Drilling stainless steel is demanding because the material combines toughness, low thermal conductivity, and a strong tendency to work-harden. A drill that is acceptable for carbon steel may produce excessive heat, built-up edge, poor hole quality, or premature failure in stainless steel. Choosing the right solid carbide drill—and matching it to the machine, coolant system, hole depth, and stainless grade—is therefore essential.
This guide explains the main selection factors for CNC drilling of austenitic and martensitic stainless steels, including drill geometry, carbide grade, coating, coolant delivery, and application setup.
Why Is Stainless Steel Difficult to Drill?
Stainless steel does not transfer heat away from the cutting zone as efficiently as many common steels. More heat remains at the cutting edge, increasing the risk of edge wear and coating damage. Austenitic grades such as 304 and 316 also work-harden quickly when the tool rubs instead of cutting. Once the surface hardens, the following revolution must cut through a more resistant layer.
- High toughness: chips can be difficult to break and evacuate.
- Low thermal conductivity: heat concentrates around the cutting edge.
- Work hardening: insufficient feed or machine dwell can harden the hole surface.
- Built-up edge: material may adhere to the cutting edge and reduce hole quality.
- Variable machinability: 304, 316, 17-4PH, duplex, and hardened stainless grades require different approaches.
A suitable carbide drill must cut positively, maintain edge strength, control heat, and move chips out of the hole before they are recut.
1. Match the Drill Geometry to Stainless Steel
Geometry is one of the most important differences between a general-purpose drill and a drill optimized for stainless steel. A sharp, positive cutting geometry reduces cutting force and limits work hardening. At the same time, the edge must remain strong enough to resist micro-chipping.
Point angle
Solid carbide drills for stainless steel commonly use a point angle around 130° to 140°. A wider point angle can improve centering and reduce the length of the chisel edge, but the best choice depends on diameter, drill design, entry condition, and material grade. Cross-hole entry, angled surfaces, and thin-walled parts may require a special point design.
Flute and margin design
Polished flutes help reduce chip adhesion and support reliable evacuation. Margin design affects guidance, friction, and hole straightness. A double-margin drill may provide additional stability and roundness in demanding applications, although it can also create more contact with the hole wall. The complete geometry should be selected as a system rather than by one feature alone.
2. Select the Right Carbide Substrate and Coating
A fine-grain or ultra-fine-grain carbide substrate can provide a useful balance of hardness, wear resistance, and edge strength. The optimum grade depends on tool diameter, machine stability, interrupted cutting, and the hardness of the workpiece.
Heat-resistant PVD coatings such as TiAlN, AlTiN, or TiSiN are frequently considered for stainless-steel drilling. The coating must work with the substrate and geometry; choosing a coating by color or nominal hardness alone is not enough.
| Selection factor | What to consider |
|---|---|
| Stainless grade | Austenitic, martensitic, precipitation-hardening, or duplex |
| Heat at the edge | Coating oxidation resistance and coolant access |
| Interrupted entry or exit | Edge toughness and tool geometry |
| Hole tolerance | Runout, margin design, machine condition, and tool holding |
| Production volume | Tool-life consistency and cost per hole, not only tool price |
3. Choose External or Through-Tool Coolant
Coolant controls temperature, lubricates the cutting zone, and helps evacuate chips. For short, shallow holes, well-directed external coolant may be adequate. As hole depth and production demand increase, internal coolant becomes more valuable because it delivers fluid directly to the cutting edges and pushes chips through the flutes.
For deeper or more demanding holes, consider a solid carbide coolant-through drill. Coolant pressure and flow must be sufficient for the drill diameter and depth. High pressure without adequate flow, or flow without a clear chip path, may still result in chip packing.
4. Match Drill Length to Hole Depth
Use the shortest drill that safely reaches the required depth. Excessive overhang reduces rigidity, increases deflection, and can amplify runout. A 3×D drill is generally more stable than a 7×D drill of the same diameter, so extra length should only be selected when the application requires it.
