# Threading Bits



## Stan (Jan 28, 2009)

Since some people are very sensitive to thread drift, I will start a new thread pertaining to the recent thread on threading. This quote came up on the previous thread.



> Brass and steel cut completely different with regards to tool rake angle. Typically brass wants less rake angle and steel wants more. I have more questions than answers at this point but I would start with the tool bit top relief angle assuming you have the rest of your settings dialed in correctly.
> 
> As far as the tool bit for steel goes: 60 degree included angle, slight radius on the end (no thread has a sharp V) and the top relief angle should be around 12-15 degrees. Stone all of the sides to remove burrs. Your tool bit appears to be mostly flat on top. Great for brass, bad for steel.
> 
> Also, is your compound set to 29 degrees? and are you infeeding with the compound and not the cross slide?



Every description of tool form that I have read calls for no rake in any direction on the top of a threading bit. On some bulletin board, a poster provided all the geometry to show how top rake changed the thread form. A statement very similar to this quote was recently made on the Logan board, so now I am questioning my own opinion. 

What do the experts say?


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## Noitoen (Jan 28, 2009)

Found this:

http://www.metalartspress.com/PDFs/60_degree_threads.pdf

Helder


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## kvom (Jan 28, 2009)

> On some bulletin board, a poster provided all the geometry to show how top rake changed the thread form.


A simple demonstration of this fact is to imagine an equilateral triangle as the lathe tool viewed from above. Now if you imagine the base of the triangle (side opposite the threading point) moved down with the point staying staionary, the apparent angle at the point increases. 

You will get the same effect on the thread form by raking the top of the tool.


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## dan s (Jan 28, 2009)

If you have your compound set to 29.5 degrees, and you have access to a surface grinder, you could make a bit like this that would work. The rake is perpendicular to the cutting edge, and thus when cutting, acts like a flat top bit.

I have never done this, because I don't have access to a surface grinder, but geometrically this will work.


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## itowbig (Jan 28, 2009)

copied for future reference Thm:
Thank you very much for posting this :bow:


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## Stan (Jan 28, 2009)

Dan: I am having trouble getting my head around your bit. Can you provide all the angles on both sides of the bit and both angles on the rake. I am visualizing a bit with 30° to each side and then 15° back and side rake. Is that correct?


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## Holescreek (Jan 28, 2009)

I have an answer to your question but I leave for work in a few minutes, so I'm hoping that another member with a copy of "How to run a lathe" published by South Bend will carry on. Pages 25 and 26 show images of threading bits with the ground back rake I described in the other thread. Page 29 shows how to grind them on a snag grinder by hand. Page 30 shows how there must be no rake for brass and bronze or it will grab.
Unfortunately I have never read HTRAL but have a copy on my desk left from my grandfather's stuff. I was sure that the info would be there though and just scanned through it. I was fortunate to have gone to a high school in the '70s with a full working machine shop where we were taught all of the machine shop basics. 4 years of machine shop and drafting made my machining career a cake walk in most regards. I really like teaching the basic processes and have a couple of friends who like to learn. Schools these days don't teach anything useful with regard to industry anymore. -Mike


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## dan s (Jan 28, 2009)

Stan  said:
			
		

> Dan: I am having trouble getting my head around your bit. Can you provide all the angles on both sides of the bit and both angles on the rake. I am visualizing a bit with 30° to each side and then 15° back and side rake. Is that correct?



The process for grinding this bit would be just like grinding a regular threading bit with one additional step. Thus you grind a standard 60 deg include angle, with 10 degrees of side relief. This would give you a "standard" flat top threading bit. 

When you turn right handed threads with the compound set at 29.5-30 degrees, you're only cutting with the left side of the bit. Thus as long as the left cutting edge stays in the same vertical plane (like a flat top bit) the geometry will be correct.

Lets take a look at how side rake and back rake is defined. side rake is relative (perpendicular) to the side of the cutting bit(green arrow), & back rake is perpendicular to the front of the bit (black arrow). 





