# Made scrap from parts due to a typo on LMS tap & clearance chart!



## finsruskw

Tapping some 1/2" 13TPI holes in 3/8" stock for Cub Cadet lift arm parts when I noticed the threads didn't look quite right, peaks were kind of flat and the bolt felt loose when turned in. This on the 3rd part out of the 6 I planned to make.

Brand new spirol tap, first use with plenty of oil, noticed it turned in easily too! Wow! these new fangled spirol 3 flute taps are really the cat's Meow!!

The tap looked good when  compared with my old Craftsman tap, although both felt loose in the bore as well.

LMS chart spec's 29/64" drill bit.
Craftsman chart that I checked after noticing this says 27/64"
Tried that bit and tapped some test holes and they turned out perfect and it turned a bit harder as well.
Glad I caught it when I did!
I think I will call them tomorrow and inform them they have a typo on that chart.
GRRRRR!!
Makes me kinda wonder if there are more than that one typo on that chart!


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## brino

@finsruskw

if it's this chart: https://littlemachineshop.com/Reference/tapdrill.php
(or the downloadable PDF version available on the same page)

You gotta be careful!
They have two columns depending on application.







Although it sounds like you used the drill they specify for steel.

I do use that chart, but often use the 75% thread column for steel....especially on thinner stock where I know I won't have much length or where I think the strength is required.

It's harder to tap, but the thread engagement feels better.

-brino


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## finsruskw

Why would they list a drill size that produces crappy threads in the material they spec it for??


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## markba633csi

Why would one want 50% threads anyhow?  I don't know what's up with that.  AFAIK it's most always 75


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## brino

My Busy-Bee Tools shop poster, my Newman Tools metal shop sign, and the Starrett wall chart and pocket versions* all show 27/64" tap drill for 1/2"-13TPI (and 29/64" drill for 1/2"-20 TPI).....but do NOT mention the thread depths.

The Machinery's Handbook also shows choices based on the thread depth you want:





.......but nothing as loose as 29/64" (50% threads).



finsruskw said:


> Why would they list a drill size that produces crappy threads in the material they spec it for??



The 50% threads are much easier to cut, and perhaps for some jobs are "good enough".

Note: I am NOT defending the LMS chart, because as I said I will often use the 75% thread column for steel.
However, even the Machinery's Handbook prints multiple options for different applications or fits.

All I can say is that when tight fit and thread strength matter, you need to know your chart.

-brino

*By the way, I believe you can get the Starrett wall chart and pocket versions for free.


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## MrWhoopee

brino said:


> *By the way, I believe you can get the Starrett wall chart and pocket versions for free.



Yes, I got my free wall chart last year.

I rarely consult a chart since it's so easy to calculate for 75% thread.
Can't say that I've ever wished for weaker threads.


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## projectnut

markba633csi said:


> Why would one want 50% threads anyhow?  I don't know what's up with that.  AFAIK it's most always 75


50% threads are sometimes used for deep holes.  When the thread depth exceeds 1 1/2 times the diameter of the fastener a 50% or 60% thread depth is usually strong enough, and allows minor misalignment of holes.  75% or larger taps are harder to turn at depths exceeding 1 1/2 times the fastener diameter and are more prone to breaking.


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## finsruskw

Well, at this point, I think the LMS chart is going in the recycle bin and I will use the old Craftsman chart as in the past even though it's tiny and hard to read.
No mention of 50% threads either.
Lesson learned, Geeesh!!


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## rwm

When tapping a deep hole in 316SS I am glad I used 50%. !
R


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## projectnut

I wouldn't necessarily scrap the parts.  If you feel the threads are too loose you can always install thread inserts.  We commonly installed them in aluminum side plates of production machines to combat the efforts of some over zealous mechanics.  There are many different styles available.  This is the style we used most often.  You don't even need a special tap. The 1/2"-13 size uses a 5/8"-11 tap.



			https://www.mscdirect.com/browse/tnpla/35475169?cid=ppc-google-New+-+Threading+-+PLA_shCddZ6el___164124448700_c_S&mkwid=shCddZ6el%7cdc&pcrid=164124448700&rd=k&product_id=35475169&pgrid=35979535606&ptaid=aud-974207103003:pla-79870179664&gclid=EAIaIQobChMI6Lyqz8Ko7wIVGx6tBh0_dwZMEAQYAiABEgLRg_D_BwE
		


They're available in different materials and from a number of sources.  I have them in the mounting flanges of the 14 hp Kohler engine in my Case 224 garden tractor.  I bought the machine used several years ago and found the engine only had 2 bolts holding it to the frame.  I put inserts in all 4 flanges and they're still holding up fine.  Just make sure to add a dab of antisieze to be sure you can remove the bolts years down the road.


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## Tozguy

I have been using the LMS chart for years and like having the two columns. I will deviate from either column if the job requires it.


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## MtnBiker

I had this question as well. In 1" steel (same 1/2 x 13) I went a little > than 50% engagement and the threads looked weird but seem to work just fine. Very informative conversation this. Can definitely see the utility in deep holes.


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## ericc

The LMS 50% thread engagement spec is good for steel IF the tap is good quality, the drill drills on size, the part is thick enough, and the screw is the correct size.  If any of these conditions are not met, the screw may not hold.  It has happened to me, so don't let anyone tell you it cannot happen.  There are two things you can do to prevent this unfortunate occurrence.  First, good taps are good economy.  Cheap taps are poor economy, but they make great boring bars.  A drill point gauge is a very useful tool to make sure that your sharpened drill makes on size holes.  Most high quality drill bits are good out of the box, but watch out for the cheap ones.  It may be useful to test their hole size before scrapping parts, especially if there is a lot of work in the part.


