# Please school me on internal threading, tool holders, inserts, HSS etc.



## kevin.decelles (Jun 3, 2021)

I'm looking for someone to educate me on the finer points of internal threading.  As a baseline, I've never internally thread anything.

I recently ordered some tool holders/carbide-inserts from Aliexpress, and the learning curve on the insert/holder naming conventions was pretty steep.  When I looked at the inserts and holders that arrived, I'm still confused.

Some base questions I have are:

*Left hand vs. Right Hand internal threading *

What side of the bore does the tool work?  I've ready about inverting the cutter and running in reverse etc.  but haven't put any of it into practice.  My right-hand holder looks to cut on the left side of the bore, the left hand holder looks to cut on the right?
Does your chuck always run in the normal (forward) rotation?
Do you thread towards the headstock or away from it?
*HSS vs. Carbide (laydown) insert*
Pros/Cons?

*ACME Threading*

Internally, assuming a larger bore (1.5" or bigger),  single-point or tap?  
HSS vs. Carbide?

I know nothing -- all opinions welcome -- you can't hurt my feelings.

Bonus Question:


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## Tom Kitta (Jun 3, 2021)

My God, you want us to write a book? 

I threaded few times internally. Once left hand (or maybe twice).

It is similar to external threading but you see less of what is going on. 

Threading is easier and faster with DRO.

Regular threading I always go towards the chuck. I never needed to play with that.

For external threads I usually use an insert. For internal HSS. Mostly b/c most of my internal threads have been small. 

With all non-standard "new" insert types as well as "faking standard" to create company proprietary inserts (like Kennametal at $10 per insert or throw your facemill at a wall) I am also confused. Why do not manufacturers put insert size on their tool? They *never* do that. They could put in something like "recommended size TNMG 322" or in metric format or both. On the flip side, why they do not even sometimes put size on a BOX the inserts are in - but "catalog" number. Trying to lock us into their insert brand? 

I only own HSS taps and dies both for regular V threads as well as ACME. I assume tapping 1.5"+ with ACME tap would be an issue - I have large V thread taps in 2" and 2.5" so I guess it is possible. Measure the torque needed and post it here, better yet make a video. Also would be of interest how straight this all went.


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## Brent H (Jun 3, 2021)

@kevin.decelles :  there are some good you tubes on the threading.  This old Tony and Blondihacks and and Vintage Machinery  have lots of help.

*Left hand vs. Right Hand internal threading*

What side of the bore does the tool work? I've ready about inverting the cutter and running in reverse etc. but haven't put any of it into practice. My right-hand holder looks to cut on the left side of the bore, the left hand holder looks to cut on the right?
For the internal  left hand threads you can cut them with the lathe running in reverse, forward feed and on the far side of the bore. Makes things easier to see.  tool facing up

 You can cut them on the nearside  of the bore with the lathe running forward and the feed  running in reverse, but you start the tool inside the bore and you cannot see what is happening as easy.  Also you will typically make a few spring passes to finish the threads and it is easier when you can see how much is coming off. tool facing up

Does your chuck always run in the normal (forward) rotation?
It doesn't have to - see above

Do you thread towards the headstock or away from it?
You can go either way depending on your set up and if you are trying to thread a landing that doesn't go through the whole way

*ACME Threading*

Internally, assuming a larger bore (1.5" or bigger), single-point or tap?
You will need to pretty much single point as a tap would be $$ .  I have single point made bushings down to 9/16 ACME LH and single pointing 1" ACME is not too stressful once you are on your game

HSS vs. Carbide?
I ground the HSS for the smaller bores and carbide insert for the larger bores.  The ACME HSS ones that you can buy are typically pretty expensive if they are one piece construction - like ground into a 1/2" piece of HSS


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## Tom O (Jun 3, 2021)

It also depends on your lathe if it unscrews don’t run it in reverse.


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## 6.5 Fan (Jun 3, 2021)

All of the above applies.  Threading isn't difficult just go slow.


