Threading tool helix angle

[T Bredehoft]

Early in my apprenticeship I made a block to hold a 3/8 HHS tool for grinding (on a surface grinder) a 60º threading tool. It had built into it 7º relief under each side of the cutting surface. This was to clear the flank of the cut thread. My toolmaker (mentor) had instructed me to study the Handbook to determine the angles. I do not remember how I arrived at the 7º angle, but it never failed me. I still have the block, but no longer have access to a surface grinder.

 

[Tozguy]

Just about the time I think I have this thing lassoed someone changes things up. My understanding is the angle increases as the TPI decreases and this would be true on standard bolts and threads we use daily, so the larger the diameter, the greater the angle, but a 1 1/2" dia. with a fine thread (I'll say 20 tpi) would have a Helix angle much less than the standard 1 1/2" UNF. I'm just looking for clarity when you commented on diameter.

To my mind the helix angle increases as TPI decreases for a given diameter.
But for a given TPI the helix angle decreases as diameter increases.

 

[benmychree]

Yes, I own a MH, the shop where I am currently employed has 3 different editions and none contain this data even in the special threads section.
The sample is likely well over 100 years old and somewhat damaged too boot, the parts were delivered today if they do not bounce it will be a miracle, I also have no idea what they do as usual.

I stand corrected; MH does not list a pitch diameter for this small of a screw with 12 pitch, however one could go by the table of thread depth (.07217" ) for a sharp V thread tool; turn a small neck on the part to diameter minus twice the depth and thread until the point of the tool touches the necked diameter.

 

[Wreck™Wreck]

Just about the time I think I have this thing lassoed someone changes things up. My understanding is the angle increases as the TPI decreases and this would be true on standard bolts and threads we use daily, so the larger the diameter, the greater the angle, but a 1 1/2" dia. with a fine thread (I'll say 20 tpi) would have a Helix angle much less than the standard 1 1/2" UNF. I'm just looking for clarity when you commented on diameter.

To throw something additional into the mix; How does the compound angle play into this? If the tool is ground to 60° and the compound is set at 29° for right hand or 31° for a left hand thread, how does the helix angle come into play if I'm advancing my compound for the depth of cut?

The angle of the compound on a lathe is meaningless for this conversation and its use is not required when threading regardless of what UTube how-to videos will tell you.
A thread is an inclined plane like a ramp or roof, helix angle is a function of lead and circumference therefore a 3/8"-16 thread has a far more acute helix angle then a 3"-16 thread as seen here with 4 TPI threads.
As mentioned above this will likely only become a problem with very course or trapezoidal threads

 

[Rustrp]

The angle of the compound on a lathe is meaningless for this conversation and its use is not required when threading regardless of what UTube how-to videos will tell you.
A thread is an inclined plane like a ramp or roof, helix angle is a function of lead and circumference therefore a 3/8"-16 thread has a far more acute helix angle then a 3"-16 thread as seen here with 4 TPI threads.
As mentioned above this will likely only become a problem with very course or trapezoidal threads

I understand this part and I'm not going off of YouTube. When you mentioned diameter increase increases helix angle increase it seemed as if you were excluding tpi. When I look at this I am looking at a side view of the thread and using the axis as a reference. I'm sure I will probably never cut a thread where helix angle becomes an issue. My comment regarding the compound was in reference to which cutting edge of the tool is doing the cutting. I'm also looking at this from my reference which is bending sheetmetal. When I'm forming a complicated piece for a hip or valley of a roof, what I'm seeing isn't always what is.
What I do understand is helix angle is a standard and when we make parts for others it becomes important, especially in larger diameters.

 

[benmychree]

The angle of the compound on a lathe is meaningless for this conversation and its use is not required when threading regardless of what UTube how-to videos will tell you.
A thread is an inclined plane like a ramp or roof, helix angle is a function of lead and circumference therefore a 3/8"-16 thread has a far more acute helix angle then a 3"-16 thread as seen here with 4 TPI threads.
As mentioned above this will likely only become a problem with very course or trapezoidal threads

I was not using compound depth of cut; the table in MH is about depth of cut (straight in) not the distance the compound is fed inwards; this is why I suggested turning a short neck onto the screw to a size double the amount stated subtracted from the nominal diameter, and feeding in with the compound until the sharp point of the tool hits the turned down portion, the compound only supplying the means of feeding into the cut. Incidentally, I have never used the 29/31 degree setting for the compound, always setting to 30 degrees off straight in. I do not know of a single journeyman in my apprenticeship shop who used the one degree difference in threading, and I was taught machine work by a shop teacher who was an apprenticeship instructor at Mare Island Navy yard and had apprenticed there himself.
I do not like chips being generated on both sides of the cut; the interference of each chip with the other can lead to galling

 

[Wreck™Wreck]

I understand this part and I'm not going off of YouTube. When you mentioned diameter increase increases helix angle increase it seemed as if you were excluding tpi. When I look at this I am looking at a side view of the thread and using the axis as a reference. I'm sure I will probably never cut a thread where helix angle becomes an issue. My comment regarding the compound was in reference to which cutting edge of the tool is doing the cutting. I'm also looking at this from my reference which is bending sheetmetal. When I'm forming a complicated piece for a hip or valley of a roof, what I'm seeing isn't always what is.
What I do understand is helix angle is a standard and when we make parts for others it becomes important, especially in larger diameters.

The helix angle is dependent on the diameter and lead where it is fixed, it is not STANDARD
I never use TPI when discussing threads with anyone, only lead, they are not always the same.

A 16 TPI thread is 1 divided by 16 or .0625" lead so one trip around a 1" diameter bar during threading is 3.141 inches per revolution X 16 times per inch or roughly 50 inches of cut length.

A 2" bar threaded 16 TPI is 2 X Pi X 16 revolutions per inch or 6.2 inches per revolution X 16 per inch = roughly 99", you have traveled the same distance in one plane yet have traveled nearly twice a far in another.

 

[benmychree]

I stand corrected; MH does not list a pitch diameter for this small of a screw with 12 pitch, however one could go by the table of thread depth (.07217" ) for a sharp V thread tool; turn a small neck on the part to diameter minus twice the depth and thread until the point of the tool touches the necked diameter.

I had a further thought on the 7/16 - 12 thread pitch; as disclosed above, MH does not list this diameter in the 12 pitch series, but what is to stop one from using the pitch diameter from a larger size of the same pitch and deducting the difference in diameter from a larger size thread, such as deducting .1875" from the pitch diameter of a 5/8-12; .5709 minus .1875 = .3843 PD for a 7/16-12 thread, although, without a calculator at hand, check my subtraction skills!