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

[Ninja]

Curiosity...what is your 'normal' tap life? 1 shift? 1 hour? 1 month?
Might be easy to set up an end of shift/month/week/whatever measurement of the tap to verify utility.
Certainly more data-centric than "it sqeals"...and might bypass/solve the issue entirely.
It would, however, require you to develop fail criteria on taps...

 

[rstocum]

Update for anyone interested. I did not have a good root cause for the customer so I called their quality engineer who agreed to let me get away with "reviewing all tap inventory for wear, discarding any worn taps found, training tapping personnel on tap wear issues, and raising awareness of the customers problem". No formal corrective action required.

I conducted the tap wear training the next day. I had pictures of wear on the lead cutting edge, buildup on cutting edges, chipping on cutting edges, cold welding, and bird nesting. Every operator recognized each of those conditions. Then I went over the various hole size and shape defects that can cause issues. That same afternoon one operator comes to me with a different part from the one that had the original issue. This part is also laser cut. The pierce point had a "blowout" which leaves the hole slightly oblong on the part face that was up when it was cut. He asked if that could be the problem. I had him tap it and check it. The nogo went in on that side about 4 turns. We sorted the whole job and found six parts with this condition, or worse. The laser chief told me that can happen if the surface of the sheet the parts are cut from has some irregularity. One area where the surface is bowed upward or downward a bit puts the focal point of the beam too deep or too shallow in the material and you don't get a good cut. His operators look for that, but don't catch them all. Now that the tap operators know what to look for, I expect to reduce this problem, but I don't see a good way to eliminate it, and I sure can't 100% inspect.

 

[Rexton]

Update for anyone interested. I did not have a good root cause for the customer so I called their quality engineer who agreed to let me get away with "reviewing all tap inventory for wear, discarding any worn taps found, training tapping personnel on tap wear issues, and raising awareness of the customers problem". No formal corrective action required.

I conducted the tap wear training the next day. I had pictures of wear on the lead cutting edge, buildup on cutting edges, chipping on cutting edges, cold welding, and bird nesting. Every operator recognized each of those conditions. Then I went over the various hole size and shape defects that can cause issues. That same afternoon one operator comes to me with a different part from the one that had the original issue. This part is also laser cut. The pierce point had a "blowout" which leaves the hole slightly oblong on the part face that was up when it was cut. He asked if that could be the problem. I had him tap it and check it. The nogo went in on that side about 4 turns. We sorted the whole job and found six parts with this condition, or worse. The laser chief told me that can happen if the surface of the sheet the parts are cut from has some irregularity. One area where the surface is bowed upward or downward a bit puts the focal point of the beam too deep or too shallow in the material and you don't get a good cut. His operators look for that, but don't catch them all. Now that the tap operators know what to look for, I expect to reduce this problem, but I don't see a good way to eliminate it, and I sure can't 100% inspect.

I am not sure how much you are able to experiment with the process, but you could drill the holes separately and then tap them, such as on a drill press or a CNC Mill. It would eliminate the variation from laser cutting and could better control the holes prior to tapping.

 

[rstocum]

I am not sure how much you are able to experiment with the process, but you could drill the holes separately and then tap them, such as on a drill press or a CNC Mill. It would eliminate the variation from laser cutting and could better control the holes prior to tapping.

Unfortunately, I am not able to change the basic process steps at all. The company is divided into two divisions. One for sheet/plate fabrication and one for machining. The machining division does not have tooling/fixturing for working on flat sheet material like these parts. The fabrication division does not have machining capability other than countersinking and tapping, and those are not done on rigid machines. I am stuck with the laser cut holes, and can only improve the process within those constraints. This is not the only problem that could be made better by being able to machine sheet parts. I can lobby for developing that capability, but there is tooling cost, and it probably won't fly.

 

[Cari Spears]

Update for anyone interested. I did not have a good root cause for the customer so I called their quality engineer who agreed to let me get away with "reviewing all tap inventory for wear, discarding any worn taps found, training tapping personnel on tap wear issues, and raising awareness of the customers problem". No formal corrective action required.

I conducted the tap wear training the next day. I had pictures of wear on the lead cutting edge, buildup on cutting edges, chipping on cutting edges, cold welding, and bird nesting. Every operator recognized each of those conditions. Then I went over the various hole size and shape defects that can cause issues. That same afternoon one operator comes to me with a different part from the one that had the original issue. This part is also laser cut. The pierce point had a "blowout" which leaves the hole slightly oblong on the part face that was up when it was cut. He asked if that could be the problem. I had him tap it and check it. The nogo went in on that side about 4 turns. We sorted the whole job and found six parts with this condition, or worse. The laser chief told me that can happen if the surface of the sheet the parts are cut from has some irregularity. One area where the surface is bowed upward or downward a bit puts the focal point of the beam too deep or too shallow in the material and you don't get a good cut. His operators look for that, but don't catch them all. Now that the tap operators know what to look for, I expect to reduce this problem, but I don't see a good way to eliminate it, and I sure can't 100% inspect.

Thanks for the follow up!!

 

[Bill Williamson]

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?​

Could you be having lubrication issues? Maybe the defective holes received no lubrication causing thread tearout. Just a missing thread on the tap can cause random threading issues. Also I have seen times that if the slug has not fallen out of the hole due to slag, the tap works like a drill until the slug pops out.

Good luck.

Bill Williamson