If this is your first visit, be sure to
check out the FAQ by clicking the
link above. You may have to register
before you can post: click the register link above to proceed. To start viewing messages,
select the forum that you want to visit from the selection below.
Cribbing from "the Starrett book for student machinists" 3/4" per foot is listed as 3 deg 34 min 44 sec. If you are turning this taper by off-setting the tail stock, your offset is equal to the taper per inch times the total length of your workpiece divided by two. Hope this helps. Jim
As you are setting it on your lathe, I'd just set the compound to 3.6 degrees and "fine-tune" it in from there.
The "half angle" as advised by arcs_n_sparks is also correct at 1.79 degrees - buy just set the compound to 1.8 degrees and fine tune from there as well.
For "creeping up to it " angular adjustment of your top/compound slide, I'd suggest putting a good (say 0.001") dial indicator on the handle end of the slide at right-angles to it. You will see instantly what has been applied. You can get very fine adjustments that way - as well as seing if you have "over-shot the mark".
For "creeping up to it " angular adjustment of your top/compound slide, I'd suggest putting a good (say 0.001") dial indicator on the handle end of the slide at right-angles to it. You will see instantly what has been applied. You can get very fine adjustments that way - as well as seing if you have "over-shot the mark".
All that math is easier than it looks when you are dealing with trig. You reduce the problem to fractions first.
The long dimension becomes 1 unit. The short one becomes a fraction of that unit.
So, 12/.75 = 16
The 12 becomes the unit we are dealing with so the fraction is 1/16 = .0625
That fraction is the sine of the angle so sine =.0625. If the taper includes both sides then divide that by 2 to get the actual sine of one side. That equals 0.03125 which is the sine of the angle of one side.
The hard part is converting that number to degrees which is why they used to publish trig tables for that purpose. These days you can just ask Google's computer.
So we just put into Google this question:
How many degrees in sine .03125
Now you know how to use a sine bar too.
Free software for calculating bolt circles and similar: Click Here
The zen answer is 3/4" per foot. Or 1/4" per 4", or 1/8" per 2", or 62.5 mils per inch. This is actually probably more useful than an answer in degrees. Why convert to a unit of measure that you probably can't measure very accurately when you can accurately measure in the original units?
Mount a dial indicator in a tool holder on the quick change post, or equivalent, and make it perpendicular to the axis of spindle rotation. With a round, non-tapered rod installed on axis (i.e. dial it in at two positions along its length), measure the deflection of the indicator for a known travel on the compound.
The catch is that the compound is the hypotenuse of the rise/run triangle. Using the Pythagorean theorem:
hypotenuse = sqrt(rise^2+run^2)
= sqrt(0.0625^2+1^2)
= 1.0019512
Thus you want to move the compound 1.0019512" inches (1.0020") and see an indicator reading of 62.5 mils.
Or you can move the compound 1.000" and, using pythagorean theorem and similar triangles, sqrt(1.0019512^2-1^2)=0.062499658", or basically the same as the original measurement in this case because the taper is so shallow.
I have used 1" because some compounds on small machines have very limited travel. Use a longer distance if you can.
Comment