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View Full Version : why is it when tapping the threads ?????????



Brett Hurt
05-20-2011, 11:10 PM
I was tapping drill rod, 3/8-16 on the lathe but have had this happen before. i was using a new dia and lube but the threads chip in spots. But I machined it to .375 just like i was suppose to, and all so I did one turn back it off and do a other turn . So very smart guys what did I do wrong Brett

Evan
05-20-2011, 11:24 PM
What lube did you use? Why back up? It isn't necessary. Also, possible misalignment of the die causing drunken threads.

Carld
05-21-2011, 12:22 AM
I would have turned the drill rod to .373". It's not uncommon to have hard material such as drill rod chip as it is threaded with a die. One cause is if the diameter is to big and another is the hardness of the metal. Both at the same time is trouble.

Is the die you used a fixed die or is it adjustable? If adjustable are you sure it is properly adjusted?

How long was the thread?

philbur
05-21-2011, 03:56 AM
A photo of the damaged threads and the tap would help. What kind of tap quality are you using.

Phil:)

PS: annealed drill rod is not prone to chip, it can "tear".


I was tapping drill rod, 3/8-16 on the lathe but have had this happen before. i was using a new dia and lube but the threads chip in spots. But I machined it to .375 just like i was suppose to, and all so I did one turn back it off and do a other turn . So very smart guys what did I do wrong Brett

38_Cal
05-21-2011, 10:05 AM
You have me confused...yep, it's my normal state of being, but this time...:confused:

Are you cutting internal threads or external? If external, you use a die if you are not single point cutting them. If internal, you would drill an appropriate hole and use a tap. So...which is it? :D For standard size external threads, I would start with a single point tool, and for a quick job, finish up with a split die.

David

Mcgyver
05-21-2011, 10:19 AM
I was tapping drill rod, 3/8-16


I machined it to .375 just like i was suppose to,


what did I do wrong Brett

You've either gone zero % thread engagement on an internal hole, or you're trying to cut an external thread with a tap :D

pulling your chain, but making an external thread you'd just call threading, tapping is reserved for internal threads using a tap. Threading would be commonly done either via single point tool in the lathe, or with a die, either in the lathe or at the bench. Also wouldn't hurt to turn the section down to say .372 or so, although it is common to just thread the nominally size rod

Is it a good quality die that's sharp, using thread cutting oil? 16 tpi is a coarse enough thread that its taking a pretty big chip and is more likely to produce tear out and burrs than finer threads. If you're using good quality stuff and its still not acceptable, here's to approaches. 1) single point the thread in the lathe then chase with the die - this makes the die like a finishing cut. The single pointing will ensure alignment and rough out the material, while the die taking a light cut will ensure the proper thread profile and good finish. 2) kind of like 1 except you use the die all the way open for the roughing cut then close it in for finishing. Needs a split die, which the good ones should be. Method 1) will produce perfect results, 2) might but you haven't that much latitude on opening/closing the die

Dr Stan
05-21-2011, 11:16 AM
Sounds like the OP is using a die to cut external threads.

1) Is it a HSS or carbon steel die? If the latter it will never produce good threads especially in drill rod.

2) 2x on turning the major diameter slightly below the nominal dimension. If you bother to check mass produced bolts you'll find them up to .010" undersized.

3) Drill rod is not the friendliest material to thread, so single pointing your threads would be your best approach. You could finish them up with a die, but once you get good at threading that should not be necessary.

Jpfalt
05-21-2011, 03:00 PM
Comments above on drill rod not the easiest to thread are correct. You do NOT want to turn the OD at .375 for 3/8 thread. The standard thread is a 75% thread which means the OD will be 12 1/2% of thread depth less than .375.

At .375, you are trying to stuff 10 pounds of thread into an 8 pound sack. The thread crests gall into the root of the thread form in the die and the thread crests tear off.

If you turn the rod to about .360, thread and remeasure the OD you will also find that in threading, the crests on the rod cold form and expand as well and you end up with a lrger OD than you started with.

