un-unr-unj-threads

INTRO.

C Nairn

Colin Nairn
Oct. 2010
colinnairn@gmail.com
(Comments/Suggestions)

REFERENCES

ASME-B1.1 (1989)UN, UNR

ISO-3161 (1977) UNJ

ABBREVIATIONS

AN= American National
BWS= Best Wire Size
ASME= American Society of Mechanical Engineers
DIA= Diameter
DOT= Depth of thread
H= height
ISO= International Standards Organization
MAX= maximum
MIN= minimum
M/C H/B= Machinery’s Handbook
MMC= Maximum material condition
MOW= Measurement Over Wires
OD= Outside Diameter
P= Pitch = 1/TPI
PD= Pitch Diameter
R= Radius
REF= Reference
TDS=Tap Drill
TNR= Tool Nose Radius
TPI= Threads Per Inch= 1/P
UN= Unified National
UNC= Unified National Coarse
UNF= Unified National Fine
~ Approximately equal to

INTRODUCTION HOME

This paper was compiled to get a clearer understanding of UN, UNR &
UNJ threads. The data was taken from ASME & ISO standards and
transposed from the function “H”, the height of the thread triangle, to "P", a
function of the pitch, so the machinist can control the 3 diameters, major
minor & pitch.
The other reason is to get a look at the root dimeter profile because when cutting a thread the root diameter is important in that it determines the strength of the thread. When cutting a thread, plunging the cutter deeper to get to size reduces the root diameter.

Summary of Contents --

All the data required, with the exception of tolerances and allowances, is in "The Easy Memory Chart".
The flank of the thread must be perpendicular as indicated by the symbol in the drawings.
The minor diameter of an external thread is controlled by the depth of cut and the pitch diameter tolerance.
The only difference between a UN internal and a UNJ internal thread is that the minor diameter of a UNJ internal is increased by .054127P X 2. (.054127 is 10% of .54127).

The section "A Practical Method to cut a Thread" is included so that even a novice could cut an accurate thread.

The section "General Notes" contains some miscellaneous observations.

Using the "Easy Memory Chart" (see the drawings to clarify), I can accurately cut a UN class 3 external thread and know that is correct and will fit without having to use a gage or any tables. I would just stay away from MMC on all 3 diameters. For a class 1 or 2 external, however, I also need to know the allowance, which is available in Machinery's Handbook.

The following “Easy Memory Chart” is a summary of all the data with the following
two points in mind—

1.) The “Best” D.O.T. was chosen so that no matter what T.N.R. is used
(within limits) the thread profile will be correct.

2.) The D.O.T. is given at MMC with no tolerances or allowances
applied. Therefore, in a pinch you could cut the thread with the
information given and “guesstimate” or look up, the tolerances
and allowance for the amount to move the three diameters away from
MMC.

EASY MEMORY CHART (NO TABLES-MMC)

BEST EXTERNAL THREAD DEPTHS (from basic O.D.)
  UN    .61343P (R 0 to .14434P)
  UNR .61343P (R .10825P to R .14434P)
  UNJ .57735P (R .15011P to R .18042P)
  (Don't confuse .10825 with .18042)

BEST INTERNAL THREAD DEPTHS (from UN basic minor dia.)
UN/UNR/UNJ = .57252P (R 0 to .072P)
then for a UNJ increase the minor diameter by (.054127P X 2).

UN BASIC MINOR DIA. = basic O.D. - (2 X .54127P).
BASIC P.D. = Basic O.D. - .64952P (UN,UNR,UNJ,External & Internal).
Max. D.O.T. is D.O.T. selected plus 1/2 the allowance plus 1/2 the P.D. tolerance.

MOW Formulas
BWS = .57735P
MOW = Nominal O.D. - (1.5155P) + 3W (W = actual wire size used).
MOW = Pitch Diameter - (.86603P) + 3W (W = actual wire size used).

No allowance
or tolerances
applied.

