What to Include on an Invar RFQ: A Drawing Checklist

By Invar Machining editorial team· Updated September 26, 2026 · How we write these guides

An Invar RFQ needs nine things the drawing usually leaves out: the alloy and its spec, the heat treatment sequence, the two or three tolerances that control function, the inspection temperature, surface finish, coating, certification, quantity and the operating environment. Two shops quoting the same Invar drawing can return very different numbers, not because one is overcharging, but because they assumed different answers to those items. A complete RFQ removes the guesswork and gets you quotes you can actually compare.

The full checklist

The table is a typical list for a precision Invar 36, Super Invar or Kovar part. Not every row applies to every part, but each one you skip is a decision the shop makes for you.

Item	Why the shop needs it	Example wording
Alloy and specification	Invar 36, Super Invar, Kovar and Alloy 42 are bought, heat treated and inspected differently	“Invar 36, UNS K93600, per ASTM F1684, annealed”
Heat treatment	Largest single cost and schedule item after material; decides the number of setups	“Stress relieve after roughing, finish after, shop to confirm cycle”
Critical tolerances and GD&T	Sets the finishing strategy and the inspection plan	“Face A flat 0.010 mm, bores ±0.010 mm true position to A, B, C”
General tolerance	Everything else, so the shop does not treat every edge as critical	“Unless otherwise noted ±0.1 mm, edges broken 0.2 mm”
Inspection temperature	Decides soak time and where the part is measured	“Dimensions apply at 20 °C, soak 4 h minimum”
Surface finish	Chooses the finishing pass and whether grinding or lapping is needed	“Ra 0.8 µm on faces marked, Ra 3.2 µm elsewhere”
Coating or passivation	Affects final dimensions and adds an outside process	“Passivate, tolerances apply after passivation”
Certification	Extra paperwork is quoted, not assumed	“Mill cert with chemistry, heat lot traceable to part”
Quantity and reorder estimate	Sets material purchase, fixture and program decisions	“10 pieces now, estimate 40 per year”
Operating environment	Changes cleaning, packaging and sometimes the process	“Vacuum service, clean and double bag”
File formats	A 3D model plus a toleranced drawing removes ambiguity	“STEP AP214 plus PDF drawing rev B”

Three notes to write on the drawing

The material note, the heat treatment note and the inspection note carry most of the information a shop needs and are the three most often missing. Sample wording for each follows.

Alloy and specification

State the specific alloy and the governing specification if you know it: ASTM F1684 for Invar 36 bar and plate, ASTM F15 for Kovar and ASTM F30 for Alloy 42. Super Invar has no single universally cited callout, so state the nominal composition, about 32 % nickel and 5 % cobalt, and the CTE band you need. If you are choosing between alloys and are not sure which fits, give the operating temperature range and the precision requirement and let the shop weigh in. The tradeoffs are covered in Invar 36 vs Kovar vs Super Invar, and the common designations are listed side by side in the Invar specs reference.

A typical material note as written on a drawing:

MATERIAL: INVAR 36, UNS K93600, PER ASTM F1684, ANNEALED CONDITION.
MILL CERTIFICATE WITH CHEMISTRY REQUIRED. HEAT LOT TRACEABLE TO PART.
SUBSTITUTION OF ALLOY OR CONDITION REQUIRES WRITTEN APPROVAL.

The last line matters. Without it a shop short on Invar 36 plate may offer a free-machining grade or a different heat lot, and the substitution may be reasonable, but you want to hear about it before the material is cut.

Heat treatment requirement

State explicitly whether the part needs stress relief between roughing and finishing, a full stabilization cycle or neither. This is the single detail most likely to be missing from a first RFQ and most likely to force a re-quote once it comes up, since it is one of the largest cost and lead time factors on a precision Invar part, covered in stress relief for Invar machined parts. If you are not sure, say so and ask the shop to recommend based on your tolerance.

A typical heat treatment note as written on a drawing:

HEAT TREATMENT: STRESS RELIEVE AFTER ROUGH MACHINING WITH 0.5 MM
STOCK REMAINING ON FEATURES MARKED (C). TYPICAL CYCLE 315 TO 345 °C,
1 TO 2 H, SLOW COOL. SHOP MAY PROPOSE AN ALTERNATIVE CYCLE FOR
APPROVAL. FINISH MACHINE AFTER STRESS RELIEF. RECORD CYCLE ON CERT.

The note fixes the sequence, rough then furnace then finish, so no shop quotes a single setup, and it leaves the exact cycle open for the shop to confirm. On a part that needs the fuller AMS 2454 style sequence, anneal, quench, stress relieve and a long low-temperature stabilize, write “full stabilization per drawing note” and attach the cycle, or reference the program spec that defines it. The temperatures above are typical; the drawing or the governing spec decides the final numbers.

Critical tolerances, GD&T and the 20 °C note

Call out the two or three features that actually control fit, alignment or function with real tolerance values and, where relevant, GD&T for flatness or position per ASME Y14.5 or ISO GPS. Leave the rest at a general tolerance. This gives the shop a realistic picture of where the difficulty is, rather than forcing every feature to be treated as critical, a point covered in Invar machining tolerances. Typical achievable values on a stable Invar 36 part are ±0.01 to ±0.025 mm, with ±0.005 mm possible on small features with extra passes and 20 °C inspection.

