Every part that comes off a CNC mill or lathe carries a price tag that was mostly decided before the machine started cutting. The geometry drawn in CAD determines how many tools are needed, how many times the part must be repositioned, and how carefully the machinist has to move to avoid breaking a cutter or leaving a bad surface.
Small adjustments to a design can cut production cost significantly without touching the part’s performance. The seven decisions below cover wall thickness, radii, hole depth, tolerances, threads, and setups, the areas where a DFM review typically finds the most savings.
Wall Thickness, Internal Radii, And Why Geometry Fights The Cutter
Working with a custom CNC machining service early in the design phase often reveals wall thickness problems before they become costly. Metal walls should generally stay above 0.8 mm and plastic walls above 1.5 mm. Thin walls vibrate under cutting forces, a phenomenon called chatter, which leaves a rough surface and can push dimensions out of tolerance. Thin sections also force slower, lighter passes, sometimes tripling cycle time for that feature. Keeping the height-to-thickness ratio under roughly 10:1 for aluminum and 4:1 for softer plastics avoids the need for internal ribs or redesigns, and a wall 20 percent thicker than the functional minimum often cuts scrap rates noticeably.
Sharp internal corners are a related and common reason quotes come back higher than expected. A round end mill cannot cut a square inside corner, so the shop either needs a smaller, slower tool or a radius matching the tool’s geometry. Internal radii of 1 mm or larger accommodate common tooling, while anything below 0.5 mm forces specialty micro-end mills that wear faster and cost more per hour.
Hole Depth, Diameter Ratios, And Drill Wander
Deep holes are judged by their depth-to-diameter ratio. Standard drills handle ratios up to about 4:1 comfortably. Beyond that, up to 10:1 or 12:1, shops need gun drills or peck-drilling cycles that retract repeatedly to clear chips. Peck drilling adds real time, since a hole at a 10:1 ratio can take four or five times longer than one at 3:1 because of repeated withdrawals. Shortening a hole or slightly increasing its diameter often moves it into a faster drilling category.
Tolerances And Thread Choices That Change The Machining Path
Tightening cnc machining tolerances beyond what a part needs is one of the fastest ways to inflate a quote. Standard tolerances around ±0.1 mm work with normal roughing and finishing passes, while ±0.02 mm or tighter typically demands slower feeds and in-process measurement. Tight tolerances also raise rejection risk from thermal expansion and tool wear, so reserving them for mating features and leaving cosmetic dimensions at standard tolerance keeps most geometry on fast toolpaths.
Tapping is fast for standard hole sizes but taps can break in blind holes, especially in harder alloys. Thread milling costs more time per hole but produces more consistent results and lets one tool cut multiple thread sizes, reducing tool changes on complex cnc machining projects. A thread depth of about 1.5 times the diameter in metal usually provides full strength without unnecessary extra cutting.
Setup Count And The Hidden Cost Of Repositioning
Every flip or re-clamp adds setup time unrelated to actual cutting. A part machinable in one setup is almost always cheaper than one needing several, because each setup requires re-establishing the part’s position relative to the machine’s reference point. Consolidating features onto fewer faces, or using a five-axis machine to reach multiple sides without repositioning, reduces labor and misalignment risk.
Turning Cost-Conscious Geometry Into Better Parts
None of these seven decisions require sacrificing a part’s function. Wall thickness, radii, hole ratios, tolerances, thread depth, and setup count are all dimensions the designer controls directly, and each has a numeric range where machining stays fast and predictable. Parts that respect these ranges move through a shop with standard tools, standard speeds, and fewer inspection delays, which keeps a quote reasonable without touching the part’s actual performance.
Before The Quote: A Quick Geometry Check
Before sending a design out for a quote, it helps to walk back through these seven areas with the actual drawing in hand rather than relying on memory of the guidelines. Check that no wall falls below the metal or plastic minimum, that internal corners carry a radius the shop’s standard tools can reach, and that any deep holes still sit within a reasonable depth-to-diameter ratio for standard drilling. Confirm that tight tolerances are applied only where a feature actually mates with another part, that thread depths aren’t cut deeper than the strength requirement calls for, and that the part can be held and finished in as few setups as possible. This kind of self-review is essentially a lightweight version of the DFM check a shop will run anyway, and catching these issues beforehand means the quote that comes back reflects the part’s real complexity rather than avoidable geometry choices, often arriving faster and closer to the final production cost.
