Odd tap

[scsmith42]

My brother recently purchased a lot of machinist tooling from the estate of a friend of his.  Included in the box was this hand tap by Greenfield.  I've never seen this thread profile before, and the numbers written on the side of the tap don't make sense to me.  The threads appear staggered so that they will cut every-other thread.

1" - 1/2 is stenciled on the side of the tap.  The sleeve that contains it has the following written on it (not all legible). Illegible parts in () below

(P)lug hand tap
No 530(3)
14314

1-1/() 7 NC
H-4

Here is a profile pic of the tap.




Anybody familiar with this?

@Old Greenhorn. Tom - do you have any ideas?

Thx.

Scott

[YellowHammer]

It's an "Interrupter" tap, most usually seen on pipe and plug taps, where the parent material can be brittle, and have a tendency to chip easily, basically making metal gravel, and so can't get ejected and clog the cutters degrading the threading process.  So it prevents the tap getting stuck, prevents the chips and gravel from screwing up the just cut threads, and prevents not being able to reverse the tap out and "pop", busted tap. 

The extra flutes in the beginning allow accurate starting and engagement. 

[doc henderson]

that's right!     I am unfamiliar but did see the thread cutters next to each other were off set.  I thought this may be to reduce power needed to tap by only doing half the cutting at a time.  thanks YH.

[Old Greenhorn]

Yes, YH nailed it. These were/are used mostly on cast iron. These days in production the best way to cut those threads is with a thread mill, but of course, you can't do that in the field. Cast iron presents a number of issues for the machinist and threading is just one of them.
 Edit to add: that's likely an 1-1/4-7 national coarse, whic is a standard thread and the fact that it says "NC" and not "UNC" tells me it is an older tap, probably pre-1980 or more. It's not a pipe tap. The H4 is a tap tolerance range.

[Old Greenhorn]

I was thinking about this some more. As best I can recall, back in the 60's most folks (that used them) called them 'cast iron taps'.
 A 1-1/4-7 tap is a honker of a tap and you would need a least an 18" tap handle to turn it, let alone keep it straight. That particular one you have looks like new and would cost a pretty penny today.

[Old Greenhorn]

Have you measured the OD of this tap? Just re-reading above you said it is marked '1-1/2' and if so, the standard pitch would be 11, if it is 1-1/4 that standard pitch is 7. To be sure, measure the OD than count how many threads (including the deleted ones) are in an inch.
 Back in the day (and still, I think) Greenfield was one of the very best tap and die makers. Top of the line in my book.

 Taps used to come in sets of 3: Taper, Plug, and Bottoming taps. A taper was used for starting the thread in a hole and keep it straight, it had a long taper on the front to make that happen. The plug tap filled out the thread and got more full thread further down the hole. The bottoming tap brought the full thread form all the way down the hole as far as geometrically possible. Obviously, using 3 taps on every hole was expensive in time and money and other methods were developed and the plug type tap became pretty much standard.

[scsmith42]

Thanks guys!

Tom - I think you're right about the 1-1-4".  That part of the number has some surface abrasion which makes the number hard to read.

Robert - thanks also for the background.

Doc - thanks for sharing your thoughts.

I have a large variety of taps (including Greenfield and plug/taper/bottoming), but had never seen the skip pattern on the threads like that.  Y'all's explanations make perfect sense.

Tom, I've been kicking around the idea of building a thread mill setup for one of my lathe's by using a router.  Seems like there is a lot of information available about doing it with good results.  My need is to thread the inside of relatively thin-wall tubing (including Ti) in different diameters and a thread mill seems to be the best way to do this.

[Old Greenhorn]

Well, that is quite a complicated undertaking. I'm sure you realize the challenges and are thinking them through. First a router turns way too fast for cutting any type of metal without burning the cutter up quickly. You will need 2,500 rpm or less on that thread mill, depending on the material.
 Second, you have to have a pretty slow rotation on the lathe chuck AND (here is the tough one) the chuck rotation and the longitudinal feed on the router have to be locked together in a very precise manner. This controls you thread pitch. I suppose you can use your gear box as if you were single pointing a thread. You have your work cut out for yourself there.
 Have you considered single pointing? Yeah, I know Ti is a challenge, but you can get single point inserts in a wide variety of grades, including ceramic for Ti. Just a thought. I know you said 'thin wall' and yeah, that can be a paint, but if you make a split collar to go around the outside and back it up to increase the wall size, single pointing may work just fine.

