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FAQs
ABOUT UN (UNS, UNEF,UNJ, UNF, ect.) & METRIC THREAD GAGES
Basic use of
Go/NoGo Thread Plugs & Rings
Using thread gages is really pretty simple if the Go thread plug goes in to the threaded hole and the NoGo does not then the product is usually considered within the tolerance limits and acceptable. The inverse is true of rings. Questions arise in some areas of how to judge if the product is not good using the NoGo gage. The specifications state that the product can be acceptable if the NoGo thread ring engages the end threads of the product being tested but should not thread over more than 3 (three) complete turns on the product. Do not force the gage on to the product and on thin walled parts, be especially aware that the gage may be more easily forced into or over the product distorting the product.
RULE # 1 through 10 "If it does not fit DON'T FORCE IT!"
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Depending on your requirements a special (Custom) thread gage will be
manufactured to your special requirements. Just let us know at
Technical@thegagestore.com .
Thread rings will require that you use a special Thread Setting Plug made
specifically for that thread size or a super micrometer with all the
balls, pins and calibration software required for this type of inspection.
FIRST RULE OF ENGAGEMENT: Thread Gages are NOT!
•Thread Chasing devices
•Lapping Or Re-Sizing Tools
•De-burring Tools
•Used to Measure Actual Size•
•Hammers (don't laugh its been done!)
•Thread Cleaning devices
CARE &
FEEDING of THREAD GAGES
•Never force a Gage into or on a Part Being Checked
•Handle gages as you would any precision tool
•Misuse or mishandling can result in nicks or other deformities which can destroy the integrity of the gage
•Store gages in a secure location, preferably in individual compartments or containers
•Gages should be dipped in an oil-wax based seal or coated with a rust preventive prior to storing
•If you must ship gages, pack them separately, coated with rust preventive, with sufficient packing material to avoid damage.
TERMS & DEFINITIONS (Yeah you probably already knew this but!)
A 60 degree thread plug is shown below
UN - unified screw thread constant pitch series
UNC - unified screw thread coarse pitch series
UNF - unified screw thread fine pitch series
UNEF - unified screw thread extra fine pitch series
UNJ - unified screw thread constant pitch series, with rounded root
UNJC - unified screw thread coarse pitch series, rounded root
UNS - unified screw thread Special diameter, pitch or length of engagement
UNJF - unified screw thread fine pitch series, rounded root
UNJEF - unified screw thread extra fine pitch series, rounded root
M - Metric Screw Threads- M profile with basic ISO 68 profile
MJ - Metric Screw Threads- MJ profile with rounded root
MJS - Metric Screw Threads- MJ profile profile special series
There are many more definitions for other thread types including National Pipe which we cover in the special section
"HOW TO USE NPT and other NP style gages, dedicated to those thread forms.
For more detailed information you can obtain the Screw Thread Specifications B1.7M 1984, directly from ASME via their web site.
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Confused about the different handle and styles of Thread Plugs?
Handle Styles can make a difference. Below you will see the two standard handle types of handles, Taper Lock and double ended reversible.
We always prefer and recommend the Taper Lock Style on thread plugs over the
Double Ended Reversible Style, because the tapered stub fits into a tapered recess in the handle
providing more surface contact between handle and thread plug member which holds the thread plug member firmly resisting rotation in the handle.
TAPER LOCK STYLE DOUBLE ENDED REVERSIBLE STYLE
Remember if your thread plug slips in the handle first check your product
to see that the thread
PD, Lead and root diameter (major) is correct, that the entry thread is not the
problem, or if the threads are not
clean. If these are okay, review your gage style to make sure it has enough
friction holding the gage member tightly for the application.
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IS IT A STANDARD OR SPECIAL THREAD GAGE?
QUESTION:
Is the gage called out with a "S" in the notation like UNS or MJS?
Do you have a non-standard Pitch Diameter called for?
If you are unsure, we have provided a PDF sheet for each ring and each plug
within the store site that will let you open and print the "Pitch
Diameter Reference Chart". You can either select your product category,
subcategory and product size at the store site or click this reference here to
see the PDF chart for standard sizes. Click on Reference Material to see
a typical PD chart.
