Easy Calculation ?

Is there such a thing for working out the depth of cut when threading on the lathe, if the thread depth is say .030thou and you are using the top slide set at 30 deg how far do you wind in the top slide to get the said 0.030thou I know you can do it on a calculator but don't know how math's is not my strong point as you may have already guessed.

Al.

I assume you really mean 30 thou not 0.03 thou. To move to a depth of 30 with the top slide set over to 30 Deg. it needs to move 30 thou divided by the cosine of 30 degrees. So 30/0.866 = 34.64 thou

Les.

Whats a cosine and how do you get it.

Al.

Sorry yes i ment 30thou

Try Wikipedia. It has a reasonably simple explanation. If you need a cosine then use a calculator. You probably only need 3 cosines, of 30, 45, and 60 degrees, which are .866, .7071, and .5 respectively. Use them as Les shows.

The easy way is to let the lathe do the maths for you!

1) Assuming the work is already turned to the thread outside diameter bring the tool up to touch the work and set both cross and top slide dials to zero.

2) Move the tool back a touch to clear the work then rack the saddle along until the point is past the end of the work.

3) Feed the cross slide forward past zero by the depth of thread to be cut.

4) Re-set the cross slide dial to zero. The tool point is now at the final depth of cut with both dials readin zero.

5) Pull the top-slide back until it clears the work then move forward to sufficiently to make the first scribe cut to verify that settings are correct.

6) After making scribe cut wind the cross slide back past zero and move saddle back past the end of the work.

7) Re-set cross slide to zero and apply first cut with the topslide.

8) Make first cut and repeat wind back cross slide, move saddle to end of work, set cross- slide back to zero and apply next cut process.

9) Repeat until both dials read zero at the end of the cut and all spring has been worked out. The thread depth will now be what you set in step 3.

If you set the book depth of cut the results will only be correct if the tool tip radius is correct or at least very close to book value. This is rarely the case in a model engineers workshop so it's usually necessary to cut a bit deeper before things screw together. I prefer to apply any extra cuts via the cross slide so when the correct fit has been achieved on either a test piece or the first of several parts I can simply re-set the cross slide zero to match the corrected final depth and all subsequent parts will be correct at zero-zero.

A major advantage of this process over mathermatical calculations it that it works fine for any reasonably sane top slide angle. If calculations are to be correct your set-over angle needs to be closer to correct than is easily achieved with the usual graduations, especially on small machines.

I was taught the method as "zero to zero" also seen it as zero-2-zero and similar variant names can be found by Google search. Geo. H Thomas has also described it both in Model Engineer and in his book "Model Engineers Workshop Manual". I imagine its described in other places. Its so easy and effective that I bemused by it not being the standard textbook way.

Clive

Edited By Clive Foster on 06/02/2014 23:00:19

Edited By Clive Foster on 06/02/2014 23:01:17

Hi Alan,
The cosine is the ratio of the length of the hypotenuse to the length of the side adjacent to the angle whose cosine we are referring to in a right angle triangle. The hypotenuse is the side opposite to the right angle. (It will be the longest side. When I went to school (About 55 years ago) you would have to use a book of log tables which also contained trig functions. (Sine, cosine and tangent.) Now any scientific calculator will have trig functions or you can use the calculator on your computer. Just open the calculator program, click on view and select scientific.

Les.

Do correct me if I am wrong, but if you set the topslide at 30deg, your depth of cut is half the movement of the topslide. Niko.

How about drawing it and using a simple scale. So draw a line at 30 degrees to a reference and measure the lines as appropriate using a simple scale like 30 mm for 30 thou.

You are either using the Sine of the angle or not setting the topslide to 30deg from the correct axis.

Edited By JasonB on 07/02/2014 07:44:20

You could clamp a dial gauge in the tool holder, put a chunk of round bar in the chuck and calibrate the the topslide by winding it in to the job.

I later realised what he is doing, but for me that is setting the topslide at an angle of 60deg from what is its normal position. Niko.

