[Newbie] Tapping holes being inconsistant.

awaqa909

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Manually and freehand tapping some 1/4-3/8 sheet? stainless. 1/2-13 or something like that, through tap. Didn't actually pay attention to what the hole diameter is, but its supposed to be good. Coolube 2210? Tapping head, 213-180rpm not sure what the tapping head reduces to. Added a chamfer that would fit a .505" pin on both sides, enough that it would just fit... sometimes a smidge more. I measured the tap at .505" but the part I could measure wasn't full diameter, so I figured it was good. (told a bad bur gets made without this)

Got a go and nogo gauge and the nogo will go, if you back it off, then go back in enough times, on some holes. (moving chips out of the way?) Other holes it won't even screw in unless cocked sideways. Some sort of clamp holds the part down to keep the tap from picking it up.

I've tried to line it up the best I can before clamping it. I already did a handful of these and they were fine, (I think they were, I wasn't checking every single hole like I am now...) but after them it was like I was trying new taps to fix burs on the threads and this problem, but have gotten nowhere. (I thought the burs were happening right before the nogo would go though)Brand new tap 3 holes done, clean, then a set of nogo holes after them. (part has several of these tapped holes) This part only has like 3-4 threads.

Someone else did 2 holes when I was having issues, they didn't even clamp it but didn't seem to have issues. They checked the holes and left. They called it good but I didn't actually get to see the turns. Later me wondering if it was just chips or if threading the gauge several times would make the nogo go. I think it ended up going at least 1/2 turn. I don't seem to have the muscle to hold it down without the clamp.

I get to go back to this tomorrow and already feel like I'm going to get fired....

Need help.

Thanks,
Awaqa909
 
Hi Awaq909,

I see you've got no replies yet......I'm no expert, but I'll try to provide some help......

1) You said:
Didn't actually pay attention to what the hole diameter is, but its supposed to be good

I would start there. Verify the holes before you tap them to determine if the previous operation is the cause of trouble.
If the hole is starts out too big, or the holes are out of round then you're already beat.
You can't fix those with the tap.
Either situation could lead to the "No-Go" thread gauge actually going into the thread hole.

Do you have a pin gauge set or bore gauge?

Also the holes may not be perpendicular to the plate to start.
The guy that succeeded on two holes may have picked two nice looking holes for his attempts.

2) Check the specifications. Do you have a drawing?
Does it spell out the tap-hole size and class of thread fit?
Does that call-out match the Go/No-Go gauge you're using?
Any tapping job will go easier if you can you can go with less thread depth. Is that an option?
(though this sounds like more like the problem here than the solution!)

3) you mention both
Manually and freehand tapping some 1/4-3/8 sheet?
and
Tapping head, 213-180rpm not sure what the tapping head reduces to.

Which is it? Is it "freehand" with a tap wrench or with a tapping head on a drill press or other machine?
Either way you need to ensure that the hole is tapped square to the plate.
Check the table for square.

4) other thoughts:
Be sure the tap isn't going too deep and the round shank of the tool throwing up a burr.
If you are simply the "operator" then also push back to the machinist/set-up person. They should be there to help guide you.

I wish you the best of luck, and please let us know how it goes.
-brino
 
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With the countersink The top of the hole will allow the nogo gauge to start but not go thru. Plus it will let it start crooked. Make sure your drill bits the right size for the tap . If you drill the holes make sure you lubricate with cutting fluid. If you force the cut it will not be the right size hole. Hand tapping without a guide can and will tap a crooked hole. Make sure the tap is the right size and you tap thru , taper taps are just that tapered up to a point. Do you have a metrology dept to Ck these things out.
Keep asking till you can find out , better to bug early then too late. Nothing makes a shop look bad more then having parts returned.
 
I'm using Coolube 2210 or something under that name and dabbing some onto the spinning tap with a pinky size brush and the hole. What I meant by freehand is it's put on a manual mill table, held down by hand and a strange clamp. Clamp is on the right side near spindle, left hand on part preventing rotation and also helps prevent the tap from pulling up the part. This part comes by shop every now and then, but it's not the squarest piece of material... It's like bent/warped slightly, even the countersinks look a bit strange. I think it was waterjet cut or something and all that needs to be done is ream and tap, but the holes I'm working on now only get chamfer and tap. I've already got the chamfer on the last parts, now just tapping and debur. There might actually be only 2-3 more parts for the tapping part.

I've complained a good few times before to the setup guy, then on the last part where it got really bad, boss man was doing his rounds and setup guy called him over and told him... Died a bit inside... Since it's a fairly expensive part vs what I normally do and I usually never have such issues. Tomorrow I will check the drawling and see what the spec is supposed to be for the hole AND tap. How much out of spec would be needed, to cause this? Will compare the drawling to parts tomorrow. I think it was a 1/2-13" NC tap. I have to tap a through hole.
 
