I have been pulling my hair out trying to tap holes to get decent
threads. Instead they are always sloppy and oversized.

I use good technique.....tap absolutely square to the hole;
I use a good cutting oil; and I back the tap out to clear
the chips; etc.

But the fact remains that a 1/4-20 tap measures .255 diameter
while a 1/4-20 bolt measures .244 diameter (threaded portion)

That is .011 difference!!! I have checked my other taps and they are
all oversized.

How can this be and is there any solution?

Thank you very much

If you *must* have tighter fitting fasteners, you may go to class 4 or
above, but that is rarely required.

Class 4 and above are undersized or closer tolerance taps, right? I was
going to suggest a slightly undersized tap for 1/4-20... I presume it is
the same thing?

Regards,
Joe Agro, Jr.
(800) 871-5022
http://www.AutoDrill.com
http://www.Multi-Drill.com

V8013

wrote in message
oups.com...
I have been pulling my hair out trying to tap holes to get decent
threads. Instead they are always sloppy and oversized.

Only by checking your tapped holes with a proper plug gage can you judge
that the thread is oversized. There must be clearance between the male and
female thread in order for them to go together, and for interchangeability
with standard fasteners. If you're using a bolt to check your threads, it's
guaranteed it will feel sloppy unless it's considerably undersized. Even a
plug gage will have a somewhat sloppy feel if a thread is on top tolerance.
Typically, a no go can go a maximum of only two turns unless it goes all the
way, but very tightly. If it spins in easily, the thread is oversized.

I use good technique.....tap absolutely square to the hole;
I use a good cutting oil; and I back the tap out to clear
the chips; etc.

Tapping is very demanding of lubrication. A good cutting oil may not be
enough. Try to use prepared tapping fluids.

But the fact remains that a 1/4-20 tap measures .255 diameter
while a 1/4-20 bolt measures .244 diameter (threaded portion)

Both perfectly normal. You can not judge a thread my the major or minor
diameter alone. As has already been mentioned, the pitch diameter is the
major feature by which threads are gauged, although all features are
important. As long as the major and minor diameter are not out of limits,
the pitch diameter is what is really critical.


That is .011 difference!!! I have checked my other taps and they are
all oversized.

As they're supposed to be, for the allowance already mentioned.


How can this be and is there any solution?

Yep, use a tapping head if possible, and don't power drive hand taps.
That's an open invitation to breakage. It's also not a very good policy
to try hand tapping with power taps. They usually don't provide the kind
of support needed for helping to keep them oriented at right angles.

Thank you very much

Not sure I helped, but you're certainly welcome.

Harold

One thing you might check is exactly what kind of tap you are using.
If you look in a tool catalog at taps, you will see that most taps are
something like 1/4-20 h3. The h3 is how much the tap is oversize.
Oversize because a tap cuts a thread that is slightly smaller than the
size of the tap. Happens because the material flexes. If I remember
correctly h1 is .0005 oversize h2 is .001 oversize. So a normal 1/4
20 tap is an H3, but you might have a tap that is a 1/4-20 h5 or even
higher. I see in the J & L catalog 1/4-20 taps are mostly h3, but you
can buy h1,h2 h3 and h5 as well as some oversize taps that are just
sold as + .005

Dan


wrote:
I have been pulling my hair out trying to tap holes to get decent
threads. Instead they are always sloppy and oversized.

I use good technique.....tap absolutely square to the hole;
I use a good cutting oil; and I back the tap out to clear
the chips; etc.

Take a close look at the threads on the bolt. The tops are probably
"flattened" a little. This is why the bolt "appears" to be undersized.
Are the bolts really sloppy in the hole or is your concern solely
based upon the measurement differences?

Pete Stanaitis
------------------------

wrote:

I have been pulling my hair out trying to tap holes to get decent
threads. Instead they are always sloppy and oversized.

I use good technique.....tap absolutely square to the hole;
I use a good cutting oil; and I back the tap out to clear
the chips; etc.

But the fact remains that a 1/4-20 tap measures .255 diameter
while a 1/4-20 bolt measures .244 diameter (threaded portion)

That is .011 difference!!! I have checked my other taps and they are
all oversized.

How can this be and is there any solution?

Thank you very much

wrote in news:1113334475.276037.242700
@z14g2000cwz.googlegroups.com:

I have been pulling my hair out trying to tap holes to get decent
threads. Instead they are always sloppy and oversized.

I use good technique.....tap absolutely square to the hole;
I use a good cutting oil; and I back the tap out to clear
the chips; etc.

But the fact remains that a 1/4-20 tap measures .255 diameter
while a 1/4-20 bolt measures .244 diameter (threaded portion)

That is .011 difference!!! I have checked my other taps and they are
all oversized.

How can this be and is there any solution?

