Thread dial indicator for a Lathe

[Johan CrousParticipant @johancrous15881]

I am the owner of a Quantum 210 X 400 mm Metric lathe.

The Leadscrew is 2.0 mm metric pith and I can cut a wide range of both metric and imperial threads.

The lathe is designed to cut an reverse, forward (cut) and reverse. There is no Thread dial.

I would like to make a thread dial, but I have now idea how it works. I am referring to the marks on the top of the dial and how many teeth the gear must have that runs on the lead screw.

I have seen some thread dials with 4 marks and even one with 12 marks (numbered 1 – 12).

Where can I get the info how to make one or the theory behind it for me to work from.

I also see that the thread dial is normally situated on the right side of the saddle. My one has got 3 screws standing out on this place, but they are meant for the gib of the half nut. However I am sure I will be able to make use of them without having to influence the grip on the gib.

I will really appreciate any help.

Edited By Johan Crous on 23/12/2012 16:50:31

[Johan CrousParticipant @johancrous15881]

[Sub MandrelParticipant @submandrel]

Hi Johan,

You will need to have a gear of the same circular pitch as the leadscrew – it need not be spiral cut if you are happy to angle it to match the helix of the leadscrew.

If the gear has 12 teeth and twelve marks, then one rotation will be 24mm. Any pitch that divides exactly into 24mm will be OK – 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2, 1.5, 2.0 but not 0.7 or 1.8 for example.

Imperial lathes with 8 or 16 tpi leadscrews use 4 or 8 marks which allow any even numver of tpi to be cut.

Neil

[AndyfParticipant @andyf]

Though your lathe will cut both metric and imperial threads, it has a metric leadscrew, so a threading dial will only work for metric threads.

 

I made a crude threading dial for an imperial lathe, shown here:

As you will see, the leadscrew is used as a worm to drive a gear, which turns the dial. For imperial leadscrews, a single gear is all that is needed , with the same number of teeth as the threads per inch of the leadscrew, or a multiple of that TPI. Thus, for my 12 TPI leadscrew, I made a gear with 24 teeth. If that dial only had one mark, I could engage the half nuts each time that mark came round, because the leadscrew would have turned 24 times, moving my carriage 2” along. If it had two marks, I could make quicker progress by engaging each time either of those marks came round and the carriage had moved 1”, because imperial threads are all expressed as the number of threads in one inch.

Life is more difficult with metric threads. Unlike imperial ones, there is no conveniently small distance into which all metric pitches will fit. If you have a gear with 15 teeth and a single mark on it, the carriage will move 30mm for one full turn of the leadscrew. The 11 standard metric pitches you might want to cut are 0.5, 0.6, 0.7, 0.8, 1.0, 1.25, 1.5, 1.75, 2.0, 2.5 and 3mm. Only 0.5, 0.6, 1.0, 1.25, 1.5, 2.0, 2.5 and 3mm pitches will divide exactly into 30mm. That leaves 0.7, 0.8 and 1.75 mm pitches; those will divide into 28mm, so a gear with 14 teeth will cover them (and would also cover 0.5, 0.8, 1.0 and 2mm pitches).

So, you will need both a 15T and a 14T gear if you want to cover all the 11 metric pitches mentioned earlier. The thread dial indicator could have interchangeable gears, or both gears could be permanently mounted on a single shaft which could be moved along to bring either of them into mesh with the leadscrew. Indeed, as the entire indicator will can be made so it can easily be attached to and removed from the apron (see my web page), it might be easiest to make two separate indicators, one with a 15T gear and the other with a 14T gear. Of course, you might decide that you could easily use dies to cut 0.7 and 0.8mm pitches, and that you wouldn't want to cut 1.75mm (as used on M12 coarse) very often, so your needs could be met with a single 15T gear.

As to the number of marks on the dial, two examples might help:

1. Imagine using a dial with a 15T gear to cut a 3.0mm pitch. The dial goes round once as the carriage moves 30mm, and 3.0 divides into 30. But it also divides into 15, so you could have two marks on the dial, and engage the half-nuts when either came round.

2. If you were cutting a 0.5mm pitch with your 15T dial, 30mm/0.5mm = 60, so you could put 60 marks round the dial and engage the half nuts on any of them.

But too many marks can be very confusing, and I have four around my dial. That means that for the finer threads, I may have to wait longer than necessasy for a mark to come round, but in reality little time is wasted.

Going back to my first remark, the reason you will not be able to use a dial to cut imperial threads is that none of their pitches will divide exactly into a conveniently small number of millimetres.

I hope this helps!

Andy.

Edited By Andyf on 23/12/2012 18:18:51

[Johan CrousParticipant @johancrous15881]

Andy,

Thanks, now everything makes sense.

I will think about this before I make a decision.

I must say that I have almost never encountered imperial threads before in about 20 years of model engineering. My first lathe had a 1/2 inch 20 UNF thread on the leadscrew and then I have encountered BSP thread in hydraulic, compressed air and water fittings.

Apart from this I only use metric thread. Life is so much easier. You only have to have a few drills and taps and dies. Not even half as much as you imperial guys need. I only have to convert if I eter an imperial plan, unless I want to duplicate precisely. Here in South Africa we grew up with metric systems and I only later in life started to encouter imperial dimensions.

You do get Milling cutters, twist drills, taps and dies, screws, nuts, etc. in imperial sizes, but it will be more expensive than metric. Metric is the norm here and therefore so much cheaper. I guess over in your country it is the opposite. We mosty use metric coarse and metric fine is more scarse and expensive (not sold in average hardware shops).

