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UNR vs. UN Screw Thread Form ANSI/ASME B1.1

UNR vs. UN
UNRS vs. UNS
UNRC vs. UNC
UNRF vs. UNF
UNREF vs. UNEF

WHY
The UNR thread designation originated during the 1950s with the demand for fatigue-resistant safety-critical fasteners.
Per ANSI/ASME B1.1-2003 para 2.3.1: "The rounded root [UNR thread] ... reduces the rate of threading tool crest wear and improves fatigue strength over that of a flat root [UN] thread."

INTERNAL SCREW THREAD
There is no UNR for internal threads. Use only UN thread.
ANSI/ASME B1.1 para 1.3: "UNR applies only to external threads"
ANSI/ASME B1.1 para 2.3.2: "(there is no internal UNR screw thread)"

EXTERNAL SCREW THREAD
Both UNR and UN are used for external threads.

External UN threads per ANSI/ASME B1.1
para 1.3: UN threads have "a flat or optional rounded root contour" at the minor diameter.
para 2.3.1: UN threads have "a flat root contour" at the minor diameter, but "a rounded root contour cleared beyond the 0.25P flat width of the basic profile is optional"..."to provide for some threading tool crest wear."

External UNR threads per ANSI/ASME B1.1
para 1.3: UNR threads have "a rounded root contour" at the minor diameter.
para 2.3.1(a): "The root contour ... shall have a smooth, continuous, nonreversing contour with a radius of curvature not less than 0.10825318P at any point and shall blend tangentially into the flanks and any straight segment. At the maximum-material condition, the point of tangency shall be at a distance not less than 0.625H below the basic major diameter."

External UNR threads per IFI, Basic Elements of Screw Thread Design
page A-3: UNR threads have a "single difference from UN threads ... a mandatory root radius with limits of 0.108 to 0.144 times the thread pitch. The minimum radius of 0.108P is the largest radius that can be fitted into the UN profile without violating the minimum material condition of the external thread. The maximum radius of 0.144P is the largest radius that can be accommodated without causing theoretical interference with an internal thread at its maximum material condition."

SCREW THREAD WORK PLUG GAGES
Because there is no internal UNR thread, use the standard UN Threaded Work Plug Gages.

SCREW THREAD RING GAGES
It is not required that the gages be marked UNR. Use the same ring gages to check either the UNR or UN screw thread form. The amount of radius at the root of the UNR thread form is too small to require any special adaptation of the standard UN thread form ring gages.

HOW TO CHECK THE ROOT RADIUS
Two methods are available. In low volume applications the use of an optical comparator may be an appropriate and acceptable method to check the UNR thread form root radius at the minor diameter of the external screw thread. In high volume applications a Gaging System 22 style Tri-Roll gage set-up to monitor the minor diameter may be more cost effective.

THREAD NOMENCLATURE
UN = Basic Unified National thread series
UNR = Basic Unified National thread series with external thread controlled root radius
UNS = Special Unified National thread series
UNRS = Special Unified National thread series with external thread controlled root radius
UNC = Unified National Coarse thread series
UNRC = Unified National Coarse thread series with external thread controlled root radius
UNF = Unified National Fine thread series
UNRF = Unified National Fine thread series with external thread controlled root radius
UNEF = Unified National Extra Fine thread series
UNREF = Unified National Extra Fine thread series with external thread controlled root radius

ADDITIONAL NOTES OF INTEREST
1. The UNK thread was initiated after the UNR thread and obsoleted in 1979. The UNK was dimensionally identical to the UNR but had additional measurement requirements.
2. In the current market place, most production made bolts 1" or smaller are by default UNR because of the almost universal manufacturing practice of thread rolling.

COMMENTS
Original Posting: 7/24/2005
Last Revision: 8/20/2012
Error corrections in or comments about the above data can be sent to: gageguy@gagecrib.com

Click here to see this article as published in the September 2005 of Shop Talk Magazine.

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