R2C3

General Notes
HOME

A PRACTICAL METHOD TO CUT A UN THREAD

1) Using the "Easy Memory Chart" calculate the following--Pitch Diameter, Depth of Thread, Best Wire Size, Measurement Over Wires and the UN basic minor diameter (if you are cutting an internal thread). Be sure to use the actual wire size used, not the best wire size, in the Measurement Over Wires formula..

For example-
1-8-UNC-3A, O.D.=1.00, PD (max.) = .9188, DOT = .0767, BWS = .072, MOW = 1.0266
1-8-UNC-3B, Basic minor dia. = .8647, DOT = .0716,
2) Machine the reference diameter, O.D. for external, I.D. for internal. Do not chamfer the leading edge, this will allow the cutter to take the initial shock on the flank as opposed to the tool tip.
3) (Important) Make sure that the threading tool has no rake, the profile is correct, it is on center and perpendicular to the work. (Use a sharp pointed cutter with a small radius honed on the point).
4) Set the lathe levers or change gears for the desired pitch or T.P.I. Then take a light cut to check it is correct and set the crosslide dial to zero.
5) To cut an internal thread first cut an external thread, to MMC, to use as a gage. Then machine the internal minor diameter (basic) as a reference for the depth of cut.
6) Set the compound parallel to the lathe axis, then feed the cutter in using the crosslide and the compound, whatever you feed in on the crosslide, feed ½ of that on the compound—for example, if you feed .012 radially (.024 dia.) on the crosslide, feed .006 on the compound. This means that most of the chip will be coming off the leading edge of the cutter preventing the chip from piling up in the middle, it is similar to feeding the cutter in at 29 1/2 degrees . When you reach the proper depth you can now move the compound back and forth to get the pitch diameter without changing the minor diameter.
7) An easy way to use the three wire method to measure the pitch diameter--

Set the mike to .025" over the O.D.

Set the mike on top of the thread.

Insert two wires between the mike anvil and the bottom of the thread.

Take the other wire and insert it between the mike spindle and the top of the thread.

Take the measurement (make sure the 2 bottom wires are both tight) and observe the reading, back off the mike to get the wires out and remove the mike.

Reset the mike to the observed reading and lock it----you now have the M.O.W. recorded on your mike.

GENERAL NOTES
Flanks of all UN, UNR, UNJ external threads to be straight above the basic minor diameter (as shown by the perpendicular sign in the drawings), for internal threads it is below the basic major diameter. (Therefore, if cutting a UN external flat root at MMC the tool must be sharp, absolutely no radius).
PD (basic) is the same for UN, UNR, UNJ, external, internal.
Allowance is always applied to external.
No allowance on a class 3 or UNJ or internal. Therefore a bolt/nut cut to MMC will be
size/size.
There is no UNR internal.
A UNJ bolt will only mesh with a UNJ nut.
UNJ is always class 3
The min. root radius of a UNJ external is greater than the max. radius of a UN, UNR.
A UNJ internal thread profile is the same as a UN except the basic minor dia. is increased
by (.054127Px2).
Threads are only rolled up to 1” diameter after which they are cut. (Rolled is stronger).
A bolt is loaded in tension, a nut is loaded in compression.
UNK- same as UNR except the minor diameter of the external thread is toleranced and
inspection of the root radii is mandatory. Became obsolete in 1979 with the use of gaging
system 22.
Do not exceed 83% AN TDS, (100% UN).
There is no class 1 for sizes below ¼ ".
P.D. tolerance internal is greater than the P.D. tolerance external. (By 30%, ASME B1.1- 1974).
UN bolts- tensile strength 60,000 psi.
UNJ bolts- tensile strength 160,000 psi.
Length of engagement

UN/UNR = 1 diameter,
UNS = 9 pitches.
UNJC, UNJF, 8UNJ – 1 diameter.
UNJEF, 12UNJ, 16UNJ – 9 pitches.

UN Definition HOME

C4 DEFINITIONS

DEFINITIONS

UN EXTERNAL

D.O.T. (From basic O.D.)

Max. = .64952P + ½ allowance + ½ PD tolerance.

Min. = .54127P (no radius) + ½ allowance.