The cut to zero method is excellent and works for both people using an angled topslide and for the rectilinear disciples of Tubal Cain.

Neil

It's worth mentioning that it's by no means obligatory or necessary to set the compound slide over at all, particularly if there is no side rake on the tool as with most modern tips. True, you get better chip clearance with the set over method, but for common modelling pitches and depths of cut, it makes little difference.

The Hardinge HLV-H manual actually recommends cross slide feed even though the compound is set over for retracting purposes. (I know GHT said otherwise but in this he was mistaken.)

Ps compound set over is more universal in the US, but historically less second nature this side of the pond. 

Edited By jonathan heppel on 07/02/2014 20:33:10

Although it is neither obigatory or necessary to set the compound slide over for threading the practice does have two not inconsiderable advantages. Especially with smaller machines.

Firstly it avoids handwheel interference between cross and top slide. On smaller machines if both are set parallel for straight in feed things can get very crowded making operation difficult. Indeed some brands actually have physical interference between the handles if the right combination of positions is selected. On larger machines the top slide can interfere with easy reading of the cross slide dial. My Smart and Brown 1024 VSL suffers in this respect, mainly due to the heroic size of the top slide which, like the cross slide, is much larger than usual for a 10 inch swing machine.

Secondly it stiffens things up as the thrust perpendicular to the tool is now divided between the top slide feed screw and the dovetails. If set for straight in feed all this thrust must be taken by the two feed screws. On smaller machines said screws are frequently somewhat slender. Wear and the associated backlash on older machines doesn't help either.

My practice has, for many years, been to use a standard top slide setting of 25°. Works fine for threading both 55° and 60° threads. Probably gets shifted only a couple or three times a year for fast tapers and similar.

Clive

Edited By Clive Foster on 08/02/2014 00:10:55

Thanks chaps I will have to learn a bit more math's I think, the reason I asked is because the top slide is all ways in the way of the tail stock when the tail stock is wound right out so I moved the top slid round to around 30deg so when I cut a thread I'm not bothered what the angle of the top slide is as long as it's some where near, I go in straight the last 10-15 thou so it works out OK.

Al.

On the subject of thread cutting I was not taught the set-over method but the straight in method, adding a little - a fraction - of a feed along with the top slide with every feed in with the cross slide.

What I have never known though is how much top slide feed to add to every cross slide feed.

Anyone have a guide on this?

Sorry if this is deviating a bit from the OP.

Chris

Hi Chris,
I've never heard of that method but the way you describe it you will need the tangent of 30 deg which is 0.5774 so for every thou you feed in you would feed along 0.5774. so in reality you would feed about 0.6 thou. I must use the tangent of of 30 Deg. much less than the sine and cosine as I could not remember its value. I had to work it out from the sine and cosine. (And confirmed it on a calculator.)

Les.

Les, you are absolutely right of course, and confirm what I had deduced from drawing, that a 5 thou cut on the cross slide would require a 3 thou ( 5 x 0.6 ) feed in on the top slide - I just didn't believe my figures! This would produce a result not a million miles away from that achieved by the set-over method.

I would like to try the set-over method, but on my lathe I'm into guess work territory as the scale on the top slide doesn't go round far enough and would be masked at the zero point on the cross slide anyway, unless I can accurately set a new zero at 90 deg to the original on the cross slide.

Chris.

Alan

A knowledge of simple trigonometry would be very useful to you. No need to explore it in depth and you may well get the basics from an internet search for SINE, COSINE and TANGENT.

N

Chris, you can see the amount of feed from my sketch above, 17.3 / 30 = 0.576 x the depth. provided you keep it a bit less than that then the back of the tool will follow the 30deg angle. This is basically the same as setting the topslide to 29.5 deg which some people do so the back of the tool just takes a fraction off but the front of the tool does most of the cutting.

J

Edited By JasonB on 09/02/2014 08:00:09