ISO 2768-mK vs ISO 2768-fH: Which Tolerance Standard Actually Fits Your CNC Parts?
Every design engineer has seen it: a drawing with a blanket tolerance block reading “ISO 2768-mK” or “ISO 2768-fH,” applied with little more thought than a default setting in a CAD template. That single choice—often made in seconds—can add 20 to 40 percent to your machining cost with zero functional benefit. In twenty-five years of running CNC production floors in Guangdong, I have watched engineers specify fH on parts that needed nothing tighter than mK, and I have seen field failures traced back to mK tolerances on critical mating surfaces. The difference between these two standards is not academic. It is dollars, lead time, and reliability.
This article breaks down exactly when to use ISO 2768-mK for general machining versus ISO 2768-fH for precision work, with real dimension examples, cost data, and practical rules you can apply to your next drawing.
What ISO 2768-mK and ISO 2768-fH Actually Mean
ISO 2768 is the international standard for general tolerances on linear and angular dimensions, as well as geometrical tolerances, unless individual tolerances are specified on the drawing. The standard has two parts: ISO 2768-1 covers linear and angular dimensions, and ISO 2768-2 covers geometrical tolerances for features without individual tolerance indications.
The letter codes tell you the tolerance class. The first letter (m or f) refers to the tolerance class for linear dimensions per ISO 2768-1. The second letter (K or H) refers to the tolerance class for geometrical tolerances per ISO 2768-2.
Here is the critical data:
ISO 2768-1 Linear Dimension Tolerances (selected range): - For nominal sizes 6 mm to 30 mm: - Class m (medium): ±0.1 mm - Class f (fine): ±0.05 mm - For nominal sizes 30 mm to 120 mm: - Class m: ±0.15 mm - Class f: ±0.075 mm - For nominal sizes 120 mm to 400 mm: - Class m: ±0.2 mm - Class f: ±0.1 mm
ISO 2768-2 Geometrical Tolerances (selected range): - For nominal sizes 10 mm to 30 mm: - Class K: 0.2 mm straightness/flatness, 0.1 mm perpendicularity - Class H: 0.1 mm straightness/flatness, 0.05 mm perpendicularity - For nominal sizes 30 mm to 100 mm: - Class K: 0.3 mm straightness/flatness, 0.2 mm perpendicularity - Class H: 0.15 mm straightness/flatness, 0.1 mm perpendicularity
The difference is not subtle. Moving from mK to fH cuts your allowable linear deviation in half and your geometrical error by roughly 50 to 60 percent. That means the CNC machine must run slower, take lighter cuts, and often require secondary operations or inspection stops.
When mK Is the Right Call—And When It Costs You
ISO 2768-mK is the default choice for parts where form and fit are not critical. This includes brackets, covers, spacers, non-load-bearing housings, and any part where two surfaces meeting within 0.2 mm is functionally acceptable. If your assembly uses through-holes for M6 bolts with a clearance of 6.6 mm, you have 0.6 mm of slop. Specifying mK at ±0.1 mm on the hole center distance is more than adequate.
The cost advantage is real. At our Andas Precision factory in Guangdong, we run mK parts on standard three-axis mills at 12,000 to 15,000 RPM with feeds of 0.1 to 0.15 mm per tooth, taking roughing passes at 2 mm depth of cut. A typical mK aluminum bracket can be roughed and finished in one setup with no inspection hold. The per-part cost is roughly 30 to 40 percent lower than an equivalent fH part, purely because we eliminate the need for second operations, slower feeds, and frequent probe checks.
But mK has limits. If you have a bearing pocket that must hold a 6202 bearing with a nominal 35 mm outer diameter, the ISO 2768-mK tolerance of ±0.2 mm on that pocket could mean a 34.8 mm or 35.2 mm bore. A 35.2 mm bore will not retain the bearing. A 34.8 mm bore will not allow assembly. In that case, mK is not just wrong—it is dangerous.
When fH Is Necessary—And When It Is Overkill
ISO 2768-fH is appropriate when your part has critical mating surfaces, press-fit features, or alignment requirements that cannot tolerate more than 0.05 mm of deviation. Common applications include precision fixture plates, optical mount interfaces, gear housing bores, and parts that must index against each other with repeatability under 0.1 mm.
