What's the difference between a UNJ and a UNR thread?

[Bill Roberto]

Group,

I've been in this business for a long time and I know what a J thread
form should look like. I ran across a customer that had a print that
specified a UNR thread. The machinery handbook says smooth transition to
a radius at the root. What is the difference between the two?

Thanks in advance.

[b***@aol.com]

On Tue, 08 May 2007 05:03:49 GMT, Bill Roberto

Post by Bill Roberto
Group,
I've been in this business for a long time and I know what a J thread
form should look like.

An R form thread looks like a J form thread, UNR form simply has a
smaller root radius than the J form does.

Post by Bill Roberto
I ran across a customer that had a print that
specified a UNR thread. The machinery handbook says smooth transition to
a radius at the root. What is the difference between the two?

UN = Basic Unified National thread series
UNR = Basic Unified National thread series with external thread
controlled root radius

UNJ applies to both internal and external threads, it has a larger
root radius and is a stronger thread than UN or UNR.

UNR applies to external thread only, for internal threads use UN.

UNR threads have a single difference from UN threads, a mandatory root
radius.

UN threads have a flat or optional "rounded root contour" at the minor
diameter. UNR threads is that optional rounded root radius.

UNR rounded root radius reduces Tool wear* over UN flat root while
improving thread fatigue strength over that of UN thread. *cut
threads

Tom

[Michael Rainey]

Post by Bill Roberto
Group,
I've been in this business for a long time and I know what a J thread form
should look like. I ran across a customer that had a print that specified
a UNR thread. The machinery handbook says smooth transition to a radius at
the root. What is the difference between the two?
Thanks in advance.

In a nutshell - UNJ external has a larger root radius than UNR, UNJ internal
has a larger minor diameter than UNR.

[BottleBob]

Post by Bill Roberto
Group,
I've been in this business for a long time and I know what a J thread
form should look like. I ran across a customer that had a print that
specified a UNR thread. The machinery handbook says smooth transition to
a radius at the root. What is the difference between the two?

Bill:

Here is some data I found on UNR threads.

===================================================================
http://www.ring-plug-thread-gages.com/ti-UNR-vs-UN.htm

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
Per ANSI/ASME B1.1-1989 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 0.108P 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
===================================================================