Morse taper

A Morse taper is a self-holding conical connection used to join cutting tools, drills, and arbors to machine spindles in lathes, drill presses, and milling machines. The taper angle is approximately 0.6 degrees, which means the cone is shallow enough that friction alone prevents the tool from slipping under cutting loads, yet steep enough that the tool can be removed by hand or with light mechanical force.

Named after Stephen A. Morse, who designed it in the 1860s, the standard comes in numbered sizes from MT0 (smallest, about 9 mm at the tip) to MT6 (largest, about 20 mm). Each size follows a specific taper ratio and length, defined in ASA B5.10. The bore diameter and depth of the spindle socket vary by number, as do the corresponding shank dimensions of the tool or arbor that fits into it. MT2 and MT3 are by far the most common in general machining shops.

The self-holding property depends on the geometry and the coefficient of friction between metal surfaces. A tool inserted at the correct angle will wedge itself into the socket under any radial cutting force; withdrawal requires either a blow from a drawbar or a mechanical spindle puller. Cleanliness is critical: dust or chips in the taper will prevent proper seating, cause runout, and reduce holding power. Most machinists wipe both the spindle socket and the tool shank with a clean cloth before inserting a tool, and some use light machine oil or a dry lubricant on the taper surfaces.

Variants and modern context

Steep-angle tapers with steeper cone angles, such as CAT (Caterpillar) and BT (NMTB) tapers, are used in high-speed milling and offer better rigidity and faster tool changes. However, Morse tapers remain standard on most conventional mills, older equipment, and cost-sensitive shops. They are also common on drilling and boring machines because they require no external clamping mechanism and tolerate moderate spindle runout better than pull-stud systems.

Damage to either the spindle socket or the tool shank will destroy the fit. Nicks, wear, or corrosion reduce holding power and cause chatter. Worn tapers are difficult to repair and usually mean replacing the spindle nose or the tool. For this reason, protecting taper surfaces and using proper pullers (rather than hammers or crude force) extends the life of the machine and its tooling.