Best DOT = .61343P + ½ allowance + ½ PD tolerance used.

ROOT RADIUS

Optional but not to exceed .14434P (@ .61343P D.O.T.).

O.D.

Max. = Nominal – allowance.

Min. = Nominal – allowance – tolerance.

O.D. Tolerance

Class 1 = .09 x ³√P²

Class 2/3 = .06 x ³√P²

P.D.

Max. = nominal O.D. – .64952P – allowance.

Min. = PD Max.– tolerance.

MINOR DIAMETER (Ref)

Max. = nominal O.D. – (.54127P x 2) no radius – allowance.

Min. = nominal O.D. – (.64952P x 2) – allowance – PD tolerance.

UN INTERNAL

MINOR DIA. P.D. Min.= Basic, all classes

Min.(Basic) = Nominal O.D. – (.54127P x 2) = O.D. – .64952P

Max. = tables Max.= Basic + PD tol. for class specified

D.O.T. (From basic minor diameter)

Min. = .54127P (Sharp tool, no radius)

Max. = .61343P + ½ the P.D. tolerance.

Best = .57252P + ½ PD tolerance (allows for a radius of 0 to .072P).

ROOT RADIUS 0 - .072P

UN Example HOME

EXAMPLE

.250 - 20 - UNC- 2A (EXTERNAL)

D.O.T.

Max.= .0325 + .00055 + .00185 = .0349

Min. = .0271 + .00055 = .0276

Best = .0307 + .00055 + .00185 = .033

ROOT RADIUS

Max. = .0072

Min. = 0

O.D.

Max. = .250 – .0011 = .2489

Min. = .250 – .0011 – .0081 = .2408

P.D.

Max. = .250 – .03248 – .0011 = .2164

Min. = .2164 – .0037 = .2127

MINOR DIA.(Ref)

Max. = .250 – .0541 – .0011 = .1948

Min. = .250 – .06495 – .0011 – .0037 = .1802

.250 - 20 - UNC- 2B (INTERNAL)

MINOR DIA. P.D.

Min. = .250 – .05413 = .1959 Min = .250 – .03248 = .2175

Max. = tables = .207 Max. = .2175 + .0049 = .2224

D.O.T.

Min. = .0271 (Sharp tool no radius)

Max. =.0307 + .0024 = .033

Best = .0286 + .0024 = .031

ROOT RADIUS

0 - .0036

UN Basic Drwg. HOME

UN External Drwg. HOME

UN Internal Drwg. HOME

UNR Definition HOME

DEFINITION

UNR EXTERNAL

D.O.T (From basic O.D.)

Max. = .64952P + ½ allowance + ½ P.D. tolerance.

Min. = .59539P + ½ allowance.

Best = .61343P + ½ allowance + ½ P.D. Tolerance used.

ROOT RADIUS (Mandatory)

Max. = .14434P (@ .61343P)

Min. = .10825P

O.D.

Max. = Nominal – allowance.

Min. = Nominal – allowance – tolerance.

O.D. Tolerance

Class 1 = .09 x ³√P²

Class 2/3 = .06 x ³√P²

P.D.

Max.= nominal O.D. – .64952P – allowance.

Min. = PD Max. – tolerance.

MINOR DIAMETER (Ref)

Max. = nominal O.D. – (.59539P x 2) – allowance.

Min. = nominal O.D. – (.64952P x 2) – allowance – P.D. Tolerance.

UNR INTERNAL (Same as UN Internal)

MINOR DIA. P.D. Min.= Basic, all classes

Min.(Basic) = Nominal O.D. – (.54127P x 2) = O.D. – .64952P

Max. = tables Max.= Basic + PD tol. for class specified

D.O.T. (From basic minor diameter)

Min. = .54127P (Sharp tool, no radius)

Max. = .61343P + ½ the P.D. tolerance.

Best = .57252P + ½ PD Tolerance (allows for a radius of 0 to .072P).

ROOT RADIUS 0 - .072P

UNR Example HOME

C10

EXAMPLE

.250 – 20 – UNRC– 2A (EXTERNAL)

D.O.T.

