Deep holes have traditionally meant two operations: drill to near-size, then bore to finish. The U-drill (spiral-flute boring tool) does both in one — it removes bulk material like a drill but machines the final wall with a carbide insert, producing a straighter, cleaner hole with less total tooling. The inserts that make it work are the SPMG, WCMX and WCMT families. Here is how they differ and how to choose.
What a U-drill actually does
A U-drill body carries spiral flutes for chip evacuation and one or two insert seats near the tip. The inserts take a radial depth of cut (typically 0.5–3 mm per pass) while the body feeds axially. Because the insert — not a high-speed steel edge — makes the finishing cut, U-drills hit diameters up to ~120 mm+ at deep ratios (12D and beyond) in a single setup, with a surface finish that would otherwise need a separate boring operation.
SPMG: the workhorse U-drill insert
SPMG U-drill inserts are the most common U-drill geometry: a U-shaped profile that holds the insert in the seat by friction and provides a full-width cutting edge for the final wall. The size code is diameter class + geometry: SPMG0502 (small body), SPMG0602, SPMG0730, SPMG0904, SPMG1104, SPMG1405 — the last two digits indicate the body size it fits. Two grade families dominate:
- DG grades: coated, for steel and stainless — the default for production parts.
- PG grades: uncoated/PCBN-ready variants for harder materials and finishing.
SPMG inserts give the highest feed rates of the three families — in steel, 0.2–0.3 mm/rev and above is routine, which is the main productivity argument for U-drilling over drill+bore.
WCMX: compact bodies, tight feeds
WCMX inserts (WCMX0302, WCMX0402 series) serve smaller U-drill bodies where SPMG geometries don’t fit. The compact size keeps the cutting edge close to the axis, which helps hole straightness in long, narrow bores. Recommended feeds are a step below SPMG — this is the trade for the smaller edge. FN-coated WCMX grades are the standard for steel and stainless work in sub-50 mm bodies.
WCMT: finishing-focused boring
WCMT inserts (WCMT05/06/08 series) sit between drilling and boring: they are used in small-diameter deep-hole boring tools and finisher bodies where the priority is surface finish and dimensional accuracy over raw MRR. The shorter, stronger profile tolerates the light radial loads of finishing passes. In finishing setups, WCMT at 0.05–0.15 mm/rev with 0.1–0.3 mm depth per pass is the typical window.
Choosing between them — a practical flow
- Hole diameter and body size set the candidate family first (SPMG for standard bodies, WCMX for compact, WCMT for finisher bodies).
- Material: steel/stainless → coated DG/FN/MV grades; cast iron → K-optimized grade; non-ferrous → sharp N-grade edge to avoid smearing.
- Ratio (depth/diameter): above 10D, check the body’s coolant pressure rating and the insert’s chip-breaker size — long chips in a deep hole are the classic U-drill failure.
- Feed target: if the cycle-time win is the point, SPMG at high feed; if the finish is the point, WCMT at low feed.
Parameters worth remembering
| Insert | Typical feed (steel) | Depth/pass | Role |
|---|---|---|---|
| SPMG | 0.15–0.30 mm/rev | 0.5–2.0 mm | Rough + finish in one |
| WCMX | 0.10–0.20 mm/rev | 0.3–1.5 mm | Compact bodies |
| WCMT | 0.05–0.15 mm/rev | 0.1–0.5 mm | Precision finishing |
(Always confirm against the specific insert’s product page — parameters vary by size and grade.)
Get U-drill support from a cutting tool supplier
AMJtool’s drilling tools and U-drill insert range covers the SPMG, WCMX, WCMT, SOMT and CCMT families with full geometry and parameter data on each page. Send us your hole size, material and target depth ratio and our application team will spec the insert, grade and body configuration for your setup — including matched tool holders from our tool holder range.