{"id":661,"date":"2026-10-22T00:42:24","date_gmt":"2026-10-21T16:42:24","guid":{"rendered":"https:\/\/amarajeweiry.com\/prong-thickness-6-5mm-moissanite-s925-cad\/"},"modified":"2026-10-23T12:59:43","modified_gmt":"2026-10-23T04:59:43","slug":"prong-thickness-6-5mm-moissanite-s925-cad","status":"publish","type":"post","link":"https:\/\/amarajeweiry.com\/ms\/prong-thickness-6-5mm-moissanite-s925-cad\/","title":{"rendered":"Prong Thickness for a 6.5mm Moissanite in S925: Where to Spec It in CAD and the Minimum to Enforce"},"content":{"rendered":"

Specify the number on a 2D section through the prong axis \u2014 Section A-A, cut through the stone center and one prong \u2014 not on the 3D mesh, and dimension the metal between the seat bearing surface and the outer face of the prong. For a 6.5 mm round moissanite in S925, put 0.95 mm minimum on the CAD, pre-polish, and 0.80 mm minimum on the finished, set and polished prong, with a one-sided tolerance of +0.05\/\u22120.00 mm. The PO should then require the factory to confirm both figures in writing before it quotes, and to prove them by sectioning one first-article casting at the seat. Below 0.80 mm finished, refuse the design: that is where a silver prong stops being a setting and becomes a wear item.<\/strong><\/p>\n

What Prong Dimension Actually Holds a 6.5 mm Stone?<\/h2>\n

A prong holds a stone with the metal behind the seat, not with its outside diameter. A 1.00 mm prong sounds solid until the setter cuts a 0.30 mm bearing seat into its inner face. The wall left behind is 0.70 mm in the CAD file and about 0.55 mm after filing and polishing. The drawing still says 1.00 mm, and the ring still fails.<\/p>\n

Scale makes it worse. A 6.5 mm round has roughly eighty times the volume of 1.5 mm melee, and its girdle presses on four small bearing surfaces. Every knock on the crown becomes a prying load at the seat \u2014 which is also where polishing removes the most metal.<\/p>\n

Silver is the wrong metal to be stingy with. Sterling is 92.5% silver; in the United States, an item may be described as silver or sterling silver only if it is at least 925\/1,000 fine, as the FTC’s consumer guidance on precious-metal jewelry<\/a> states. The 7.5% copper adds hardness, but cast sterling still creeps under steady load and wears against a hard girdle. Thickness is the only lever that costs nothing in alloy.<\/p>\n

Why the 0.5 mm and 1.0\u20131.2 mm Figures Do Not Answer This<\/h3>\n

The 0.5 mm figure is not a seat-wall specification. At its source it is a tip-base rule: for the same 6.5 mm round, the prong-bend guidance<\/a> asks for “a wall at the base of the tip no thinner than about 0.5 mm” \u2014 the metal above the girdle that a re-tip has to rebuild. The figure is also the one usually quoted for micro-pav\u00e9 claws on 1.0\u20131.5 mm melee, where the stone weighs a fraction of a point. Those are different measuring points: the tip base sits above the girdle, the seat wall sits between the bearing surface and the outer face of the prong, and a 6.5 mm stone can pass a 0.5 mm tip-base check with 0.55 mm left at the seat. The 1.0\u20131.2 mm figure is usually a prong diameter, quoted without reference to seat cut, stone size or alloy. A factory can meet it and still deliver a 0.55 mm wall at the seat. Neither number tells a CAD operator what to dimension, or a buyer what to reject.<\/p>\n

Where Exactly Should the Prong Thickness Appear in the CAD?<\/h2>\n

A 2D Section, Not the 3D Mesh<\/h3>\n

STEP and STL files carry geometry, not tolerances. A factory quotes from the mesh \u2014 stone count, weight, metal price \u2014 and nothing in that file states a minimum wall. Worse, a general \u00b10.05 mm tolerance applied to a 0.95 mm wall lets it read 0.90 mm with no deviation flagged. The number has to live on a drawing sheet issued with the model, repeated in the quote, and attached to the PO.<\/p>\n

Section A-A Through the Stone Center and One Prong Axis<\/h3>\n

Cut a plane through the stone center and one prong axis, and section the assembly with the stone as reference geometry. This is the only view where the seat cut, the bearing ledge and the wall behind them appear in true profile. Dimension the shortest distance from the bearing surface to the outer prong face, the radial seat depth, the ledge width, the prong diameter at the girdle plane, and the tip diameter with the height available for a re-tip. Add Section B-B at girdle height and a 5:1 detail; the isometric render is for approval only.<\/p>\n