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## DPittman (Jun 3, 2021)

Kevin,
I've gotten confused many of times also with left/right hand inserts, threading towards/away from headstock, internal/external threading and reverse spindle directions.  What I did learn is that there is more than one combination possible to get the job done.  Remember compound angle changes with a different setup.

I have both carbide inserts and still like and use hss occasionally also for threading.  Small internal threading I do must be done with hss in boring bar holder.  I have a "Mesa tool" carbide insert grooving and threading tool that allows you to do left/right hand and external/internal threading that I like very much.


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## PeterT (Jun 3, 2021)




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## PeterT (Jun 3, 2021)




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## PeterT (Jun 3, 2021)

upright & inverted tool positions


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## PeterT (Jun 3, 2021)

The main reason for not threading towards headstock is the pucker factor when the tool is approaching a dead end. If you cant disengage or retract within the relief then you risk a collision. So alternately you start the tool in the relief groove, engage thread & go from HS to TS. But you need the tool orientated properly & fwd/rev selected properly. That's why the mention of screw mounted chuck vs camlock/pin style because it could unscrew because load is in loosey-lefty vs righty-tighty direction. if there is no dead end issue than conventional threading is from TS to HS. I think most lathes can change chuck direction independent of feedscrew direction but I may be wrong. In which case you have less options


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## RobinHood (Jun 3, 2021)

That is an excellent summary of threading - covers all the possibilities. Thanks for posting, Peter!


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## PeterT (Jun 3, 2021)

If you are making a dedicated pair from scratch, meaning threaded shaft & nut, make the external thread shaft first & internal nut tread second. Why? Because you have the means to measure the external thread either with 3-wire method or thread mic or whatever else is appropriate to verify the pitch diameter & OD is within class target tolerance. Then machine the nut to fit y creeping up on depth. Reason is there isn't a great way to measure internal pitch diameter. Unless you're Tom Lipton & who probably has some tricked out indicator from a 1955 Skunkworks tool shop. 

Even if you have a tap & intend to use it, there still could be case for machining the threads to 80% depth & let the tap do the finishing work because the load on it will be substantially less. Here I'm talking big TPI threads or tough materials where taps are spendy.


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## ShawnR (Dec 3, 2021)

Thanks Kevin for starting this thread. I searched for acme threads and found this one. As much as I feel ok about cutting internal threads, finding the cutter is proving to be a difficult or expensive, if I understand correctly. But I think I might have what I need already, with some coaching....that is where you all come in...

I want to cut 3/4" diameter or more internal acme threads. 

Is this a start? 





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						TMX,TMX 5/8 SIR-0500-L11 RH INT THRD BOREBAR,6-821-009,KBC Tools & Machinery
					

TMX,TMX 5/8 SIR-0500-L11 RH INT THRD BOREBAR,6-821-009,KBC Tools & Machinery




					www.kbctools.ca
				




I have watched a couple of Tony's videos on it and in one, he just says he shaped an old end mill and then he brazed it into a piece of steel rod. I am good with everything except shaping of the cutter. Perhaps my sharpening equipment is what is holding me back....a misshapen green wheel . Tony does show a neat little guide. I made one up years ago that could probably be tweaked. Here is what I have to work with currently. Perhaps the next project is sharpening equipment, then the tool, then the job....

Here is my grinder used only for tools. The left is the standard wheel. The right is a green wheel. I have wondered about changing one out for a "white" wheel but then the question of straight, cupped, etc.






If I was to be able to shape a profile, then these are a couple of boring bars. The larger is a 1/2" and the smaller, a 3/8". I would think that the 1/2" would be good for a 3/4" acme tool bit? Wondering about how much tool has to stick out but that info can come from the Machinist's handbook. 






I don't have an acme thread gauge. Is that where I am stumbling cause I cannot see how I can shape a tool based on only dimensions. But I suppose if I use the same tool bit for internal and external, then it will work but that sounds like a poor practice.

Suggestions?


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## kevin.decelles (Dec 3, 2021)

I’m on the same journey, I ended up buying a gauge like this one https://www.kbctools.ca/itemdetail/1-999-031

I also picked a trapezoidal one from aliexpress.  

