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!

 

[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

 

[finsruskw]

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

 

[markba633csi]

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

 

[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).

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.

 

[MrWhoopee]

*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.

 

[projectnut]

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.

 

[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!!

 

[rwm]

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

 

[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.