Centerless Grinding Delrin Guide Rods: OD Tolerance in Microns
Most shops will happily turn you a plastic rod. Very few will promise you one ground to a micron-class tolerance, and fewer still can mill features onto that ground surface without leaving a mark. The guide rod that Dylan Mercer ordered in May 2025 asked for all three: a slender black Delrin body, 180 mm long, centerless ground to Ø16.00 mm ±0.005 mm, with a milled eyelet end carrying an Ø8 mm transverse bore and a milled longitudinal flat along the body. His company in Australia builds finishing equipment for print media, where these rods guide a carriage that must glide with no measurable play — which is why the diameter tolerance, not the material, was the real specification.
Why not just turn it?
A good turning pass on acetal will hold ±0.02 mm and give you a surface that feels smooth. This part needed more for two reasons:
- The rod is long and slender (an 11:1 length-to-diameter ratio), and a slender part deflecting in the lathe chuck shows up as taper and lobing that no tool path can fully remove.
- The carriage bearings in Dylan's design run directly on the rod surface, so roundness and diameter consistency along the full length matter more than average size.
Centerless grinding removes both problems — the part is supported along its full length between the grinding wheel, the control wheel, and the workrest blade, with no chuck to introduce deflection.
Stage one — turning the blank
Each rod started as black acetal homopolymer bar stock, turned to Ø16.20 mm with the eyelet-end profile roughed in and both faces squared. The turning stage did the heavy lifting on geometry: the Ø8 mm eyelet bore was drilled and reamed while the blank was still held on the main axis, so the bore and the outer diameter shared one datum before grinding ever started. Tooling was sharp two-flute carbide with positive rake, and an air blast kept the chips from re-welding onto the surface — acetal chips carry static and will stick to the part if the air is not there.
Stage two — what centerless grinding added
The ground stage took the rods from Ø16.20 mm to Ø16.00 mm — only 0.20 mm of stock, but removed in a way that turning cannot match:
- Throughfeed grinding on a centerless machine with a 60-grit silicon carbide wheel, running under flood coolant.
- Stock removed in two light passes rather than one aggressive pass; plastics do not tolerate heat, and a single deep cut will melt and smear the surface instead of cutting it.
- The wheel was dressed every 400 pieces, and the first article after each dressing was checked for roundness and size before the batch continued.
The measured result across 2,000 rods: Ø16.000 ±0.005 mm held throughout, roundness 0.001–0.002 mm, surface finish Ra 0.4 µm — a surface that a bearing can ride on for years.
Stage three — milling on a ground OD without marking it
The milled features — the eyelet profile, the longitudinal flat, and the radial holes — all had to be cut after grinding, because grinding needs a full round surface to support the part. The challenge flipped: how do you hold a part with a finished ground surface without damaging the very surface you just paid to create?
- The rods were clamped in a precision V-block lined with a soft film, sized so that the clamp force was just enough to resist the cut — never enough to indent the OD.
- The flat was milled along the body in a single light climb pass with a sharp end mill, no re-cutting, to avoid smearing the acetal.
- All radial holes were drilled through the same fixture positions, so hole-to-flat relationships stayed inside ±0.1 mm.
Every tenth rod was checked for clamp marks under magnification. The answer was zero — the ground OD left the milling stage exactly as it entered it.
Качество Китая хорошее Части CNC поворачивая Поставщик. © авторского права 2022-2026 Shenzhen Perfect Precision Product Co., Ltd. . Все права защищены.
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