Hi Martyn,
The direction of travel of the top slide is the hypotenuse of a triangle. If we measure the angle between this and the axis of the lathe (0 Deg when it is parallel to the axis.) and multiply the amount the top slide moves by the sine of the angle it gives the effective distance moved at right angles to the axis. As the sine of 30 Deg. is 0.5 then if the top slide moves 10 thou then the component of this at right angles to the axis is is 5 thou. (So you would reduce the diameter by 10 thou.) If you only want the tool to move in by 1/4 of the amount the top slide moves the look up the angle who's sign is 0.25 this turns out to be 14.477 Deg. They do teach useful stuff a school even if you don't think so at the time.

Les.

If you consider a right angle triangle where one side is parallel to the centreline of the lathe, the base side is the direction of the cross-slide and the hypotenuse is the angle of the compound then extending that hypotenuse at the apex and dropping a new perpendicular to the base has moved the base towards the lathe centreline… but by less than the distance the hypotenuse was extended.

Play with simple trig and you will see how that changes for a change of angle between base and hypotenuse.

If we play with very simple 3,4,5 triangles and change that hypotenuse from 5 to 10 then it has increased 5 units for a 3 unit change on base distance (from 3 to 6). in the case of a 3,4,5 triangle the angle between base and hypotenuse is just over 53deg.

In the case of a 60deg triangle all sides are equilateral of course.. say 10 units. To turn that into a rt triangle we cut it in half. Hypotenuse remains 10, base becomes 5

Now use an equilateral triangle of say 12 units and do the same…hypotenuse remains 12 and base becomes 6

So for 2 units increase in cross slide you have 1 unit increase towards lathe centreline

.

Martyn,

That has to be my "Quote of the Day"

… You are absolutely right.

MichaelG.

For small angles the set over angle you want is 1rad x dividing factor. 2Pi rads in 360deg so a radian is 57.29deg.

If you want 10 thou travel on the topslide to 1 thou on the radius you need 57.29/10 which is 5.729 degrees. (somewhere around 6 will do)

1:20 is 2.8 degrees

To simplify and so you can remember it's Heinz 57 x 10 for tenths ( one thou on the cross slide dial is a tenth on the radius)

regards Martin

I have found that the most inspirational teachers I have had are ones who have worked in industry, I went to a grammer school where all the teachers had degrees from top 10 universities except for the metalwork teacher. He was a great guy who had started work aged 14 hand sanding buses in a factory, I still have the passion from those lessons 47 years ago. At tech college I had a lecturer for electronics who had worked in industry, his anecdotes about the practical applications brought the subject alive and made his lectures ones not to miss. Trigonometry has been the most useful thing I ever learned in maths, finding almost daily tasks for it in the workshop. I think it would be a very good thing if the teaching profession were able to attract more people from industry or business to inspire the young with their experience.

Mike

If you want best control over tight clearances, set the topslide at 6 degrees (or just under).

That way, the geometry works out that every .001" graduation on the topslide collar moves the tool inwards by one tenth, or .0001".

Depends on what lathe you are using and its condition, though. Generally on a good lathe, you should be able to machine a spigot to within half a thou or less using the conventional cross slide in the normal manner. Trick with spigots is to machine the hole first, then turn the OD of the male spigot to fit the hole. Machining the OD is easier to do accurately than machining the ID.