Consider a simple example: a 50 mm diameter shaft hole for a precision dowel pin. The dowel pin has a nominal diameter of 50 mm with a tolerance of h6 (0 to -0.016 mm). If you specify fH on your hole, the linear tolerance is ±0.075 mm. That still leaves a potential interference of 0.075 mm or a clearance of 0.091 mm. For a true press fit, you need an individual tolerance—typically H7 (+0.025 mm / 0 mm). The fH block tolerance alone is insufficient. However, if you specify fH and then add individual tolerances only on the critical features, you save money because the rest of the part can run at the fH standard without individual callouts.
The cost penalty for fH is significant. At our facility, fH parts require slower spindle speeds—8,000 to 10,000 RPM—and lighter finishing passes at 0.3 mm depth of cut with feeds reduced to 0.05 mm per tooth. We run in-process probing after every critical operation. For a 200 mm x 150 mm aluminum plate with multiple bored holes, an fH part typically requires 1.5 to 2 times the machining time of an mK part. That translates directly to a 25 to 35 percent higher unit price.
The trap is specifying fH on parts that do not need it. I have seen drawings for simple sheet metal brackets—0.5 mm thick, formed, with punched holes—that call out fH. Sheet metal forming cannot hold ±0.05 mm on hole positions across a bend. The drawing is impossible to meet, or it forces the shop to add CNC machining to a stamped part, quadrupling the cost. If your part is going to be laser cut, punched, or 3D printed, fH is almost always wrong.
Practical Rules for Choosing Between mK and fH
After reviewing thousands of drawings and running millions of parts, here are the rules I use to decide which tolerance block belongs on a drawing:
Rule 1: Apply the “Worst Case Fit” test. Take your largest nominal dimension and calculate the worst-case stack-up using mK tolerances. If the assembly still functions—if the bolt goes through the hole, if the cover closes—use mK. If the stack-up could cause binding, leakage, or misalignment, move to fH or add individual tolerances.
Rule 2: Use fH only when geometrical tolerances matter. The “H” in fH is often more important than the “f.” If your part must have flat surfaces within 0.1 mm or perpendicular holes within 0.05 mm, you need Class H geometry. But if you only need tighter linear dimensions and your geometry is non-critical, consider using ISO 2768-fK instead. That combination gives you fine linear tolerances with medium geometrical tolerances, saving cost.
Rule 3: Never use fH for sheet metal, plastic, or cast parts. These processes have inherent variability that cannot meet fH without secondary machining. For sheet metal, use ISO 2768-mK or even cK (coarse). For plastic injection molding, use ISO 20457 or a custom tolerance block based on shrinkage data.
Rule 4: Add individual tolerances on critical features even with fH. A blanket fH block does not guarantee press fits or sliding fits. If you need a 25 mm shaft to fit an H7 hole, write “25 H7” on the hole dimension. The fH block will handle the rest of the part, but the critical feature gets its own tighter control.
Rule 5: Communicate with your shop. If you are unsure, send the drawing with a note: “Tolerance block is preliminary—confirm with manufacturing.” A good CNC shop like Andas Precision will run your part through our Project Review analysis, which checks every feature against machine capability and tolerance class. We will tell you if mK is sufficient or if fH is overkill, often before you cut a chip.
The Bottom Line: Pick the Loosest Standard That Works
ISO 2768-mK is your default for 80 percent of CNC machined parts. It keeps cost low, lead times short, and shops happy. ISO 2768-fH is a precision tool—use it only when your assembly demands it. Over-specifying fH on a part that could run mK is not being careful; it is being wasteful.
Before you send your next drawing, run this quick checklist:
- Does the part have any press fits, bearing bores, or precision alignment features? If yes, add individual tolerances.
- Are all surfaces non-critical for flatness or perpendicularity? Use mK.
- Is the part made from sheet metal, plastic, or cast metal? Use mK or coarser.
- Will the worst-case stack-up at mK still allow assembly? Use mK.
- Are you unsure? Send the drawing for DFM review.
Your CNC parts will fit, function, and cost less. That is the standard you should aim for.