for machining prior to heat treatment. Maximum decarburization limit, 80% of machining allowance.
Table notes
Standard machining allowances for hot-rolled square and flat bars: the recommended total stock to remove from the as-supplied mill form to reach a surface free of scale, seams and other imperfections, and suitable for heat treatment and service.
Bar weight for a given size, before any allowance is removed, is in the steel weight calculator for round bar and plate.
In addition to these standard allowances, extra stock must be provided for the cleanup of any decarburization or distortion that occurs during final heat treatment — the amount varies with the type of tool steel and the hardening treatment.
Allowances for other shapes and other methods of forming and finishing are given in ASTM specifications A 600, A 681 and A 686.
Metric and imperial allowance values are as printed in the source; the inch values are the standard figures (for example 0.025 in. ≈ 0.64 mm).
Related reference pages
D2 tool steel · O1 tool steel · H13 tool steel · Tool steel machinability rating chart · Tool steel properties chart · Tool steel density and thermal expansion
Source: ASM Handbook, Volume 1 — Properties and Selection: Irons, Steels, and High-Performance Alloys, Table 17 “Standard machining allowances for hot-rolled square and flat bars” (10th ed., pp. 1833–1836). Reference data for comparison — verify the applicable heat treatment before design use.