Drilling and tapping hard metal

[eagle24]

A few weeks ago, I broke a HSS tap of in the bolster of my lockplate.  Good lesson, and more of that experience you gain about 5 minutes after you needed it.  Anyway, after every attempt at getting the tap out and salvaging the lockplate, I finally ground it out with a die grinder and welded the hole up with my wire welder.  I dressed the weld back down with a file and polished the lockplate good as new, right?  Well when I redrilled the lockplate and tapped it, I found out that the weld was much harder than the original metal I replaced.  My question is this.  Can you anneal a weld just as you would any other hardened steel?  If so, i'm thinking about welding it up again and annnealing before I drill and tap it.  I think some of the threads broke out when I tapped because it was so hard.

[northmn]

Taps can be frustrating as I have had one break and then used another and had it tap smoothly to finish the job after getting the old one out.  Some are dull when you get them.  I just had that experience with a new one that broke and finished the job with an old rusty looking one that cut like a dream.  What you can do in annealing depends a lot on what you welded it in with and how you cooled the weld.  Annealing is the slow cooling of a metal and sometimes requires an insulation like wood ash or other mediums to permit the slow down.

DP

[Acer Saccharum]

I think you will need to heat the whole plate and let it cool slowly. If you get it red in a wood fire, then bury it in ash to let it cool slowly.

If you just try to spot anneal, the surrounding cooler metal may draw the heat away too quickly from the spot you're trying to anneal, causing it to stay hard.

Tom

[eagle24]

Will an oven get it hot enough to do anything?  I thought about sticking it n the oven and heating it as hot as the oven will get and then just turn off the oven and leave the part in there until the oven cools down.

[Dphariss]

The plate is probably high enough in carbon that welding it served to harden the part at that point. The rest of the part acts as a heats sink and "quenches" the part.
You will have to anneal it.
Heat to high silver, past blue, or dull red and let it cool. Should be OK since you used a wire welder. TIG can produce hard spots that will not anneal.
I always use a Moly or other high tech tapping fluid. It will sometimes allow taps to survive  where other stuff will not.
I have some Castrol moly tapping oil that has worked very well for me and a pint (?) will last for years.

Dan

[Stophel]

Now you have learned why you should avoid HSS taps as much as possible!  When they break you're screwed!  A carbon steel tap will  (usually...) shatter when you hit it with a punch and you can break it out of the hole.   Generally, you can just drive the broken tap right back out of the hole, assuming it is a through hole.

 A stuck HSS tap just laughs at you.

[eagle24]

Now you have learned why you should avoid HSS taps as much as possible!  When they break you're screwed!  A carbon steel tap will  (usually...) shatter when you hit it with a punch and you can break it out of the hole.   Generally, you can just drive the broken tap right back out of the hole, assuming it is a through hole.

 A stuck HSS tap just laughs at you.

Yep, it was a good lesson learned the hard way.  I could have avoided it if I had remembered how brittle those taps are.  Guess I pushed it a little too hard, trying to save a minute cost me about 8 hours.

[davec2]

When I break a tap in a part that I don't want to make over or lose, I send it out to have the broken tap EDM'd out.  For the money it costs, you are way ahead of the game in your time and, usually, money.  It almost always costs me a $#*! of a lot of time and money to save a few bucks by trying to Mickey Mouse a fix.

[Steve-In]

For through holes go to a 2 flute gun tap.  The 2 flutes are hard to break, and the gun tap has lots of lead to make tapping easy.  The more rigid you are, the better.  Also watch your drill speed in work hardening materials.  Turn them slow and use constant pressure.

[Acer Saccharum]

Also, guide your tap from above, using a point in the drill press chuck, or a reverse cone if the tap has a pointy end.

Any side loading is real hard on taps, especially the small ones.

I love the two flute gun taps. HSS. The teeth chip before the tap breaks, so you you know when to toss them. I get hundreds of holes before they get dull. Greenfield Tap and Die makes killer taps.

BUT, as Davec says,  if they break off in a hole, you have to grind or EDM them out. However, that said, they are really tough, and I have never broken one off in a lockplate. I check the cutting edges first when I feel they are starting to cut with resistance.

[T*O*F]

if they break off in a hole, you have to grind or EDM them out.

Dremel and others sell and 1/8" straight, double cut carbide bit.  You can chuck it in your drill press, running on its slowest speed, and grind out broken taps, hammer screws, etc.  It will leave threads intact if the item is larger than the bit.  Once thru, an easy out or Torx bit driven into the hole will remove it; or sometimes you can hit it with a bit of heat and back it out with a small chisel.