In order to have rake on a threading bit, You can only have one angle (blue arrow), and it must be relative to the cutting edge. Since the rake angle is relative to the cutting edge, you need a jig and a surface grinder so that you can present the bit to the grinder properly. Does that make scene, if not let me know because I have considering publishing a blog post on this topic.


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## Paolo (Jan 28, 2009)

Hello dan's
copied for future reference 
Thank you very much for posting this Thm:
Paolo


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## Stan (Jan 28, 2009)

> When you turn right handed threads with the compound set at 29.5-30 degrees, you're only cutting with the left side of the bit. Thus as long as the left cutting edge stays in the same vertical plane (like a flat top bit) the geometry will be correct.



Does this mean that a 60° point is not required. Is all you need a 30° angle on the left edge and anything less than 30° on the right side? If that is true, then the angle of the compound is immaterial. What cuts the back side of the thread?
 I have been cutting threads for thirty years but finding out how little I know about it.


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## Holescreek (Jan 28, 2009)

Dans, I love the graphics! I'm learning Argon5 myself for just the same purpose. The back rake I use for threading is just a 12 degree straight back, no side rake. The one you've drawn would work just as well, maybe better. I'll have to try it sometime. graphics reallly take the guesswork out! 

Stan, each time you infeed the compound you are cutting from the tip and along the left side, infeeding at 29.5 degrees. The short answer is that the tip is cutting the right side of the V.
-Mike


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## dan s (Jan 28, 2009)

Stan,

As Mike said, the tip does the cutting see graphics in the upper right and Bottom left of this image.


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## Stan (Jan 28, 2009)

The picture shown for the 30° infeed is exactly the way I think the system works and they point out the problem of drag, which is overcome by setting at 29° and going straight in with the crossfeed for the last .001 to remove the error but 29° also takes a little more cut on the right side of the bit.

Mu concept, is as they show. Assuming a flat topped cutter set on centerline at 90° to the work, on the first pass, both sides of the bit cut an equal amount. On the second pass the left side cuts approx twice as much as the right side of the cutter (at 30° the compound moves approx twice as much to the left as toward the work). Assuming equal advance of the compound on each pass, on the third pass the left side cuts four times as much and on the four pass it cuts eight times as much etc.

This is all clearly visible in the picture shown. However the crux is that if there is any rake on the right side of the cutter, it is not cutting at the same angle as the left side.

 If you have the compound set at 29° you cannot go straight in for the last cut or you will make a big change in the angle of the back of the thread if there is any rake.

I have cut a lot threads with a flat top bit but I am always willing to learn new techniques. Certainly rake will make the bit cut deep threads much easier. Do you have any published references on this method?


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## dan s (Jan 28, 2009)

ask and you will receive. ;D

http://www.mmsonline.com/articles/threading-on-a-lathe.aspx

In my eyes, the issue with going straight in with a flat top bit is that, it creates a lot of deflection forces, due to the size of the cutting edge.


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## Holescreek (Jan 28, 2009)

> C. Modified flank infeed (recommended)
> 
> This method is similar to flank infeed except that the infeed angle is less than the angle of the threadthat is, less than 30 degrees. This method preserves the advantages of the flank infeed method while eliminating the problems associated with the inserts trailing edge. A 29½-degree infeed angle will normally produce the best results, but in practice any infeed angle between 25 and 29½ degrees is probably acceptable.



Dan, where do you find time to look all this stuff up? :bow: All of this (maybe without the scientific jargon on the rest of the page) is in HTRAL written back in the '20's. I'm thinking you should go ahead and put together your presentation. You certainly have the data all together, and the information is quite correct. I'm sure there are lots of lurkers gaining knowledge from your posts. -Mike


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## dan s (Jan 28, 2009)

Holescreek  said:
			
		

> Dan, where do you find time to look all this stuff up?