Max.= .0325 + .00055 + .00185 = .0349

Min. = .0298 + .00055 = .030

Best = .0307 + .00055 + .00185 = .033

ROOT RADIUS

Max. = .0072

Min. = .0054

O.D.

Max. = .250 – .0011 = .2489

Min. = .250 – .0011 – .0081 = .2408

P.D.

Max. = .250 – .03248 – .0011 = .2164

Min. = .2164 – .0037 = .2127

MINOR DIA.(Ref)

Max. = .250 – .0595 – .0011 = .1894

Min. = .250 – .06495 – .0011 – .0037 = .1802

.250 - 20 - UNRC- 2B (INTERNAL)

MINOR DIA. P.D.

Min. = .250 – .05413 = .1959 Min = .250 – .03248 = .2175

Max. = tables = .207 Max. = .2175 + .0049 = .2224

D.O.T.

Min. = .0271 (Sharp tool no radius)

Max. =.0307 + .0024 = .033

Best = .0286 + .0024 = .031

ROOT RADIUS

0 - .0036

UNR External Drwg. HOME

UNJ Definition HOME

C12

DEFINITIONS

UNJ EXTERNAL

D.O.T. (From basic O.D.)

Max. = .60766P @ R .15011P + ½ P.D. tolerance.

Min. = .57735P @ R .18042P max.

Best = .57735P @ R .15011P to R .18042P + ½ P.D. Tolerance used.

ROOT RADIUS (Mandatory)

Max. .18042P @ .57735 d.o.t. Do not confuse UNJ .18042 root radius

Min. .15011P with UNR .10825 root radius.

O.D.

Max. = Nominal

Min. = Nominal – tolerance.

O.D. Tolerance = .06 x ³√P²

P.D.

Max. = Nominal O.D. – .64952P (same as UN ).

Min. = PD Max. – tolerance.

MINOR DIAMETER (Toleranced)

Max. = Nominal O.D. – (.57735P x 2) @ R .18042P.

Min. = Nominal O.D. – (.60766P x 2) @ R .15011P – P.D. tolerance.

UNJ INTERNAL

MINOR DIA. P.D. Min.= Basic

Min.(Basic) = Nominal O.D. – (.48714P x 2) = O.D. – .64952P

Max. = tables Max.= Basic + PD tol.

D.O.T. (From UNJ basic minor diameter)

Min. = .48714P (Sharp tool no radius).

Max. = .55931P + ½ the P.D. tolerance.

Best = .51839P + ½ PD tolerance (allows for a radius of 0 to .072P).

ROOT RADIUS 0 - .072P

UNJ Example HOME

C13

EXAMPLE

.250 – 20 – UNJC – 3A (EXTERNAL)

D.O.T.

Max. = .0304 + .0014 = .0318

Min. = = .0289

Best = .0289 + .0014 = .0303

ROOT RADIUS

Max. = .009

Min. = .0075

O.D.

Max. = = .250

Min. = .250 – .0081 = .2419

P.D.

Max. = .250 – .0325 = .2175

Min. = .2175 – .0028 = .2147

MINOR DIA. (TOLERANCED)

Max. = .250 – .0578 = .1923

Min. = .250 – .0608 – .0028 = .1864

.250 – 20 – UNJC – 3B (INTERNAL)

MINOR DIA. P.D.

Min. = .250 – .0487 = .2013 Min = .250 – .03248 = .2175

Max. = tables = .2121 Max. = .2175 + .0036 = .2211

D.O.T.

Min. = =.0244 (Sharp tool no radius).

Max. = .028 + .0018 = .0298

Best = .0259 + .0018 = .0277 (Or cut a UN internal then increase

the minor dia. By .0054)

ROOT RADIUS 0 - .0036

UNJ Basic Drwg. HOME

C14

UNJ External Drwg. HOME

C15

UNJ Internal Drwg. HOME

C16

UN/UNR
Tolerances
Coarse HOME

UN/UNR
Tolerances
Fine HOME