How to: Specify Tolerances in Sheet Metal RFQ

Start by separating critical dimensions from general ones

Do not apply the same tolerance to every edge, hole, and bend. A bracket that locates a motor shaft needs tighter control than a decorative panel or a non-structural cover. Before you write the RFQ, walk through the part function and identify which dimensions actually matter.

Ask three questions for each feature:

1. Does this dimension determine whether the part fits the mating component?
2. Does this dimension affect how the part performs, such as alignment, load path, or sealing?
3. Does this dimension control assembly sequence, such as bolt hole spacing or bend clearance?

If the answer is yes to any of those, the dimension belongs in your critical tolerance list. Everything else falls under general tolerancing. This separation keeps your drawing clear and stops the fabricator from over-inspecting low-value features.

Consider a chassis rail for a light vehicle. The distance between the two main mounting holes must align with the frame crossmember. If that distance is off by 1 mm, the welder may have to stretch the metal or drill new holes. That is a critical dimension. Now look at the decorative vent opening on the side panel. If that opening is 2 mm off from the nominal size, the part still looks correct and functions perfectly. That is a general dimension.

The distinction matters for cost. Tight tolerances require slower machine speeds, more frequent tool checks, and sometimes additional inspection steps. If you mark every hole with a tight tolerance, the shop may charge for a CMM inspection of every part, even though only three holes actually matter. By isolating the critical features, you signal where the shop should spend its time and money. You also avoid the common trap of over-specifying, which can make a part unmanufacturable or prohibitively expensive.

Choose tolerance values based on material and thickness

Sheet metal behaves differently from machined solid stock. Bending, punching, and forming introduce variation. A 1.5 mm bend in 2 mm stainless steel will not hold the same nominal angle as the same bend in 10 mm mild steel, and hole edge quality changes with material hardness and tool condition.

General practice in sheet metal fabrication:

• Hole diameters: plus-minus 0.2 mm for standard punched holes, tighter for drilled and reamed holes.
• Bend angles: plus-minus 1 degree for typical formed angles, tighter for precision fixtures.
• Flat-to-flat lengths: plus-minus 0.5 mm for general parts, plus-minus 0.2 mm for precision brackets.
• Thickness: usually specified as a range, for example 1.8 to 2.2 mm for 2 mm nominal plate.

These are starting points, not rules. If your part is a press-fit bearing seat, a seal groove, or a mating surface for a gasket, you will need tighter values or a different approach, such as machining after forming. State the material grade and thickness range explicitly, because tolerance capability depends on both.

Material hardness plays a direct role in how cleanly a hole is punched. Aluminum 6061 is softer than carbon steel. When a punch hits aluminum, the material flows more easily around the punch, which can create a cleaner edge with less burr. When it hits hardened spring steel, the punch may shear the material, leaving a rougher edge and a more pronounced burr. If you need a hole that will accept a pin without play, you might specify a reamed hole in aluminum, but in spring steel, you might need to specify a drilled and tapped hole instead, because reaming hardened steel is difficult and expensive.

Thickness variation also affects flat-to-flat lengths. A sheet that is nominally 2 mm thick might be 1.9 mm at one end and 2.1 mm at the other due to mill variation. When you bend this sheet, the bend line shifts slightly because the neutral axis moves closer to the thicker side. If you specify a flat-to-flat length of 100 mm plus-minus 0.5 mm, the shop needs to know if that 0.5 mm includes the thickness variation effect or if it is purely a cutting tolerance. It is safer to specify the bend deduction and the flat length separately, or to accept the combined result.

State the tolerance class or general tolerance standard

If you do not list every dimension individually, reference a general tolerance class. Many sheet metal shops follow internal standards or industry common practice, but you should not assume the fabricator will match your expectations.

In the RFQ, write something like: “General tolerances per ISO 2768-m, unless otherwise noted on the drawing.” If the shop is unfamiliar with that standard, ask them to confirm which standard they use. The goal is a single agreed reference so that unmarked features have a defined acceptance range.

When mixing standards, say so. For example, “Hole diameters per drawing, bend angles per ISO 2768-m, all other linear dimensions per ISO 2768-m.” This avoids ambiguity when two different standards appear on the same document.