I pulled that one from my bookmarks, I have a good indexing scheme. :big:


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## Stan (Jan 28, 2009)

Dan: I read through that page and didn't see anything about rake on threading bits. Did I miss it or is somewhere else on the site?


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## dan s (Jan 28, 2009)

Stan  said:
			
		

> Dan: I read through that page and didn't see anything about rake on threading bits. Did I miss it or is somewhere else on the site?



The link I posted is mainly about carbide inserts that have it built in, thus the reason they don't mention rake. However the in feed angles will work regardless of cutter material.




 However


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## Stan (Jan 29, 2009)

Dan: If I cut fine threads on ABS plastic, I cut them in one pass. If I use a bit such as you show in your first post, it is obvious that the back of the thread would not have the correct form (the angle is less than 30° as it drops down the side relief). Is there some specified number of passes you have to take in order to get the correct thread form?

Do you have any catalog references to the carbide inserts where they show all the rake angles? I would expect that these angles would be very specific for the metal being cut, or does it not matter?

Do you have any references for grinding HSS bits with top rake for threading bits?


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## dan s (Jan 29, 2009)

Stan  said:
			
		

> Dan: If I cut fine threads on ABS plastic, I cut them in one pass. If I use a bit such as you show in your first post, it is obvious that the back of the thread would not have the correct form (the angle is less than 30° as it drops down the side relief). Is there some specified number of passes you have to take in order to get the correct thread form?



Not that I'm aware of most people, very the number of cuts and the depth of each cut, to keep cutting forces constant. In other words each cut is shallower than the last, because it is wider than the last.




			
				Stan  said:
			
		

> Do you have any catalog references to the carbide inserts where they show all the rake angles? I would expect that these angles would be very specific for the metal being cut, or does it not matter?
> 
> Do you have any references for grinding HSS bits with top rake for threading bits?



Sorry, I don't have any references.

I did find these photos that might help
http://img.directindustry.com/images_di/photo-g/threading-insert-36216.jpg
http://www.ststooling.com/images/sp-tc.gif
http://www.jlindustrial.co.uk/ProductImages/large/VTH-16385.jpg
http://www.iscar.com/ProductUpdate/Img/IscarThread_84_2.jpg



for ABS, I bet you could get buy with something like this in one pass.
http://www.carbidedepot.com/SearchResult.aspx?CategoryID=1744

note the graphic is wrong on the page, and, these inserts are not really designed for threading, but ABS is so soft, I don't think it would phase the inserts at all.


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## Stan (Jan 29, 2009)

I never use inserts so I know almost nothing about them. I looked at the pictures of the inserts in the last link and what I see are flat top inserts with chip breakers. They have to be flat top when they can be used either right or left hand and can be rotated to three different positions, so no back rake is possible.

Your quote earlier that the slope on the insert was built in would make me think that the insert would be thinner on one side than the other and that should be noticeable on the picture of the insert with two adjacent cutters

Further on my example of a bit with a side rake having the right side at the wrong angle and give an incorrect thread form if you cut the full thread with a single pass. If you cut that same thread with two passes you reduce the error by cutting with shorter portion of the sloping edge. But, no matter how many passes you take, if the cutter angle is wrong the thread form will be wrong except it will be in small steps.

I know that back and side rake improves the performance of normal turning tools but you also have to have both left and right hand tools. I am just having a hard time getting to the idea of cutting with both sides of a tool with a sloping top.

The only other approach I can see is to take very small DOC and then ignore the small error.


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## dan s (Jan 29, 2009)

Stan  said:
			
		

> Your quote earlier that the slope on the insert was built in would make me think that the insert would be thinner on one side than the other and that should be noticeable on the picture of the insert with two adjacent cutters



I couldn't find a good picture of a threading insert, with rake on both sides. They can do it though because the inserts are molded (I have seen them before, ISCAR I belive), so they don't have to grind them, thus just about any shape is possible.

take a look at these two inserts rake all the way around.















