Spot drills
Spot-drill purpose, point-angle compatibility, shallow-spot geometry, and differences from center drills and pilot holes.
Tags: tooling, drilling, holemaking
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An ideal 120° spot drill makes a 3.00 mm spot after 0.866 mm of travel from first apex contact on a flat surface. Another 0.10 mm adds 0.346 mm to the spot diameter. Check the real tip flat, length reference, work surface and following drill geometry.
Purpose and terminology
A spot drill makes a shallow entry feature for a following drill. On curved or inclined surfaces, Harvey notes that a flat-bottom tool may be needed. Harvey Performance, Spot Drilling: The First Step to Precision Drilling, displayed publication date August 25, 2022
The included point angle is the full angle between opposite sides of the conical point, not the angle of one side to the tool axis. The distinction matters in both tool comparisons and depth calculations below. A nominal tool diameter and the diameter of the shallow spot are also different quantities: a partial plunge can make a spot smaller than the cutter.
Related tools and features
| Term | Distinction |
|---|---|
| Spot drill | Creates the entry depression for another drill. It does not imply a full-depth pilot hole. |
| Center drill / centre drill | Produces a center hole with a drilled portion and conical seat for supporting a workpiece between machine-tool centers. ISCAR describes it as combining drilling and countersinking, rather than being synonymous with a spot drill. |
| Pilot hole | A preceding bore with a defined diameter and depth; this article uses the term for a guiding hole, rather than a shallow cone alone. |
| Countersink or chamfer | Describes a conical seat or edge feature. A tool suitable for spotting is not automatically suitable for every specified seat angle, diameter or finish. |
The center-drill distinction comes from ISCAR, Reference Guide for Machining Titanium, “Center Drilling” section. The cited hosted edition is undated; the filename alone does not establish its issue date. The pilot-hole and feature distinctions above are explanatory terminology used here, not a dimensional standard.
Some spot drills also support edge chamfering. For example, Titan USA's solid-carbide family lists 90°, 120° and 142° included angles and describes chamfering the top of drilled holes as an application. This is a capability of that family, not permission to substitute any spot drill for a countersink. Titan USA, Spot Drills—Carbide, undated product page.
Point-angle compatibility
For carbide drills, the contact sequence matters. A spot cone narrower in included angle than the following drill can make the outer cutting edges contact before the center, risking shock loading and chipping. Harvey recommends a slightly larger spot angle and also accepts equal angles. Harvey Performance, angle-selection discussion.
Guhring's guidance is more specific to drill type:
| Spot angle | Manufacturer's stated application |
|---|---|
| 90° | HSS, cobalt high-speed steel (HSCO), or powder-metallurgy subsequent drills; cutting-lip contact occurs first. Generally unsuitable for carbide drills. |
| 120° | Subsequent drill with a smaller chisel edge and point angle below 120°; point-first contact is intended. |
| 142° | Illustrated with 135°–140° subsequent drills to achieve point-first contact, particularly important for carbide. |
Guhring, NC Spot Drills, printed page 12, undated catalog. The third heading says 140°; its diagram and text use 142°, matching the series on printed page 10. This table follows the diagram and text.
Synthesis: “Always use a 90° spot” and “all spotting requires a larger angle” both omit important conditions. Compatibility depends on the following drill's geometry and its manufacturer's application guidance; the 90° HSS case and point-first carbide case describe different entry conditions.
Depth, construction and operating limits
Guhring states that its spot drills lack a margin and are not intended to drill deeper than the height of the point. Its HSS, HSCO and carbide series have product-specific coatings and operating tables, not a universal speed or feed. Guhring, printed pages 3–10 and 12–15.
Spotting is not a mandatory step for every drill. In its February 13, 2024 training session, Sandvik Coromant describes CoroDrill 460 and 860 as self-centering products that do not need prior spot drilling. This is a product-specific exception, not a rule for all carbide drills. Sandvik Coromant, Solid Round Tools—session 7, drilling discussion.
Worked example: shallow-spot diameter
Assume a sharp ideal cone and perpendicular flat surface without runout. Measure depth from first apex contact. Let:
α= included point angle, in degrees;z= axial penetration, in millimeters;d= spot diameter at the surface, in millimeters.
A right-triangle section gives d = 2z tan(α/2), or z = d / [2 tan(α/2)].
For a hypothetical 120° point and desired 3.00 mm spot:
z = 3.00 / [2 × tan(60°)]
= 0.866 mm
With a hypothetical 6.00 mm cutter, the ideal cone reaches full cutter diameter at 6.00 / [2 × tan(60°)] = 1.732 mm. The example's 0.866 mm plunge is below that geometric limit. An additional 0.10 mm plunge increases spot diameter by 2 × 0.10 × tan(60°) = 0.346 mm, showing why a small depth change can materially change the entry feature.
These results do not establish compatibility with a subsequent drill. A real tip flat, web, edge preparation, tool-length reference and surface condition can change the relationship between programmed depth and measured spot diameter. The calculation also does not predict final hole location, diameter tolerance, tool life or cycle time.
Coverage limits
The sources describe manufacturer guidance and selected product families, not an acceptance standard for hole position or a qualified machining process. No material-specific operating window, holder-runout allowance, inclined-entry limit or minimum spot diameter is established here. Those quantities remain dependent on the actual drill system, workpiece, setup and drawing requirements.
For the wider tooling context, see Cutting tools and tooling references.