Finally, use cutting oil on drill rod. The sulfur in it acts like a dry film lube at the cutting edge and helps prevent the galling. Chain saw bar oil or way lube will work, but not quite as well.

DATo
05-21-2011, 03:22 PM
I teach a course in machine shop and I tell my students that unless the strength of the thread is essential to drill a slightly larger tap drill hole. In your case you'd normally use a 5/16 drill (.312) for the 3/8-16 so you might want to up that to (off the top of my head) a letter 'O' drill which is .316 .

EDIT: I almost always drill slightly oversized tap drill holes and seldom break a tap. Now when you get to tapping a hole in titanium all bets are off .... Ti is a MOTHER to tap. Work hardens if you breathe on it.

philbur
05-22-2011, 03:00 AM
Don't know where you get this from but it is not correct.

Phil:)



1) Is it a HSS or carbon steel die? If the latter it will never produce good threads especially in drill rod.

jugs
05-22-2011, 04:18 AM
When you single point screw cut in the lathe & want a good finish, first thing you do is look @ tool, if it's worn you touch it up or replace it.

A die is a series of single point tools arranged in a helix ..... they wear too - like all cutting tools they are disposable - NOT a tool for life.

For the thrifty - careful of a die grinder will extend the life of a die. :D

john
:)

Brett Hurt
05-22-2011, 09:20 AM
I read the threads and went back to try it under cut cut it to .373 and it still hates me. so I cut the tread off machined it flat drilled a hole .250 then took a 3/8 ss bolt left 1/2 in. of thread macined it to .251 put it in and pined it know it works. thanks for the info Brett

Carld
05-22-2011, 09:27 AM
Drill rod is not the easiest metal to thread. You may have had better luck single point threading rather than using a die.

Depending on the load you may have to silver solder the threaded piece in the rod.

Dr Stan
05-22-2011, 01:08 PM
Don't know where you get this from but it is not correct.

Phil:)

go to: http://www.nedians.8m.com/ferrous.html and read the section on high speed steel

Mcgyver
05-22-2011, 01:53 PM
go to: http://www.nedians.8m.com/ferrous.html and read the section on high speed steel

Its a case of excellent correlation but no causality; most (all?) hand threading tackle made today from carbon steel is poorly made so people say don't use/buy carbon steel taps/dies, but that it is made from carbon steel is not why its performance lacks. Saying don't buy carbon is probably good advice, but saying they perform poorly because they're carbon is incorrect, hence you're both right, in a way :)

philbur
05-22-2011, 03:10 PM
What do you think industry used for a hundred years, until the invention of HSS. I make special cutting tools from carbon steel all the time and it performs as well if not better than HSS. The weakness of CS is it loses its heat treatment at relatively low temperatures and it is not as wear resistant as HSS. However it will take a keener edge which means it's gives a better finish and cuts better while sharp.

By your approach if you compare HSS with carbide you would never use HSS either.

The issue is the quality of manufacture not the material from which it is made.

It is still possible to buy well made carbon steel taps and dies as it is possible to buy poorly made HSS taps and dies.

Phil:)


go to: http://www.nedians.8m.com/ferrous.html and read the section on high speed steel

Evan
05-22-2011, 03:48 PM
There is nothing wrong with carbon steel tooling. It just can't stand getting hot or the temper is drawn.

Here is the finish on regular 1018 steel that so many complain about. The tool is a sharpened allen wrench. The lube is bacon fat.


http://ixian.ca/pics8/1018finish.jpg

Mcgyver
05-22-2011, 06:06 PM
It is still possible to buy well made carbon steel taps and dies

I have some old BA ones that I think are quite good, but who's still making good carbon taps/dies?