			
				Stan  said:
			
		

> The only other approach I can see is to take very small DOC and then ignore the small error.



It's all relative, without breaking out the graphing calculator and doing a few hours of derivations, I can say two things with certainty.

1. the closer the compound is to 30 degrees the smaller the error will be.
2. the less rake you have the small the error will be.

The other thing to our benefit, is that the threading system has tolerances of a few thousands so I bet a standard bit with a few degrees of back rake will make a thread that is still within tolerance.


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## tel (Jan 29, 2009)

These?


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## rake60 (Jan 29, 2009)

I'm loving this thread!

Now my 2¢ worth...

At work I use the the multi tooth inserts. They are run at 300SFM,
.005 depth of cut and flooded with coolant.




The machine is programed to advance the tool on a 29° angle and it works 
beautifully for production manufacturing.

At home the rules change.
Depending on the thread depth and size, I use one of these tools.




Surface speeds are half, 150SFM and the cutting fluid is nothing more than
a little cutting oil brushed on between cuts.
For model scale threads I have never found a need for the 29° compound advance.
I go straight in with no problem.

The very first single point threading job I did at home was making a new
mixer needle valve for a 60 year old hit and miss engine. It turned out to be
this:





Single point threading is not a Black Magic craft!
If you can grind a piece of HSS to a 60° included angle, you can cut model
scale threads.

The information that has been posted here is dead on PERFECT for job shop
machines and practices. It all converts very well to the home shop when it
is scaled down to hobby machine capabilities.

Great information Dan!
Please keep it coming!

Rick


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## dan s (Jan 29, 2009)

Stan,

I found the image I was looking for (make sure you exapnd it to full view).
http://news.thomasnet.com/images/large/006/6025.jpg

As you can see in the image it's a threading insert with rake on both sides. This can be accomplished with inserts, because they are molded. Unfortunately we can't easily grind a HSS bit like this. We can however make one side of the the bit have rake (like the one I posted above). 

If your interested, Sandvik made the video below showing how inserts are made.
[ame]http://www.youtube.com/watch?v=al0xryL1vjI[/ame]


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## Holescreek (Jan 29, 2009)

A couple of things have transpired regarding my confusion about tool bit rake on threading tools in th elast couple of days. I have always put back rake on my threading tools, and guessed that everyone else did. To that regard I discovered that I was very wrong (after lots of reading and searches). I also discovered another South Bend publication in my grandfather's stash, Bulletin #35 "How to grind lathe tool cutter bits" printed in 1936. I took the time to scan the entire phamplet and upload it to Photobucket tonight. I will be forwarding it to the WE Williams site for everyone who wants a copy. http://www.wewilliams.net/SBLibrary.htm He has the most Southbend publications I have ever seen on one site and provides them for free. In the meantime, I also scanned the 7 or so pages from the 1935 HTRAL publication relative to tool bit sharpening. My take on both publications is that they lack much info. on threading considering the questions that still arise 75 years later. They can be seen here:
http://i2.photobucket.com/albums/y29/Holescreek/Scans/HTRALP24P25.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/HTRALP26P27.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/HTRALP28P29.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/HTRALP30P31.jpg
Check out the reference on page 30 referencing bronze and brass, not necessarily with regard to threading, but interesting to many none the less. 

For those not wanting to wait for the files to be converted to PDF format here is the phamplet on lathe tool grinding:
http://i2.photobucket.com/albums/y29/Holescreek/Scans/FrontCover.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/InsideFrontCover.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/Page1.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/Page2.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/Page3.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/Page4.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/Page5.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/Page6.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/Page7.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/Page8.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/Page9.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/Page10.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/Page11.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/Page12.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/InsideBackCover.jpg
http://i2.photobucket.com/albums/y29/Holescreek/Scans/BackCover.jpg

-Mike


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## Stan (Jan 29, 2009)

Rick: Is the tool you show threading at 29° have both side and back rake?


