Threading issue

[Dale Allen]

Hello all. First let me say up front that I am not a machinist and would starve if that were to be my chosen profession. But really, this doesn't seem right.
Both of these were cut with tools marked as 1/2-20 threads. There is absolutely no thread engaging when put together.
However, the tools are from a 'cheap' offshore set.
Can they be that bad and that far off or am I missing something here?

Note that the internal threaded part was about 3/16" longer and the tap was well into the major threaded part. I cut off some to see if it would make a difference.

Thanks.

 

[Displaced Canadian]

Not completely sure what happened, you could use a 1/2" nut and bolt to see if one or the other side is bad. I also found I had better success using finer threads.

 

[KBs Pensnmore]

It is quite possible that both tap and die are the problem.
1)try running a 1/2x20 nut on the tap. A nice firm fit is required, if that is OK,-
2)is the die adjustable?? If not a hacksaw cut (may require a double blade to thicken the cut) down the side into one of the swarf clearance holes will make it adjustable (a dodgy way of doing it), the holders adjusting screws will bring it down to size, check on a bolt, that it is a firm fit also.
3)if the tap is loose on the nut, (and you don't want to purchase a better set) try cutting a thread with the tap in a piece of metal and adjust the nut to cut a thread on some metal so that they match?
When you have it set, leave it at that. After you have cut a thread, unscrew and turn the handle around and thread as far a required,as the dies are usually tapered. Another thing that helps to get a good fit, is to turn a small groove against the shoulder (1/16"x1/16") for the die to cut square against.
Cheap imports are usually made from soft material, breaking teeth easily, always use a cutting fluid/oil, especially on top quality taps and dies (makes them last longer, and a cleaner cut).
Cutting threads on acrylics, use "PAM", I think it is over there, a type of cooking spray.
When purchasing taps and dies, I always get the best I can and from a reputable manufacturer.
I have found buying cheap, is false economy, as it will have to be replaced sooner that later!!!!!

Kryn

 

[Johnny westbrook]

Did you use a bottoming tap are starter tap

 

[Dale Allen]

Well, thanks all for the information and suggestions.
I could not find a 1/2-20 bolt and nut around here to check.
Basically this tap and die set needs to be become scrap metal.
Below is a close up of the die, which is the poorest excuse for a tool that I have ever encountered. I would never have imagined these could be this bad so I never checked until this morning.
I'm going to get a good tap and die of one size to practice with and maybe I'll have better results.

 

[mredburn]

your hole may be two large for the tap your using. It doesnt look like you have any proper threads formed. Your may be stripping them out as you go. Too large of hole is just as bad as too small of hole. We do have pam or spray canola oil. Mineral oil works as well.

 

[Dale Allen]

The 1/2-20 calls for a 29/64 bit. I didn't have that size so I used a 7/16, which is smaller!

 

[mredburn]

A smaller hole can force the tap to pull harder against the threads its cutting to force itself into the hole. That can lead to stripped threads on softer materials.

 

[mredburn]

Also did you use a tap guide in a drill press or your lathe or did you try and cut them by hand?

 

[dogcatcher]

Is the small part just sliding into the larger part? Like a plug? No catch of the threads at all? Or is the small part so large it won't fit into the hole?

 

[Dale Allen]

No catch of the threads at all. That is why I cut off some of the internal threaded part thinking that because of is not a bottoming tap, it may not have threaded far enough.
The plug just drops into the hole with absolutely no resistance.

 

[frank123]

Can you give a detailed description of the exact technique you are using to do the threading?

 

[redneckmedic]

Can you give a detailed description of the exact technique you are using to do the threading?

Might be easier to actually just repeat the process and catch it on video, then post it.

 

[frank123]

Can you give a detailed description of the exact technique you are using to do the threading?

Might be easier to actually just repeat the process and catch it on video, then post it.

Hadn't thought about that but it is an excellent idea, no mistake about what is being done or not done and needs to be changed or not.

(FWIW, a 20 pitch if pretty coarse in a half inch, I'd think a 28 pitch would work better in plastic. That isn't necessarily related to the problem though)

 

[randyrls]

I could not find a 1/2-20 bolt and nut around here to check.

Dale; The local big box store has some kind of fixture that you can use to check the threads.
this is also an option:
Screw Checkers - Sizing Gages | MSCDirect.com

 

[Dale Allen]

OK, let's get a few things cleared up. Even though I said I was not a machinist, I am not completely lost when doing this.
I use a die holder in a piece of hard plastic that has been turned to use on the lathe with the live center pushing it so everything is nice and straight. Threading is done by hand, lathe off, with slight tightening of the tailstock with each 1/4 turn. Also, the tap is held between the workpiece and the tailstock also using the live center. Again everything is straight. Hole sizes and OD of the stock are per spec and measured with a digital caliper. Also, the ends were chamfered to aid the thread starting.

