Mounted Point — Abrasive Bit on Steel Mandrel for Lapidary Work
Mounted Point — Abrasive Bit on Steel Mandrel for Lapidary Work
Carving and shaping bit for rotary tools, available in dozens of shapes and abrasive types
Mounted point is the lapidary trade term for a small abrasive bit — typically diamond, silicon carbide, or aluminium oxide — bonded to a hardened steel mandrel for use in rotary tools, flexible-shaft machines, and drill presses. Mounted points are the principal hand tool for shaping, carving, and detailing gemstones in the small-to-medium size range, particularly for softer materials such as opal, turquoise, malachite, and the various jadeite and nephrite jade carving traditions. The category is broad: dozens of shape profiles (cylinder, cone, ball, flame, tapered, inverted) and several abrasive types and grit ranges are catalogued in the major tool suppliers' inventories, with each combination optimised for particular material types and operations.
Abrasive types
Diamond mounted points are the standard for hard materials (above approximately 6 on the Mohs scale) and are essentially universal in contemporary professional practice for shaping work in any moderately hard gem material. The diamond particles are bonded to the steel mandrel either by a sintered metal matrix (more durable, more expensive, generally preferred for production work) or by an electroplated nickel layer (less durable, less expensive, often adequate for occasional use). Diamond points come in grit ranges from coarse (60 to 120 grit, for rough shaping) to fine (600 to 1200 grit, for pre-polish refinement).
Silicon carbide mounted points are the conventional choice for softer materials (below approximately 6 on the Mohs scale) and for specific finishing operations on harder materials. Silicon carbide cuts more aggressively than diamond on softer materials but does not have the durability of diamond on hard materials. Aluminium oxide points fill a similar role to silicon carbide for some applications and are common in metalworking applications where mounted points see crossover use between lapidary and metal finishing.
Use
Mounted points are run in flexible-shaft machines (a foot-pedal-controlled motor with a flexible drive shaft and a handpiece holding the bit), in compact battery-powered or mains-powered rotary tools (Dremel and equivalent products), or in fixed drill presses. Speed and pressure are critical: each material has an optimum surface speed range, with too-high speeds producing heat damage or chipping and too-low speeds producing inefficient cutting. Light pressure and constant motion are the standard technique; heavy pressure causes chipping and rapid wear of the bit.
Lubrication and cooling are essential for most lapidary applications. Water is the conventional cooling medium; some applications use light oil. Dry running is occasionally permissible for very brief operations on tough materials but is generally avoided.
Application contexts
Mounted points are central to lapidary applications including carving (figurative and abstract sculpture in jade, lapis lazuli, malachite, and similar materials), cabochon shaping for unusual outlines that don't fit a fixed templated cut, intaglio and seal work, repair of damaged gemstones (chip removal and re-shaping), and detail finishing on a wide range of work. The Sinkankas reference work Gem Cutting describes mounted point applications in detail, alongside the various alternative tool categories (laps, wheels, drum sanders) that complete the lapidary toolkit.
In the trade
Mounted points are inexpensive supplies in the lapidary trade, with quality and price varying considerably between low-cost imports and premium professional brands such as Diamond Pacific and Lortone. For occasional or amateur use, the inexpensive lines are typically adequate; for production work, the premium brands offer significantly better durability and consistency. The major suppliers maintain extensive catalogues with detailed specifications for each shape and grit combination.