718 Roll Thread Lap Forming

[Cup909]

We're working on a project to roll thread some solution annealed 718 and have run into issues with lap formation. The parts are roll threaded in an infeed setup using a 2-die roller machine (setup picture below.) The dies are 6" long and close in on a 5" machined blank with chamfered ends. After completing threading, we ran liquid penetrant testing and noted issues on the threads flanks. A lap has folded halfway up the thread flank and wraps all the way around the thread for the full length of the 5" threaded region. The lap doesn't show on all pieces (approximately 20% defect rate) nor is it consistent on what side of the flank the lap shows up on (forward or aft of machine.)

I took a cross section of the thread and examined it under magnification. Below are pictures at 50x and stitched together thread profiles. The lap appears to begin to form in the chamfered region of the machine blank and wraps around to the fully formed thread. I'm at a bit of a loss as to what could be causing the lap to form in the chamfered region then roll up. Historically, I've seen issues in thread mismatch of the dies, thread blank leveling, die condition, etc. but these variables have all been checked.

718 is known to form 'funny' compared to our bread & butter low alloy steels. In low alloy it's pretty consistent that if you push the thread with a die, material moves in a manner that you would expect. 718 we have found the material will resist forming and, when filling voids like on a roll thread or forming die, will sometimes burst causing issues. In previous projects, I've learned that 718 resists forming at speeds that typical low alloy steel would have no issues with forming. That said, I'm still having difficulty imagining the force profile that would create the below laps.

What thoughts are out there that could be forming this?

Does anyone have any literature out there?

50x metallography, material chamfer begins on left side and grows upward. -


2-die infeed setup (no affiliation, borrowed from google images)

 

[enginesrus]

Rollers not tracking the same? Just a guess maybe end play issue?

 

[3DDave]

More or different lubrication?

 

[mfgenggear]

OP
generally precision die holders are designed to hold the thread dies. and generally not adjustable. and require rigid setup.
look into flat die thread rolling. much better and easier method.
while cylindrical thread rolling works well is done on a cnc lathe. and requires adjustments to develope the correct setup.
the difference between flat die vs roller dies.
is flat dies Don't require a chuck. the dies support the part. the dies will rotate the part in one second. it is a fast method for medium volume production.
on the roller dies the manufacture supplies a hand book with causes and effects , and correction.the external forces and the runout
will effect double tracking. thus mutilated
thread.

 

[Cup909]

Rollers not tracking the same? Just a guess maybe end play issue?

The machine is setup with built in end play to allow for the dies to move in forward/aft movement.. Internally the spindles are on a spring and the machine rests at a '0'. The spring is stretched/compressed to give the dies ab it of forward/aft movement.

The dies themselves are keyed to the spindles of the machine to address rotational movement. It's possible the key has slop, but they are precision ground on a surface plate as part of setup so it should be minimal. The machine gets started by running a test piece before production. I imagine this test piece would favor the die against the key to address any remaining slop. A large nut (2") torqued to (heavy roll thread operator * 6ft wrench) foot lbs of torque holds the dies on the spindle. Once the die engages against the key from machine loading, I don't think it would become un-engaged.

More or different lubrication?

Different lubrication is an area we're looking at. The flood system completely covers the part, so I think we're good there.

OP
generally precision die holders are designed to hold the thread dies. and generally not adjustable. and require rigid setup.
look into flat die thread rolling. much better and easier method.
while cylindrical thread rolling works well is done on a cnc lathe. and requires adjustments to develope the correct setup.
the difference between flat die vs roller dies.
is flat dies Don't require a chuck. the dies support the part. the dies will rotate the part in one second. it is a fast method for medium volume production.
on the roller dies the manufacture supplies a hand book with causes and effects , and correction.the external forces and the runout
will effect double tracking. thus mutilated
thread.

This type of thread rolling is done between 2x roll thread dies. The thread diameter is larger (2"+) so flat dies aren't a standard setup at this diameter and the lengths associated with it.

Tesker posts a good video of a similar machine in action. Ours is a bit bigger and we're performing a manual operation with lower volumes. Ours runs much slower rpm wise.