Now, I have thrown all but 3 pieces of the set in the trash can and the 3 I kept out will soon be there too.
I decided to try this again with a size for which I had a nut and bolt for checking, 3/8-16. I tried twice to use the tap and in both cases the threads seemed to distroy themselves before I got very far. I also tried the die 3 times and could not get it to cut past about 2 threads before it destroyed the threads.
I nealy gave up but then remembered that I had a really old plastic container around that contained a few taps. As luck would have it there were 2 of them that were 3/8-16.
So, just for giggles and grins, I decided to try one of them. Neither had the hole in the back to use the live center so I just put it in a tap handle and tried to keep it as straight as possible. It sliced through the hole with ease and left a threaded section that would make you proud. The bolt threads in with hardly any sideways play. What an absolutely night-and-day difference.

Then, the quest became that of figuring out why the cheap one would not work. I have a partial answer but at this point I don't think it matters.
Below is an image of both taps used, the good one is on the top. I can now see that the thread pitch is not true to spec and that within 1/2" in length the cheap one looses almost an entire thread. What does that mean and is this part of why it will not thread? I don't know. Seems to me that even with the pitch being off it would still make threads, albeit not the correct size.
BTW, you can check this tap against a bolt or a thread gauge and it would look pretty close.
The moral here is, don't depend on the cheaper stuff to work.

I'm getting out of this twilight zone and going to buy some real tools!

 

[frank123]

If the tap and die threads are true to each other and cutting reasonably smoothly they should still provide a working threaded piece. If they are cut so poorly that they are tearing out the threads instead of cutting them then they will never produce anything workable. Since the threading is not right (probably on the 1/2 ones) then there is the possibility the diameters are not right either and the right hole and tenon sizes for the 1/2 inch would not be right for whatever size the die and tap are actually made for. Sounds like maybe a low end foreign machine shop made them using metric machinery and not figuring the right compensation for it to make an inch thread.

A couple suggestions that may make accurate threading easier (at least from they way I do it, not necessarily the exclusively right or best way):

Make a die holder that slides on a shaft chucked up in the tailstock drill chuck. An easy proposition, just a tube of anything reasonable strong (plastic, aluminum, brass, steel -whatever you like working with) bored on one end to tightly fit the die with enough depth behind it larger than the diameter of the die size to allow for deep threading through the die and a further bore all the way through in the shaft size you want it to slide on. This allows finger starting and threading with the minimum pressure needed so you don't have to keep adjusting the tailstock and deal with varying pressures and clearances on the die while doing it. It also allows you to turn the chuck while rapidly moving the die forwards and backwards an eight of a turn or so till the critical starting threads are fully formed and drive the die forwards on their own. (You can use a set screw to hold it if you intend to remove the die between uses or just locktite in place if you want to leave it in and make a different holder for each die)

For tapping, do it immediately after drilling without removing anything, just unchuck the drill bit and chuck the tap then use a very light advance of the tailstock till the starting threads are cut after which loosen the tailstock and keep just enough forward pressure to keep it sliding effortlessly forwards (just to keep it from hanging up on something, not to actually advance it) and let the tap advance itself on its own threads.

If you're working with a drill press instead of a lathe, just adjust the procedure accordingly.

I find lubricant helps on the harder more brittle plastics and not really needed on the softer ones. A light gentle touch allowing the tap or die to drive itself forwards on its own threading instead of trying to force it in is what gives me best results, and plastics seem to like more tolerance than metals since they are a bit more elastic while cutting.

 

[IPD_Mr]

This is part of the reason that so many recommend against buying a tap and die set. Many of the sizes will never be used for our purposes. Yes the price is good but the quality usually is not. Dale I am sorry you had to learn this the hard way. The next issue is drill bits. Here is another tool that skimping on for what we do, does not pay off. You really need a complete set or two. You would be amazed at how many cheap bits that are not straight. The worst part is when you start to question your ability when it is not you but the tools. The other thing I would suggest is to get some of the cheap clear acrylic rod and practice. The clear stuff is great to help you see any problems you might have. I don't have the link to the company that sells it but should be easy to find doing a search on here. Oh and pick up a can of buttered Pam for your shop. It is a must tool for me when cutting threads and it cleans them as it goes. Best of luck to you.

 

[Paul in OKC]

Most of the 'cheap' sets are carbon steel, not high speed steel (HSS). the only thing any of those are good for is to rethread something that is already threaded, such as a boogered thread on a bolt or nut. They are not made or ground to cut new threads. The ones pictured don't look the same size. Were both marked 3/8?

 

[Dale Allen]

Another interesting thing about this 'cheap' tap.
Anyone see anything wrong with this picture?
BTW, the opposite end is the reverse of this issue.
I don't even understand how it was possible to make it this way?