Screw cutting

I apologise in advance as I'm guessing most of you are properly bored with this topic but it is a very long time since I have cut a screw thread on a lathe.

So had my first go at the weekend. I cut a M20 male thread. It was the biggest nut i had. I started by turning two 'blanks' onto the same bit of steel. So I had a small section of minor diameter between the major diameters. I started with the compound at 29.5 method but it was like I ended up cutting into the screw ahead before I got down to depth (not sure if that makes sense). I'm using a chipmaster so I cant disconnect the lead screw. After making the cut I would stop the machine take the cut off with the cross slide, run the machine back to the start go back to zero on the cross slide and put the next cut on with the compund.

Second attempt was back to zero degrees and just put the cut on with the cross slide and it worked fine. I was using the same dimensions on the bar so i am sure the issues was with my technique.

Any thoughts on what I was doing wrong?

There was a lengthy discussion recently on the two methods ( angled compound and zero degrees via cross slide ) and opinion was divided. Personally I find simply feeding via the cross slide perfectly OK and much less prone to ' operator miscalculation ' , its so very easy to forget to adjust both slides correctly every time or as you may have done not zeroed the cross slide correctly due to backlash . Could be any number of common causes.

if one method works and you're happy with the result - stick to it.

Edited By Mike Hurley on 20/09/2021 10:27:54

I find the 'straight in' feed the easiest to use and it always seems to work just adjust the amount of in-feed, less as the depth increases.

With a lathe such as yours where the lead-screw cannot be disengaged a flip-up tool holder can be useful:-

See *** Flip-Up Tool holder *** for more details.

John

Pick a pitch (and diameter) like M16 x 1.5mm such that you can disengage your halfnut.

You would do well to mention in this conversation that it is a native metric lathe rather than us finding this out by chance from reading your other recent contribution.

Give some numbers: what minor diameter are you using; what stock diameter are you using; what cutting tool are you using; what is your total infeed in both straight ahead and in angled mode?

Just like John, I usually cut straight in.

When cutting this M20 thread

I had to offset the top slide to avoid the tailstock centre(the 7/8" square bar didn't quite fit into the headstock). Doing it this way requires more concentration to turn each wheel the correct amount/direction. A flip-up holder would have made the 100mm long thread a much quicker job.

Appologies for the lack of info. I'm not sure what a native metric lathe is. I'm not sure if it really is metric. If it was I thought I would get a dial indicator? I was guessing it still had an imperial leadscrew but the gearbox was set up for metric ( a sort of cross over from imperial to metric) but I'm a proper novice hence asking the question on this particular forum. I didnt include some of the specifics since I was successfully on one of the methods but all the dimensions etc were the same for both. For both attempts my OD was 20mm my minor diameter was 16.93mm it was a M20 x 2.5mm external thread in mild steel. I used a home made 60 degree hss tool. I was cutting at 35rpm. Sorry I cant remember my cut sizes. 

Edited By colin hamilton on 20/09/2021 11:48:29

Thanks for the pointer

OK. Your Continental Chipmaster is a native metric lathe. Its leadscrew and all its feedscrews are metric, as are the dial graduations. A native imperial one would have the corresponding items in imperial units.

It is possible to cut metric threads on a (native) imperial lathe and imperial threads on a (native) metric lathe using a translation gear. When you do this, it is generally necessary to leave the half nuts engaged for every pitch you cut.

When you are cutting a thread in the same units/language as the leadscrew, there are instances where you can disengage the half nuts even without a leadscrew indicator and other instances where you need the indicator to tell you where to re-engage them.

It is worth looking at the ISO metric thread diagram here: https://en.wikipedia.org/wiki/ISO_metric_screw_thread

It does not have sharp crests or a sharp root. If your home-made tool has a sharp point, the infeed distance after touching off on the stock will be different to if it has a flat end. The total infeed distance for a metric thread is never 'H' in the diagram above as 12.5% of that H lies outside your stock. The minor diameter you have calculated is possibly for an internal thread and the diagram shows the relationship (or difference) between the minor diameters of external and internal threads.

For an imperial lathe, one, single thread dial indicator will work for all pitches you would normally cut. For a metric lathe, this is not true. At this stage, let us just say that metric thread dial indicators are more complicated than imperial ones and that is why many manufacturers did not chose to fit them.

It would be wise to read the Chipmaster manual very well and understand what bits are inside your Continental version. Martin Cleeve's book is a goldmine of information of screwcutting, including tool grinds.

Colin's actual question is nothing to do with 'which method is best/preferred' and all that baloney. He is puzzled as to why he didn't get to depth when using the topslide set over before the V of the thread was such that both flanks had met, no uncut max diameter bar left, yet he wasn't at full depth.

