Tapping problems - 3/8" hot roll pickled & oiled steel - seeking expertise

[rstocum]

My company makes a high volume part where we tap M8 x 1.25 (6) places in 3/8" hot roll pickled & oiled steel. We laser cut the blank including the hole pattern that gets tapped. We tap the part while its flat and then form it in a press brake. No bend lines occur near the hole pattern. We recently had a return from the customer of 16 pieces for oversize thread. I have inspected the return parts and found 5 of them oversize. The nogo goes more than 2 turns into the hole, or goes all the way through. The other 11 parts pass in the sense that the nogo either does not start, or goes no more than 1 turn into the hole. I am confident that the pilot hole size is correct. The laser has failure modes that can cause the holes to be oversize, but if those failure modes occur, all the holes on every part after the failure would be oversized. I know that I am not dealing with that scenario.

I have two concerns that my own experience does not address:

1) That 2 turns max (for metric threads) of the nogo is permissible is tribal knowledge as far as I know. I was trained to this "standard" in a previous life. I thought it was common knowledge in fabrication, but it may not be for the customer. I plan to ask the customer to verify their gage is in calibration, and let them know the parts pass my gage, which is brand new. If it becomes a matter of interpretation of the nogo entering the hole at all, do I have a leg to stand on by saying the nogo is allowed to start, but not go further than two turns?​

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2) All 6 holes are tapped in succession by the same tap, unless the tap squeals from wear or breaks. Broken & worn taps are replaced immediately. Some of the parts that failed have holes that failed and holes that passed on the same part. I have no knowledge of a failure mode that can account for this. One part has 5 failed holes and one good one, another part has 1 failed hole and 5 good ones, yet another has 3 failed and 3 good. The failed parts all came from the same batch, shipped on the same purchase order, so I know they were tapped on the same shift. Needless to say, I am having trouble formulating ideas to test for root cause. Does anyone here know what kind of tap related failure could account for this?​

 

[Golfman25]

Without seeing our print or forming setup, my gut feeling is the forming may be causing a problem. Of course that would probably make the hole oval which would interfere with your gage?

 

[Rexton]

How are you tapping the parts? Are you fixturing the plate on a CNC mill or are you using a Drill Press of some sort to tap? Are all the holes tapped on the same machine? Is it a forming or cutting tap? Does you tap have an H value (H3, H7, ....) for cutting oversize? Are chips building up on the tap causing issues? This is some of the things I thought up quickly that may cause oversized tapped holes, but I can't say anything without a print or something.

 

[rstocum]

None of the bend lines are near the holes. The forming is not distorting the holes. The parts are large enough that they don't move during tapping, so they are not fixtured. They rest on a 2x4 frame and are tapped by a mobile tapping unit called a flexarm. The motive force for the rotation of the tap is hydraulic, and the initial downward pressure is supplied by human arm power. The flexarm unit holds the tap vertical and does not allow the tap to drive at an angle. The class of fit is 6H. We use a standard cutting tap, TiAlN coated, supplied by MSC. All holes are tapped on the same machine. I regret that I cannot share a print due to confidentiality agreements, but the screen snip attached shows the hole detail. There are no notes or special instructions on the print regarding the threaded holes.

I see your point about chip buildup, Rexton. If chips are dragged into the cut during tapping, perhaps they are causing the thread form to cut oversize? I will be inquiring about tapping fluid to see if it is the right substance to use in our material, with our current taps.

I asked our operators if the part could be at an angle to the tap arm and cause this problem. They said the frame holds the part in the correct orientation, and if the part were not located correctly in the frame so that the tap approached the hole at an angle, the tap would squeal and the problem would be immediately obvious.

 

[Golfman25]

None of the bend lines are near the holes. The forming is not distorting the holes. The parts are large enough that they don't move during tapping, so they are not fixtured.

I guess I would ask, "how do you know?" Has it been tested and verified?

None of the bend lines are near the holes. The forming is not distorting the holes. The parts are large enough that they don't move during tapping, so they are not fixtured. They rest on a 2x4 frame and are tapped by a mobile tapping unit called a flexarm. The motive force for the rotation of the tap is hydraulic, and the initial downward pressure is supplied by human arm power. The flexarm unit holds the tap vertical and does not allow the tap to drive at an angle. The class of fit is 6H. We use a standard cutting tap, TiAlN coated, supplied by MSC. All holes are tapped on the same machine. I regret that I cannot share a print due to confidentiality agreements, but the screen snip attached shows the hole detail. There are no notes or special instructions on the print regarding the threaded holes.

