Help!! Problem

[bspooh]

I have got a major problem..I hope someone can help me..

well I made a lot of parts that are threaded on the I.D...I sent them off to heat treat and now the threads don't fit!!

The part is a cast 4140..The thread is 1 3/4-12..The bore is 1.665"..They are heat treated to 44-46 Rc...so just rethreading them is not fun...When I machined them, each part was checked with a thread gage..beautiful, i thought...But now the thread gage won't fit...The bore stayed the same diameter before and after heat treat..But the major diam. of the thread must have shrunk...right?? These threads were single pointed, because tapping is a bear, plus the cost of the tap...Is there any way I can heat these parts up so the threads will expand and stay there, without annealing the part?? To try and chase these threads with a tap will be a bear and really expensive..I would go through a lot of taps because of the hardness...any Ideas??? I have 91 parts....don't you just love problems...I sure do!!!!

thanks

brent

[bspooh]

Just thought I'd mention that I have a total of 38 hours invested in these parts...I can't scrap them...It would be too hard to hold on to because they are held on to bored out 3 jaws...there is no way to hold them so they run true...they are shaped funny, so a 4 jaw is out of the question,,,I really want to know if heating them would do anything...

brent

[SGW]

Your heat-treat shop might have some ideas. I doubht you'd be the first person they've run across with this problem. I sorta suspect there isn't any way to fix 'em by reheating, without messing up the heat treat, unless the shop could locally heat the threaded section maybe.

Or you might try this: make up a threaded plug of appropriate dimensions out of brass or similar, and lap the threads a bit with some fine grinding compound. You might wear your way through a couple of laps, but they're something you can make yourself, vs. shelling out bucks for a tap that size. Maybe thread the end of the lap for a pipe plug, then split the lap, so you can expand it a bit as it wears.

[C. Tate]

Could it be that the scale from the heat treat is the problem? Have you tried a small wire brush on a die grinder to clean off scale. You might be able to fix with a dip in nitric acid unless you are working with very close tolerances. The acid will attack the threads and remove material in an even fashion. Last but not least is a carbide tap which will be expensive but quick and you will have the tap if you ever make the parts again.

[kap pullen]

brent,
threads do funny things in heat treat.
they act like cooling fins and your part dosen't cool evenly.
your part may have thick and thin areas and cause this also.
I've seen threaded parts bust in half at the quench operation.
maybe the heat treater could have forseen this and advised you
I had the same problem years ago and had to tap them out.
Some materials shrink or expand a little in heat treatmant.
Machinery's handbook may say somethimg about it.
i have no solution except run a trial batch next time
sorry about your situation
kapullen

ps
etching is an interesting idea.
at work the platers etch a couple tenths off so to mantain size after plating.



[This message has been edited by kap pullen (edited 02-07-2002).]

[snorman]

Maybe you could run a hard bolt thru the threads and loosen them up enough to fit your gage. I remember having to do that pretty regularly after heat treating die sections and the like. A little penetrating oil helps too. Hope you don't have a lot of holes to do 'cause it isn't much fun. Good luck anyways.

[halfnut]

I hate it when things like this happen, got my sympathy.

What might work is this, make up a male thread to match, if you make it out of O-1 you might get by with flame hardening just the threads since this is of good size. Just run torch around threads, turn them red, parent metal will do the quench.

Wire brush off any scale, grind off any burnt sections to below contact surface. Take this plug and oil up hole, screw in as far as you can by hand, take smallish hammer and rap the outer end of this tool you made. See if you can screw it in some more, smite it some more. Repeat as necessary.

Sometimes this will iron things out, it has worked for me. It also astounds maintenence men. Machinist using a hammer instead of a tap.

Good luck, hope you get these threads straightened out.

BTW, 44-46 RC can be single pointed, takes carbide or cobalt tooling, regular HHS will work but it's hard on it.

[bighammer]

Try a pickle of salt and vinaigr if you have scale, degrease first, or use a lap made from aluminum (holds more grit) or brass, split and use wedge as lar wheres.

[bighammer]

P S use as mutch salt as will disolve in the vinaigar.

[bspooh]

Salt and Vinegar...sounds interesting, what exactly does it do..does it just clean out the crap, or what?? I do appreciate all the people that has responded..I can single point it, but i have to make a special fixture for the 4 jaw, and then dial in the part...a little time consuming, so i am trying to use that as a last result...about the pickling,,do i heat up the salt and vinegar solution?? How long should I soak it...if i heat up the part and then try to tap it would it make a difference...This is usually no big deal to me, when this happens it only happens on a few parts,, but this time it happened on all 91 parts...Sometimes being a machinist is tough, but I still wouldn't trade my job for anything in this world..mistakes make you learn more, correct?? thanks all........

brent

[JCHannum]

Salt and vinegar makes a real nasty smelling acid solution. Good for removing scale and polishing brass as well.

Pickling in any acid may help as much as lapping. It will also reduce the OD as well though. This may not be desireable.

Idea of straightening out with hammer has a lot of merit. It sounds like you have only a bit of metal to move if bore size is OK.

[crypto]

Sandblasting with rough grit and then with fine grit????????

[Thrud]

Try a powdered metal tap for refactory metals. I have used these to clean up Brown & Sharpe's pathetic "Ultra Precision 1-2-3" blocks. Don't believe the name - holes are not drilled the same in each block and off by 1/16" - could have done them better with a blind guy drilling them! Anyway, the tapped holes had the same problem - they sucked. Got a powdered metal tap and that fixed them. Should have bought $12 Chinese blocks (much better!) A carbide tap would also work, a Diamond coated tap might work - get in touch with Nachi and ask them - they might be able to help.

Dave

[docn8as]

Thrud....bummer ...sometimes u DONT get what u pay for! but if ur B&S blocks are square , ur qhead of me & my 9.99 blocks...probably should have bought the $12 ones ....case was pretty....oh well, i need practice on the surface grinder.....just wish they had advertised it as a kit! grinder is 70 + yrs old ,no dials (rigged w/ indicators) runs in large split/tapered bronze bushes ,& 2+ turns b/lash on x/feed screw,but still grinds square,& by reversing pieces on table grinds flat to .0001.....found out last week what happens when u forget to turn on the mag. chuck!!!!was lucky ,...only one scar on chuck & one hole in the wall!
best wishes
docn8as

[Thrud]

Doc
Yup, I got a pretty case too! That case will be nice for those Erons from Japan (nice as Starretts for 1/3 the price). The B&S are not parrallel either - you may want to check yours on a granite plate if that is important for what you machine. Guy Lautard's second book tells how this was corrected on his toolmaker's cube.

I would plunk down $20 for the chinese blocks long before B&S "Ultra-Precision" blocks. B&S thought it was funny when I told them they had a alot of gall to call them that. I told them "at least the case is nice" - even if the blocks are crap. They sent me a free B&S 6" shock proof caliper for my "troubles." Don't know if I can trust it...

Dave