The speed of a drill is usually measured in terms of the rate at which the outside or periphery of the tool moves in relation to the work being drilled. The common term for this is "Surface Feet Per Minute", abbreviated to sfm. The relation of sfm and Revolutions Per Minute, or rpm, is indicated by the following formulas:
sfm = .26 x rpm x Drill Diameter in Inches
rpm = 3.8 x sfm / Drill Diameter in Inches
In general, when operating a drill at a speed anywhere within its range for the particular material involved, reductions in speed result in more holes before regrinding is necessary. On every job there is a problem of choosing a speed which will permit the most economical rate of production which is also determined by drill costs, pieces produced in a given time and down-time for tool changing. The most economical speed for operating a drill will depend on many variables, some of which are:
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1. Composition and hardness of material 2. Depth of hole 3. Efficiency of cutting fluid |
4. Type and condition of drilling machine 5. Quality of holes desired 6. Difficulty of set-up |
Speeds shown in the table below include a range in feet-per-minute. On most jobs, it is usually better to start with a slower speed and build up to the maximum after trials indicate the job can run faster.
| Material | Speed in feet per minute (fpm) |
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| Aluminum and its Alloys | 200 - 300 |
| Brass and Bronze (ordinary) | 150 - 300 |
| Bronze (High Tensile) | 70 - 150 |
| Die Castings (Zinc Base) | 300 - 400 |
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| Magnesium and its Alloys | 250 - 400 |
| Monel Metal or High-Nickel Steel | 30 - 50 |
| Plastics or Similar Materials (Bakelite) | 100 - 300 |
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| 5AI-2Sn, 8AI-1 Mo-1 V (Annealed) | 30 - 40 |
| 2 Fe-2Cr-Mo (Annealed) | 30 - 40 |
| 6AI-4V, 4AI-4Mn, 7AI-4Mo (Annealed) | 25 - 35 |
| 6AI-4V, 4AI-4Mn, 7AI-4Mo (Solution Treated & Aged) | 15 - 20 |
| 2Fe-2Cr-2Mo (Solution Treated & Aged) | 15 - 20 |
| Wood | 300 - 400 |
| (1)Austenitic (2) Ferritic (3) Martensitic | |
