Threading Bits

[Stan]

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?

 

[Noitoen]

Found this:

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

Helder

 

[kvom]

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.

 

[dan s]

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.

 

[itowbig]

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

 

[Stan]

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?

 

[Holescreek]

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

 

[dan s]

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.

 

[Paolo]

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

 

[Stan]

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.

 

[Holescreek]

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

 

[dan s]

Stan,

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

 

[Stan]

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?

 

[dan s]

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.

 

[Holescreek]

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

 

[dan s]

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:

 

[Stan]

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?

 

[dan s]

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

 

[Stan]

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?

 

[dan s]

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.

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.