threading question.

[jrlenz]

I'm working on the 1/4 scale Galloway. Some dimensions are missing on the oiler assembly. Normally I can figure it out with the help of the internet but this one has me stumped. The bottom of the base assembly calls for a 9/32-40 thread. I bought the tap and die but I cannot find the dimension I should turn the part to in order to accept the 9/32-40 die. being an unusual thread the data its not to be found on the internet. Any ideas?

Regards,
Jeff

 

[chipenter]

Found this on the internet .
--------------------------------------------------------------------------------
Thread data with tapping and clearance drill sizes for the following commonly
used types.
* ME (Model Engineer) series.
* BA (British Association) series.
* BSP (British Standard Pipe) series.
* BSC (British Standard Cycle) series
* BSB (British Standard Brass) series.

--------------------------------------------------------------------------------
Model Engineer - ME

Size T.P.I. Tapping Drill Size.
mm Number/Letter/fraction
1/8 40 2.55 38
5/32 40 3.25 30
3/16 40 4.00 21
7/32 40 4.8 11
1/4 26 5.3 4
1/4 32 5.5 7/32
1/4 40 5.5 7/32
5/16 32 7.00 J
5/16 40 7.00 9/32
3/8 32 8.7 11/32
3/8 40 8.7 11/32
7/16 40 10.3 13/32
1/2 40 11.9 15/32
1/2 32 11.9 15/32
1/2 26 11.8
9/16 26 13.5 17.32
5/8 26 15.0 19/32
3/4 26 17.8 45/64

 

[jrlenz]

Yea. I found that kind of data. My problem is the unusual thread, and the fact that I'm using a die and need to know the diameter to turn instead of drill. But thanks for the quick response.

 

[dman]

I believe the major diameter goes by the formula (n*.013)+.060 so a 9 is .177 for the rod that goes through the die to become a bolt.

the minor is a formula I'm a little rusty on. but major dia -(1/tpi) will get you close. I can get a bit more accurate with more math but there is a general formula for this that escapes me at the moment.

 

[kvom]

Personally I would just use an 8-32 or 10-32. On models you can be pretty free with fasteners.

 

[va4ngo]

my 1934 machinerys handbook says that for U.S.S. Threads, the depth of thread equals 0.6495/threads per inch. so for 40 threads = 0.6495/40 . just double the answer and subtract from major diam

 

[dman]

my 1934 machinerys handbook says that for U.S.S. Threads, the depth of thread equals 0.6495/threads per inch. so for 40 threads = 0.6495/40 . just double the answer and subtract from major diam

if threads were an ideal isosceles triangle it would be sine 60 which is about .866. but some threads are 59deg and they are never completely pointed. so yeah .65 could be right. so major dia - (1.3/tpi) may be the best answer. you may want to round up on the tap drill from that to make tapping easier. especially for cast iron.

 

[sssfox]

Why wouldn't it be a couple of thousandths less than the diameter of the tap?

 

[dman]

Why wouldn't it be a couple of thousandths less than the diameter of the tap?

that would work for the bolt. but he need both the bolt and the tap drill size. gotta know how deep the threads are for that. I was gonna suggest that but some taps have 3 flutes and can be awkward to measure.

 

[Mike N]

that would work for the bolt. but he need both the bolt and the tap drill size. gotta know how deep the threads are for that. I was gonna suggest that but some taps have 3 flutes and can be awkward to measure.

The answer is 9/32nds:wall:

 

[sssfox]

that would work for the bolt. but he need both the bolt and the tap drill size. gotta know how deep the threads are for that. I was gonna suggest that but some taps have 3 flutes and can be awkward to measure.

There isn't anything I could find in the op about drilling a hole for the tap, only sizing the rod to fit the die.
Am i missing something?

 

[dman]

The answer is 9/32nds:wall:

oh wow totally missed that. thought he meant a 9-32 or 9-40. a 9/32-40 is way larger and that is quite a fine thread for that diameter. so yeah the bolt is uhm 9/32 or .281 and the tap drill is .281-(1.3/40) = ~.250

 

[dman]

There isn't anything I could find in the op about drilling a hole for the tap, only sizing the rod to fit the die.
Am i missing something?

maybe I read into it. he said he bought a tap and a die. the tap drill is the one that's harder to find. maybe I'm tired but I swore I read things that are not actually there. lol...

 

[sssfox]

Don't sweat it. It happens to the best of us.

 

[dman]

yeah I can't believe I got the size wrong as well as the question. I mean 9/32-40 is weird but 9-40 is equally weird so I'm not sure why I assumed that's what he meant. oh well I think the question was answered pretty thoroughly. to the op the diameter for the thread size is always the nominal major diameter. the actual major diameter is slightly less but depends on the class of fit. if you subtract .003-.005 it's close enough but by the time the die runs over it and you deburr you probably don't need to take it into account.

 

[bazmak]

Any thread size is the od of the bolt nothing else matters.9/32 x 40 is an ME thread Whit form but fine
therefore shaft should be 9/32 before threading. Not complicated BAZMAK

 

[jrlenz]

Ouch. Well, ain't I the noob. Sorry for the silly question. But thanks for the replies.

 

[cncjunior]

As a quick answer take a few though off of the 9/32 diam. You can leave the daim as 9/32 and the die will take it down to the right major diam. All depends on how good your die is. Cheap dies will not work well taking off from full diam. A quick look at the MHB for a similar size 1/4 x 40 gives a dia. range of 0.2491 to .244". This is for the 1/4" size but the range is the same regardless if it 12"x40 or 3"x40. Use the range. In this case it would be 0.2804 to 0.2753" for the 9/32" diam shaft.

 

[Lew Hartswick]

If it works for all Metric threads why not for ALL threads???
Diameter of the basic thread minus the pitch.
ie in this case 9/32 - 1/40 inches. Works for all the Imperial threads I've checked.
...lew...