Anyone ever see internal threads like this? 5

[heloman78]

I'm reverse engineering this part and I sectioned the tube these (internal) threads were housed. I put it under my digital microscope to inspect the threads and I found these groves machined into the tip of each thread. I've never seen anything like it. I'm guessing their their to provide a tighter fit by deforing upon assembly, Can anyone identify this?

 

[heloman78]

whoops, they're there

 

[berkshire]

Looks like a thread forming tap was used i.e. roll forming.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[TVP]

berkshire is correct, that is what the thread crest looks like when it has been tapped using a cold forming tap. Look at page 15 of this .pdf from Emuge on their InnoForm product line:

http://www.emuge.com/technical/pdf/Threading/Tap/zp10019_gb.pdf

FWIW, this is essentially identical to what happens when rolling external machine threads as well, although it is dependent upon the exact shape and penetration rate of the dies during the rolling process.

 

[rb1957]

if this is related to your other thread (about T3 and T42 tubes) ... i supposed at the time that the thread might be rolled (as the wall thickness is pretty small, 0.07")

 

[heloman78]

You were right RB, I've never seen that little groove in the peak of the thread until I magnified it. The picture is at 45x magnification. TVP, your attachment clearly shows the groove on page 18 lower left figure. I've never see that before on rolled threads, any idea how common it is?

 

[dgapilot]

Told you we used a roll tap.

 

[berkshire]

It is very common on rolled and cold formed threads.
It gets worse if the " tapping Drill " is a little oversized.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[TVP]

Very common, especially in softer materials like aluminum.

 

[dgallup]

TVP's attachment shows it on the front cover. Nice PDF, I saved it for future reference. LPS.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[monkeydog]

Good pics of the rolled threads, but the cut threads sure look a lot "cleaner" then what is normal.

 

[rb1957]

i haven't looked at the pix (work site blocks 3rd party file sites), but rolled threads shouldn't have grooves as discussed. the point with rolled threads is that they have better fatigue properties, 'cause they don't go along cutting across grain flow; they reform the grain flow to fit the threads. that and of course rolling the thread doesn't remove material, and places it into compression ... all good things. "grooves at the root of the thread" doesn't sound like a good thing.

 

[TVP]

rb1957,

The "grooves" are not in the root but at the crest. They are sometimes referred to as fish lips, bunny ears, or crest craters and are schematically depicted in discontinuity standards like ISO 6157-3 or SAE J123 and fastener procurement standards like ISO 7689, SAE AS7466, ASTM F835, etc.

 

[rb1957]

my interpretation of "peak of the thread" ... whatever, doesn't sound like an intended result