# Single point cutting at 60 degree compound.



## Todd Adams (Jun 2, 2020)

I haven't received  my new lathe yet, but have been studying every nite in preperation.
If you set the compound at 60 degrees and advance the compound on every cut, wouldn't that position the cutting tool closer to the chuck creating a wider valley every time you go deeper? I'm missing something. I would think you would want to go parallel with the chuck, straight in.       Confused.


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## Ed ke6bnl (Jun 2, 2020)

Todd Adams said:


> I haven't received  my new lathe yet, but have been studying every nite in preperation.
> If you set the compound at 60 degrees and advance the compound on every cut, wouldn't that position the cutting tool closer to the chuck creating a wider valley every time you go deeper? I'm missing something. I would think you would want to go parallel with the chuck, straight in.       Confused.


I believe that it does cut to one side to help ease the cut of the thread forming tool. Some will just go straight into thread with the cross slide on smaller threads. Also for my lathe the compound is set for 29.5 degrees, yours maybe different. I am still a newbie to machining as well.


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## Todd Adams (Jun 2, 2020)

No you are correct  ,  1/2 of 60


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## Ed ke6bnl (Jun 2, 2020)

Todd Adams said:


> No you are correct  ,  1/2 of 60


BUT I believe some lathes will show 60 degree mark for cutting at an angle of 30 degrees just have to check that out.


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## Todd Adams (Jun 2, 2020)

Ok. If it ever gets here. Thanks


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## benmychree (Jun 2, 2020)

Some lathes mark the degrees for the compound from a different zero point, but the compound should be 30 degrees off of straight in, to the right for RH threads and to the Left for LH threads.  Some folks do the 29 1/2 deg. thing, that is not what I have done now since about 1962.


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## Todd Adams (Jun 2, 2020)

Thank you.


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## Todd Adams (Jun 2, 2020)

Todd Adams said:


> Thank you.


So if I advance 010 it will advance at  a 30 degree angle, using the compound. And if that's true how is using the cross slide , straight in ,not create a different thread?  Still confused.


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## BtoVin83 (Jun 2, 2020)

Since the tool follows the flank of the thread it cannot be any wider than the tool bit unless you keep missing the mark on pulling out of the cut.


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## Todd Adams (Jun 2, 2020)

What do you mean by missing when pulling out of the cut.


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## Tozguy (Jun 2, 2020)

This video might help explain things


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## Todd Adams (Jun 2, 2020)

Now I get it! Sorry that was so painful. I'll sleep better. This has been bugging me for a week.
Thankyou very much.  Peace out


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## RandyWilson (Jun 2, 2020)

The idea behind advancing at an angle is so that the brunt of the cutting is done by the leading edge of the tool, the part advancing into the work.Plunging can lead to more drag and chatter. I plunge fine threads, say 20 tpi and up. Angle cut the courser ones.

Say you have a 2 foot by 2 foot hole in the ground. Now you want to make it 4 foot by 4 foot. Does it matter whether you tale 1 foot off of each wall, or just gouge 2 feet of the North and east walls?  The hole will be the same in the end. Yes, it's position will move a bit, if it matters. In threading most times the position doesn't matter. You are cutting from the end to some set point. By coming in at an angle, you do advance the thread at that instant diameter down a bit, but all the really means is the thread moves around the circumference.


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## BtoVin83 (Jun 2, 2020)

Most hobbiests will either end in a groove  or let the tool cut a groove at the stopping point of the thread. If you stop early or late then the groove will get wider but not the thread because you are following the flank of the too bit as you go deeper. I do not use a groove to end the thread as it creates a stress riser and weakens the fastener.


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## mikey (Jun 2, 2020)

BtoVin83 said:


> Most hobbiests will either end in a groove  or let the tool cut a groove at the stopping point of the thread. If you stop early or late then the groove will get wider but not the thread because you are following the flank of the too bit as you go deeper. I do not use a groove to end the thread as *it creates a stress riser and weakens the fastener*.



My understanding has been that a thread relief (groove), being only 0.002-0.005" deeper than the Minor Diameter, does not appreciably weaken the part. A thread relief is a common feature in many turned threaded parts in the industry and not just hobby guy parts, although admittedly not common in mass produced fasteners that are not single-pointed. My question is how significant a stress riser and weakening feature is a thread relief? Is there a source I can look at to guide me in this? I've been threading parts for 30 years and have never had a part fail at the thread relief  but that doesn't mean it can't happen so I want to learn more about this.


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## BtoVin83 (Jun 3, 2020)

My experience comes from oilfield related machining and I  cannot recall any oilfield application where a relief groove was allowed. I have made draw works shafts  that could not have any sharp shoulders or keyways. All diameter transitions were large radius and gradual changes. Even the sprockets we shrunk on the shafts had special tapers so as to relieve the stress at the faces of the sprocket hubs. When you have hundreds of thousand pounds suspended above a hole in the earth you had better have it right.


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## mikey (Jun 3, 2020)

I can certainly see how drills that go thousands of feet into the earth have to sustain very high loads so it makes sense to be very strict re stress risers in that situation. Thanks for clarifying.


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## Tozguy (Jun 3, 2020)

My experience has been similar to mikeys. None of the times a relief groove was used for cutting to a shoulder did it result in a failure. Not to say that relief grooves are always OK. It depends on the application. 
As a hobbyist, we generally know very little about the stresses in the parts we make and what our safety margins are. So it becomes difficult to choose which of the specialized applications in industry we should blindly follow, if any.