 I once had a project dropped in my lap that 10 other large shops had tried to make and failed. It was a development for Sikorsky Helicopter. I forget the material, lets just say aircraft grade tool steel heat treated, and it was basically a torsion spring cut through a solid blank, but it had a variable pitch that changed as you progressed along the axis. Nobody could figure out how to make it and they tried everything they could think. Basically they dropped it in our lap on a provisional level. "If you can make these, we will pay your price, whatever it is". We kicked it around among the senior guys for a week or so bouncing ideas around. Nothing stuck. I had an idea. We had a waterjet machine, the issue was a water jet cuts straight through anything in front of it, all the way through. That would not work for this part, but I figured out a way to divert/diffuse the jet right in the center of the part before it could damage the far side. The other part was similar to yours. How to time the part rotation with the (X axis) movement on the waterjet? The answer was to time the rotation increments and changes directly into the waterjet machine control based on the axis feed rate vs, the rotation rate. I called the machine builder and got their schematics and a little help and picked off some spare M codes in the control and tied them into the rotary head we were using. Lots of programming changes, adjustments, and testing with dummy parts, I used wood, then later aluminum dummy blanks until we got it right. (A waterjet doesn't care if it's cutting wood, carbide, granite, or glass, it's all the same.)
 Now if you have ever seen one of these machines run you know it is one of the loudest, dirtiest, sloppiest, most miserable machines you can work on when you are doing anything but flat plate work close to the water tank. We covered the rotary head in plastic to try and keep it dry and keep the garnet media out of it. All that did was make it last long enough to do about 20 parts. 
 When we proved out the theory we quoted the job and gave it a "we really, really, really do want to do this" price. We added in the cost of replacing our rotary head with each order ($3k), added a day's labor just for setup and cleanup, and some other stuff. Then we added 80% on top of that price just to be safe. We got the job, we made the parts, and as far as I know they are still making them 25 years later and they have refined all aspects of the operation to make it routine in those years. When we did the first parts, they sent 4 engineers and 2 inspectors in to check them out. They actually didn't think we had done it 'to print'.
 So as I say, you have some challenges, but there's no reason you can't figure it out.

[scsmith42]

Great background info Tom!  I'll sketch out some ideas of the router based thread mill and follow up later.

Back to the tap - here is a pic of the writing on the tap.  I swear that it appears to read 1" 11-1/2 NPT.  Now this doesn't make sense to me but that's what it appears to read!

[scsmith42]

Here is the printing on the sleeve that the tap came in.

[DHansen]

A 1" NPT would have 11 1/2 threads per inch and tap should tapper slightly.  I suspect the package was not originally for that tap.  Package appears to be for a 1 1/4" 7 Nation Coarse plug tap. (7 threads per inch)

[doc henderson]

another fine example of all the knowledge we have in the FF bank.

[Old Greenhorn]

OK, so now we know that the tube you have is not the one the tap came in, somebody just grabbed another tube to protect the tap. Your tap is actually what it says: It is a 1" pipe tap (NPT means National Pipe Thread) with 11 and 1/2 threads per inch. Yes, that pitch seems odd, but it's a standard, believe it or not. So if you take a pair of calipers and measure from crest to crest (or root to root) along the thread length, you should find that there is .087" between threads.
Yeah, it's still a honker of a tap to turn into any material by hand.

[DHansen]

That tap was meant for black pipe used in gas and air lines or could be used on aluminum.  Softer aluminum or steel pipes that cut fairly easy.  Could be used on galvanized pipe with care.

[Larry]

Scott, some years ago I designed and built a thread mill but instead of a router I used a cheap Harbor Freight pneumatic die grinder. I used the thread box on my metal lathe to advance it. I made it to thread wood boxes on a production bases and never used it on metal. The reason I chose a die grinder over the router was size. A friend liked my design and built one using a Foredom flexible shaft tool. If I was planing on threading metal they do make slow speed die grinders $$$$'s.

One could also do it backwards. Put the cutting tool in the chuck and make something to hold the work on the compound. This way you could dial in any cutting speed you desire.

I'm looking forward to see what ya come up with.

[Machinebuilder]

Tom that's an impressive story about the Sikorsky part

It reminds me of this saying

"but it looked good on paper"

now days "it looked good on the computer"

[Ed_K]

Both of Rita's an I mother's worked at Greenfield Tap & Die. Rita's mother worked off the farm in the Sales Dept and my mother worked grinding taps. I have 2/3s of a set of 1" 11-1/2 -NPT, the camera batt needs charged so I'll have a pict in a little bit.

[Old Greenhorn]

