Thread Dial Indicator Chart?

Good morning,

My Warco WM250 has a chart attached to the chip guard, indicating which thread dial numbers can be used with various metric thread pitches. Also, there are three boxes along the top, one of which says Mn=0.95. Anyone know what it means, or if it's important? Nothing in the handbook (no surprise there), and Warco don't seem to know.

On industrial metric lathes the TDI has a choice of gears that mate with the leadscrew as metric thread pitches are not all simply related. May be it's something to do with selection of the appropriate gear, if the lathe has them?

Andrew

Thank you Andrew, but that's taken care of by the box that says Z=30T. The other box is the leadscrew pitch (P=3) in case you're wondering.

It is the module (gear pitch) of the gear mating with the leadscrew. Probably if the factory has a box full they need to select the right one for the lathe depending on its leadscrew. Pi x .95 = 3

Just to be clear I'm talking about gears that fit on the TDI, nothing to do with the gears between spindle and leadscrew. A picture of the chart would be useful.

Andrew

Hi Bo'sun,

I agree with Bazyle, when I made a 28T gear for my 290 lathe to cover pitches not covered by the 30T standard wheel I used a Module 1 gear cutter. That worked well, I guess my depth of cut was slightly deeper than usual.

Thor

Thanks Bazyle,

That makes sense. Mn presumably stands for "module number".

Andrew,

The chart is simply shows which TDI numbers can be used with which metric thread pitches.

Ok, so if you've only got one gear on the TDI then you must be limited to a few pitches?

Andrew

Hi Andrew,

According to the chart. 0.2, 0.3, 0.4, 0.5, 0.75, 1.0, 1,25, 2.0, 2,5 & 3.0mm pitches are possible (depending on the relevant TDI number/numbers) with the 30T gear, unless I'm reading it wrong. Sorry, but I can't post a pic of the chart.

That is true but it is a case of marketing trumping engineering.

For a 3mm pitch leadscrew, assuming the necessary leadscrew gears are present, the following can be cut with no indicator at all:

0.2, 0.25, 0.3, 0.5, 0.6, 0.75, 1, 1.5, 2, 3.

The thread dial indicator is required only for 0.4, 1.25 and 2.5.

Any pitch that is a factor of the leadscrew pitch does not need an indicator.

As Thor says, the addition of a 28t gear (and dial) is the best 'bang for your buck' in adding missing pitches. Potentially, you could cut it on a blank sized for 30t then you do not need to adjust the meshing centre distance when changing gears. Alternatively, recut both gears on a 29t-size blank.

Edited By DC31k on 23/12/2020 14:45:07

Edited By DC31k on 23/12/2020 14:49:47

Sorry, on re-reading above post, there is an error. For 2mm pitch on a 3mm leadscrew, the indicator IS required.

Although its true that any pitch on the (revised) list given by DC31k can be cut without a threading dial (indicator) the dial does make it much easier to judge when to re-engage the half nuts.

The basic case being always to re-engage at the same point on the dial which works for all threads being a factor of the leadscrew pitch. But, as with imperial, there are satisfactory sub divisions for other pitches so you don't have to wait so long for the dial to come round.

A leadscrew of 3 mm pitch is good in terms of the number of threads that can be cut with only one gear for the threading dial. My Smart & Brown has a 4 mm pitch screw and that needs 4 gears. A fairly typical complement for ex-industrail lathes.

Clive

Edited By Clive Foster on 23/12/2020 18:16:32

I am having some difficulty understanding the above sentence.

When the thread being cut is a factor of the leadscrew pitch, a dial is not needed. An alternative way of saying that is "you can engage at any point the half nuts drop in".

If you choose to use a dial for these threads, you most certainly do not need to engage at the same point on the dial. You can engage at any point at all on the dial and at any different point on the next pass. Since the graduations on the dial are frequently less than the number of teeth on the gear, you can even engage at a point in between graduations with no ill effect. If the nut goes in, the threads will align.

Taking his 30t dial on a 3mm pitch screw as an example, Cleeve would call this a 90mm indicator. If you wait for the same point on the dial each time, you are effectively saying that all threads that are a factor of the leadscrew pitch have a minimum synchronisation distance of 90mm, whereas the true figure is 30 times lower than that at only 3mm. Say, for example, that the carriage is advancing at a rate of 1mm in 1 second. For the 'same point' method of working, you are waiting for 90 seconds whereas you only need to wait for 3 seconds. 87 seconds _each_ _pass_ are wasted.

Hi All,

Thank you for all your input, I didn't realise the question would invite so many comments. I now need to digest the above and maybe undertake some practical exercises to help me get my head around it.

Thanks again

DC31K is correct there is no need for a threading dial for any metric thread than is an integer of a metric leadscrew pitch. No need for waiting for a dial either, all you need is a little light pressure on the lhalf-nut lever until it drops in.

It's easy to explain. Any thread pitch that will divide into 3mm will always line up every 3mm. So let's say you're cutting a 0.75mm thread. there are four of those every 3mm. If you had your tool engaged in the thread and stopped the machine, released the half-nuts and moved one pitch along the screw (3mm) then re-engaged the half-nuts the tool would now line up perfectly four threads from the previous. Do it again and it'll line up 8 threads away, again you'll be 12 threads away. It's literally impossible to get it wrong.

This is why some metric threading dial charts have a lot of pitches with no gear specified - it's not necessary.

There's some more info on it here:

https://www.model-engineer.co.uk/forums/postings.asp?th=131889