Tap calls for .4219" but at least one hole was .439... for whatever reason that hole tapped fine. But most of them averaged .015+ above tap hole size spec. What kind of tolerance is there before issues happen?
 
You said:

This part comes by shop every now and then, but it's not the squarest piece of material... It's like bent/warped slightly, even the countersinks look a bit strange. I think it was waterjet cut or something and all that needs to be done is ream and tap, but the holes I'm working on now only get chamfer and tap.

and I usually never have such issues.

This makes me wonder even more about the quality of the holes you are starting with.......

I would think that water-jet would leave the plate totally flat; but plasma torch or ox-acetylene cuts could warp the parts.
Any idea how the hole is made?

If the part is not flat, then it's hard to know what side was up when the hole was made.
If you flip it over or use a different size table when tapping it then the plate is no longer at the same angle as when the hole was made.
That would make the tapping job harder and could lead to more broken taps and scrapped parts.

Reaming would make the hole a very accurate size, but again only if it's reamed at the same angle as the drill.

What kind of tolerance is there before issues happen?

That depends on the class of thread being produced.
Certainly an over-sized hole could produce what you see; the "no-go" thread gauge will go thru.

I'll do some research......
-brino
 
Maybe if I invoke the names @Bob Korves and @RJSakowski we'll get some input from some more experienced guys with their suggestions.

Stay tuned!
-brino
 
according to:
http://www.newmantools.com/tapdrill.htm
the tap drill size for a 1/2-13NC tap is 27/64" = 0.421875

see here:
https://www.threadcheck.com/1-2-13-...ug-gage-w-handle-class-x/thread-gages-gauges/
for a bunch of different go/no-go thread gauges.
Scroll down and see how many there are for 1/2-13NC.
They show both "go", "no-go" and "double-ended" gauges for "2B" and "3B" classes of fit.

@awaqa909 , I respect the work ethic you have to try to get this figured out on your own time.
Hopefully this kind of commitment to quality will NOT go unnoticed.

-brino
 
Thanks for the vote of confidence, Brino. When I read the original post, I thought that the poster had already exceeded my level of expertise. There are a few thoughts though. What is the stainless alloy? Stainless has a tendency to work harden. Once the surface has work hardened, it will dull the tap which will make the work hardening worse. The OP said that he was able to get three good holes before things went south. I suspect that the tap had dulled slightly. When the tap dulls, it takes more pressure to cut the threads which will result in a distortion of the material and an undersized hole. I have experienced this with stainless.

I wouldn't think that burrs would be left in the threaded holes. At least not to interfere with the thread gage. Running the tap through should clear them. I suspect that the thread gage is burnishing the hole slightly, enough to allow it to eventually feed through.

I have never felt a need to ream a hole that I was going to tap.IMO, it is more important to have the correct hole diameter. Is there as spec. for hole diameter? Opening up the diameter slightly will make tapping much easier without reducing thread strength significantly. It will also extend the tap life and reduce any work hardening. Recommended tap drill sizes are based on 75% threads but a 60% thread will perform almost as well. A 75% thread calls for a .422" hole while a 60% thread calls for a .438" hole.

Finally, to the OP, you shouldn't be afraid to go to your supervisor when you run into problems. Trying to solve a problem by yourself is commendable but lost time and scrap parts cost your company money. A good supervisor will appreciate your request for assistance and chances are that he may have run into the problem himself and already knows what the solution is. Additionally, he will recognize your desire to improve your skills and knowledge.

Parts drawings are often done by people with little or no machining experience. The hole call-out is the textbook value for a 78% thread and, if fact, what a CAD program like SolidWorks will call out as a default. 60% threads are often recommended for difficult to machine materials.
 
Maybe if I invoke the names @Bob Korves and @RJSakowski we'll get some input from some more experienced guys with their suggestions.

Stay tuned!
-brino
I have you fooled, too, Brino? ;) You seem smarter than that...

It is a good thing that RJ jumped in and answered the OP's question, and quite well, I might add from my limited experience. Stainless is sometimes a B*tch to drill, tap, or otherwise machine, with some alloys much worse than others. I rarely machine stainless steel, much less tap it, but it has the same issues as other steels can have, just sometimes worse. Giving the OP instructions to change hole diameter and other changes is probably not a good idea in a production environment where the numbers should be called out on the prints and cut just how they are spec'd. Before being a maverick, ask the foreman or other person in charge for help, and definitely get permission before going experimental in that environment. The new guy usually gets some latitude, but not so much for screw ups of his own design.
 
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