Drilling Techniques Guide
Eight practical lessons for drilling cleaner holes, protecting expensive drill bits, and working safely. Read the main lesson, then open either explanation if you want more detail.
8 LessonsRPM ChartTroubleshooting
8 Drilling Lessons
Do not enlarge existing holes with split-point drill bits.
Split-point drill bits are designed to start holes in solid material. Using them to enlarge an existing hole can cause grabbing, chatter, chipped edges, or breakage.
Want to know why?
With the center already removed, the drill point cannot engage normally. The cutting lips may bite unevenly, particularly when the drill is hand-held or the workpiece is not rigid.
A better way to do it?
For sheet metal, consider a step drill rated for the material and thickness. For precise enlargement, use an appropriate boring operation or a reamer with the correct amount of stock left for finishing. Not every reamer is intended for removing large amounts of material.
Choose the correct drilling speed.
Larger drill diameters need lower RPM at the same cutting speed. Tougher metals often require slower cutting speeds. Too much RPM produces heat and shortens tool life.
Want to know why?
The outer cutting edge travels farther per revolution as diameter increases. At the same RPM, a larger bit has higher surface speed, generating excess heat if the material and tool cannot handle it.
A better way to do it?
Check the manufacturer’s speed recommendation for the exact bit and material. Use the RPM chart below only as a general HSS starting reference; adjust for rigidity, coolant, and results.
Use the right cutting fluid.
Many metal-drilling jobs benefit from an appropriate cutting fluid. It helps reduce friction, heat, and cutting-edge wear.
Want to know why?
Heat and friction accelerate tool wear and can encourage chips to stick to the cutting edge. Some alloys, coatings, and drill types require different lubrication practices.
A better way to do it?
Use a cutting fluid recommended for the workpiece and drill type. Apply it safely, follow the product label, and keep it away from electrical components. Some operations are designed to run dry.
Apply steady, appropriate feed pressure.
Let the cutting edges make chips. Too little feed can cause rubbing; too much can overload or break the drill. Ease pressure near breakthrough.
Want to know why?
When a drill rubs instead of cuts, friction builds heat. In stainless steel, rubbing can contribute to work hardening. Excess force can bend the bit or cause a sudden grab.
A better way to do it?
Use firm, controlled feed that produces chips without forcing the drill. Keep the drill aligned, clear packed chips, and reduce feed near exit.
Pay attention to the chips.
Chip shape, heat, and color are clues. Squealing, dust instead of chips, or packed flutes signal a problem.
Want to know why?
Chips carry away part of the cutting heat. If the drill rubs or chips cannot escape, temperature and cutting forces rise. Chip appearance varies with the material.
A better way to do it?
Stop and inspect the bit if cutting changes suddenly. Adjust speed, feed, fluid, and chip clearing. Use a brush to remove chips only after the spindle has stopped.
Secure the workpiece and drill bit.
Clamp the workpiece. Keep hands, gloves, sleeves, and loose clothing clear of rotating equipment. Remove chuck keys before starting.
Want to know why?
A drill can catch as it breaks through or encounters a snag. An unsecured workpiece can spin violently, and a poorly seated bit can slip or wobble.
A better way to do it?
Use a vise or suitable clamps and back up thin material when appropriate. Seat the drill straight, tighten the chuck per its instructions, wear eye protection, and never hold small parts by hand.
Keep drill bits sharp.
A dull bit makes more heat, requires more force, and often produces poor holes.
Want to know why?
Rounded cutting edges rub rather than shear material. Unequal or damaged lips can cause wandering, oversized holes, and chatter.
A better way to do it?
Inspect edges for chips, rounding, or discoloration. Resharpen with the correct geometry when practical, or replace the bit. Do not continue forcing a damaged drill.
Choose the right bit for the material.
HSS, cobalt-alloy HSS, and carbide drills have different strengths. Geometry matters as much as material.
Want to know why?
Harder or hotter-running materials may benefit from higher hot-hardness or specialized geometry. Carbide can cut effectively but is brittle and typically needs a rigid setup.
A better way to do it?
Match the drill material, coating, point geometry, and machine to the job. Follow the manufacturer’s application guidance, especially for stainless, hardened materials, and specialty split-point drills.
Starting RPM Reference: HSS Twist Drills
Approximate starting speeds for conventional HSS drills, based on illustrative cutting speeds of 80 SFM (mild steel), 40 SFM (stainless steel), and 200 SFM (aluminum). Rounded to the nearest 25 RPM. Actual speeds depend on drill geometry, coating, machine rigidity, coolant, and material grade. Follow the drill manufacturer's recommendations when available.
| Drill diameter | Mild steel | Stainless | Aluminum |
|---|---|---|---|
| 1/8" | 2,450 | 1,225 | 6,100 |
| 3/16" | 1,625 | 825 | 4,075 |
| 1/4" | 1,225 | 600 | 3,050 |
| 5/16" | 975 | 500 | 2,450 |
| 3/8" | 825 | 400 | 2,025 |
| 1/2" | 600 | 300 | 1,525 |
| 5/8" | 500 | 250 | 1,225 |
| 3/4" | 400 | 200 | 1,025 |
Formula: RPM = (SFM x 3.82) / drill diameter in inches. These values are starting estimates, not universal settings. A handheld drill may not reach the calculated speed, particularly for small bits in aluminum.
What the Chips Are Telling You
| What you notice | What to check |
|---|---|
| Bit squeals or rubs, little chip produced | Dull cutting edges, too little feed, or too much RPM |
| Bit gets very hot or discolors | Reduce speed, check lubrication and chip evacuation, inspect sharpness |
| Chips pack into flutes | Peck to clear chips; check flute capacity and cutting fluid |
| Hole oversize or bit wanders | Check chuck runout, bit sharpness, workholding, and starting position |
| Stainless suddenly stops cutting | Possible work hardening; stop, inspect bit, and reset speed/feed |
Safety: Clamp work securely, wear eye protection, and stop rotating equipment before clearing chips. Match tools and techniques to the material and machine. Follow the drill manufacturer’s instructions for specific products.
Crowbar's shop reference guide. General drilling guidance, not a substitute for product-specific KnKut instructions. RPM values are illustrative starting estimates for conventional HSS drills, not KnKut specifications.