What makes high-speed steel different
High-speed steel, often shortened to HSS, is a family of tool steels designed to keep cutting at the temperatures that friction creates at the cutting edge. It is used for twist drill bits, lathe tool bits, router and shaper cutters in some designs, taps, reamers, hole saws and some planer or jointer knives. Compared with plain carbon steel, it typically holds a cutting edge better when hot, which is why it is the common choice for tools that spin or cut at speed.
The same traits affect how you sharpen it. HSS is hard and wear-resistant, so it takes longer to abrade by hand than a typical chisel, but it can also be damaged by careless grinding. The goal is to remove material from the cutting faces while keeping the steel cool enough that you do not undo the heat treatment the manufacturer built in. Some HSS tools also carry surface coatings, which are discussed below.
Heat is the main risk
Grinding creates heat, and a thin cutting edge has very little mass to absorb it. If the edge gets too hot, the steel can lose hardness. Warning signs include discoloration, such as straw, bronze, purple or blue tints at the edge, which indicate high temperatures. A tool that has been overheated may seem sharp but can dull quickly. Opinions vary on how much heat damage matters for HSS compared with plain carbon steel, but keeping it cool is a safe, widely accepted habit.
Good habits for heat control include:
- Using light pressure and brief passes rather than pressing hard.
- Dipping the tool in water frequently, if the manufacturer’s instructions and the tool’s design allow it.
- Using a sharp, clean wheel. A glazed or loaded wheel burnishes instead of cutting and produces more heat.
- Choosing a wheel type suited to the steel. Many suppliers describe specific wheel compositions for HSS, so check what your grinder’s maker recommends.
- Stopping to let thin edges cool rather than powering on.
If you use a bench grinder, keep the tool rests adjusted close to the wheel, wear eye protection and follow the grinder’s safety instructions. Grinding sparks and metal dust are hazards, and a face shield is a sensible addition. A bench grinder should be set up and used strictly to its manual.
Sharpening twist drill bits
Twist drill bits are the most commonly sharpened HSS tool. Their cutting geometry has several parts: two cutting lips, a chisel edge at the center, and relief angles behind the lips so the edge can bite into the material. A point angle of around 118 degrees is a common general-purpose figure, with flatter or steeper angles used for particular materials. The lips need to be equal in length and angle, or the bit will drill an oversized or wandering hole.
Sharpening by hand on a grinder takes practice. The basic idea is to present each lip to the wheel at the right angle, rotating the bit slightly as you pull it across so that relief is ground behind the cutting edge. Many people instead use a drill bit sharpening jig or a purpose-built sharpener that holds the bit in a fixed position. These fixtures are often more reliable than freehand work for a beginner, and they are available in styles from simple attachments to benchtop units.
After sharpening, compare the two lips by eye, and drill a test hole in scrap of the same material you plan to use. A properly sharpened bit produces two equal curls of chip and a clean hole. A bit that cuts on only one lip, grunts or wanders needs another look.
Different bit types and materials affect what is worth sharpening, since drill bit types and bit material both change how a dull bit should be treated.
Other HSS tools
- Lathe tool bits are usually ground from blanks to a shape that suits the job, and re-ground when dull. They are often ground freehand with light pressure and frequent cooling, then finished with a fine stone.
- Router and shaper cutters vary. Some are made with carbide inserts that are generally sharpened by a professional service, and others are solid HSS. Profile cutters are difficult to sharpen at home without altering the shape, so many people hone only the flat faces lightly, if at all. The shape of router bit profiles is what makes them hard to restore.
- Planer and jointer knives must be sharpened evenly and matched in weight and height, otherwise vibration and uneven cuts result. These are often best sent to a sharpening service.
- Taps, dies and reamers have precise geometry and are usually replaced rather than sharpened by hobbyists.
- Hole saws and step bits can sometimes be touched up with a small file or stone, but cheaper ones are normally replaced.
Hand-honing HSS
Hand tools made of HSS, like some plane irons, can be honed on diamond plates, which cut hard steel efficiently. Conventional oil or water stones can handle HSS, but they may wear faster or take longer. Ceramic stones and diamond abrasives are common choices. Honing lightly between grindings helps keep the edge alive without removing much steel.
Coatings
Some HSS tools carry a surface coating, such as a gold-colored or dark finish, intended to reduce friction or wear. Sharpening grinds away the coating on the faces you touch, so a re-sharpened tool may behave like an uncoated one. That is usually acceptable, but it means the benefits of the coating are partly lost. Check the maker’s literature if the coating matters for your application.
When sharpening is not worth it
Not every dull HSS tool deserves the effort. Consider replacement when:
- The bit is cracked, bent or chipped deeply, since removing enough metal to reach sound steel may ruin the geometry.
- The tool is small or inexpensive, for example a tiny drill bit, where your time and the risk of poor results outweigh the cost.
- The bit has been overheated, showing blue or purple discoloration along the flutes, which suggests the temper may be affected.
- The cutting geometry is complex, such as profile cutters or taps, and you cannot restore it accurately.
- You are cutting something where a poor edge could be dangerous, such as work held at the limit of its capacity.
A sharpening service can handle many of these cases economically, especially for larger cutters. For routine drilling, a good bit set with replacements is often the most practical approach. If you drill hard material such as stainless steel, expect bits to dull sooner. Slower speed, steady pressure and lubricant when drilling metal help protect them.
Safety
Always wear eye protection when grinding or filing, and dust protection if the grinding produces fine particles. Tie back loose hair and clothing, secure the work, and never use a grinder with a cracked or damaged wheel. Unplug the grinder before changing wheels or making adjustments. This article is general information. Follow the tool and grinder manufacturers’ instructions, because their recommendations take priority over any general guideline here.