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## pacifica

A 27/64" drill with a R/O of .015 and a 29/64 with R/O of .001 won't be giving you a hole much different in diameter , and 29/64  will be easier to thread especially in hard steel. In a 1/2" hole with 5 threads engaged- in most situations this is a strong assembly.
If you need class 3 or 4 thread then this is a different issue and more difficult to atttain.


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## aliva

I always use 75% even in SS


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## walz10

I’ve found that depending on the material you’re working with and the size of thread your trying to tap that 50% thread depth can be troublesome to tap at best. I’ve worked with some materials that getting a 1/2-13 tap through 1/2” tick material with 50% thread depth would break a good sharp tap every time. Keep in mind most of what I’m talking about are automotive castings. I’m those cases the solution is to keep enlarging the hole size until the tap will go in without breaking. Often times this means less than 50% thread depth. In the instances I’ve had to do this the tapped hole has always served it’s purpose. My point is that the LMS drill chart isn’t necessary wrong or right but also that we need to be aware of what we are doing when making things and when things don’t turn out the way we want them to we should learn from that so we’re less likely to do it again. For me in the OPs situation I’d leave that chart on the wall as a reminder of what happened when I used it that one time. But I’m forgetful so I need those reminders hanging around. 


Sent from my iPhone using Tapatalk


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## pacifica

_According to a study done by U of Tulsa engineering department_:

Internal threads, 5/8-18 in .26 % carbon mild steel, precision bored holes. Cut threads maintained their strength from .578 to .597. 35,000-lb strip out strength was used with  a 5/8-18 ,.5" long grade 8 steel setscrew . Setscrew was  pressed  out with a steel rod while recording the required force(35k lbs).
In fact tap holes must be increased to .602( about 33% thread engagement) before the  strip out force needed dipped below 35,000. both spiral and plug type taps were used.

test done in Jan of 2011.


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## higgite

finsruskw said:


> Tapping some 1/2" 13TPI holes in 3/8" stock for Cub Cadet lift arm parts when I noticed the threads didn't look quite right, peaks were kind of flat and the bolt felt loose when turned in.


Would 50% thread engagement not work for your application or were you just not happy with how it felt?

Tom


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## JimDawson

One other thing to concider is what size hole did the drill actually make?  A 1/2-20 thread drilled at a true 29/64 diameter will give a nice thread fit, if the hole is oversize then it can be a problem.  I use 29/64 all the time in SS, saves taps, important when doing 100's of holes.  It only makes the thread crests a bit flat, the major diameter is controlled by the tap.


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## matthewsx

Thread inserts are a lot easier to install than broken taps are to remove.

John


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## mattthemuppet2

I use the LMS chart all the time. I like having the 2 columns, as others have mentioned, as it gives you different options. I've tapped holes where I've had to go below 50% thread engagement to be able to tap it without risk of breaking the tap, I've tapped others (especially thin material) at 80%. Then there are thread/ roll form taps. Sometimes I split the difference between the two columns, depending on material and thickness. Thread engagement doesn't need to be a fixed value, use what is appropriate for the material, thickness and application.


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## rwm

I thought I had 50% engagement but then I ended up married...
Robert


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## Tozguy

We all know the feeling when a tap starts getting tight and springy........but we are almost there so try for just one more turn.....pink!


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## mattthemuppet2

yup, I've done that. It's pretty much guaranteed to be the last op on a part too!


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## brino

Tozguy said:


> We all know the feeling when a tap starts getting tight and springy........but we are almost there so try for just one more turn.....pink!



Don't do that!
My heart was racing and I started to sweat as I read it.

You're damned if you go forward and you're damned if you go back........

A dreadful place to be!

-brino


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## finsruskw

Not happy with the feel or the look.
Did some more today with 27/64 and all is well once again.
Hole size was spot on in both instances as well


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## brino

finsruskw said:


> Did some more today with 27/64 and all is well once again.
> Hole size was spot on in both instances as well



Can you save the ones you're not happy with?
-brino


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## finsruskw

Possibly, if i went with 9/16" but then mods would have to be made in the OEM part it mates with., don't think anyone would want to do that.

Not a big deal, small part and I'm not out that much in the way of material.
Will chalk it up to training!!

This part has to do with fore and aft movement and part of a lift rod ass'y for a hydraulic cylinder under load and I feel, it should have full thread engagement and stay tight in the sleeve it goes through.
It is also secured with a jam nut for added rigidity.
The piston clevis is pinned to it in this photo and moves in the same arc on the same axis with the white lift arm so has to remain tight.


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## projectnut

finsruskw said:


> Why would they list a drill size that produces crappy threads in the material they spec it for??


The threading charts on LMS are only basic references for the most common types of threads in the most common materials.. They are  meant to be quick references for those knowledgeable as to where and when each apply.  Much more comprehensive information about thread styles,  forming, and cutting can be found in the Machinery's Handbook.  Even then each edition doesn't cover all the thread types and methods to create them.  Each edition covers those that were popular at the time it was printed.  As time moves on some styles and methods are considered obsolete and deleted while others previously not covered are added.  Editions from the 1930's and 40's barely touch on metric threads, and later editions largely pass over British Standard threads.

Looking at the 22nd (1986) edition there are nearly 230 pages covering thread types and methods of cutting or forming.  As Mentioned previously this section only covers those deemed to be widely used in industry at the time of printing.  I currently have over a dozen different editions of this publication to cover the types, styles, and methods of creating the ones I deal with on a regular basis.  Thread cutting and/or forming is far from a one size fits all scenario.  There are many variations depending on the material, method, and type of thread needed.


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