ISO 2768 defines general geometric tolerances for parts. The “m” version is medium. It is often the default for general engineering parts. However, some shops use ANSI Y14.5 or their own internal tables. If you are working with a shop in a different region, ask them to confirm their interpretation. For example, ISO 2768-m allows a total tolerance of 0.14 mm for a 40 mm to 120 mm dimension. If your shop uses a different standard, that same dimension might have a tolerance of 0.2 mm. That 0.06 mm difference can be the difference between a part that fits and a part that scrapes.

Do not rely on the drawing title block alone. If the drawing says “General Tolerances: ISO 2768-m,” but the RFQ says “Per Shop Standard,” the RFQ usually wins in a contractual sense, or it creates a dispute. Be explicit. Write the standard name and version in the RFQ notes. If the shop cannot meet the standard, ask them to propose an alternative before you release the order. This prevents the situation where the part is built, inspected, rejected, and then rebuilt.

Define how each tolerance will be measured

A tolerance without an inspection method is a negotiation waiting to happen. The same dimension can be checked with a ruler, a digital caliper, a CMM, a gauge, or a visual inspection, and each method gives a different level of confidence.

In your RFQ, assign an inspection method to each critical dimension. For example:

• “Hole location: CMM or master gauge, plus-minus 0.1 mm.”
• “Bend angle: go/no-go angle gauge, plus-minus 1 degree.”
• “Face flatness: three-point contact or granite plate, max 0.2 mm.”
• “Edge burrs: visual, no sharp edges detectable by fingertip.”

This protects both parties. The fabricator knows what to prepare, and you know what you are accepting. If you only need a functional check, say “must assemble without force” or “must seat on mating surface without gaps visible to the eye.” Precision language saves time.

Consider a part with two holes that are 50 mm apart. If you specify “plus-minus 0.1 mm,” the shop might measure the hole centers using a digital caliper. The operator places the caliper jaws on the hole edges and subtracts the hole diameter. This method is quick but introduces error. If the holes are not perfectly round, or if the caliper jaws are not square, the measurement is off. If you specify “CMM or master gauge,” the shop will use a machine that measures the coordinates of the hole centers directly, or a physical gauge that only fits if the holes are within tolerance. These methods are more reliable but take longer.

For bend angles, a go/no-go gauge is often the best method for production. The gauge has two arms set at the nominal angle and a tolerance angle. If the part fits between the arms, it is good. If it does not fit, it is bad. This is fast and consistent. If you specify “plus-minus 1 degree” but do not say how it is measured, the shop might use a protractor. A protractor is less accurate and slower than a go/no-go gauge. By specifying the method, you control the quality and the cost.

Include notes for special features

Some sheet metal parts have features that do not fit neatly into standard tolerance tables. Examples include:

• Spring fingers or snap-fit tabs: specify deflection or gap rather than just length.
• Sealing surfaces: specify flatness and surface finish, not just thickness.
• Painted or coated edges: specify edge burr limit, because coating thickness affects fit.
• Asymmetric bends: specify the bend angle from the neutral axis or state the tolerance on the included angle.

Add a notes block on the drawing or in the RFQ. For example: “Top flange must not interfere with housing lip. Max gap 0.3 mm at full assembly.” This kind of functional note often matters more than a decimal dimension.

Spring fingers are a common example. You might have a 3 mm wide, 50 mm long tab that needs to snap into a slot. If you specify the tab length as 50 mm plus-minus 0.5 mm, that is not enough. The critical dimension is the gap between the tab tip and the slot edge when the part is in the neutral position. If the gap is too small, the part will not snap in. If the gap is too large, the part will not hold. You need to specify the gap tolerance, or the deflection of the tab under a specific load. For example: “Snap-fit gap: 0.5 mm to 0.8 mm at neutral position.”

