What is correct? 29.5 vs 30 vs 59.5 vs 60 vs 60.5 compound setting?

[12bolts]

...So now what is the magic number 60.5 or 59.5 and what is the difference in why you would use either/or?

Schemer, if you are going to use the compound for threading then you need 29.5 or 60.5. Using 59.5 would be the equivalent of 30.5
The reasoning is here...

The relationship of movement established by having the compound set at 29.5*. The tool form does establish the thread profile, but by having the tool cut progress at not quite 30* it takes the majority of the cut on the left (leading) edge, and the right, (trailing) edge takes very light cuts. In the pic the V cut by the tool form is 60* but as can be seen by the dashed line the movement of the tool when set at less than 30* causes it to move in a marginal amount as well as left a greater amount. Causing the tool to take a heavier cut on the leading edge and a very fine cut on the trailing edge.


As the sketch illustrates, having the tool move in at an angle less than 30* we do maintain the included angle of the form tool it just cuts less on the trailing edge. If the angle is set at or greater than 30* then the tool will only cut on the leading edge as it is sliding along or moving away from the trailing edge as it cuts progressively deeper, and this will result in a staircase effect on the trailing edge.

The point of it is to eliminate the staircase effect and reduce tool loading. The drastic and perceivable difference of leaving the staircase effect is that the right thread flank in most cases is the load bearing side of the thread and needs to be as smooth and accurate as possible.

Cheers Phil

 

[Don B]

Don,
I will try what you said and in the end I guess it will all make sense and one way will be either quicker or better than the other. More experience cutting threads is what I need. I usually use bolts and screws from the hardware store. :whistle:
Thanks,
schemer

I think you hit the nail on the head, I'm not sure there is a definitive better way to cut threads, it's just through experience and experimentation that each person develops there own method/procedure of thread cutting that is efficient for them.
If your more conformable always putting your cut on the compound and leaving the cross slide at 0 then thats the way you should do it, there's all kinds of ways you can get to the depth but what's most important is the tool shape being correct and the tool being set correctly to the workpiece.
As far as the 1/2 or 1 degree one way or the other set on the compound its just not that critical, in my opinion the angle will never be bang on anyways just reading off the compound scale, to shallow of an angle on the compound means a little will come off the back side of the thread each cut, to steep will create steps on the backside of the thread, in the latter case that's the importance of plunging at the end to ensure both sides have been cleaned up.
I alway buy hardware store bolts when possible, for me cutting threads is a last resort when there's no other option.)

 

[schemer]

If your dial reads 0 degrees when the compound is perpendicular to the long axis of the lathe, the angle is 29.5. If your dial reads 90 when perpendicular, the equivalent reading would be 60.5. My understanding is the reason for the 1/2 degree is to keep the back side of the cutting tool in contact with the thread, so it would be cutting a miniscule amount on the back side. Probably the biggest difference between feeding with the compound at 29.5 or feeding with the crossslide is that the latter takes more power, significantly so when you are deep into the thread.

Craig,
The dial reads "0" when when the compound is parallel so I guess that is the same as reading "90". ) Sometimes it takes a day for it all to sink in and make perfect sense. Parallel, perpendicular, to the face of the chuck or to the bed of the lathe etc...had to think extra hard a few times. And the hard part was and is why did they do it that way, and how is that going to affect any other angles I use with the compound??
Thanks,
schemer

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Schemer, if you are going to use the compound for threading then you need 29.5 or 60.5. Using 59.5 would be the equivalent of 30.5
The reasoning is here...


Cheers Phil

Thanks Phil for the added reassurance on the 60.5 setting and the reasons why.
schemer

- - - Updated - - -

I think you hit the nail on the head, I'm not sure there is a definitive better way to cut threads, it's just through experience and experimentation that each person develops there own method/procedure of thread cutting that is efficient for them.
If your more conformable always putting your cut on the compound and leaving the cross slide at 0 then thats the way you should do it, there's all kinds of ways you can get to the depth but what's most important is the tool shape being correct and the tool being set correctly to the workpiece.
As far as the 1/2 or 1 degree one way or the other set on the compound its just not that critical, in my opinion the angle will never be bang on anyways just reading off the compound scale, to shallow of an angle on the compound means a little will come off the back side of the thread each cut, to steep will create steps on the backside of the thread, in the latter case that's the importance of plunging at the end to ensure both sides have been cleaned up.
I alway buy hardware store bolts when possible, for me cutting threads is a last resort when there's no other option.)

Don,
I guess I missed that "plunge" part at the end of the operation in the video's I watched. But now I know why and when to use it. I sure have learned a lot about thread cutting in this thread.
Thanks,
schemer

 

[schemer]

Just a followup. I cut the same threads again on a new case 5 bolt that I am using for stock(turned down from 1/2 to 3/8)and they turned out excellent. I knew something didn't look/feel right about the 29.5 setting (on this particular brand-model lathe) when I made the first attempt. Although the threads accepted the nut ok, they were rough looking and sloppy. Now I am going back to try the same on a case 8 bolt like my original but I feel confident the results will be great. Thanks again to all that chimed in on my thread. It is much appreciated.
schemer

 

[machinist18]

I cut threads quite often and I always set the compound rest at 29 degrees. At this angle one side will always scrape a little bit as you advance, keeping that side a clean cut.

 

[chuckorlando]

I was taught that it just has to be close to yet under 30. Just to keep the back side from cutting much untill the last pass.

I set my compound at 60.5 or so. Then I zero the compound. I feed with the cross slide, retract with the compound, move the tool back to the start, re-zero the compound, feed with the cross slide. On the last few thou I feed the cross slide to clean the back side of the thread.

It just helps to keep your numbers straight if your not retracting the tool with the same dial your counting

 

[schemer]

I was taught that it just has to be close to yet under 30. Just to keep the back side from cutting much untill the last pass.

I set my compound at 60.5 or so. Then I zero the compound. I feed with the cross slide, retract with the compound, move the tool back to the start, re-zero the compound, feed with the cross slide. On the last few thou I feed the cross slide to clean the back side of the thread.

It just helps to keep your numbers straight if your not retracting the tool with the same dial your counting

Chuck,
Are you sure you said that the way you wanted to? From all the reading and video watching on the subject, and watching it be done and actually doing it many years ago, I thought zeroing the cross slide and feeding with the compound was the way to do it. Then clean up the last cut with the cross slide. Or there is yet another method to thread cutting I was unaware of. :thinking:
Thanks,
schemer

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I cut threads quite often and I always set the compound rest at 29 degrees. At this angle one side will always scrape a little bit as you advance, keeping that side a clean cut.

Well, I guess that answer is why I asked the question in the first place. It seems like others have pointed out, that the angle doesn't really matter, especially if you use just the cross slide to move the tool, or to clean up on the last cut to get both sides of the thread.
Thanks,
schemer

p.s. Reminds me of a "best oil" thread on a motorcycle forum. :roflmao:

 

[chuckorlando]

Yeap you are correct. I said them backwards. The last cut with the cross slide so it feeds straight in.