Is a tolerance of plus or minus 0.05 millimeters too tight to ask for?

Not automatically, but it is the point where cost and risk start climbing fast in both CNC machining and injection molding. In CNC it typically adds 15 to 25 percent to cost. In injection molding, most molders treat anything tighter than that as a high-risk flag they want to discuss before quoting, since plastic shrinkage makes it hard to hold consistently. Reserve it for the one or two dimensions that actually need it.

Why does a molded part's tolerance depend on where the dimension sits relative to the parting line?

A dimension that stays entirely on one side of the mold, in a single cavity half, only inherits variation from that one half. A dimension that spans the parting line also picks up variation from how precisely the two mold halves align when they close, which is a separate source of error on top of shrinkage. That is why the same nominal size can carry a tighter or looser realistic tolerance depending on which side of the parting line it falls on.

What tolerance can a CNC shop or injection molder actually hit?

The question people usually ask is what tolerance to put on a dimension. The more useful question is which tolerance band that number falls into, since the band, not the number itself, is what actually moves cost.

CNC machining tolerance bands

A standard tolerance around plus or minus 0.13mm is close to free on most CNC parts, since it sits within what the process holds without extra setup or inspection. Ask for precision grade, around plus or minus 0.05mm, and expect roughly 15 to 25 percent added cost. Go to high precision, around plus or minus 0.025mm, and it climbs to roughly 30 to 50 percent more. That last band is meant for actual mating surfaces, bearing bores, press fits, not every dimension on a part.

Injection molding tolerance bands (DIN 16742)

Injection molding follows its own standard, DIN 16742. For a typical mid-size dimension, the baseline sits around plus or minus 0.1 to 0.2mm, and where exactly it lands depends on material and whether the dimension crosses a parting line, since parting-line dimensions inherit extra variation from mold alignment. Ask for anything tighter than about plus or minus 0.05mm and most molders will flag it as high-risk before they even quote it, because plastic shrinkage is not perfectly predictable, and forcing a tight number on an unpredictable material mostly just raises the reject rate.

The habit that keeps a quote from spiking

Only tighten a tolerance where the part genuinely needs it to fit or seal against something else. Everywhere else, leave it at the process default. That single habit is one of the fastest ways to keep a quote from coming back higher than expected, since tight tolerance across an entire drawing gets priced as if every dimension mattered equally.

Checking tolerances before a file goes out

A machinist or molder can tell you fast whether a called-out tolerance is routine or a red flag, often before a formal quote. Comparing quotes on the same part with two different tolerance callouts also shows the cost gap directly. Tools like fabdose can help catch some manufacturability issues on a CAD file before it goes out for quotes, worth having as an early pass alongside a real quote conversation, not instead of one.