View source for Spotface

{{short description|Localized flat region on a machined workpiece}}
[[File:Machined Holes.svg|thumb|Spotfacer is marked '''F'''. Similar to an endmill but with flat-bottom plunging ability. The tool at '''E''', a [[counterbore]], is also often used for spotfacing in manual machining (not so much in CNC machining).]]
A '''spotface''' or '''spot face''' is a [[machining|machined]] feature in which a certain region of the [[workpiece]] (a spot) is [[facing (machining)|faced]], providing a smooth, flat, accurately located surface. This is especially relevant on workpieces [[casting|cast]] or [[forging|forged]], where the spotface's smooth, flat, accurately located surface stands in distinction to the surrounding surface whose [[surface roughness|roughness]], [[flatness (manufacturing)|flatness]], and location are subject to wider [[engineering tolerance|tolerances]] and thus not assured with a machining level of precision.
The most common application of '''spotfacing''' ('''spot facing''') is facing the area around a [[Bolt (fastener)|bolt]] hole where the bolt's head will sit, which is often done by cutting a shallow [[counterbore]], just deep enough "to clean up"—that is, only enough material is removed to get down past any irregularity and thus make the surface flat.<ref>{{harvnb|Degarmo|Black|Kohser|2003|p=606}}.</ref> Other common applications of spotfacing involve facing a pad onto a [[boss (engineering)|boss]], creating planar surfaces in known locations that can orient a casting or forging into position in the assembly; allow part marking such as [[Stamping (metalworking)|stamping]] or [[nameplate]] riveting; or offer machine-finish visual appeal in spots, without the need for finishing all over (FAO).
The cutters most often used to cut spotfaces are [[counterbore]] cutters and [[endmill]]s. In manual machining especially, the former is useful because its pilot guides the cutter into the correct location (established by the bolt hole), and its cutting lips are perpendicular to the hole axis with no relief angle, meaning that a plunging cut, moving in only the Z-axis, will leave a flat surface. In contrast, most general-purpose endmills have a relief angle such that a plunging cut (Z-axis-only toolpath) will leave a very slightly convex surface. But in [[numerical control|CNC]] machining this is irrelevant, because the flat spotface is left by [[circular interpolation]] of the endmill as it traces a circular toolpath in the XY plane. Zero-angle cutters obviate this, functioning similarly to the traditional counterbore cutters of manual machining but needing no pilot. Spot facing of larger planar surfaces sometimes employs [[milling cutter#Face mill |face mills]].
'''Backspotfacing''' ('''back spot facing''') is analogous to backboring, meaning that the tool manages to reach to the far side of the workpiece (away from the spindle side) and cut back toward the spindle. Such operations can be done with [[boring bar]]s reaching through a hole while shifted off-center, then shifting onto center for the cut (for example, in the [[G-code#G76_bore_on_mill|G76 fine boring cycle]]), or with back-deburring style tools whose cutting edges open and close in umbrella-like or check-valve-like fashion. Such back-cutting operations can obviate second operations,<ref name="Hider_2018-03-15">{{Citation |editor-last=Hider |editor-first=Julia |date=2018-03-15 |title=Back-spotfacing tool frees production from costly bottlenecks |journal=[[Modern Machine Shop]] |url=https://www.mmsonline.com/blog/post/back-spotfacing-tool-frees-production-from-costly-bottlenecks |accessdate=2018-03-16 |postscript=.}}</ref> with their attendant setup time and part rehandling time. Sometimes the limiting factor in applying this advantage is the difficulty of [[swarf|chip]] clearance; the operator needs to intervene because chip nests prevent the tool from passing freely and opening and closing freely.<ref name="Hider_2018-03-15"/> In such cases, improved chipbreaking via different [[tipped tool|inserts]] in the main operation may help, and backspotfacing tools powered by [[cutting fluid|coolant pressure]] and using through-tool coolant feed may succeed where others cannot (opening, cutting, and retracting despite some chips),<ref name="Hider_2018-03-15"/> not only obviating a second operation but also making the main operation less reliant on operator intervention and program stops ([[G-code#M00|M00 or M01]]) than a lesser backspotfacing tool would.<ref name="Hider_2018-03-15"/>
Some fastener designs can obviate spotfacing for some applications. For example, a [[self-aligning nut]] allows the nut to find its own perpendicularity to the thread axis by floating via a pair of spherical bearing surfaces (a ball-in-cup arrangement).
[[Technical standard|Standards]] exist for the sizes of spotfaces, especially for fastener head seating areas. These standards can vary between corporations and between [[standards organization]]s. For example, in [[Boeing]] Design Manual BDM-1327 section 3.5, the nominal diameter of the spot-faced surface is the same as the nominal size of the cutter, and is equal to the flat seat diameter plus twice the fillet radius. This is in contrast to the [[ASME]] Y14.5-2009 definition of a spotface, which is equal to the flat seat diameter.
==See also==
*[[Counterbore]]
*[[Countersink]]
==Notes==
{{reflist}}
==References==
*{{Citation | last = Degarmo | first = E. Paul | last2 = Black | first2 = J T. | last3 = Kohser | first3 = Ronald A. | title = Materials and Processes in Manufacturing | publisher = Wiley | year = 2003 | edition = 9th | isbn = 0-471-65653-4}}.
[[Category:Machining]]
[[Category:Metalworking terminology]]