The difficulty isn't so much temps in use, that's easily controlled by surface speed and I've never hand tapped fast enough that its an issue....the problem more is that you have to be sooooo careful grinding them. After a very light grind I stone the last bit figuring any drawing of temper at point of contact with the wheel gets stoned away.....or more commonly on homemade cutting don't grind at all; just stone after quenching. The difficulty in grinding is probably a compelling enough reason for not commercially making high end carbon steel cutting tools, but if you do get one or make one they can be every bit as effective as hss is, used at the right speed of course

Carld
05-22-2011, 11:38 PM
The issue is getting quality carbon steel taps and dies. Today all you can get is crap carbon taps and dies.

philbur
05-23-2011, 03:14 AM
http://bbs.homeshopmachinist.net/showthread.php?t=25517

http://bbs.homeshopmachinist.net/showthread.php?t=33696

Phil:)

Mcgyver
05-23-2011, 09:27 AM
The issue is getting quality carbon steel taps and dies. Today all you can get is crap carbon taps and dies.

That's been my experience, Phil claims different -I asked for but haven't seen who .... maybe there are some exceptions. It doesn't matter though.....the discussion is little more than a parlor game insofar as going to the catalogue or store is concerned where one will find as you say the good stuff is all hss. Where it might help is someone coming across an old set and not immediately concluding it was junk (although it may be) because it wasn't hss

bottom line is, buy good stuff (poorly made hss is still crap), the rest is just gum flapping :)

Dr Stan
05-23-2011, 09:53 AM
bottom line is, buy good stuff (poorly made hss is still crap), the rest is just gum flapping :)

On this we agree. However, Phil & I seem to agree that HSS is the way to go.

BTW, HSS is all we used in industry when I was working as a machinist, tool maker and industrial engineer.

Mcgyver
05-23-2011, 12:09 PM
all I use as well (except for my BA ones) cuz its all there is in quality tackle, ...but that its hss doesn't matter or do you any good, it was just easier for the manufacturer to grind it. might even be cheaper to make good hss because of that, but that's just speculation on my part.

philbur
05-23-2011, 03:17 PM
I've used Dormer carbon steel taps before, but not from this supplier:

http://www.mptools.co.uk/sp/cat/Engineering+Tools/sub/Tap+and+Die+Sets/sb4/Dormer.html

The first time of use, after a number of years of using the junk HSS and chrome steel variety, the tap ran in so easily I thought I had completely stripped the thread, the cut thread (mild steel) was perfect. A warm knife through butter.

My conclusion was/is that it's not so much what they are made of but who made them.

Phil:)

tmbg
05-23-2011, 04:56 PM
I have a box of Sossner 10-32 spiral point taps, elektralube coated. They are AMAZING... I keep waiting to break one so I can move on to a fresh one, and it simply won't happen. I typically use them to power tap thru holes in aluminum, but they breeze through even stainless in a hand drill.


I was screwing around with my CNC lathe a couple days ago, trying to set up some rigid tapping for a small part that I want to make. I was trying to tap 1" deep with one of these 10-32 taps, and they're spiral point, so not really optimized for blind tapping deep like this. I knew full well it was a bad setup, and likely to break the tap, and my friend that was helping me with it wanted me to let him know before I started the program so he could watch the tap break.

I ran it, it got close to the bottom of the hole, full up with swarf, and spun the tap in the holder and pushed the stock back in the collet, but did not break. I was able to back it out and it's still 100% usable.

spope14
05-23-2011, 09:00 PM
Machinery's Handbook has the OD for an external Thread (2A) at between .3737 to .3643. Drill rod likes to "expand" or burr a bit when worked, so I would hit this OD to .370 or even .368 for Drill Rod (probably .371 or so for other metals). Besides drill rod not liking to be worked, the burring tends to clog a die and eat upon itself for lack of better words when the chip goes through the die and stays there as you move ahead.

One question, besides having the diameter a bit OS, are you using a die that has a start side and finish side? Many new people do not check for this, a taper for the start side and an instant start on the opposite side (kind of like taper tap and botton tap), so they start on the finish to shoulder side and this chips things up regardless of material.

Finally, check for lube and do 1/4 turns for chip break and blow out the chips.