My only suggestions are 1) you got mixed up between set over at 60 and 30 degrees. 2) you forgot to realign teh tool so it wasn't at the right angle.
Maybe old fashioned drawing it on paper and moving a cardboard cut out tool into the 'cut' would suddenly show you what was happening wrong.

As Bazyle stated it's going to be 'operator error' some where along the line.

Tony

Tony and Bazyle - I think you have hit the nail on the head operator error. Not sure where but I'm going to buy a hss thread cutting tool and at least that will remove one (big) variable - my grinding!

You really need to know the pitch of the Leadscrew on your lathe. You may well need this information at some time in the future.

What follows may be teaching granyy to suck eggs, but is intended to be helpful, by explaining the basics..

In essence, to cut any thread, you need to set up a gear ratio which rotates the Leadescrew at such a rate, relative to the Chuck, so that the tool travels the correct distance for each revolution of the Chuck.

If you are trying to cut a Metric thread on a lathe with an Imperial Leadscrew, or an Imperial thread on a lathe with a Metric Leadscrew, , the gear ratio will need to allow for the fact that there are 25.4 mm to an inch. (Hence you will see references to 127T gears. 127 = 25.4 x 5 ).

Putting simply, if the lathe has a Leadscrew with a 3mm pitch thread, to cut a 3 mm pitch Right hand thread, the Leadscrew needs to rotate at the same speed as the Chuck.

If you wanted to cut a 1.5 mm pitch thread, the leadscrew would need to rotate at half the speed of the chuck, to move the tool towards the chuck..

Chuck makes one turn, Tool moves 1.5 mm. So using changewheels rather than a norton gearbox, the mandrel would carry a gear with, say, 30T, with a 60T gear on the Leadscrew, with an Idler gear to fill the gap, The Idler will not affect the ratio, so the tooth count will not matter, as long as the gear mesh is correct, so could be a 55T if it transfers the drive correctly..

If you wanted to cut a Left hand thread, the gear train would need an additional Idler, to reverse the direction of rotation of the Leadscrew, relative to the chuck, to move the tool AWAY from the chuck

The tool has to be ground to the correct angle for the thread being cut, 60 degrees for metric or Unified threads, 55 degrees for Whitworth or BSF threads, and to have the correct clearance angles ground into the flank,

If everything else is correct, are you trying to take too deep a cut? You will need to take several mpasses, with the depth of cut decreasing as you go in deeper. A M20 x 2.5 mm thread is 1.5336 mm deep,tol so may need a lot of shallow passes, and should have a 0.3125 mm flat on the crest of the thread.

As an example, the Cri Dan industrial screw cutting machines, took at least 20 dry passes to cut a 1 inch BSW thread. The thread depth is 0.080" (so a little over 2 mm ) with the swarf coming off blue at each pass..

You might start with a 0.1 mm DOC but as the width of the cut increases, the depth needs to decrease.

HTH

Howard.

I think from the description of the tool "cutting into the thread ahead" that Bazyle has the problem pinpointed. It is easy for a new operator to assume that the compound slide parallel to the spindle centre line is 0° when it is in fact 90°. You need to move 60.5° away from this parallel to spindle position to be at 29.5°. This means that the compound slide leadscrew is 29.5° away from the cross slide leadscrew. You are probably at 90-29.5 = 60.5° in your setup.

Martin C

The least ambiguous description of setting the top slide that I’ve seen is at 5:06 in this video:

(Yes, the video is a bit wobbly, and he talks funny, but he explains things very well - the whole series is worth a watch.)

(Hopefully link works now)

Edited By Andy_G on 21/09/2021 09:19:15

Duplicate post!

Edited By Andy_G on 21/09/2021 09:08:55

Thanks for the input and video. I didnt realise there could be an issue with setting 30 or 60 degrees. I'll be checkingvtgat next.

I had a quick flick through the responses and, although I may have missed it, I don't think anyone has mentioned that you don't need to calculate anything when you use the set-over top-slide method. Simply put the tool in contact with the work and then zero both the cross-slide and top-slide dials. Move the saddle to the right so that the tool point is just past the end of the work. Wind the cross-slide in, and hence the tool point, to the correct depth of thread to be cut and then zero the cross-slide dial again. Wind the tool out using the top-slide until the tool point is just clear of the work. Start the lathe and put the cuts on using the top-slide dial. Use the cross-slide to withdraw the tool at the end of the cut and return it to read zero before you put on the next cut with the top-slide. When the top-slide dial again reads zero the thread will be at the correct depth.

Edited By Chris Crew on 21/09/2021 22:41:20

By the time you have worked your way through Sparey,s chapter on screwcutting, I think you should have a fairly good idea of how to proceed.