Coated edges also require attention. If you specify a hole that will accept a pin, and the hole is painted, the paint adds thickness. If the paint is 0.2 mm thick, and you specify the hole diameter as 10.0 mm plus-minus 0.1 mm, the effective diameter after painting is 10.2 mm plus-minus 0.1 mm. If the pin is 10.1 mm, it will not fit. You need to specify the hole diameter before coating, or the clearance after coating. It is clearer to say: “Hole diameter: 10.0 mm plus-minus 0.1 mm before painting. Ensure 0.2 mm clearance after coating.” Or, if the coating is not uniform, you might need to specify the minimum clearance.

Review the drawing with the fabricator before issuing the RFQ

The most expensive tolerance mistake is one made after the shop has already cut, bent, and finished the part. Before sending the RFQ, have a quick call or video review with the fabrication team.

Ask them to flag:

• Dimensions that are too tight for the process, such as a 0.05 mm hole location on punched 1.5 mm plate.
• Features that change shape after forming, such as a bend that shifts the flat length.
• Inspection points that are inaccessible after assembly or finishing.
• Any tolerance that conflicts with the material or thickness you specified.

This review does not require a formal engineering meeting. A thirty-minute walkthrough of the drawing often prevents a costly change order later. The fabricator will tell you what is realistic, and you will know where to spend tolerance budget.

During this review, show the shop the part in context. If you have a 3D model, show it. If you have a prototype, show it. Ask the shop to walk you through their process. How will they cut the part? Laser, plasma, or waterjet? How will they bend it? Press brake, roll, or hammer? Each process has different capabilities. A laser cutter can hold a hole position to 0.05 mm, but a plasma cutter might hold it to 0.5 mm. If you specify 0.05 mm hole location on a plasma-cut part, the shop will either reject the drawing or charge a premium for manual rework.

Also ask about the bending process. Press brakes have a minimum bend radius and a maximum bend angle. If you specify a bend angle of 89 degrees, and the press brake is not capable of holding that angle without springback correction, the shop will need to adjust the tooling or the bend sequence. If you do not discuss this, the part may come back with a 1 degree error, and you will have to reject it. The review is your chance to catch these issues before money is spent.

Final verification before releasing the RFQ

Before the RFQ goes out, run through this checklist:

1. Every critical dimension has a tolerance value.
2. Every critical dimension has an inspection method.
3. General tolerance standard is named or every unmarked dimension is specified.
4. Material grade and thickness range are stated.
5. Special features have functional notes, not just dimensions.
6. The shop has confirmed the tolerances are achievable and costed.

If any item fails, fix it before release. A clear RFQ is not just a document. It is the first layer of quality control. The more precise your tolerance specification, the less room there is for misinterpretation, and the higher the chance your part arrives ready for assembly without a second visit to the shop.

Common mistakes to avoid:

• Putting a tight tolerance on every hole because it looks “safe.”
• Forgetting that coating adds thickness, which changes fit.
• Specifying a tolerance without saying how it is measured.
• Using a tolerance class that the shop does not follow.
• Leaving “as per drawing” without defining what “as per drawing” means for unmarked features.

Each of these creates room for interpretation. In manufacturing, interpretation is where rework starts.

When you run through the checklist, pay attention to the inspection method. If you say “plus-minus 0.1 mm” for a hole location, but you do not say how it is measured, the shop might use a different method than you expect. If you say “CMM,” the shop will budget for CMM time. If you say “visual,” the shop will not. The inspection method is part of the tolerance specification. It defines what “plus-minus” means in practice.

Also, check the material grade. “Mild steel” is not a grade. “A36” or “S235” is a grade. The grade affects the bend deduction and the surface finish. If you specify “mild steel,” the shop might use a cheaper grade that has more variation. If you specify “S235,” the shop knows what to expect. The material grade is part of the tolerance capability. Do not leave it vague.

Finally, confirm the cost. If the shop says the tolerances are achievable, ask how much more it costs. A tighter tolerance usually costs more. If the cost increase is too high, you may need to relax the tolerance or change the process. For example, if you need a tighter hole location, you might change from punching to drilling and reaming. That costs more but gives better accuracy. If you do not discuss the cost, you might receive a part that meets the tolerance but has a price that blows your budget. The review and the final verification are your chances to balance quality and cost before the order is locked in.