Single point thread cutting

I'm trying to cut an M16 x 1.5 thread on my Warco 180. The leadscrew is 2mm and the gear train set up as per the plate on the cover. The problem I am having is that I cannot pick up the cut on successive passes even though I'm using the same point on the thread dial indicator. I am ending up with a cut every .5mm or so instead of the single thread at 1.5mm. Am I doing something wrong or is it likely to be a problem with the lathe? Any thoughts would be welcome before I tear my hair out in frustration.

For certain pitches you can only use particular numbers on the dial, should be in the manual but if not have a practice using a different number. A sharpie in the toolpost will save wasting metal.

Keep the half nuts engaged and reverse the motor to traverse back to the start. Then it doesn't matter what number you engage on.

Phil. Keep it simple dept.

I'd take Phil's advice. I need to cut the exact same thread you want an M16 x 1.5 on a WM280, the issue with it is my lathe is imperial, the thread dial will not match, so it's half nuts on and reverse to play it safe.

Certain metric threads will not work with the indicator dial unless you change the gear that meshes with the leadscrew. Some lathes offer a set of three gears but not the Warco It is possible to make an indicator dial that has all three gears fitted and you select the one needed.

Easier is to keep the half-nuts engaged all the time as Philip suggest above. If you make a flip-up tool holder the process is much easier. Notes on how I made mine here *** Flip-Up-Toolholder *** it is quite a simple and quick project.

John

That's what I do, too. It's simple, as sure as can be, and wastes little time unless the thread's unusually long.

Well that answers one question. On the metric version of my industrial lathe (Harrison M300) the thread dial indicator has a choice of 5 gears, and even then there are some finer metric pitches it can't cut. However, it seems the Warco doesn't offer that facility. So, as said, you'll need to keep the half nuts engaged.

The need for multiple gears on metric lathes is to do with the fact that metric pitches are not always nice multiples of each other or of the leadscrew. The imperial measure of tpi is much easier to deal with in terms of creating gear trains.

Andrew

Normally I delight in pointing out the many ways in which the metric system is effortlessly superior to the inconsistent alternative. This isn't one of them!

For reasons I don't understand metric threads are defined in terms of pitch rather than turns per unit of measure. Although the two methods of defining threads are mathematically equivalent - pitch is the reciprocal of turns per unit measure - they are not mechanically equivalent.

In the imperial system (turns per inch) it's easy to arrange a gear against the leadscrew to drive a dial showing when the cutter is correctly placed to cut a thread. Not so on a metric lathe. Worse, the reason it doesn't always work isn't obvious, at which point hair tearing starts! On a metric lathe a single gear cannot represent all the required pitches.

Some metric lathes, like my WM280, come with a single gear on the thread dial that doesn't cover all the possibilities.

Note the common pitches 0.7 and 1.5 aren't in the table. The lathe is perfectly capable of cutting them, what the table means is that the thread dial can't be used to reset the cutter, b*gg*r. A different technique has to be used. The easiest is to leave the half-nuts permanently engaged and drive the lathe in both forward and reverse as per Phil's post. Following advice received from Jason, I always cut threads in reverse because it avoids smacking the tool into the chuck at end-of-cut. The method works well when rewinding with closed half-nuts.

Some metric lathes don't bother fitting a thread dial at all, others come with a set of gears to cover all the common pitches (don't forget to fit the right one), some come with just one gear to cover common examples. Chinese lathes of which the WM family is typical might be a third case. The metric versions can cut a range of imperial threads and the imperial lathes can cut a range of metric threads. Such flexibility is very handy but a side effect is that application of the thread dial is less straightforward. As soon as you go metric watch out special cases and exceptions that the thread dial can't cope with.

Not your fault, or the lathe, but whoever decided to specify metric threads in terms of pitch. Does anyone know of a practical advantage gained by defining threads by their pitch? In comparison cutting TPI is a doddle.

Dave

You should have the "indicator chart" on the front of the machine though not all have it.

This shows what numbers you can use for each pitch and I also think there are two options using either a 40T or 42T in the drive train. Can't find an image on line big enough to enlarge.