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## vtcnc (Jun 3, 2020)

Tozguy said:


> My experience has been similar to mikeys. None of the times a relief groove was used for cutting to a shoulder did it result in a failure. Not to say that relief grooves are always OK. It depends on the application.
> As a hobbyist, we generally know very little about the stresses in the parts we make and what our safety margins are. So it becomes difficult to choose which of the specialized applications in industry we should blindly follow, if any.



Yeah although we are comparing oil rigging and drilling to steam engine and hobby shop fasteners the same principles apply when it comes to stress and strain. The difference being that stress risers really, really matter in the field and are really, really insignificant for the hobbyist. Until it isn’t - but that is a matter of knowing how much tension your fastener will be subjected to and then sizing the diameter and creating stress risers to compensate.

Hobbyists are primarily going to use relief grooves for clearance and ease of machining, not for stress reduction in the part.


Sent from my iPhone using Tapatalk


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## Bob Korves (Jun 3, 2020)

Todd Adams said:


> I haven't received  my new lathe yet, but have been studying every nite in preperation.
> If you set the compound at 60 degrees and advance the compound on every cut, wouldn't that position the cutting tool closer to the chuck creating a wider valley every time you go deeper? I'm missing something. I would think you would want to go parallel with the chuck, straight in.       Confused.


Believing the compound rest to cross slide numbers cut into the lathe does not necessarily give you that number in the completed part, sometimes by much more than you might guess.  Be careful about the "measure it with a micrometer, mark it with a crayon, and cut it off with an axe" approach that we often use unintentionally, because we believe too many scales at face value.  Trust, but first verify...


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## RandyWilson (Jun 3, 2020)

Any sudden section change is a stress riser. The thread root itself is a prime offender. The key is gradual section transitions. Surely a threading gully that is a sharp 90 degree trench is bad. But a gully with a gradual change can actually be a stress reliever. Common practice with highly stressed engine bolts is to have the shank at the threaded end be smaller than the thread minimum diameter, then gradually taper back up to full diameter.

 I dunno. Maybe I've read the Caroll Smith books too many times. I'm always watching stress patterns. If I decide I want a gully in  a place that matters, I use a rounded form tool to cut it.


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## Flyinfool (Jun 3, 2020)

The difference between a relief groove and a thread run out is very small. Yes no groove is stronger. The only time it comes into play is when the threaded joint will be used right at the failure points. In many if not most cases for hobby use a thread is used because of its dimensions and not its ultimate strength. We will often use a 1/4 inch fastener because it is a convenient size when a much smaller fastener would still have plenty of strength. Hobbyists love overkill


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## RandyWilson (Jun 3, 2020)

Sometimes hobbyists are making plant stands. Sometimes they are making race car parts.  Stress concentrations are of most concern with cyclic loads.


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## Tozguy (Jun 3, 2020)

I have nothing against stress analysis in parts, my son earns a good living as a stress expert in the aviation industry.
But when I am at my lathe making things for my own pleasure, stress reduction in the operator comes first.


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## mikey (Jun 3, 2020)

I agree that for highly stressed components, a rounded or tapered relief profile is superior to a simple squared thread relief. This is standard practice for power transmission components and high tension fasteners. Such relief profiles require a deeper cut at the bottom of the radius than a typical thread relief so you have to consider that reduction in cross section in the design.

I suppose the lesson in this digression is to be aware of stress risers in stressed components as BtoVin83 pointed out and profile it properly if the situation calls for it or not use it altogether. With that said, a rounded thread relief will be wider and deeper than a typical squared edge thread relief and, as hobby guys in a non-critical situation in most cases, we just may not have room for that width and depth. I've decided that I won't worry about it and continue to cut thread relief grooves with a parting tool as I've done in the past unless I have a situation in which the part is highly stressed - then I'll worry about it. 

Good discussion.


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## aliva (Jun 3, 2020)

I remember watching Mr. Pete explain thread cutting. If my memory serves me correctly when cutting threads with the compound at 30 or 29.5 degrees. When you advance the compound by say .010 on the dial the actual depth is increased by only .05. This on direct read dials.
Could someone confirm this. I can't find the particular video.


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## RandyWilson (Jun 3, 2020)

With the compound at 30, to keep it simple,  advancing the compound by X will move the leading edge X into the work. Your DoC will be X. Transferring this movement to relative to the work, the tool will advance cos(30) into the work and move sin(30) towards the chuck. Sin(30) is 0.500. Cos(30) is 0.866-something, if I remember.


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## Janderso (Jun 3, 2020)

Watch the series of 4.
As stated above there are some very good instructional videos on Youtube.
You are on your way my friend.
 Have fun and be careful. A lathe can be deadly.


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## BtoVin83 (Jun 3, 2020)

I didn't mean to stress everyone out just pointing out how I was taught. Old habits die hard and as pointed out the best was to reduce stress is go out and take something big and make it smaller


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## Todd Adams (Jun 4, 2020)

I understand the how's , I just didn't understand the compound vs cross feed. Now I do . Thanks every one. Lathe got shipped today, finally!


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## Mitch Alsup (Jun 6, 2020)

Ed ke6bnl said:


> I believe that it does cut to one side to help ease the cut of the thread forming tool. Some will just go straight into thread with the cross slide on smaller threads. Also for my lathe the compound is set for 29.5 degrees, yours maybe different. I am still a newbie to machining as well.



Using one side to perform the cut is stiffer than using both sides, and thereby creates less chatter.

However, going in at a 30º (or 29.5º) cause the trailing edge of the tool to burnish that edge which will be "taking" the stress of tightening a nut. This leads to more reliable torque readings on the TW of the thread after being cut.


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