General tolerances for SMW Fixture Plates are given below.

Parallelism

Parallelism determines the deviation between the top and bottom surfaces of the fixture plate when constrained to a flat surface. This is often confused for flatness, which controls only the top or bottom surface of the plate (while parallelism controls the relation between them).

• 1/2" and M12 SMW Fixture Plates are held to a parallelism of 0.0005" per 18" [0.013mm per 457mm] of plate length.
  • Dimensions under 18" [457mm] are held to a total parallelism of 0.0005" [0.013mm]. For example, an 18" x 9" plate would have a global parallelism tolerance of 0.0005".
• 1/4" and M6 fixture plates are held to a total parallelism of 0.001" [0.025mm].

Flatness

We do not qualify plates in an unconstrained state. However, we do evaluate and process the raw material in a specific way so that when the fixture plate is secured to the machine table, it's "flatness" will be the result of the parallelism tolerance plus any deviation, level, or tram error in the machine table. 

The below example shows how a fixture plate could be perfectly flat on both edges (blue) but would sit incorrectly on your machine table. In a free state plates will be parallel but will have some inherent 'bow' (yellow). Once secured to a flat surface, e.g. a machine table, any bow is pulled out and you are left with a plate that is both flat and parallel (green).

*examples are exaggerated for emphasis

Matched Set Tolerances

• For sets of multiple fixture plates the plate thickness is matched at the seam between plates. Each plate is then individually subject to the standard parallelism tolerance. Matched thicknesses are as follows:
  • Sets of 2-3 plates: 0.0005" [0.013mm]
  • Sets of 4+ plates: 0.001" [0.025mm]
  • 6061 aluminum fixture plates are matched +/- 0.001" [0.025mm] from their nominal thickness if intended to be used as a set.
• The continuity of the fixturing hole pattern across multiple plates is dependent on careful setup of the plates. These sets have a small gap (~0.008" [0.20mm]) designed between fixture plates, allowing for some adjustment of the hole pattern alignment. We recommend using our Fixture Plate Set Alignment Jig for installation of these fixture plate sets.
• As part of the manufacturing and handling process, multi-plate sets may exhibit minor cosmetic variations, marks, or porosity that do not affect accuracy, function, or performance.

Plate Thickness

• We prioritize parallelism over the final thickness of the fixture plate. In general, thickness may vary +0.01" [0.25mm] -0.04" [1.02mm] from the nominal value given. This does not mean a single plate will have 0.05" [1.3mm] variation in the thickness.
• For example, the measured thickness of our standard 0.950" nominal plates may come out at 0.930". Each point measured along the plate will be within  0.0005" of this 0.930" dimension (the parallelism tolerance).
• Some fixture plates, such as those sold individually but intended for use as a set (e.g. Shapeoko 4, Tormach 24R) are subject to a tighter thickness tolerance so that plates can be purchased and used interchangeably.

Hole Spacing & Bore Specifications

1/2" Fixture Plates

• Each fixturing hole is tapped 1/2-13 UNC and counterbored approx. 0.25" deep for a 1/2" pin
• 1/2" precision bores are machined to ⌀0.5005±0.0002"
• Fixturing holes are spaced 1.25" on center
• All 1/2" fixture plates have a double-offset hole pattern (two patterns, both on 1.25" centers, with one pattern offset 0.625" in X and Y  to maximize fixturing locations)

M12 Fixture Plates

• Each fixturing hole is tapped M12x1.75 and counterbored approx. 6.35mm deep for a 12mm pin
• 12mm precision bores are machined to ⌀12.013±0.005mm
• Fixturing holes are spaced 30mm on center
• All M12 fixture plates have a double-offset hole pattern (two patterns, both on 30mm centers, with one pattern offset 15mm in X and Y  to maximize fixturing locations)

1/4" Fixture Plates

• Each fixturing hole is tapped 1/4-20 UNC
• 1/4" fixture plates do not have a precision counterbore, instead they are countersunk for use with our SMW Fixturing Pins.
• Fixturing holes are spaced 0.75" on center
• Most 1/4" fixture plates have a single-offset hole pattern

M6 Fixture Plates

• Each fixturing hole is tapped M6x1.0
• M6 fixture plates do not have a precision counterbore, instead they are countersunk for use with our SMW Fixturing Pins.
• Fixturing holes are spaced 20mm on center
• Most M6 fixture plates have a single-offset hole pattern

We have found these bore tolerances to be the ideal fit when using a mix of dowel pins, gauge pins and accessories like the SMW Mod Vise.  Keep in mind, for a dowel pin to have a "slip" fit (e.g. not require hand tools, vice grips, etc. to remove), there must be some gap or "slop" in the fit between the pin and the hole.