Fixture Materials
Aluminum Tooling Plate Fixtures: MIC-6 and ATP-5
The cast plate that starts flat, stays flat after the pockets go in, and does not need a grind to get there.
Cast aluminum tooling plate is the default fixture base when flatness carries the datum. MIC-6 is published at a maximum deviation from flat of 0.005 in on plate 3/4 in and thicker and 0.015 in on 1/4 to 5/8 in; ATP-5 guarantees 0.005 in over 0.500 in thick and 0.015 in at 0.500 in and under. Both are stress relieved, so a base stays flat after pockets and dowel holes are cut. Send the fixture print; a person quotes in 24-48 hours.
Updated September 2026
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Cast plate versus rolled plate, and why fixtures use cast
Rolled aluminum plate such as 6061-T651 is rolled and then stretched, which leaves internal stress distributed through the thickness. Machine a pocket into one face and that stress is no longer balanced, so the plate moves, sometimes days later. That is the classic story of a fixture base that was flat on the surface plate at the end of the week and bowed by the following one.
Cast tooling plate is made and treated to avoid exactly that. The ATP-5 data sheet (public.vistametals.com/products/atp-5-precision-tooling-plate.pdf) describes a 5083-based alloy that is homogenized and stress relieved, and MIC-6 is sold on the same property: a fine-grained, cast, stress-relieved plate machined flat on both faces. The practical result is that pockets, dowel holes and cutouts can go into one face without the plate arguing about it, which is the whole reason it is under fixtures.
The published flatness numbers, by thickness
MIC-6 publishes flatness by thickness band. The MIC-6 brochure (howardprecision.com/wp-content/uploads/2017/08/Mic-6-brochure-1-1.pdf) gives a maximum deviation from flat of 0.005 in, or 0.127 mm, on plate 3/4 in and thicker, and 0.015 in, or 0.381 mm, on 1/4 to 5/8 in. So the thin gauges are three times looser than the thick ones, which is worth knowing before a 3/8 in base is specified for a datum surface.
ATP-5 is specified differently and that is its selling point: the data sheet gives 0.005 in, or 0.127 mm, over 0.500 in thick and 0.015 in, or 0.381 mm, at 0.500 in and under. Clinton Aluminum’s side-by-side comparison (clintonaluminum.com/atp-5-vs-mic-6/) sets the two out together. Either way, those are as-supplied plate numbers, not a promise about the finished fixture: what the fixture is held to is what its own print says.
When the plate number is not the fixture number
A base cut from 0.005 in plate is not automatically a 0.005 in fixture. Cutting deep pockets, bolting the plate down hard, or building a tall fixture on a thin base all move it, and a datum surface that has to be flatter than the plate arrives needs to be machined or ground flat as a separate operation with that callout on the print. State the flatness and parallelism you need on the fixture print, and it is quoted from the print rather than from the mill certificate. Every part is inspected against your print before it ships.
The same goes for the reverse case. If the print says nothing, the quote assumes the plate’s own condition is acceptable and the fixture is checked as machined, which is often exactly right for a nest or an assembly plate. Precision grinding is where a base goes when the callout is tighter than a milled finish gives, and it is quoted on the same order.
Machining behaviour, and what it means for the fixture
Cast tooling plate machines easily and predictably, which is part of why fixtures are built from it. It does not take an anodize as evenly as a wrought grade, because the cast structure and the alloy behave differently in the bath, so a cosmetically uniform anodize is not what cast plate is for. Where appearance matters, that face is usually a separate wrought detail, and the finish is specified on the print and quoted with the part.
Threads in cast plate hold fine for fixture duty, but a hole that gets assembled and disassembled hundreds of times is better as a threaded insert called out on the print than as a tapped hole in aluminum. Dowel holes are reamed to fit class, and the fit class belongs on the print rather than being left as "dowel". 6061-T6 is the right choice for the risers, blocks and details that bolt to the base.
Where tooling plate belongs, and where it does not
It belongs under check and inspection fixtures, CMM holding fixtures, assembly plates and nests, and any base where a datum surface has to stay put. Those are the fixtures where flatness is doing real work, and the extra cost over 6061 plate buys something measurable. It is also the sensible base for a multi-station build plate, where a single flat reference keeps every station honest. A plate that guides a drill through hardened bushings is covered on drill and trim jigs.
It does not belong next to an arc. Weld fixtures take localised heat, spatter and hard clamping, and a steel base in 1018, A36 or 4140 is stiffer, cheaper to replace and more forgiving; weld fixtures and jigs covers that case. It is also overkill under a simple hand nest that only has to be square and light, where plain 6061 plate does the job for less.
What to send, what is inspected, and when it ships
Send the fixture print and STEP model naming the plate grade and thickness, the flatness and parallelism required on each datum face, dowel and locating hole fit classes, thread and insert callouts, and the hardware list. Any file type is accepted, STEP plus PDF is ideal, and the quote lists anything missing. If you are unsure whether MIC-6 or ATP-5 suits, say the thickness and the flatness you need and the quote will name which one meets it.
Every fixture is inspected against its print before it ships: flatness and parallelism on a surface plate with a height gage, dowel and locating holes with pin gages, hole position by CMM where the print calls it out. One fixture or a set is a normal order with no minimum, standard lead time is 4-6 weeks from approval, and rush at 1-2 weeks is quoted when a date is fixed, and extended is 6-8 weeks when timing is flexible.
Questions
Before you send a job.
01 MIC-6 or ATP-5: which should the print call out?
They are close. MIC-6 publishes 0.005 in maximum deviation from flat at 3/4 in and thicker and 0.015 in at 1/4 to 5/8 in; ATP-5 guarantees 0.005 in over 0.500 in thick and 0.015 in at 0.500 in and under, so ATP-5 holds the tighter number down to a thinner plate. State the thickness and the flatness you need and the quote will name the one that meets it.
02 Why not just use 6061 plate for a fixture base?
Because rolled and stretched plate carries internal stress, and machining a pocket into one face unbalances it, so the base can move after it is cut. Cast tooling plate is homogenized and stress relieved for exactly that reason. 6061-T6 is still the right choice for risers, blocks and details that bolt to the base.
03 Will my finished fixture be flat to 0.005 in?
Only if the fixture print says so. The published number describes the plate as supplied, not the fixture after deep pockets, hard clamping and a tall build on it. State the flatness and parallelism you need on each datum face and it is quoted from the print; where the callout is tighter than a milled finish gives, the face is ground on the same order.
04 Does cast tooling plate anodize well?
Not evenly, compared with a wrought grade, because the cast structure behaves differently in the bath. For a cosmetically uniform finish, the visible face is usually a separate wrought detail. Any finish is specified on the print and quoted with the part, and parts come back finished and inspected.
05 Should tapped holes in tooling plate be threaded inserts?
Where the hole is assembled and disassembled hundreds of times, yes, and the insert goes on the print by part number. Ordinary fixture duty holds fine in tapped aluminum. Dowel holes should carry a stated fit class on the print rather than just being called dowel holes.
06 Can you use tooling plate for a weld fixture?
It is the wrong choice next to an arc. Localised heat, spatter and hard clamping favour a steel base in 1018, A36 or 4140, which is stiffer and cheaper to replace. Tooling plate belongs under inspection, CMM, assembly and build fixtures, where flatness is doing real work.
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