Doc is right, there is an amazing amount of knowledge and history in these pages that come from 'our people'.
 Ed although I could not wait to leave the metals manufacturing trades behind after 50+ years I do have fond memories. I grew up sitting on a shop stool in my Dad's basement shop watching him run his lathe and asking and endless stream of questions beginning at age 5. That never ended (just changed format a bit) until my Dad passed at 96. He had stories from when hea blacksmith's helper boy in the 20's, then later was an oiler, then a machinist apprentice in the 30's. Later he was a chief machinist mate during WW2 and his career brought him up to the executive level doing designs and building a company or two. He knew a lot of 'stuff' and could make anything and often did. SO he taught me a lot about 'the philosophy of work, design, and quality'. He always strove to get the best tools when he could make the tool he needed. Companies like Greenfield, Hardinge, Brown & Sharpe, and Starrett were always highly regraded, even a little revered. I carried that forward whether I knew it or not. One of my career highlights was working directly with Hardinge to have them make one of our parts in various sizes and in large quantities. I picked them because they were the best in the world at making collets and they were right in NY, 3 hours drive from us. The project ran about 5 years and thy made a few million in billables from us (they were happy), and I worked side by side with their management from the VP on down to spending time with the engineers and shop personal teaching them how to deal with the material which was extremely frustrating to work with. At first they blew me off, but when they failed as I had told them they would, they came back and we started over. Once they got the fundamentals don, they were off and running and making dang good money. That was right around the 2008 crash and they had been laying folks off. The work we brought to them was a big shot in the arm and kept a bunch of folks working when everybody was having a tough time of it. That might have been the best project I managed in my whole career. Unfortunately those companies are now being run by folks who never worked in a shop or actually made anything more complicated than a paper spreadsheet. They are all in decline because the principals they were based on have been forgotten. I have a great story about an interaction I had with Hardinge where in I left a voice mail message for the VP of sales for North America and it was ranting and angry talking about  major decision they had made that could damage their reputation forever in the collet world. I can't give the particulars here, but I do remember saying something like "I don't know who or how your company made this decision, but if the President knows about it and is allowing it to happen, you have that needle necked, pinheaded, poor excuse of a man give me a call at any hour and I will explain to him in simple terms how this decision could run your whole company down the toilet in 2 years time." Then I left my number, just in case. 
Well it as a voice mail I had left for the VP. As it turns out, he felt exactly the same as I did (he was also a shop rat). He listened to my VM, smiled, and then forwarded the message to the President. I thought I just blew up our relationship when I found out the next day. I never heard from the President, but my little tirade served to make them change their mind. About 6 months later I met the President in Chicago and my VM came up in the conversation and I wanted to crawl under the carpet. I apologized for my language and tone. He just laughed. He said, 'you know, you were right of course, I had my head up my ash and what I really needed was a lifelong customer who cared as much about our company as I do to set me straight. Don't ever apologize when you are taking the high road and are right." Fun times.
 Sorry, I got off there for a bit.  I also got on them a bit when they decided to stop making Toolroom Lathes. Still think it's a bad idea.

 I wish I could remember more of the details about those springs. I know we had to make an arbor for each part because they were destroyed in the process and there was a bit of tweaking for several days. We too thought it was a crazy design but the Sikorsky guys explained to me what they were doing and it actually made sense what they needed. They knew what they were doing, just didn't know how or even if, it could be made. Handling that progressively changing pitch was the real tricky part with the transitions between each pitch change. I think that was around 2003 or so.

[Ed_K]

Let me ask a questions about that spring, was there any thought about turning the water jet head a few degrees so the cut would go into the top of the thd pitch and out the bottom of thd without cutting into the sides of the thread.

Finally got the camera to work.


I'm going to have to find the post about adding picts.

The pictures are of a 1" NPT 11-1/2 thd the small one is a # 2-56 from Greenfield, my mom hated grinding these I have a few left but their easy to brake. I have some taps made at Vermont tap and die.

 

[Old Greenhorn]

The short answer is No, Ed. That wouldn't work. It was not a thread. The cut sides of the spring had to be perpendicular to the cent axis of the spring. But these are the things we were trying to wrap our heads around as we tired to create an approach to makin the thing. If it was easy, any one of those other shops would have made them an not failed. If you ever want to see me motivated, give me a job and tell me nobody else can figure out how to make it. That was my specialty. If I recall, we mounted these on an arbor which we treated to 62Rc with the center drilled out and put a loose fitting carbide pin in the hole. When the jet hit the pin it would move around and disperse a lot of the energy in the jet stream, but my memory is foggy on these details. I do know it was a new arbor and pin for each part. Amazingly we only lost one part out of the 16 piece prototype run for application testing. On a job like that with al the variables and weak points in the process, I would have expected to loose half or more.

[gspren]

Another 40 some year machinist with a bit different opinion on why some pipe taps are skip tooth. Unlike straight threads, on tapered pipe threads all the threads are still cutting utill you stop and back out, that leaves a visible line on the cut thread where the tooth stopped, by skipping teeth the stop marks don't line up with each other making high pressure fittings seal better. Single pointing and thread milling are even better. This was explained to me by an engineer when I worked at Borg Warner making high pressure compressors back in the late 70s and early 80s before I went to the Ballistic Research Lab which later merged and became Army Research Lab.

[scsmith42]

WOW!

The aptitude, knowledge and skill level from the responders is incredible!

Tom - thanks for sharing some great stories.

GSPren - I've seen the lines that you're referencing and your explanation makes perfect sense.

Larry - as I get further into the project I'll be sure to start a thread about it.

All - many thanks for the stories and insight.

Scott