DFM Checklist for CNC, Stamping, Sheet Metal and Springs

CNC machining

• Keep internal corner radii at least one third of the pocket depth - small radii need small tools and slow passes.
• Avoid pockets deeper than four times the tool diameter where you can.
• Make wall thickness uniform; thin walls chatter and need extra passes.
• Put tolerances and Ra callouts only on functional faces.
• Design a clear fixturing face so the part can be held for the tightest cut.

Metal stamping

• Hole diameter should be at least the material thickness.
• Keep the distance from a hole to an edge at least one and a half times the thickness.
• Add a generous bend radius - at least the material thickness, more for high-strength alloys.
• Avoid sharp outside corners: they crack and wear the die.
• Tolerances across bends are looser than in-plane features; keep tight dimensions flat.

Sheet metal

• Use a standard thickness from the mill's range to avoid special orders.
• Design bend relief notches so the bend does not tear.
• Keep holes at least two and a half times the thickness from a bend line.
• Allow for springback - the shop compensates, but very tight bend angles cost time.
• Specify the finish after the bend, since forming can mark a coated surface.

Springs

• Spring index (mean diameter / wire diameter) of 5 to 12 gives the most stable coiling.
• Keep the number of active coils above three.
• Avoid index values below four - the spring is hard to coil and cracks easily.
• Specify load at a length, not only free length, so performance is measurable.
• Name the material and the expected life; stress relief depends on both.

Heat sinks

• Keep fin thickness and spacing within the extruder's die limits for the profile size.
• Allow a machined face for the thermal interface if a flat base is critical.
• Tapped mounting holes need a minimum wall around them; design them into the base, not a fin.
• State the airflow direction so fin spacing matches the application.

Frequently asked questions