Design for Turning

Length-to-Diameter Ratio

This is the single most important design consideration for turned parts. A long, thin part deflects under cutting forces, which makes it impossible to hold tight tolerances. The rule of thumb: unsupported L/D ratios above 3:1 require a tailstock for support. Above 6:1, consider a steady rest or redesign.

A 1" diameter part at 6" long (L/D = 6:1) will deflect measurably under normal cutting forces. The shop can compensate with lighter cuts and slower feeds, but cycle time goes up and the tolerance they can reliably hold goes down. If you can increase the diameter or reduce the length, the part becomes easier and cheaper to turn.

Internal Features

Through-bores are easier than blind bores. A through-bore allows chips to evacuate freely and the boring bar to pass completely through. A blind bore traps chips at the bottom, requires a plunge-and-retract cycle, and is harder to measure in-process.

Bore depth matters. Deep bores require long boring bars that deflect more under cutting forces. Bore depth-to-diameter ratios above 4:1 start compromising tolerance. Above 8:1, specialized boring bars (carbide shank, damped) are required and cost goes up significantly.

Threads

Single-point threading on a CNC lathe produces superior threads to tapping or die-cutting. The thread is cut in multiple passes with precise depth control. Standard thread forms (UNC, UNF, metric) add no special cost. Non-standard threads, multi-start threads, and buttress threads require custom programming and add time.

Internal threads in blind holes need a relief groove at the bottom — the threading tool can't cut to a shoulder without one. Design the relief groove into the part or call it out on the drawing. Don't assume the shop will add one.

Grooves and Undercuts

O-ring grooves, snap ring grooves, and thread relief grooves are standard operations on a CNC lathe. Specify groove width as a standard tool width (0.062", 0.093", 0.125") and the shop can use an off-the-shelf grooving insert. Non-standard widths require custom-ground tooling, which adds cost and lead time.

Surface Finish Callouts

Don't specify a surface finish tighter than you need. Ra 32 μin is a standard turning finish achievable in normal production. Ra 16 μin requires a finishing pass with controlled feed and a wiper insert. Ra 8 μin may require polishing as a secondary operation. Each step tighter adds cycle time and cost. Only call out the finish requirement on surfaces that actually matter — sealing surfaces, bearing surfaces, sliding fits.