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## Stan (Jan 29, 2009)

Dan: Apparently there is a lot of difference in terminology. The first pictures you posted (the red toolbit) had what I call rake. None of the insert pictures have that shape but have chip breakers. The cutting edge is the same height on both sides of the bit.

The last insert you show I think would be for straight in feed.


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## lathe nut (Jan 29, 2009)

I took Rick's advise a while back, ground my tool, did some reading and got in front of the lathe, first terrible, looked like a meat grinder did it, on about the tenth one it was starting to look like threads, that was 1/2X13, now I am able to cut with ease, this past weekend cut 32 TPI inside and out, that is the most rewarding thing I have done on the lathe and proud to say learned it here, Love this group, thanks all, Lathe Nut


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## dan s (Jan 29, 2009)

Stan  said:
			
		

> Dan: Apparently there is a lot of difference in terminology. The first pictures you posted (the red toolbit) had what I call rake. None of the insert pictures have that shape but have chip breakers. The cutting edge is the same height on both sides of the bit.



I think the terms rake and chip breaker get abused, and I'm probably just as much to blame as anyone else. Take a look at following image. I just made it, it's suposed to be insert cross sections.

the red insert - has no rake or chip breaker
the green insert - has no rake but does have a chip breaker
the blue insert - has rake and a chip breaker


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## radfordc (Jan 29, 2009)

Really great information. I recently made a project that required making threaded parts two inches in diameter with 20 TPI inside and outside threads. I was very happy when I screwed them together and they fit perfectly.

Charlie


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## AlanHaisley (Jan 29, 2009)

It seems obvious that if you use a thread center gauge (fishtail) to check the actual cutting edge on the tool, even with severe back rake you can get the cutting edges to an exact 60 degrees.


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## Stan (Jan 30, 2009)

Dan. This would have been so much easier if you had posted picture of an insert in you first post. The pictures in you first post show a classic HSS bit ground for turning (not thread cutting) with angles referred to as rake in the instruction books. I have had that bit in my mind while you have had an insert in your mind.

In order to get a correct thread form, the cutting edges of the tool have to be on center and parallel to the horizontal center line of the work. This is easily accomplished by a simple grind (angle and relief) on each side of a HSS bit.

I should rephrase that last paragraph. It is not necessary to have the cutting edges parallel to the horizontal center line if you make a bit with angles that compensate for the helix of the thread. I presume this is possible with molded carbide bits and I don't know if they do it or not. If is also possible to grind a HSS bit that way but takes a lot of math to arrive at the angles and there is no reason to do it if you just leave the top flat.

The end result is I think we would agree if we were talking about the same thing.


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## dan s (Jan 30, 2009)

Stan  said:
			
		

> The pictures in you first post show a classic HSS bit ground for turning (not thread cutting) with angles referred to as rake in the instruction books.



Stan, The first images I posted are of a HSS *ground for threading*. As I said in my first post a bit of that type will cut perfect threads, if it is feed into the work at 29.5 degrees. The reason it will, is because only the left side of the bit is doing the cutting.


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## Stan (Jan 30, 2009)

Dan:


> The reason it will, is because only the left side of the bit is doing the cutting.



That is something we apparently disagree on. Take the first pass and look at the shape of the cut you made. Making small cuts reduces the error. Here is your quote:



> 1. the closer the compound is to 30 degrees the smaller the error will be.
> 2. the less rake you have the small the error will be.



Everything I make has errors in it, some have monstrous errors, but I don't grind tools with a built in error on purpose.

Let us just leave it there and let everyone grind their thread bits the way they like.


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## hssmike (Feb 5, 2009)

Just a note:
  If you grind a tool to 60°, THEN grind a back rake on it you have changed the 60°. 
Hold a triangle up looking straight on the cutting point. Now tip it away from you. The 60 included angle becomes smaller.


Mike Warner


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