I see your point about chip buildup, Rexton. If chips are dragged into the cut during tapping, perhaps they are causing the thread form to cut oversize? I will be inquiring about tapping fluid to see if it is the right substance to use in our material, with our current taps.

I asked our operators if the part could be at an angle to the tap arm and cause this problem. They said the frame holds the part in the correct orientation, and if the part were not located correctly in the frame so that the tap approached the hole at an angle, the tap would squeal and the problem would be immediately obvious.

I guess I would ask, "how do you know?" Has it been tested and verified? You're at the point where all the "easy" stuff seems to been accounted for. You may want to go back an independently verify to see if any of the assumptions may be wrong.

 

[Rexton]

I guess I would ask, "how do you know?" Has it been tested and verified?


I guess I would ask, "how do you know?" Has it been tested and verified? You're at the point where all the "easy" stuff seems to been accounted for. You may want to go back an independently verify to see if any of the assumptions may be wrong.

I agree with Goldman, I believe doing some testing, such as checking all the tapped holes with a gage after tapping and then again after forming.

Also to clarify, I did not mean the thread class 6H, but the tap class of cut. I am not sure if this is used outside the US, but taps are readily available here that cut oversized in order to account for plating (and are designated with H#, e.g. H6). If an oversized tap was accidentally purchased it could cause issues with thread size.

As for 2 turn NOGO standard, ISO 1502 should cover it. (It came up in a quick google search and I don't have a copy of the standard.)

 

[outdoorsNW]

How often is the tap changed?

Even if you did not order the wrong tap by mistake, could mixed stock from the supplier cause the problem?
I have seen enough mixed stock for other types of items to think mixed stock might have occurred.

Also, is the squealing a reliable indicator of an angled tap 100% of the time, or does the tap squeal only in some circumstances?

 

[Al Rosen]

7.2.2 NOT GO screw plug gauges

c) The NOT GO screw plug gauge, when screwed by hand without using excessive force, may enter into both ends of the threaded part, but by not
more than two turns of thread. If it can be screwed in by more than two turns of thread, the workpiece thread does not comply with the specification. The NOT GO screw plug gauge shall not pass completely through a workpiece with a length of thread of three threads or less.

 

[rstocum]

I agree with Goldman, I believe doing some testing, such as checking all the tapped holes with a gage after tapping and then again after forming.

Also to clarify, I did not mean the thread class 6H, but the tap class of cut. I am not sure if this is used outside the US, but taps are readily available here that cut oversized in order to account for plating (and are designated with H#, e.g. H6). If an oversized tap was accidentally purchased it could cause issues with thread size.

As for 2 turn NOGO standard, ISO 1502 should cover it. (It came up in a quick google search and I don't have a copy of the standard.)

The holes are verified with the go/nogo at the threading operation, and I am verifying them myself post forming. The closest bend is 4 inches away from the nearest hole. Steel in this thickness does not move in any way we can measure that far from a bend.

A quick double check of our taps shows they are all the same Guhring tap in the proper size. Some have a bit of wear, but nothing drastic.

I was completely unaware of an ISO standard governing thread acceptance. Thank you.

 

[rstocum]

How often is the tap changed?

Even if you did not order the wrong tap by mistake, could mixed stock from the supplier cause the problem?
I have seen enough mixed stock for other types of items to think mixed stock might have occurred.

Also, is the squealing a reliable indicator of an angled tap 100% of the time, or does the tap squeal only in some circumstances?

Yes, at this time I cannot rule out operators using taps that are close to their "end of life", but not obviously done yet. We have never done any kind of study to determine when taps should be discarded, or how many holes our preferred tap can do in the materials we use them in. I will see if I can production on board to look at that. Thank you.

The tap squeals when it encounters too much resistance to engagement. This happens when the tap is presented to the hole at an angle, and when it has too much wear on the cutting edges. If the holes were undersize, or the edges of the holes were hardened due to improper laser cut settings the tap would also squeal. Sometimes the tap simply breaks before making any unusual noise at all.