I see my lathe appears basicly the same than your one in the link that you have shown. My one have three screws for the halfnut gibs where yours have four. You gave me quite a good idea how to mount the indicater, should I decide to make it.

My lathe have a set of gears to make up the gear train for screw cutting but I don't have such a big gear as a 127. The gear set is made up of odd and even numbered toothed gears with 90T the biggest. The imperial threads range in 12 from 8 – 44 TPI and the metric in 14 from 0.4 – 3.5 mm pith. I gues you can do more imperial ones by playing around with the gear set-up.

One thing I do not know: With a metric leadscrew, does my lathe simulates imperial thread like the imperial lathes simulates metric threads. I know it is so close that the human eye will not know the differsence. Yoy may pick it up if you count the pitch on a 100 yard threaded rod.

Anyway, thanks again for your information.

[AndyfParticipant @andyf]

Hi Johan,

Yes, the imperial threads you cut with your metric leadscrew will be almost, but not quite, correct. But using the chart supplied with my imperial lathe for (almost) metric threads, the error was often around 0.5%. Another page on ny website shows how I used the gears supplied with my lathe to get much closer metric threads, when I need more accuracy. Of course, the error figures rely on my Chinese leadscrew being accurate!.

The only way you can get them exactly right is to use a 127T gear (but a threading dial still won't work).

Have a nice warm Christmas in South Africa, and spare a thought for us in the UK, where the rain is so heavy that serious flood warnings have been issued!

Andy.

[Johan CrousParticipant @johancrous15881]

Thanks Andy and a blessed season for you too.

Today is cool but the past two weeks, temperatures approached and reached 40 degree C. Hot season will continue at least until May next year.

[Another JohnSParticipant @anotherjohns]

Johan;

This is an interesting thread, thank you for starting it.

FYI, I do everything in metric, but my machine tools are mainly inch-based but with DROs or dial indicators. Not usually a big issue, but did notice cutting a M9x1mm pitch thread about 30mm long, that my tap-cut female thread and my "use the supplied change wheels for metric" on my british Kerry 1124 lathe with quick change gearbox did not mate.

Drilling the threads out of my handle, leaving about 10 mm or so was fine – it is a knurled handle for hand-tightening, so 10mm of threads is still about 10x the amount needed for hand-tightening.

I mention this, as we all know that metric threads on an imperial lathe without a 127 tooth gear has to be a bit of a bodge, but usually not an issue.

Another JohnS

[AndyfParticipant @andyf]

John, I needed to make an pretty accurate metric LH tap for a one-off purpose, and that started me off on some pretty boring and repetitive maths to see if I could do better metric threads than those on the chart supplied with the lathe, using two sets of compounded gears between the spindle and leadscrew gears, rather than one set plus an idler. There are programs around to help with the calculations, but I couldn't find any which dealt with two compounded pairs rather than one. Then I found that many theoretically good combinations wouldn't work because gears would foul one another, or the overall length of the gear set was too long to fit on the banjo. It was irksome trying each combination to see if it would work, and (if not) whether rearranging the order of the drivers and drivens would help. In the end, I devised an Excel spreadsheet containing the relevant dimensions which, when I entered each theoretical gear set, would test the six possible arrangements of it and tell me if any of them would work in practice.

Anyway, my 1mm pitch threads now have an error of about 1mm in 100 metres.

Maybe I should get a life…

Andy

[BazyleParticipant @bazyle]

Nice explanation and website Andy.

As no power is transmitted the gear can be a thin brass sheet and the teeth cut with a file if necessary as all they have to do is keep in step. Then no axle and dial is needed, just a pin through the disc and the indications a dob of paint on it.

[AndyfParticipant @andyf]

Thank you, kind sir!

Nothing at all wrong with your idea, but a combined gear/dial down at leadscrew level might strain my poor old eyes. Lifting the dial up an inch helps them a bit.

Andy

[Les Jones 1Participant @lesjones1]

Hi Johan,
On 19/09/2011 at 11:47 Ady posted an idea for an electronic alternative to a thread dial indicator. It was in a thread called "modern Digital aids" I think it is a good idea worth considering.

Les

[Another JohnSParticipant @anotherjohns]

Posted by Andyf on 24/12/2012 17:19:54:

…Anyway, my 1mm pitch threads now have an error of about 1mm in 100 metres.

Maybe I should get a life…

Andy;

I'm hoping to CNC-ize one of my 2 Emco Compact-8 lathes in the new year, so that should possibly help. (purchased the 2nd expressly to CNC, and use same accessories as my manual one).

So, I'll trade simple, easy to figure out change gears for a computer and stepper motors, and something that looks like something out of a sci-fi movie.

I don't know who's smarter here! (FYI, North American basement has room for my workshop, but it is getting full. Have a smallish CNC mill, now looking at the lathe)

Here's to a good holiday season to all!

Another JohnS.

[AndyfParticipant @andyf]

Oo-er, John. I hope it works. As a (long non-practising) radio ham, digital electrickery scares me to death! I'm better with cogwheels.

All the festive best to you and everyone else on the forum.

Andy

[Sub MandrelParticipant @submandrel]

A 63 tooth gear in the gear train will allow metric to be cut on impetrial leadscrew or vice versa with errors smaller than industrial tolerances.

Neil

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