I bought some carbide cutters and holders too but just have to get some cycles to try it


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## ShawnR (Dec 3, 2021)

I should clarify...shaping a tool cutter is fine except that it would be so darn small...

So as I continue to search, I think I am coming up with it. Does means spending a bit, (see what I did there?..   ) but I will be versatile then for other diameters.

Get the gauge Kevin refers to
and




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						TMX,TMX 5/8 SIR-0500-L11 RH INT THRD BOREBAR,6-821-009,KBC Tools & Machinery
					

TMX,TMX 5/8 SIR-0500-L11 RH INT THRD BOREBAR,6-821-009,KBC Tools & Machinery




					www.kbctools.ca
				



and




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						VARDEX,6ACMEX3/8IC EXT. RH VARDEX INSERT VK2,1-2892-287-C2,KBC Tools & Machinery
					

VARDEX,6ACMEX3/8IC EXT. RH VARDEX INSERT VK2,1-2892-287-C2,KBC Tools & Machinery




					www.kbctools.ca
				




That does limit me to larger diameters though. How small of a holder can you go to in order to cut a 3/4" thread? Would a 3/8" do it?


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## ShawnR (Dec 3, 2021)

I just realized that the insert IC does not match the holder so need to adjust that


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## YYCHM (Dec 3, 2021)

This states External.... https://www.kbctools.ca/itemdetail?itemcode=1-2892-287-C2


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## ShawnR (Dec 3, 2021)

Ah...yes. Missed that. 

I cannot seem to find a tool suitable for cutting the whole internal length, which I would like to be at least 1 1/2" to 2".

I think I am missing some simple aspect of this. It should not be this hard. Waiting for that Ahhh moment..


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## RobinHood (Dec 3, 2021)

Please refer to this link:





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						ACME General Purpose Thread Design Calculator
					

This ACME Thread General Purpose Design Calculator will determine the three classes of General Purpose, 2G, 3G, and 4G, for the external and internal threads.



					www.engineersedge.com
				









From that you can see that the cutting tool needs to stick out a minimum of 0.090” from the edge of the boring bar (0.08915 min thread depth).

With a max bore size of 0.5917” a bar whose diameter is 1/2” will be very tight (0.5917 - 0.5000 = 0.0917”, but the tool needs to stick out 0.090” leaving you only 1.7 thou to maneuver the bar).

So perhaps a custom boring bar of ~0.400” would make life easier. I suggest a long 7/16” grade 8 bolt. Drill a cross hole to accept 1/8” or 1/4” HSS round tooling. You’ll also need a way to retain the tool bit (a system like you have in the boring bars you have should work).

Shape the cutting tool according an acme gauge (you can use a piece of paper shaped as per machinery’s handbook  [or any other favourite source] as a guide to check geometry with a magnifying glass). Use your grinder with the rest using a 14.5* grinding guide. If you leave the tool blank long, you have a way to hold it. Cut it to length (so it fits the width of the boring bar and sticks out the required amount) after you have shaped it.

I would recommend you use a regular 60* internal threading tool to remove the bulk of the material. Then finish your part off with the acme shape. Doing this imitates what a 3/4-6 acme tap does. If you don’t have a 60* internal threading bar, just make an approximate bit for the 7/16” boring bar you made above.

You could try and use the existing 3/8” boring bar. You may find with a bore 2” deep that it flexes too much. But as always, until you try, you don’t know. It may well work fine…


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## 6.5 Fan (Dec 4, 2021)

As RobinHood suggests you can make a gauge to help shape your acme cutter or buy a gauge, much cheaper than buying special tools that may only be used once. Back before the days of internet and information over load i had to cut a few acme threads for a buddy, internal and external, my teacher was a well thumbed copy of How To Run A Lathe a great book Southbend published. It isn't hard once you get started.


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## ShawnR (Dec 4, 2021)

Thanks @RobinHood for the explanation. You addressed my hesitation issues well. I have ordered an ACME gauge but might do some internal threading today for practice until it gets here. I can shape some HSS bits as you suggest. 