..244" is within the major diameter tolerance for a 1/4-20 thread. Your tap
appears to be in tolerance as well. As others have pointed out it's the
pitch diameter that counts. Bolts are always going to be somewhat on the
low end of the allowable tolerance for the major diameter. The reason is
that the bolts are thread rolled. Rolling a full form greatly reduces
rolling die life and doesn't really increase the strength of the thread. As
far as your threaded connection goes the same thing applies. It's unlikely
that having a greater percentage of engagement will increase the strength
of the connection enough to warrant the extra headaches that come with
making tight fitting threads. Here is a link:
http://www.precisiontwistdrill.com/t..._of_drills.pdf
If you are using the thread for adjustment rather than fastening and want a
tight fit, you would be better off using an external thread that you cut
yourself. If you are trying to use the thread for locating as well as
fastening, you would be better off designing your part with a pin or key
for locating.

--

Dan

Thanks guys for all your help. I still have some more questions.
First of all to answer Pete's question, the fit of the bolt is really
loose, especially the first 3 or 4 threads. If put to the test
they usually will strip out.
Question: Does the class of fit (1 thru 4) refer to the tap or the
bolt?
In all of my catalogs and all my internet searches I have never seen
a class 4 bolt. Usually they are referring to the hardness of the
bolt.
In any event, does anyone know of a source for class 4 bolts? Also
a source for quality taps?
Finally, what do you guys think of a spiral pointed tap for open-ended
holes?
Thanks so much.

Class of fit is between the two components. Essentially class 1 is a loose
fit.
Class2 is a free runnng fit. Class 3 is light interference fit. Class 4 is
a wrench fit.

If specifying a tapped hole, it is usually like .... 1/4-20 2B
The threaded component would be... 1/4-20 2A

Looking at apppropriate references (Machinery Handbook) gives specs for
these
classes of internal/external threads.

Then you select necessary tools to achieve these various spec's.
For example, you may use a "F" drill as a tap drill, but if it drills
oversize, you may have to select a smaller drill so you end up with
the correct sized hole prior to tapping.

The specs for bolt strength is an entirely different subject.

Good luck!










wrote in message
oups.com...
Thanks guys for all your help. I still have some more questions.
First of all to answer Pete's question, the fit of the bolt is really
loose, especially the first 3 or 4 threads. If put to the test
they usually will strip out.
Question: Does the class of fit (1 thru 4) refer to the tap or the
bolt?
In all of my catalogs and all my internet searches I have never seen
a class 4 bolt. Usually they are referring to the hardness of the
bolt.
In any event, does anyone know of a source for class 4 bolts? Also
a source for quality taps?
Finally, what do you guys think of a spiral pointed tap for open-ended
holes?
Thanks so much.

The class of fit is for the hole and bolt.

wrote in news:1113431278.985045.226760
@f14g2000cwb.googlegroups.com:

Thanks guys for all your help. I still have some more questions.
First of all to answer Pete's question, the fit of the bolt is really
loose, especially the first 3 or 4 threads. If put to the test
they usually will strip out.

What material are you tapping? Are you hand tapping? And most important
have you checked your hole for size and taper? Also, how many threads are
engaged between the bolt and your part?

snip (already answered)
Finally, what do you guys think of a spiral pointed tap for open-ended
holes?

Depends on the answer to the above questions. If you have a drill press,
knee mill, lathe, etc. and can power tap, rather than hand tap, you may
want to consider a roll form tap. Formed threads are stronger than cut
threads. If you are tapping a soft material and are using hardened bolts,
then it wouldn't take too much torque to strip out the threads with a small
thread like a 1/4-20, particularly if you only have a few threads engaged.



--

Dan

I read your first post and it begs the question....What size drill bit did
you use? Some of the 1/4-20 bolts here from Homedepot measure as low as
..236" and while they do fit sloppier than those in the .245" region they can
still hold without any inclination of stripping and that's in aluminum and
cast iron(albeit in tapped holes 1/2" deep or more). Assuming you're thread
cutting and not forming then your drill bit should either be #7 or 13/64",
that's .2010 and .2031 respectively. You shouldn't have a problem with the
bolt and tap diameters you mentioned provided the correct drill bit was
used. Heck, one of the 1/4-20 tap/drill bit combos from Homedepot should
work just fine if you live in the US. I've used them without complaints.

wrote in message
oups.com...
Thanks guys for all your help. I still have some more questions.
First of all to answer Pete's question, the fit of the bolt is really
loose, especially the first 3 or 4 threads. If put to the test
they usually will strip out.
Question: Does the class of fit (1 thru 4) refer to the tap or the
bolt?
In all of my catalogs and all my internet searches I have never seen
a class 4 bolt. Usually they are referring to the hardness of the
bolt.
In any event, does anyone know of a source for class 4 bolts? Also
a source for quality taps?
Finally, what do you guys think of a spiral pointed tap for open-ended
holes?
Thanks so much.