This is an explanation I found that applies to my WM250 with a 3mm pitch leadscrew. It may may (or may not) make things a little clearer, the maths will need re-doing for the WM180 2mm pitch leadscrew:-

The screw-cutting thread indicator dial meshes with the leadscrew via a 30 tooth gear. To get the indicator dial to rotate once the carriage has to move 90mm (number of teeth x leadscrew pitch). Only those metric pitches that divide exactly into 90 will be able to use the dial when screw-cutting. Thus pitches of:-
0.5, 0.6, 1.0, 1.25, 1.5, 2.0, 2.5 and 3 will work, pitches of 0.7, 0.8 and 1.75 won′t work.
Replacing the 30 tooth gear with a 28 tooth gear would work for the missing pitches but not for all. With a 28 tooth gear the carriage has to move 84mm for a full turn of the indicator and 84 is divisible by 0.7, 0.8, and 1.75. So either change the gear, make an indicator that has both gears or just leave the leadscrew engaged and reverse the lathe.

Hope that helps

John

You can disengage the half nuts, Stop the spindle quickly, reverse the direction watching the TDI and when the original mark comes back, re engage the half nuts again. Put a red sharpie mark on the TDI line or number, lot easier to see.

The metric system was designed by scientists, not engineers. So there's no convenient unit of length that allows a useful count of threads. The metre is too large, and the millimetre is too small. In a similar way I find DP is a more intuitive way of defining gears than module.

Andrew

There is a sequence you can use to disengage the half nuts when cutting metric threads on an imperial lathe, I have posted this before. I am not sure it will work in this instance but you could try the method "dry" i.e. after the initial cut with the tool close to the work to observe whether its path matches the thread : -

Screwcutting Metric / Imperial

When screw cutting a metic thread on an imperial lathe you can use the chaser dial and disconnect the half nuts when cutting up to a shoulder by doing the following : -

First make sure you engage the half nuts with the chasing dial on a particular number — number 1 is best

Then make your first cut and at the end of the cut disengage the half nuts and stop the lathe and retract your tool exactly as you would if it was an imperial thread — DO NOT move the carriage — this is very important.

Now the thread dial will have passed number 1 so start the lathe in reverse and re-engage the half nuts on number 1 and run it back to the start point of the thread. No need to disengage here just stop the lathe apply the second cut and start the lathe forward then repeat the above at the end of the cut. This allows you to finish at a shoulder or undercut without fear of a disaster !

Above is a copy of instructions I sent to another person on the forum, Logic says to me it will work but the proof of the pudding !!!

I do have a metric lathe, Boxford, and cut metric threads using the dial but always use the numbers indicated on the chart and it works fine up to now.

Hope this helps regards John

Thanks to all who responded to my plea. My lathe has the plate fitted but I didn't have a clue how to read it - I do now! As the 42 tooth gear wheel is about as rare as rocking horse poo I'll just leave the half nuts engaged and reverse the lathe. It's only a short length of thread to be cut and there's no panic to get it done so I can use the rest of the piece to try it and get some experience before doing it in anger.

I would think that if the gear on the thread indicator had a multiple of 3 teeth AND the markings on the indicator corresponded to 3 tooth spacing the thread indicator should work. The reasoning is that 4 x 1.5mm = 6mm so that any multiple of 6mm should line up as three teeth move along a 2mm pitch leadscrew is also 6mm.

Les.

Is the 42T not supplied as part of the gear set or is it already on the indicator? Sometimes they bot fit on the end and you just swap them round or move the indicator so the correct one is engaged.

Would make sense that 42 is needed going by what has been posted above: 42 x 2mm pitch = 84 which can be divided by 1.5.

Watch this video, about 2mins in he shows the 42T gear which is included when you buy the machine, finer pits and slightly angled.

26mins in shows the chart with 1.5mm pitch needing the 42T gear and then drop in on No 4 or 8

Edited By JasonB on 08/12/2018 16:24:15

Sparey mentioned in his book,many moons ago,that it was becoming standard practice,even then,to leave the half nuts engaged,even where there was an indicator. That way there was no way to go wrong.

I removed the indicator fro my lathe,since I never used it,I just cut the thread,move out,reverse,and move in again for another cut,no problem.

Some threads you will need the indicator.

Whether imperial cutting imperial or metric and vice versa.
Have imperial lead screw cutting 19tpi all dialled in but need indicator on 1,3,5 or 7 says so on indicator. Dare say if disengaged and wound out would have to wait an eternity for the same number to come round before re engaging each time.
Metric can be anything 1-8 or in between left engaged.

Previous metric could wind out and re engage any where on 'some' metric threads but not all think 1.75 was one whereas 1 and 1.25 ok.