Fixtures
Custom CMM Holding Fixtures, Machined to Print
Hold the part one way, every time, with the probe paths open and the setup repeatable across shifts.
A CMM holding fixture presents a part in one repeatable orientation so a measuring program runs without re-probing or re-aligning. The job it does is mechanical: seat the part on its datums, hold it without distorting it, and keep the probe paths clear. Machined fixtures are dowelled to the granite or to a grid plate so the same setup returns after teardown. Send the part model and print plus the plate pattern; a person quotes in 24-48 hours, standard lead time 4-6 weeks.
Updated September 2026
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Why a CMM setup gets its own fixture
A coordinate measuring machine (en.wikipedia.org/wiki/Coordinate-measuring_machine) measures against a coordinate frame it establishes from the part, so anything that changes where the part sits changes the run. Clay, modelling wax and a stack of parallels get a part measured once. They do not get the same part measured the same way next month, and they do not survive a program written against them. A fixture is what turns a one-time measurement into a repeatable inspection step.
ZEISS, writing on CMM fixtures (zeiss.com/metrology/us/explore/topics/cmm-fixtures.html), frames the fixture’s job as stabilising and orienting the part for unobstructed probe access and repeatable positioning, and Renishaw’s metrology fixturing range (renishaw.com, “Metrology fixtures for CMM and gauging”, page 48219) is built on the same idea with modular columns and clamps. Modular kits cover a lot of parts. A machined fixture is what you reach for when the geometry, the cycle count or the probe access will not fit the kit.
When modular stops and a machined fixture starts
Modular fixturing runs out on a handful of recognisable cases: a contoured or castinglike surface with no flat to rest on, a thin part that a standard clamp deflects, a deep bore or an undercut where a column would sit in the probe path, a compound angle that needs the part tipped and held there, or a part that goes on the machine hundreds of times a year and needs a setup nobody has to rebuild. In each of those, a machined nest that matches the part surface does what a stack of standard parts cannot.
The other trigger is throughput. Measuring four or eight parts per run needs a plate that holds all of them in known positions so one program indexes across the set. That is a machined plate with repeated nests and a dowel scheme, not a rebuilt modular stack. The fixture plate and workholding page covers grid and sub-plates; this page covers the part-specific fixture that bolts to one.
Holding without distorting: the rule that drives the design
A measuring fixture has a constraint a machining fixture does not: clamping force that pushes a part into spec is a measurement error, not a good hold. So a CMM fixture holds on the datum features, constrains six degrees of freedom and no more, and applies the lightest clamp that keeps the part from moving. Three rest points define the primary plane, a round pin the secondary, a diamond pin the tertiary, and the clamps push toward the rests rather than across them.
Thin and flexible parts get supported rather than squeezed: adjustable rests under an unsupported span, a nest that carries the part on area instead of points, or a low-force toggle clamp with a soft pad. Toggle clamps and fixture pins are ordinary catalog hardware (mcmaster.com/products/toggle-clamps/, mcmaster.com/products/fixture-pins/) and go on the print by part number so the right hole is made for them.
Probe access, and the part of the print worth marking
The fixture has to stay out of the way of every feature the program touches, including the ones probed from underneath. Mark on the print which features are measured, from which side, and with what stylus length, and the fixture can be cut away, stepped or opened up where a probe needs to reach. Relief under a measured face, a clearance window through the base, and rests moved off a measured edge cost nothing at design time and are expensive to add after the first run.
Repeatability comes from dowels. The fixture is located to the plate or the granite by two dowel holes and clamped, so it comes off for another job and returns to the same coordinates. Give the plate or table hole pattern with the order, including hole size, spacing and fit class, and the mounting scheme is quoted with the fixture rather than fitted on arrival.
Materials and what they are held to
Bases are cast aluminum tooling plate where flatness carries the datum: the MIC-6 brochure (howardprecision.com/wp-content/uploads/2017/08/Mic-6-brochure-1-1.pdf) gives 0.005 in maximum deviation from flat on plate 3/4 in and thicker, and ATP-5 (public.vistametals.com/products/atp-5-precision-tooling-plate.pdf) guarantees 0.005 in over 0.500 in thick, stress relieved so it stays flat after pockets go in. Rests and locating pins that see repeated loading are hardened and ground A2 or D2; Delrin and UHMW go where a finished face must not be marked.
