Complete Guide to Drilling and Tapping Stainless Steel

Non-corrosive and easy to clean, stainless steel is revered both for its strength and its appearance. A go-to material in transportation, consumer goods, food production, construction, and a variety of other industries, stainless steel plates and tubes are popular work materials for drilling and tapping applications. While a common material, drilling and tapping stainless steel requires the proper tools and precautions to ensure efficient and safe operations. Before discussing the proper taps and drills for working with stainless steel, it is important to understand the material’s characteristics in order to extend tool life and achieve quality workmanship.

Properties of Stainless Steel

Though noted for its strength, stainless steel does not rank among the hardest materials you are likely to encounter in your metal work. The problem is that many stainless steels – 304 and 316 in particular – can get harder and tougher to cut as you work if the cutting tools are improperly applied. This phenomenon, known as work hardening, occurs because of the heat generated if the incorrect speed and feed rates for drilling and tapping stainless steel are applied. If the cutting speed (RPM) is too high and the feed is too slow, the drill will “rub” rather than cut, which will create heat and actually toughen the work material. Adding to the challenge, the heat is generated as work progresses, hardening the bottom and sides of the hole just before they are about to be cut. Drilling stainless steel that is susceptible to work hardening makes the drill work harder, shortening its life through wear and increasing the chance of breakage. And because the material stays hard, it has the same effect on taps employed later in the machining process.

Stainless steel possesses a few other traits that affect the way it should be drilled and tapped:

Ductility – Austenitic, duplex, and other stainless steels stretch more than most other metals when subjected to tensile stress. They “give” or yield to a drill’s pressure before the tool’s cutting edges can gain purchase. This makes it difficult for the tool to break off chips.
Strength – Even when the drill or tap bites into some stainless steel varieties, the material’s strength resists the forming of chips, causing built-up edges which lessen hole quality and thread formation.
Low Thermal Conductivity – The material cannot dissipate heat quickly. The high temperatures generated when drilling or tapping stainless steel remain concentrated in the tool tip, which can cause it to deteriorate. High-speed steel components will lose hardness and carbide taps and drills can develop microscopic cracks on cutting edges when exposed to high, sustained heat.

Keys to Drilling and Tapping Stainless Steel

As discussed above, stainless steel’s great strength and elasticity causes difficulty in chip breaking, formation, and evacuation. Chips clogging in the flutes typically will cause catastrophic tool failure but can also cause the tool and the workpiece to vibrate excessively which is not good for the cutting tool, hole finish, and thread quality. Securely clamping the workpiece and stabilizing the drill in the correct tool holder will help, as will reducing the tool overhang as much as possible.

Using sharp tools also will help reduce vibrations as well as work quality and heat reduction. But running work at proper feeds and speeds is the best way to cut clean holes and tap stainless steel most efficiently. While narrow-diameter drill bits must maintain a minimum speed to be effective, wider bits, especially when used to bore deeper holes, must be dialed back to lessen the risk of breakage. However, the recommended surface footage per minute (SFM) must be maintained regardless of the drill diameter. While different applications and grades of stainless steel require different speed and feed parameters, maintain the recommended speed. Feed rates can be relatively high, so long as you’re careful to guard against breakage and tool weakening owing to high heat. High feeds will help the tool cut, evacuate chips and to cut through work-hardened material. Maintain the feed rate throughout the process and take care not to allow the tool to dwell or rub against the side of the hole. This procedure will allow the heat to escape with the chip and not build up in the tool or the workpiece. Ideally the goal is to see a chip that has turned blue. A black or brown chip indicated there is too much heat; reduce your speed (SFM) and increase the feed. A shiny chip indicated the lack of heat; increase the speed and feed.

Heat is the enemy here, so it may be advisable to drill stainless steel in stages, allowing time in between for the tool to cool down. Make it a series of sprints instead of a marathon. The drilling sessions should be short, with adequate pressure. Be sure to keep the drill at full feed when exiting and re-entering the hole.

Don’t forget to lubricate while working with stainless steel. Flood throughout the work, if possible. If not, stop regularly to reapply oil or coolant.

Metal Cutting Tools for Stainless Steel

Tool geometry, design, and metallurgy have created drills and taps for stainless steel that incorporate features that make the work go more smoothly:

Cutting Edges – Split points have cutting edges that are often sharpened at more acute, narrow angles at the tip, which prevent the drill from walking. Cutting angles of 135 degrees cut thinner chips that can be evacuated more easily.
Tool Metal – The higher purchase price for cobalt drills may prove a worthwhile investment for those who work extensively in stainless steel. The cobalt alloy runs completely through the bit – it’s not merely a coating - so they maintain cobalt’s hardness and heat-resistant properties through multiple sharpening’s. High-speed steel tools can be used on stainless steel, but shorter drills are the most effective as they are less susceptible to deflection. The best HSS drills for stainless steel are alloyed with tungsten or molybdenum. Carbide tools can be used on long runs without the need for frequent tool changes, making them ideal drills and taps for stainless steel.
Coatings – Coating drills in titanium nitride makes them more resistant to heat. They can stand up to the high feed rates that stainless steel requires, while increasing hole integrity and a smooth finish.

Regal Cutting Tools carry an extensive line of taps and drills perfect for every application and workpiece material. Our tools come in every machine screw, fractional and metric size imaginable. Can’t find the exact cutting tool for your need? Regal specializes in custom special taps, to ensure you have the exact tool you need for your application. For more information, contact our experts today or request a quote.