Anyone with advice and experience dealing with galvanizing steel with internal threads. On larger sizes of threaded holes we have used oversized taps ( 3/4 inch) On this last batch of 3/8 on size threaded holes we filled the holes with silicon sealant/caulking. Will the holes have to cleared out one more time in order for the bolts to thread in after galvanizing ??? Randy
It was common practice to do all tapping after parts were finished (usually anodizing) when I worked in the missile industry. If your parts lend themselves to that idea, you might consider tapping after galvanizing.
I have had weldments with tapped holes hot-dip galvanized several times. I designed them so that the threads in all the holes could be accessed sufficiently to allow chasing the threads by hand with a tap and tap wrench. The zinc is soft and this task is done quickly compared to the original tapping, plus it leaves as much zinc as possible in the area.
I would talk to the foreman at your galvanizing company. He gets paid for knowing stuff like this.
I know they make taps that cut slightly larger than spec. If you did this and then retapped with a true size tap after you get the parts back from the galv shop then you might get the best of both worlds.
I think I would take a bar and drill and tap three or four holes, one to spec and then the rest one size over and tow sizes over etc.
Then have the shop that does your work give the test plate a dunk with the same process they will be using on your parts. This should tell you if you can over shoot the size reliably or if you need a second op.
I can tell you don't get a lot of stuff hot-dipped, running a test plate through would likely take 3 weeks and then your parts another 3. A whole lot of guys couldn't wait that long, especially Randy Z ..
I will let you know how it comes out if the parts come back to us before the customer. The caulking idea came from the galvanizing service. Apparently the silicon seal withstands the acid dip and the molten zinc . I still don't know what the result will be. Does the piece end up with a carbon slug left in the hole or does it fall out? Over 400 pieces have already gone for dipping. I was told there are around 3000 pieces total. Handling time becomes the main concern. Randy
came in late on this thread, and had already accidentally deleted all the beginning of it, but I'll slip in with some hopefully relevant comments~ Ive had a fair bit of expereince with having things hot dip galved, if you're hoping to retain tapped threads, there are a few approaches;
tap after hot dipping; nice and easy! doesnt work though if you are usuing an automatic tapper in a drill press on parts by the thousands then welding them in. the final product probably wont fit in to this type of semi automated process
put a bolt in the holes; can work, but then you have to weigh up the cost/time of bolts vs. the cost/time of the next option.
clean them back out with a tap after dipping. depends on how thick the dip is, or if you go in for a second dip. if its a light dip, and the tapped hole goes right the way through to the other side, then you'll come out pretty clean. the resultant re-tapping will be very quick and easy, if the operator has some skill fit the tap to a cordless drill and you'll zip through thousands with only the occasional snapped tap.
if its a heavy dip, then you're probably looking at other clean up issues too, then the re-tapping will also be harder, but in perspective maybe les sof an issue. for items like ballistrading etc where a certain leverl of finish is required, or if they are to be powdercoated after hot dipped then you will need to take to them with a file to remove dags/lumps. Then you
*will* have a time of removing the galv from the threads! the bolt option starts looking more attractive, as long as the bolts are cheap.
if the hole is blind, then you are pretty much in the poo. bolts are the only successful option ive ever used, followed by a retap to boot. I've heard of people trying to fill the holes up with gunk, but no galv shop i know would let you put that stuff in their dip, then you've got to get it out anyway.
consider re-engineering future products to not need tapping, then allow enough margine in the bolt hole that you still have clearance after the dip? thats what we always tried to do.
Nor do I, but I have seen stuff that is hot dipped and some times it seems to get pretty gloppy. This is why I sugested the test piece. After all if the end result does not function the way you want it to, doing the job over is going to take a lot more time than getting it right the girst time.
I have no idea what happens to the plug. I have seen plugs used in electroplating, and anodizing, but that is a lot cooler process. Zinc melts at 785°F and my gut tells me that silicone would cook at that temp. Usually when there is some impurity dunked into molten metal, it turns to smoke and slag PDQ.
If the plugs burn off, then what keeps the zinc from buggering the threads? Even if it does not make a really good bond, then time will still be expended in removing the slag.
Come to think of it, I do not recall ever seeing anything hot dipped that had been threaded before dipping. Usually the holes are through holes and a nut and bolt are used to secure the pieces together.