Knowledge Base

SMW Fixture Plate Tolerances

General tolerances for SMW Fixture Plates are given below.

Parallelism

  • For dimensions over 18", 1/2" and M12 SMW Fixture Plates are held to a parallelism of 0.0005" per 18" of plate length [0.013mm per 457mm].
  • For dimensions under 18" [457mm], 1/2" and M12 SMW Fixture Plates are held to a total parallelism of 0.0005" [0.013mm]. For example, an 18" x 9" plate would have a 0.0005" parallelism spec along both the long and short edges.
  • 1/4" and M6 fixture plates are held to a parallelism of 0.001" [0.025mm] across the entire plate.

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. 

Matched Set Tolerances

  • In general for matched sets of 2+ plates, thickness at the plate seams is matched within 0.0005" [0.013mm]. Each plate is then individually subject to the standard parallelism tolerance (i.e. each plate in the set will be parallel within 0.0005" per 18" of length [0.013mm per 457mm] and have a thickness variation of no more than 0.0005" [0.013mm] at the edges where plates meet). Bare 6061 aluminum plates, such as those for the Tormach 24R, are matched within 0.001" [0.025mm] at the plate seam.
  • 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.

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 the 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. See product drawings for these tolerances.

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.

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