The 20 °C note is the part most drawings miss. ISO 1 sets 20 °C as the reference temperature for all geometrical product specifications, and on an Invar part the temperature matters less for the part than for the measuring machine around it. A typical inspection note:

INSPECTION: ALL DIMENSIONS APPLY AT 20 °C PER ISO 1. SOAK PART AND
FIXTURE MINIMUM 4 H AT 20 ±1 °C BEFORE MEASURING FEATURES MARKED (C).
REPORT MEASUREMENT TEMPERATURE ON THE INSPECTION RECORD.

Finish, coating, certification and environment

Surface finish and coating

Note any specific surface finish requirement on critical faces and whether the part needs a coating for corrosion protection, appearance or a functional purpose like emissivity control. If a coated dimension is critical, state whether the tolerance applies before or after coating, covered in surface finish and coating options for machined Invar.

Certification and documentation

Specify whether you need a standard mill cert, which most shops include by default, or more: measured CTE data for your heat lot to ASTM E228 or E831, full traceability for an aerospace or space program or a specific inspection report format. Asking for documentation after a quote is issued usually means a revised price and schedule, covered in Invar material certification.

Quantity and expected reorders

State the quantity for this order and, if you expect to reorder, a realistic estimate of future volume. This lets the shop plan material purchasing and decide whether holding fixtures and programs for a repeat order makes sense.

Operating environment

If your part will see cryogenic temperature, vacuum, humidity, repeated thermal cycling or another specific service condition, say so. These details, covered in Invar for cryogenic and vacuum applications, can change cleaning, packaging and sometimes the process itself.

Common omissions and what they cost

The table lists the items most often missing from a first Invar RFQ, what a shop typically assumes in their absence and the typical consequence. Costs are illustrative; the shop’s quote governs.

Omission	What the shop assumes	Typical cost of finding out later
No heat treatment note	No furnace step, single setup	Re-quote, plus one to two weeks for a furnace run and a second setup
No general tolerance	Its own default, often ±0.1 mm or looser	A dispute at inspection over features you thought were tight
No inspection temperature	Measurement at the machine	A report you cannot compare to your own incoming inspection
No spec, only “Invar”	Whatever low-expansion stock is on hand	A substitute alloy or condition, discovered after cutting
No reorder estimate	One-off job, no fixtures	Higher unit price on the second order than it needed to be
No cert requirement	Standard mill cert only	Revised price when CTE data or traceability is requested
No coating note	Bare part	Dimensions out of tolerance after a coating you added later
2D drawing only	Setups and datums inferred	Assumptions that differ between shops, so quotes do not line up

Worked example: one hypothetical lens cell, quoted three ways

Take a hypothetical Invar 36 lens cell, 80 mm outside diameter, 40 mm long, turned from bar, with a bore at ±0.010 mm and a mounting face flat to 0.005 mm. Three shops receive it.

Shop A gets a STEP file and the word “Invar”. It assumes a general tolerance, no furnace and inspection at the lathe, and quotes the lowest number with the shortest lead time.

Shop B gets a STEP file and a PDF with the two tight features toleranced but no heat treatment or inspection note. It adds stress relief on its own judgment and a second setup, and quotes a higher number without saying which line covers the furnace.

Shop C gets the STEP file, the PDF and the three notes above: material, heat treatment and inspection. Its quote lists the furnace run, the 4 h soak and a CMM report per part as separate lines.

Only Shop C’s number tells you what you are paying for. Shop A’s cell will hold its bore at the lathe and may not a month later. Shop B’s cell is probably fine, but you cannot see its assumptions. Illustratively, if the three quotes come back at 100, 135 and 140 on a relative scale, the 40 % gap between A and C is not a markup. It is the furnace, the second setup and the inspection time that Shop A did not include. The same effect on a milled part is worked through in why Invar 36 parts are costly.

Files and sending your RFQ

A STEP file (AP203 or AP214) for the model and a PDF for the toleranced drawing is the combination that gets the most accurate read. IGES works for the model but loses assembly and color information, and native CAD files should only go to a shop that has confirmed the same software version. DWG and DXF are fine for flat parts and sheet, but a 2D file alone leaves setups, datums and hidden features to interpretation. Name files with the part number and revision. If the drawing changes after the quote, send a new revision rather than a marked-up copy, as covered in engineering change requests on Invar parts.

Filling in these details up front means the quotes you receive are quoting the same part, not different assumptions about it. Attach the STEP file and the PDF drawing, paste the material, heat treatment and inspection notes if they are not on the drawing yet, and give the quantity now and the reorder estimate.

Send your drawing through the RFQ form with this checklist covered, and we match you with up to three shops that quote based on what your part actually needs. If you are still qualifying shops, how to find a machine shop with real Invar experience lists the questions to ask before the drawing goes out.

For the whole path after the RFQ, from shortlist to acceptance, read our guide to buying custom Invar parts.

Frequently asked questions

Reviewed against

• ASTM F1684, Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications
• Wikipedia, Geometric dimensioning and tolerancing

Values in this guide are typical handbook figures for planning. The mill certificate for your heat lot and the shop's quote for your geometry govern.

Keep reading

• Stress Relief for Invar Machined Parts: When and How Shops Do It · Process
• Invar Machining Tolerances: What a Shop Can Hold and What It Costs · Process
• How to Find a Machine Shop That Has Really Machined Invar · Choosing a Supplier
• Invar Mill Certifications: What a Certified Material Report Should Show · Choosing a Supplier

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