Does the gage require special treatment such as a UNJ with rounded root radius?
Are you measuring the part before a plating operation "Pre-Plated" that
requires a custom thread PD?
TAPS & DIES, HOW DOES THE TAP CLASS CHANGE THE
PDs?
What PD is generated by your tap operation? See the Tap class
example below. Remember these classes of taps determine the PD sizes they will
cut. You will need to specify the PD of the gages you need based on your
experience and these target sizes before a gage OEM can manufacture your
specific thread gage.
Note: We do not provide advice on taps and machining processes, you should consult your local expert if ever in doubt.
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OOPS! I DROPPED MY THREAD GAGE
So you have used the gage to inspect a lot of parts and have concerns if the gage is worn or has been
damaged! Lets take this issue logically and break it down into two basic functions which are geared towards, first, a Thread Plug gage and second a Thread Ring gage.
When your Thread Plug gage was originally manufactured the OEM had the luxury of having an entire quality department set up to
measure the gages and confirm that they were in compliance with the prevailing standards and checking methods. If you are like most users you probably did not prepare
to replicate their quality department.
%$#@!. Thread gages will wear out, they will get dropped or stored in such a way as to cause
damage on the very surfaces you
are depending on to qualify your products. As such you should always have back
up gages on hand and be prepared to replace your gages periodically.
How do you do a quick, seat of the pants, confirmation after dropping the gage or finding out that you are all of a sudden rejecting everything you are making
(or maybe worse yet passing everything a little too easily)?
If you prepare a little in advance, you can make a quick confirmation if the gage is seriously damaged
(or not) by saving one or two of the original parts you passed (or did not pass)
when you first received the gage.
You might consider keeping one or two parts around that
originally passed & failed if possible just for this type of sanity check.
NO, DO NOT GO TO THE LOCAL HARDWARE STORE AND USE ONE OF THEIR PARTS AS A REFERENCE!
Second if you are making and inspecting threaded components you should
make preparations to periodically inspect your plugs for wear and damage. Worn
or damaged thread gages need to be replaced! Your parts
are not much good to your bottom line if they are sent back due to
nonconformance issues.
A simple method to check if the thread plug is due
to be replaced from wear is the method shown below. This is a relatively simple
method to confirm the PD on a plug but a word of caution. If you do not have the
"Best Fit" thread wires for your exact thread PD your results may be invalid. A
small difference in the thread wire diameter can make a large difference in the
measured PD values due to chordal error as the thread wire contacts the PD
surface of the gage.
It is very strongly recommended that you purchase the setting plug with
your thread ring. We recommend that you purchase the setting plug 100% of the time
for any custom thread rings when you order the thread
ring. Rings can get dropped on the job site, jarred in shipment and may need to
be reset periodically, so consider the small investment for the setting plug as
insurance that your products are leaving your shop with confidence.
The picture shown
depicts the use of a micrometer and 3 thread wires used to establish the pitch
Diameter of a traditional 60 degree gage plug.
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This is pretty simple and most of you experienced
thread heads know
these facts, but bear with me, since some of us have not had a lot of exposure to threaded parts and using thread rings to measure them.
First, as
most of you know, there are 2 (two) types of thread rings produced to measure the
functional fit of an external thread on a bolt or stud. You will still find the
occasional solid thread ring being used or called out but the predominant style
of ring in the USA is the split (or adjustable) thread ring. The exception is
for National Pipe rings to be explained in a different Technical section on the
site.
The split style of ring
was accepted many years ago by the regulating bodies as it became apparent that
solid ring costs both in manufacturing and end user durability were fast
becoming cost prohibitive. You can imagine what it might take to make a
"Perfect" thread form on the ID of a solid ring of steel cost
effectively, not to mention the potential scrap the OEM
could generate just to produce one saleable thread ring.
The split ring
however, which is most common for 60 degree thread forms, allows some small
amount of adjustment by the OEM, (and in special cases the end user or
calibration labs) based on a split adjustable design. This permits the OEM to
fine tune the ring to a SETTING PLUG. The setting plug lets the user do periodic checks of the ring
and determine if the it is time to replace the thread ring due to wear or
damage.
