Hey! I’m looking for a bit of advice from anyone that has had to re-tap threaded holes in metal over stripped threads.
How much larger do I need to re-tap to get stable threads? They were M4 screws, would M5 be fine?
Also, do you have any tips on tapping holes in metal? I mostly work with wood.
Lastly, do you have any recommendations on brand to buy/where to source the tool. I only need it for the one job, so (absolutely no expectation of this) if some one had a correct size tap that I could lend off of you, I would be interested and very thankful.
A bit of background: I work in a research lab with multiple -80 C freezers. One had a screwed up door when I joined, and I have the door realigned and clamped shut, but the screw holes where the handle goes are completely stripped. They fit M4x.7 screws. They want me to try to get the original hardware back on, so I want to re-tap a larger size.
If you want to be pedantic about it, the new screw should have a tap drill size equal or greater to the clearance drill size of the stripped hole. Consulting a metric thread chart:
Looks like M5 would work out just fine, assuming the old holes are just stripped and not mangled or oval or some other kind of nastiness you need to work around.
Are these through or blind holes? If you’ve got plenty of back clearance, one of these guys might be just the ticket: Amazon.com
There is a tap and die set in the metal shop available for members for use at asmbly. Presumably Drew needs to tap this hole in situ.
My advice would be chamfer the hole after you drill it to the new size with a countersink bit. That will help start the tap. Use a lubricant oil and go slowly, a quarter-half turn at a time, then reverse the tap to clear the chip. If you force it you’re likely to break the tap, which are hardened and brittle, and a much bigger pain to remove that a stripped screw.
IMO, You have to be fairly careful on the quality of taps, particularly for steel. You can probably get away with a cheap tap on aluminum. I tried to use the taps at asmbly on some stainless, and they were a no-go, it would have broke. I bought my own set (greenfield threading is the brand I used), and it easily did the job in the exact same hole.
There’s also several types of taps, which vary in terms of the taper on the tap. Usually you want the taper tap, unless you are tapping a blind hole, in which case you want a bottoming tap.
Yeah, the tap set we have is just average quality. They should be fine for aluminum and mild steel, but I’m not surprised they’re not adequate for stainless. (They’re better than the previous set, which literally could not cut aluminum.)
A single high quality tap shouldn’t be too expensive. The real trick in a situation like this will be keeping the drill bit and tap straight. You may want to make some sort of guide blocks, I.e., a thick block with a flat surface and perpendicular hole exactly big enough for the bit/tap to just slide through freely.
It is unfortunately a blind hole. I’ll need to research what kind of metal, I’d assume it’s steel, there’s a lot of pressure put on the door frame to keep it shut tightly.
I do unfortunately need to tap it on site. They’ve gone ahead and filled the freezer before I finished the repair because another one went down haha.
My plan is to get a high quality bottoming tap. Since there is already a hole for the M4, should I have to drill it out before using the tap? Or do you think I’d be fine to just countersink over the existing M4 hole then go in with a quality M5 tap?
A bottoming tap usually only works after you’ve cut a few threads with a normal taper tap. I would still pre drill it with the closest bit size you can find, especially if it is steel. I can lend you an M5 taper tap and #16 drill if M5x0.8 hardware will work, then you would just need to buy the bottoming tap (or grind the end off of a cheap taper tap)
@ewei that hardware would absolutely work! If that’s ok with you, borrowing the tools would be amazing. I’ll look into the bottoming tap. My number is 703-232-5639 if you want to reach out, I can swing by and pick the stuff up or whatever is most convenient for you.
Another option that could work for you if you haven’t already drilled and tapped the next larger size is to use a metal epoxy like JB Weld to epoxy a stud into the stripped hole and then use a nut. I think this works best when the threaded hole is fairly deep and not a blind hole.
Sorry I’m late. I have become a huge fan of spiral taps. They are similar to bottom taps in terms of when the 1st threads start. The flutes are an upcut helix. I like them for quick chip evacuation. It allows for slightly faster tapping than a neutral flute tap.
I wouldn’t use a cutting tap at all. Get a roll tap aka forming tap aka threadformer. It makes excellent quality threads, better than you can get with cutting taps but the initial hole size can’t be too small, it would break the former.
How good are they? I had a big complicated 3/8" 6061 aluminum plate when I was building the CNC way back which has a mix of inch and metric components. It was late.
I drilled a hole for an m12 bolt ran an metric threadformer. Nice thread feel by hand. Then… oh crap that was supposed to be an inch measure
Without drilling anything a second time, I ran a 1/2" threadformer through it. Took the bolt just fine.
Then I realized I was wrong, it was right the first time. So I ran i roll tapped the hole for a third time. Back to m6. Again, still surprisingly high quality
Ok Danny, admit it…you’re just trying to demonstrate the interchangeability and compatibility of two sides in biggest argument in modern maker history. Which is better, metric or imperial? Clearly you’ve found the perfect solution
Thanks y’all (and especially @ewei for the tools!)
The freezer is back up and I’m finally able to take my wood clamps home, the handle is on and the temperature is holding at -80 C! Again, thanks for all of the help. Lots of happy grad students because of this, and I was able to make a good impression on the lab I just joined haha.
