Design
CNC Design Mistakes That Raise Your Quote
The design choices that quietly raise a quote.
Most of what raises a CNC quote is not the part itself but a handful of design choices that force slower cuts, special tooling, or extra setups: sharp internal corners, thin walls, deep pockets, tolerances tighter than ±0.005 in where the function does not need them, and odd hole sizes. Every one is easy to design around once you know it. A real person flags them in a free DFM review with every quote. Send your part through the quote form and the notes come back in 24-48 hours.
Updated October 2026
Design review · cost drivers
Toggle a feature to see what it adds: these are the things the team flags when reviewing a design.
Illustrative example. Your actual quote comes from a real person in 24-48 hours.
Sharp internal corners and the tool that has to cut them
End mills are round, so a perfectly sharp inside corner is impossible to mill: the tool always leaves a radius equal to its own. When a print shows a dead-sharp internal corner, there are only two ways to honor it, and both cost money. Either the corner gets cut on slow wire EDM, a separate process billed on top of the milling, or the part is reworked by hand. Neither is what you want on a bracket pocket.
The fix is free and almost always invisible to function: add an internal radius at least as large as a standard end mill, roughly 1/8" (0.125") on most pockets, larger on deep ones. A generous corner radius also lets a bigger, stiffer tool run faster, so it often makes the part cheaper than a tighter radius would. If a mating part truly needs a square corner, undercut or relieve it instead of specifying zero radius, and say so on the print so it gets quoted right the first time.
Thin walls, tall ribs, and deep narrow pockets
Thin walls and tall thin ribs chatter and flex away from the cutter. To hold them in tolerance, the cut has to slow down and take light passes, which adds time, and a wall that’s too thin can still warp or scrap out late in the job after most of the cost is already in it. As a rough guide, keep walls at or above 0.030" in aluminum (thicker in plastics like Delrin, which moves more), and give tall ribs some width rather than knife-edging them.
Deep, narrow pockets have the same problem from the tool’s side. To reach the bottom the cutter needs a long, skinny shank, and long tools flex and cut slowly, so the deeper and narrower the pocket, the more time it eats. A practical rule is to keep pocket depth under about four times the tool diameter; past that, the part climbs in price fast. If a deep feature is doing nothing structural, open it up, split it, or ask whether it can be shallower. The geometry, not the material, is what drives the cost here.
Tolerances and finishes tighter than the part needs
This is the single most common quote-inflator, and the easiest to fix. A standard milled feature holds about ±0.005" with no special effort. Call out ±0.0005" across the whole part and you’ve added precision passes, slower cuts, and tighter inspection on dimensions that never needed it. Tolerance is not a quality setting; tighter than the function requires is pure added cost. Put the tight numbers only on the features that actually mate or seal, and leave the rest at standard. The full picture lives on the tolerances page.
Surface finish works the same way. A fine finish callout where it doesn’t matter, like the back of a bracket or a non-sealing face, forces extra finishing passes for nothing. Spec a tight finish only on sealing surfaces, sliding fits, and faces that show. Everywhere else, an as-machined finish is fine and saves both time and money.
Odd holes, tiny features, and parts that need many setups
Standard drill and tap sizes are cheap because the tooling already exists. A 1/4-20 tapped hole or a #7 drill is a stock tool off the shelf; a non-standard diameter or an oddball thread means a special tool, a reaming operation, or single-pointing the thread, all of which add time. Unless a hole has to be a specific size to mate with something, round it to a standard drill or a common thread and the quote drops.
Tiny features are their own cost: a 0.020" slot or a delicate engraving needs a micro tool that runs slow and breaks easily, so the part gets babied through the cut. Keep small features as large as the function allows. The other quiet multiplier is setups. A part with features on one or two faces is fast; a part with features on all five or six faces has to be flipped, re-fixtured, and re-indicated for each one, and every flip adds time and a chance for misalignment. Keeping most features on a couple of faces can cut the number of setups in half, and these are exactly the things a person catches in the DFM review before quoting.
Why design mistakes are cheap to fix here
The point isn’t that your design is wrong. It’s that a few choices have outsized effect on price, and most designers were never told which ones. Every quote here starts with a real person reading your STEP file and PDF, and if something will cost more than it needs to, you hear about it: a radius to add, a wall to thicken, a tolerance to loosen, a hole to round to standard. That review is part of the quote, it’s free, and it happens before any metal is cut, so there’s time to tweak the model and requote.
These design mistakes tie straight back to what actually drives a quote: machining time, setups, tooling, material, and finish. Fix the design-for-manufacturing issues and you’re pulling the levers that matter. Send your file and you’ll get the numbers plus the notes in 24-48 hours, with no minimum order and every part inspected before it ships.
Questions
Before you send a job.
01 Why can’t a CNC mill cut a perfectly sharp internal corner?
Because end mills are round, so the tool always leaves a radius equal to its own in any inside corner. A truly sharp internal corner has to be cut on slower wire EDM or finished by hand, both of which add cost. Add an internal radius of at least 1/8" and the corner mills cleanly, often faster and cheaper than a tighter one would.
02 Do tighter tolerances really make a part more expensive?
Yes, and it’s the most common quote-inflator. A standard milled feature holds about ±0.005" with no special effort; calling out ±0.0005" everywhere adds precision passes, slower cuts, and tighter inspection on dimensions that never needed it. Put the tight numbers only on features that mate or seal and leave the rest standard. The tolerances page walks through it.
03 How thin can a wall be before it raises the price?
Thin walls chatter and flex, which forces slow, light cuts and risks scrapping the part late in the job. As a rough guide, keep walls at or above 0.030" in aluminum and thicker in plastics, and give tall ribs real width rather than knife-edging them. If a thin wall isn’t doing structural work, thickening it usually costs nothing and machines faster.
04 Will someone tell me if my design is driving up the cost?
Yes. A real person reads your files and a free DFM review rides along with every quote, before any metal is cut. If a corner radius, wall thickness, tolerance, or hole size is costing more than it needs to, you hear about it with a specific suggestion, so there’s time to tweak the model and requote.
05 Why do standard hole sizes and threads cost less?
Standard drills and taps are stock tooling that’s already on hand, so a 1/4-20 tapped hole or a common drill size cuts with no extra setup. A non-standard diameter or an oddball thread means a special tool, a reaming step, or single-pointing the thread, all of which add time. Unless a hole has to be a specific size to mate, round it to a standard and the quote drops.
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