SOLID THREAD RING SHOWN
SPLIT RING
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HOW TO USE THE
SETTING PLUG
We
will limit our explanation to the Truncated setting plugs we would normally
supply for standard and special 60 degree thread forms.
We use the term "truncated" to denote the leading threads of a special setting plug that has had the thread crests ground down as prescribed in standard ASME practices. In this example below we show the Full Form Thread and also the Truncated (ground down ) thread form.
The reason to have 2 forms on this setting plug is to give the user the ability to check for wear on their thread ring both in the leading threads and also with full engagement. A procedure follows directly from the manufacture on setting your thread ring to the setting plug. The basics are as follows:
Adjusting a THREAD RING GAGE to a TRUNCATED SETTING PLUG
Make sure the setting plug being used is in good condition, proper PD not damaged or worn.
The following description will guide you to setting your split ring to the master setting Go or No/Go plug gage.
When the ring gage is set to the setting plug with uniform feel over the entire length of the plug, including no end thread shake, it is correctly mated with the plug. However, it is well to remember that because of tolerances in the manufacture of both the ring and the setting plug, and that unless they are made at the same time or ordered to be mated with each other, allowances must be made in the assembly feel or fit to compensate for slight differences. For instance, a slight difference in angle and/or Lead cause more of a tightening or drag at full engagement than over partial engagements. This is not a serious condition as both the ring and setting plug may well be within specifications.
If the ring gage is set to the truncated portion of the setting plug first instead of the reverse procedure described above, and it fails to pass over the first full thread of the full form section of the plug, there is major diameter interference in the ring and it should be replaced.Step 11 above will provide you an indication that the lead threads on your gage are worn but how do you check quickly if the entire thread form is worn and beyond adjusting range. Keep in mind that the adjusting screw is to "TWEAK" as we put it to make a very minor adjustment in the ring. This adjustment is not meant to change the PD of the ring to correspond to a custom undersized external thread nor to correct for excessive wear.
CAUTION: TRYING TO TIGHTEN THE RING ADJUSTING SCREW TOO MUCH WILL DISTORT THE RING CREATING AN INCORRECT FUNCTIONAL FIT EITHER PASSING NON-CONFORMING PARTS OR REJECTING PERFECTLY GOOD PRODUCTS.
A quick way to check if the ring is worn excessively is to use the truncated Setting Plug as follows.
1. Perform steps 4 & 5 above but thread the ring on only the truncated portion of the setting plug.
2. Perform steps 7 & 8 above until you achieve the "light drag" as required.
3. Now thread the ring the rest of the way over the Full Form threaded portion of the setting plug. You may feel a bit more drag but the ring should thread over the full form portion without any binding. DO NOT FORCE IT! If your ring is worn too much the ring will not easily screw onto the full form portion of the setting plug and needs to be replaced.
Even thought these rings are usually made of very hard material ( in some cases they are hard chrome plated) they do wear out so don't let yourself in for a customer reject, or worse yet, a field failure of the threads you have manufactured. Keep your gages in good condition check them periodically to make sure they are not worn beyond their wear limits, replace them when necessary.
A WORD ABOUT METRIC THREAD RINGS & PLUGS
The basics covered above apply to both UN and Metric thread plugs and rings. You will find that the typical classification call outs are different from the UN style but are similar. You will have classifications for Metric Thread Rings of 6g and 6g 4g which require a small difference in these two classes of rings. The Plugs are classed as 6H which also applies to the Thread Ring Setting plugs as well. Reference ASME B 1.16M for more complete information on metric thread gaging.
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NOTICE: All information provided on this reference sheet is intended to be for general guidelines and may not represent the full
specifications or instructions as outlined within ASME requirements or other technical specifications which govern the manufacturing and use of thread measuring gages.
thegagestore.com/LEECO INC, does not represent this information as anything
except a common sense general set of guidelines. You must always follow
the technical specifications relative to the gage, thread design or
other related gage attributes and usage that apply. When in doubt always acquire
the current specification from
http://www.asme.org/ codes & standards.
IF YOU
HAVE QUESTIONS OR COMMENTS, E-MAIL
Technical@thegagestore.com
OR FAX US AT 860-404-8903
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