The Dimensions the Drawing Sheet Needs<\/h3>\n\n\n\n\n\n\n\n\n\n\n\n
Dimension<\/th>\nCAD, pre-polish<\/th>\nFinished minimum<\/th>\nHow it is checked<\/th>\n<\/tr>\n<\/thead>\n
Seat wall: bearing surface to outer prong face<\/td>\n0.95 mm MIN<\/td>\n0.80 mm<\/td>\nSectioned finished piece, 10\u00d7<\/td>\n<\/tr>\n
Seat cut, radial depth<\/td>\n0.25 mm MAX<\/td>\n0.25 mm<\/td>\nSection A-A against the stone<\/td>\n<\/tr>\n
Bearing ledge width<\/td>\n0.25 mm<\/td>\n0.20 mm<\/td>\nSection A-A, 10\u00d7 loupe<\/td>\n<\/tr>\n
Prong diameter at girdle plane (Section B-B)<\/td>\n1.20 mm MIN<\/td>\n1.05 mm<\/td>\nCaliper plus section<\/td>\n<\/tr>\n
Tip diameter above girdle<\/td>\n0.95 mm<\/td>\n0.80 mm<\/td>\nCaliper<\/td>\n<\/tr>\n
Prong height above girdle for re-tip<\/td>\n0.60 mm<\/td>\n0.45 mm<\/td>\nSection A-A<\/td>\n<\/tr>\n
Voids on the sectioned face<\/td>\nNone at 10\u00d7<\/td>\nNone at 10\u00d7<\/td>\nVisual, 10\u00d7<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Every finished value is the CAD value minus the 0.15 mm finish allowance, with two exceptions: the seat cut holds at 0.25 mm MAX, because a deeper cut only eats the wall, and the bearing ledge loses 0.05 mm, from 0.25 to 0.20 mm, not 0.15. A polishing route that removes more does not get to eat into the 0.80 mm floor.<\/p>\n

\n\"Side
A side profile is where prong shoulders read at actual size; in CAD, the same view becomes Section A-A, the only place the seat wall can be dimensioned and checked.<\/figcaption><\/figure>\n

Dimension as a Minimum, With a One-Sided Tolerance<\/h3>\n

Write 0.95 MIN, not 0.95, and add a title-block note: “PRONG SEAT WALL MINIMUM, PRE-POLISH; FINISH ALLOWANCE 0.15 mm ON EXPOSED PRONG SURFACES; TOLERANCE +0.05\/\u22120.00.” State the finished value separately \u2014 “FINISHED MIN 0.80 AT SEAT” \u2014 because a polisher cannot infer the allowance on a curved, hand-worked surface. Two numbers, two stages, one section.<\/p>\n

Model the stone from the parcel you will actually set. A 6.5 mm moissanite is not the same body as the CAD library’s generic diamond: girdle thickness, crown angle and pavilion depth differ, and the seat is cut to the girdle. A wrong reference stone makes the setter deepen the seat, and the wall goes with it. One formula keeps this honest: minimum prong diameter = 0.95 mm + seat cut depth.<\/strong> At a 0.25 mm seat, that is 1.20 mm.<\/p>\n

Why Is 0.80 mm the Floor, and What Should a Buyer Refuse?<\/h2>\n

No published standard fixes a prong wall for a 6.5 mm stone in silver. The 0.80 mm figure is a purchasing floor sized for finishing, wear and one re-tip \u2014 not a claim that thinner prongs always fail tomorrow.<\/p>\n

The Stiffness Math<\/h3>\n

Treat the wall behind the seat as a short beam of constant width: bending stiffness rises with the cube of its thickness, and bending stress at the same load falls with the square. A 0.80 mm wall is not 33% stiffer than 0.60 mm; it is 2.4 times stiffer (0.80\/0.60)\u00b3, and bending stress at the same load is about 44% lower, 1 \u2212 (0.60\/0.80)\u00b2. At 0.55 mm \u2014 what you get from a 1.00 mm prong with a 0.30 mm seat and normal polishing \u2014 the section keeps about a third of its stiffness (0.33), and stress more than doubles (2.1\u00d7). That is the difference between a prong that springs back and one that stays bent.<\/p>\n

The Re-Tip and Porosity Allowance<\/h3>\n

Budget 0.10\u20130.15 mm per re-tip and re-polish. Starting at 0.80 mm, one re-tip leaves 0.65\u20130.70 mm and a second leaves 0.50\u20130.60 mm \u2014 rebuild territory, not shave-again territory. Starting at 0.55 mm, there is no first re-tip. Porosity compounds it: a 0.80 mm wall with a third of its section in voids behaves like 0.53 mm, and the seat is where feed metal arrives last in a thin prong. So the wall dimension is only half the specification; the other half is a sectioned face with no voids visible at 10\u00d7.<\/p>\n

The Refusal List<\/h3>\n

Refuse the quote or the design when:<\/p>\n