Compound Slide Angle

KevinMaiorka said:
Please explain further as my wife and I are going crazy now trying to understand this. She is a math teacher with many years under her belt.

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Kevin, the paths of the cross slide and compound describe a right triangle with the com[pound path being the hypotenuse. From trigonometry, the cosine of the include angle is the side adjacent/hypotenuse. For every .001" that the compound is advance, the tool is advanced .001 times the cosine of the included angle. Because congruent triangles maintain the same ratio between the lengths of the sides, the ratio holds regardless of distance traveled. Here is a drawing of some of the more popular compound angle settings.
Lathe Compound Angles.JPG
 
RJ,

That is a nice and simple illustration of the compound slide angle.
 
Okay so yesterday evening after work I went to the lathe to see for myself and put a dial indicator on the ways and turned the quick change tool post so that the dial was contacting a flat area on it. Left the compound at the 45° angle and sure enough it was off! Man y'all are smart! I believed y'all and my wife as well but I just couldn't get it through my thick skull that it was the way it was. Moved it to 30° on the compound and sure enough it is exactly half of the movement on the compound that I thought I was getting at the 45° before.

Still blows my mind that it works the way it does but I learned something and I will be forever grateful for yalls help with this. I had always thought my lathe was just off or something and that was all I could do about it. Thank you again and thanks for adding me to the group and I look forward to learning more.

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Okay so one more question. Why is it that we have to set the compound angle at 29.5° setting for cutting threads? On my smithy I have cut threads once and it was a pain but I did it. It does not have a thread dial so it was quite an adventure.

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It keeps the tool cutting on the leading edge as you advance the compound for each pass.
If you just feed in with the cross slide the point of the tool move forward and both side of the point cut. If you feed in with the compound at 29.5deg (just under half of the 60deg thread angle) the right hand side of the tool follows the thread side and all the cutting is done on the left hand side of the tool.
ThreadingCompoundAngles_zpsd1f35179.jpg
 
Thank you for explaining it to me. I always wondered why it was done that way and didn't quite understand it.

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