- Short holes: prioritize rigidity and simple chip evacuation.
- Medium-depth holes: verify coolant delivery and chip shape.
- Deep holes: use a purpose-designed drill, suitable coolant supply, and a controlled entry strategy.
KY Cutting Tools supplies solid carbide drills in standard and customized geometries, including different diameters, lengths, point angles, coolant configurations, and coating options.
5. Check Machine Rigidity, Runout, and Tool Holding
Even a well-designed drill cannot compensate for poor setup. Carbide is stiff and wear-resistant but less tolerant of impact than high-speed steel. Excessive runout causes unequal cutting-edge load, oversized holes, rapid corner wear, and possible breakage.
- Inspect the spindle, holder, and collet for contamination or damage.
- Measure runout as close to the drill point as practical.
- Minimize tool overhang.
- Secure the workpiece and avoid vibration.
- Confirm that coolant reaches the cutting zone before production begins.
For small-diameter drills, minor runout can represent a large percentage of the tool diameter. Setup accuracy becomes increasingly important as diameter decreases.
6. Start with Reliable Cutting Parameters
Cutting speed and feed must be selected for the exact stainless grade, drill diameter, coating, hole depth, coolant delivery, and machine condition. Avoid copying parameters from a different drill geometry without adjustment.
In stainless steel, feeding too lightly can be as harmful as feeding too aggressively. If the cutting edge rubs instead of forming a controlled chip, the material may work-harden and cutting temperature can rise. Begin with the tool manufacturer’s recommended range, monitor chip formation and spindle load, and make controlled adjustments.
What healthy chip evacuation looks like
- Chips leave the flute without packing around the tool.
- Chip form remains reasonably consistent from hole to hole.
- There is no severe discoloration, welding, or heavy built-up edge.
- Spindle load and hole size remain stable during the production run.
7. Evaluate the Application by Cost per Hole
The lowest-priced drill is not always the lowest-cost choice. For production machining, compare the number of acceptable holes, cycle time, tool-change frequency, scrap risk, and consistency between tool batches. A stable drill that supports predictable tool changes may create more value than a tool with an occasionally long but inconsistent life.
Record the failure mode when testing: flank wear, corner chipping, built-up edge, chip packing, breakage, or poor hole size. Each failure mode points to a different corrective action. Changing speed alone will not solve every drilling problem.
Solid Carbide Drill Selection Checklist
- Exact stainless-steel grade and hardness
- Hole diameter, tolerance, depth, and surface-finish requirement
- Through hole or blind hole
- Flat, angled, curved, or interrupted entry and exit
- Machine spindle condition, power, and maximum speed
- Tool holder type and measured runout
- External or internal coolant, including available pressure and flow
- Required production quantity and target cycle time
- Any secondary operation such as reaming, threading, or countersinking
Frequently Asked Questions
Is a solid carbide drill suitable for 304 and 316 stainless steel?
Yes. A solid carbide drill with suitable geometry, coating, coolant delivery, and cutting parameters can machine both 304 and 316 stainless steel efficiently. The complete setup must prevent rubbing and support reliable chip evacuation.
Should I use peck drilling in stainless steel?
Pecking is not automatically required. A properly applied carbide drill with effective through-tool coolant may drill continuously within its designed depth range. Unnecessary pecking can add cycle time and may allow rubbing during re-entry. Follow the drill supplier’s recommendation for the specific depth and coolant setup.
When should I request a custom carbide drill?
A custom drill may be appropriate for non-standard diameters, tight tolerances, stepped holes, special point forms, unusual reach, combined operations, or a recurring production problem that a standard drill cannot solve efficiently.
Need Help Selecting a Drill?
Send KY Cutting Tools your stainless grade, hole diameter and depth, tolerance, machine details, coolant conditions, and expected quantity. Our engineering team can recommend a standard tool or review a custom carbide drill design for your application.
