While spot-facing and counterboring of holes are geometrically identical operations, the spotface provides a flat landing surface, where the counterbore fully recesses the flat-faced fastener fitted to the hole so treated. This article will further discuss spotface holes, how to drill one, how to choose the right tap, as well as the applications and uses of a spotface hole.

Spotdrillangle

The ISO drawing callout symbol for a spotface hole is a subset of the callout for a counterbore, with the letters SF superimposed:

There are no universal standard charts that define spot-facing of parts. It becomes the responsibility of the designer and/or machinist to correctly size both the diameter and the net depth of the spotface operation to suit the needs of the part.

Spot hole angle fixtool

For a through hole, the selection of taper or middle taps depends on the depth of the hole to be tapped. There must be enough total length of unaltered tap and shank combined to allow the tap to penetrate deeply enough to complete the thread through the part.

Spot hole angle fixapp

A spotface is a flat-faced, notionally cylindrical but very shallow hole that is larger in diameter than the hole that it is concentrically cut into. It is cut to impose a flat landing surface for contacting/mating parts, such as washers or fastener heads. The primary reasons for spot-facing holes are:

Spotdrill vs center drill

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Where the hole is blind, tapping requires a two-stage operation. The thread is initiated with a taper or middle tap and then completed with a bottoming tap to the required depth. The choice between taper and middle taps is driven by the blind hole depth; where its use is possible, the taper tap gives a softer and more certain start to the thread cutting. But, if the taper excludes effective (full depth) cutting as it is longer than the blind hole is deep, it is necessary to use a middle tap.

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This article presented spotface holes, explained what they are, and discussed when to best drill this type of hole. To learn more about spotface holes, contact a Xometry representative.

Spotdrill bit

Spotface holes are blind or through holes that have had a perpendicular and flat-faced shallow shoulder cut at the material face. This design provides a high-quality surface on otherwise rough-surfaced parts or provides a perpendicular landing surface on otherwise curved-surface parts.

In either case, the spotface serves the purpose of providing a flat landing onto a controlled and precise surface. It imposes both flatness and precision onto otherwise imprecise or unsuitably shaped parts.

140 degreeSpotDrill

Spot hole angle fixreviews

The purpose of spot-facing a hole is to provide a smooth, perpendicular/flat landing for a component to be fixed down to the surface. Such a component may be a post/standoff held down by a threaded fastener, or a fastener holding other parts on the reverse side of the spotface.Â

In general, spot-facing is sized to be larger than the part that is designed to land upon it. It should have sufficient depth to ensure a fully annular flat surface, removing all curvature or roughness from the landing surface.

Spotface holes are rarely made by hand. Spot-facing cutters exist, with a cylindrical peg to locate into the drilled hole before cutting. Although, more often, spot-facing is performed on a mill or CNC machine.

Larger companies such as Boeing have design manuals (BDM-1327) that specify the nominal diameter of the spotface as the matching size of the cutter to be used (to reduce unnecessary machine time). As a guide, the appropriate diameter is approximately the flat seat diameter required, plus 2x the corner radius of the cutter. The ASME Y14.5-2009 guide for sizing a spotface specifies it as equal in diameter to the required diameter with no safety allowance for radii.

Where the hole to be spot-faced is formed/drilled through to the other side of the part, the tap required for threading such a hole is a taper or middle type. These two forms of tap have the initial threads ground away, allowing the tap to enter the hole and correctly center from the start of tapping/thread cutting.