I will report back. 

Thanks @6.5 Fan


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## Susquatch (Dec 4, 2021)

kevin.decelles said:


> I'm looking for someone to educate me on the finer points of internal threading.  As a baseline, I've never internally thread anything.
> 
> I recently ordered some tool holders/carbide-inserts from Aliexpress, and the learning curve on the insert/holder naming conventions was pretty steep.  When I looked at the inserts and holders that arrived, I'm still confused.
> 
> ...



I know this response is late to the thread, and I know there are some excellent responses already posted. But I thought you might appreciate seeing a copy of my own notes written for myself about 15 years ago when I first wanted to try cutting threads from the backside of the lathe. I too found the LH/RH outside/inside tool thing confusing.

Cheers!

The only difference between Right and Left Hand Threading Tools is the angle that the trailing edge of the cutter makes so that it can follow the thread groove without scraping on the edge of the grooves.

1. For Right Hand Threads, the same RH tool geometry will cut both outside threads (front in forward and rear in reverse), and the opposite trailing edge angle tool (LH tool) will cut both inside threads (front inside in forward and rear inside in reverse).

2. Left hand threads are the opposite of #1 above.  LH tool tips cut outside LH threads, and RH tool tips cut inside LH threads.

3. Within limits, thread pitch and size affect the clearance requirements of the cutting tool bit both in terms of the tip shape (which affects the cut quality and the root of each thread) and the angle of the edge trailing away from the tip (which is mostly determined by the thread diameter and the thread pitch).

4. Theoretically it is possible to cut threads with an upside down tool. But practically speaking, upside down tools don't work well. There is no rigidity in the tool holder because it is lifting away from the compound. To work best, they should push down to the compound in reaction to the force of the work spinning against them.

5. Preground hss or carbide inserts are only sold as RH and LH tips. But RH outside = LH inside, and LH outside = RH inside.

6. Care must be used when cutting inside threads in small diameter holes to ensure there is enough clearance to clear the trailing edge. Basically, the radius of the trailing edge must be less than the hole radius.


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## Susquatch (Dec 4, 2021)

@kevin.decelles thanks for posting this question. To everyone who contributed to this thread (@Brent H , @Tom Kitta ,@Tom O, @DPittman , @PeterT, @RobinHood , and many many others, I wish to extend a HUGE thank you! I wish I had been able to read all your posts back when I first started threading.  I especially like the charts that @PeterT had provided. Every once in a while I have to hold my finger like a cutter and twist my other hand like a turning part. On bad days, when I have an extra bad brain fart, I have to grab a big nut or a bolt..... I wonder how many others can relate...... The info in this thread will "clear the air"!

@Janger - should this thread be a sticky?


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## Johnwa (Dec 4, 2021)

@Susquatch the lack of rigidity is one of the advantages of the upside tool.  With it upside down the tool springs away from the work.  With it right side up it springs towards the work and digs in.  This is more noticeable on a lightly built lathe.


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## YotaBota (Dec 4, 2021)

As mentioned before one of the biggest things (besides avoiding the crash) is to have the clearance from the bottom of the boring bar to the work so the you can cut the depth of thread required. That was one of my biggest hurdles, the thread got to the point where it would start curling over and then would not track the previous cut. The bottom of the bar was holding the cutting edge from making the thread any deeper.
Just a suggestion, instead of wasting steel and dulling cutters trying to get it right, I bought a bunch of cheap plastic pipe fittings for practice cuts. I can't remember what size the adapters were but eventually used them as a spindle protectors on the Logan and the SM9.


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## Susquatch (Dec 4, 2021)

Johnwa said:


> @Susquatch the lack of rigidity is one of the advantages of the upside tool.  With it upside down the tool springs away from the work.  With it right side up it springs towards the work and digs in.  This is more noticeable on a lightly built lathe.



I agree with you. However, I don't ever thread that way. 

Sometimes I do deliberately use a parting tool upside down and run the lathe in reverse. This stops the tool from grabbing and makes impossible parting possible.


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