Fixtures
Custom Assembly Fixtures and Machined Nests
Present the part at one height and one orientation so a screw, a press-fit pin or a label lands in the same place every cycle.
An assembly fixture holds a part at a set height and orientation so the next operation happens the same way every time, and a nest is the machined pocket that matches the part shape so an operator can only seat it one way. Contact faces are Delrin or UHMW where the part is finished, aluminum or steel where it is not. Send the part model and print with the assembly operation described; a person quotes in 24-48 hours, standard lead time 4-6 weeks.
Updated September 2026
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What a nest is, and why it beats a mark on the bench
A nest is a machined pocket cut to the part outline so the part drops in one way and sits there. That does two jobs at once. It sets position, so the drill, the press, the screwdriver or the label applicator meets the same point on every part, and it makes the wrong orientation physically impossible, which is the cheapest error-proofing there is. A part that can only go in one way does not get assembled backwards on a Friday afternoon.
Wikipedia’s Fixture (tool) entry (en.wikipedia.org/wiki/Fixture_(tool)) describes the same three elements a nest uses: a base, location elements and, where needed, clamps. Most assembly nests need very little clamping. Gravity and a close pocket do the work, and a clamp only appears where the operation pushes the part sideways or lifts it, in which case toggle clamps are ordinary catalog hardware (mcmaster.com/products/toggle-clamps/) called out on the print by part number.
Clearance: the number that decides whether it works
A nest cut exactly to nominal part size does not accept the part. A nest cut generously lets it rattle and loses the position you built it for. The useful clearance depends on how the part itself is made: a machined part at plus or minus 0.005 in will seat in a much tighter pocket than a molded or cast part carrying draft and shrink variation, and a formed sheet metal part varies more again across the bend. Molded parts from a press are the common case, and fixtures for molders and stampers covers the nests and check fixtures a launch needs.
So the print should state the part tolerance the nest has to swallow, and the fixture print should state the pocket clearance rather than leave it to be inferred. Where the part varies a lot, the pocket takes a lead-in chamfer and contacts at three or four defined pads rather than around its whole outline, which locates the part without fighting its variation. Where the part varies a little and position matters, the pocket is close and the lead-in is small.
Non-marring: what the part touches, and what that costs
Assembly usually happens after finishing, so the part in the nest is anodized, powder coated, painted or polished and any scratch it picks up is a reject. That pushes the contact faces to plastic: acetal, which is dimensionally stable and machines cleanly (en.wikipedia.org/wiki/Polyoxymethylene), or UHMW, which is softer and more slippery (en.wikipedia.org/wiki/Ultra-high-molecular-weight_polyethylene).
The common build is a metal base carrying plastic inserts or a plastic top plate, so the fixture keeps its stiffness and its bolt pattern while only the wear faces are soft and replaceable. Note on the print which faces touch a cosmetic surface, and those faces come back in the softer material with the rest in aluminum or steel. Delrin and UHMW fixture details covers the trade in full.
Set height, access and the operation around the part
A nest that holds the part perfectly and blocks the tool is a failed fixture. Whatever happens at the fixture, a press stroke, a screw run, a pin insert, a label, a glue bead, needs a path to the feature and clearance for the tool body and the operator’s hands. Marking the operation on the print, including the approach direction and the tool envelope, is what lets the pocket be cut away, stepped or opened where it needs to be.
Height and angle are part of the same question. Presenting a part tipped 15 degrees can be the difference between an operator working comfortably and one leaning over the bench all shift, and a fixture built at a set height means the press or the driver is set once. Where several nests sit on one base plate for a build sequence, give the order of operations so the layout follows the work rather than crossing it.
Sets, duplicates and pallet-mounted nests
Assembly work rarely needs exactly one fixture. A build cell needs a nest per station, a second line needs a duplicate set, and a conveyor or indexing table needs a run of identical pallet nests that all have to locate the same way to the pallet. Those are quoted together as one order, which is also the only way to be sure they are made from the same setup and interchange.
Pallet and table mounting is a print question: give the pallet or plate pattern with hole size, spacing and fit class, and the mounting scheme is quoted with the nest instead of fitted on arrival. Automation cells that also need gripper and wrist plates will find those on end-of-arm tooling plates.
What to send, what is inspected, and when it ships
Send the part model and a PDF print with the part tolerances, a description or sketch of the assembly operation and its approach direction, the pocket clearance if you have a number in mind, which faces touch cosmetic surfaces, the set height and angle you want, and the pallet or bench pattern if it mounts to one. Any file type is accepted and STEP plus PDF is ideal; the quote lists anything still missing.
Every fixture is inspected against its print before it ships: base flatness and parallelism on a surface plate with a height gage, pocket dimensions and pad positions with gage pins and a height gage, hole position by CMM where the print calls it out. One nest or a set is a normal order with 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 build date is fixed, and extended is 6-8 weeks when timing is flexible.
Questions
Before you send a job.
01 How much clearance should a nest pocket have?
It depends on how much the part itself varies. A machined part at plus or minus 0.005 in seats in a much closer pocket than a molded part carrying draft and shrink, or a formed sheet part that moves across the bend. State the part tolerance on the print and the pocket clearance you want on the fixture print, and both are quoted to; leave them off and the quote will ask.
02 Will the fixture mark an anodized or painted part?
Not where the print says it must not. Contact faces on finished parts are machined from Delrin or UHMW, usually as inserts or a top plate on a metal base, so the fixture keeps its stiffness and only the wear faces are soft. Mark on the print which faces touch a cosmetic surface.
03 Can one fixture hold several parts for a build sequence?
Yes. A build plate with several nests laid out in the order of operations is a common ask, and it is quoted as one fixture. Send the sequence with the order so the layout follows the work rather than crossing it, and say which stations need clamping and which only need the pocket.
04 Do you make pallet nests for an indexing table or conveyor?
Yes, and a run of them is quoted together so they are made from the same setup and interchange. Send the pallet or plate pattern with hole size, spacing and fit class, the part model and the pocket requirement, and the mounting scheme is quoted with the nests.
05 What is the difference between a nest and a check fixture?
A nest holds a part so work can be done to it; a check fixture holds a part so it can be judged pass or fail against its datums. They often look similar and are built the same way, but the tolerances differ sharply: a check fixture locator is held to about a tenth of the tolerance it checks, a nest only has to locate well enough for the operation.
06 How long does an assembly fixture take?
Standard lead time is 4-6 weeks from approval and the quote comes back in 24-48 hours. Rush at 1-2 weeks is quoted when a build date is fixed. A single aluminum nest with a plastic top sits toward the front of the standard window; a multi-station build plate with clamping and hardware sits toward the back. Extended is 6-8 weeks when timing is flexible.
Related
Where to go next.
Custom machined fixtures
The hub: every fixture family, quoted from your print.
View →Delrin and UHMW fixture details
Non-marring contact faces, and where each one belongs.
View →End-of-arm tooling plates
Gripper and vacuum plates drilled to the robot flange.
View →Get a quote
Any file type; STEP + PDF ideal. A person replies in 24-48 hours.
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