Tolerances follow the measurement, not the fixture. Locating pins and dowel holes are called out to fit class, rest heights and pin positions to the band the datum scheme needs, and flatness and parallelism on the base to what the print states. Put those callouts on the fixture print and they are quoted from it; leave them off and the quote will ask rather than guess. Every part is inspected against your print before it ships. CNC machining tolerances covers the general bands.
Who orders CMM fixtures: plants, labs and university groups
Two very different buyers order the same thing. A plant metrology group wants a fixture per part number so a program runs unattended and the lab is not the bottleneck, often several at once when a family of parts launches. A research lab or university group wants exactly one: a sample holder, an instrument mount, a rig fixture that exists so one measurement can be repeated between runs. Quantity of one is a normal order here with no minimum, and it is quoted by a person reading your files rather than by a size-and-material calculator, which matters when the geometry is unusual or the drawing is a sketch.
Lab work also brings its own material constraints, driven by the environment rather than by load: vacuum work rules out anything that outgasses (en.wikipedia.org/wiki/Outgassing) and favours stainless and aluminum with no plating, magnetic work rules out ferrous grades, and cryogenic or high-temperature work rules out most plastics. Name the grade on the drawing rather than describing the environment, or say the environment on the form and the quote will name what it is pricing. University purchasing is ordinary here: vendor setup takes a day, with W-9, NDA and net terms available, and a quote can be held valid as long as your purchasing system needs. Scientific parts machining covers lab parts beyond fixtures.
What arrives, and how it is checked first
Every fixture is inspected against its print before it ships: base flatness and parallelism on a surface plate with a height gage, dowel and locating holes with pin gages, rest heights and pin positions by CMM where the print calls them out. Hardware is installed and the fixture is assembled where the print shows an assembly, so the box holds a fixture, not a kit of details.
One fixture or a set for a family of part numbers is a normal order, and there is no minimum. Standard lead time is 4-6 weeks from approval, the quote comes back in 24-48 hours, and rush at 1-2 weeks is quoted when a program date is fixed, and extended is 6-8 weeks when timing is flexible. Labs and metrology groups ordering a single fixture will find the same terms on one-off CNC machining.
Questions
Before you send a job.
01 When is a machined CMM fixture worth it over modular columns?
When the part has no flat to rest on, when a standard clamp deflects it, when a column would sit in a probe path, when the part needs holding at a compound angle, or when the setup runs often enough that rebuilding it by hand costs more than the fixture. Below that, a modular kit from ZEISS or Renishaw is usually the right answer and this page is not.
02 How do you keep the clamping from changing the measurement?
By holding on the datum features, constraining six degrees of freedom and no more, and pushing the clamps toward the rests rather than across them. Thin spans get supported rather than squeezed, with adjustable rests or an area nest and a low-force clamp with a soft pad. Clamp positions and force expectations go on the print so they are quoted, not improvised.
03 Can the fixture hold several parts for one program?
Yes, and that is a common reason to have one made. A machined plate with repeated nests and a dowel scheme holds four or eight parts in known positions so a single program indexes across the set. Give the part count, the spacing you want and the plate hole pattern with the order.
04 How does the fixture locate back to the machine?
Through two dowel holes and bolts into your plate, sub-plate or granite pattern, so it comes off between jobs and returns to the same coordinates. Send the hole pattern with hole size, spacing and fit class and the mounting scheme is quoted with the fixture instead of being fitted on arrival.
05 What should the print mark that is easy to forget?
Which features are measured and from which side, the stylus length the program uses, the datum scheme, fit classes on every dowel and locating hole, base flatness and parallelism, and the plate pattern the fixture bolts to. Probe access is the one that costs the most to add after the first run.
06 Do you quote a single CMM fixture?
Yes. One fixture is a normal order and there is no minimum; a lab or university group ordering one is quoted the same way as a plant ordering a set. Send the part model and print, the fixture print if one exists, and the plate pattern through the quote form, and a person replies in 24-48 hours.
07 I have a sketch, not a fixture print. Is that enough?
Enough to start. A dimensioned hand sketch, a CAD screenshot with the critical dimensions marked, or photos of what you are replacing with a ruler in frame all work, and the quote lists what is still missing before a build-to-print number can be given. No design service is offered here, so the quote will say which numbers it needs rather than choosing them.
08 Can you make a fixture for vacuum or cryogenic use?
Yes, when the drawing names the grade. Environment drives the choice: vacuum work rules out outgassing materials and plating, magnetic work rules out ferrous grades, and cryogenic or high-temperature work rules out most plastics. If you know the environment but not the grade, say so and the quote will name what it is pricing rather than choosing quietly.
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