Calculating Thread Dial Chart

Hi I’ve got a Bailiegh GH1340 lathe but the thread dial chart is incomplete! Is there an easy way of working it out?

lead screw pitch is 3mm

Dial divided into 12

The gears for the dial are 28, 30 and 32

thanks Ian

Here’s a photo of the chart

I doubt the indicator works on all pitches. My metric lathe is the same. There are a number of pitches where dial indicator works usefully, but several others where it's useless.

The underlying problem is gearing the thread dial to the lead-screw. Not difficult when threads are specified in ''Turns per", but hard when the thread system is defined by pitch. Metric!

When a pitch can't be synchronised with a TDI, the technique is to maintain sync by not disengaging the half-nuts, always powering in and out. Although this makes threading slightly slower, the method avoids thread mangling mistakes!

How much slower is moot! I often thread in reverse, cutting away from the headstock at high-speed. This is safe because there's no danger of crashing the saddle into the chuck. Although the forward return is done slowly with the cutter disengaged, the method might be faster than a TDI: I've never measured it.

The right way to do drive an imperial fixed speed belt lathe with a screw on chuck, may not apply to a metric variable speed lathe with a bolt-on chuck.

Dave

It's usual for metric thread dial indicators to have 5 gears. By chance i have just bought a metric thread dial indicator for a Harrison M300, as part of a job lot where what I really wanted was the multi-position saddle stop. I'll be selling it on as my M300 is imperial.

The metric M300 TDI has five gears; 14, 16, 18, 20 and 22 teeth and the lathe has a 6mm pitch leadscrew. It is relatively poor for pitches below 1mm, doing 0.225, 0.25 and 0.75mm. I suspect your lathe would do similar and it certainly ought to be able to do 1mm pitch, but you may need different gears.

Andrew

Have a look at this Understanding the threading dial as a guide. You should then be able to work out your own details.

Martin C

There is some misinformation posted above, including the allegation in the original post that the table is incomplete.

Most importantly, and this applies to both imperial and metric lathes, any pitch that is a factor of the leadscrew pitch does not need an indicator at all. You can drop the halfnuts in anywhere they engage and the threads will be synchronised.

For a 3mm pitch leadscrew, this will be 0.25mm, 0.5mm, 0.75mm, 1mm, 1.5mm, 3mm.

(For an 8tpi leadscrew, this would be 8tpi, 16, tpi, 24tpi, etc.)

Thus any row of the table above that fits the above metric pitches should be blank to signify 'thread dial not needed'.

Unfortunately, the table itself is not consistent in its use of blank lines. For a 5.5mm or 11mm pitch, you need a gear that has a factor of 11 in its tooth count (e.g. 22). No such gear is available so for those two pitches, the only option is keeping the halfnuts engaged and full reversal.

The above is just a flavour of the problems with the table. At this point we can give you a fish by providing a correct table or we can teach you to fish so you can draw up the table yourself not only for this machine but for any machine that you will ever own.

The model engineering equivalent of 'Teach yourself to fish' is Martin Cleeve's 'Screwcutting in the lathe'. You can fast-track your angling education and read just one chapter of the whole book: the part sub-headed 'problems of repeat pickup. In that section, he discusses 'minimum synchronisation distance'. Understand that concept and you will understand the table above, where it is correct, where it is incorrect and where it is incomplete.

There are at least two other, newer screwcutting books but neither of them deal with this issue. Cleeve is the only place you will find enlightenment.

Anyone remember the recent posts on the forum regarding a screwcutting tool with a loose lifting piece which automatically lifts and retraces it's path when the lathe is reversed ready for the next cut?

The video of this shows the thing in use and it looks a lot easier than the usual methods when cutting a thread for which the half nuts must stay closed. I can't remember what it was called or find it via the search facility, but I am sure someone can

Rod

it's odd that the metric TDI for the Harrison M300 lists some pitches that are integer factors of the leadscrew, albeit any number can be used but with a specific gear. But other pitches, such as 0.5mm, are not listed although the lathe will cut them. Some pitches that are not factors, such as 6.5 and 11.5mm are also not listed on the TDI. Presumably one has to keep the half nuts engaged. I suspect that the inconsistency is due to lack of space on the front of the TDI.

The imperial TDI from my lathe may have the same inconsistencies, but it's not fitted as I don't use it and I'm not going to climb up to the shelf it is sitting on to have a look.

Andrew

I must have missed the post, but it sounds like you mean one of these:- Flip-up Toolholder

If not, thanks for reminding me, hopefully the workshop will be moved in January and this will be another one for my "to-do" list.

Brian G

Manufacturers in general do not seem to acknowledge that a number of pitches can be cut without a TDI. Maybe to them, it is something so obvious that it is not worth mentioning. This is true of imperial pitches as well.

Roughly speaking, for ISO metric pitches up to the pitch of the leadscrew, 50% do not need a TDI. I once did an exercise to see if there was a particular metric leadscrew pitch that maximised the number that could be cut without a TDI, but there is no clear winner.

It is also strange that for metric machines they seem to list every pitch available from the gearbox whether it is actually used or not (you see 1.1mm , 1.6mm and the 2.25mm and 2.75mm above). With an imperial machine, if a particular gearbox setting is a non-used tpi, the chart is usually left blank.

For a 6.5mm pitch on a 6mm leadscrew, the minimum synchronisation distance is 78mm (13 leadscrew threads equal 12 workpiece threads). Thus a TDI gear would need 13 as a factor of its tooth count.

For an 11.5mm pitch on a 6mm leadscrew, the MSD is 138mm (23 leadscrew threads equal 12 workpiece threads). Thus the TDI gear needs 23 as a factor.

As these threads are so rarely used, maybe it is not economic to provide a gear. The 22t gear that is more common is only necessary for 5.5mm and 11mm pitches and they are not cut every day.

It is interesting that in the OP his dial is divided into 12 and numbered 1-12. Note that 2, 6, 8 and 12 are never used, so a dial with 8 numbers (it would look somewhat like the red bits of a union flag) would suffice.

For Rod.

https://www.model-engineer.co.uk/forums/postings.asp?th=52176&p=1

Tony

Brian and Tony,

That's the thing, I had forgotten the Flip- up name. Thanks for doing the research. Very simple and looks very useful.

There are lots of hits once you have "flip -up toolholder" to search for and I would suggest anyone who has not seen them to look at the videos of the toolholder in action. The user only has to run the lathe spindle forward and back while watching the tool traverse the work and add the cut at the end of each pass. There is no need for a Tubal Cain style dead stop on the cross slide or a George Thomas style retracting topslide, and no thread dial indicator in sight!

Rod

Thanks everyone I’m all sorted now it’s just a matter of mathematics!

As long as the pitch will divide into the distance between the marks on the dial it should work! As long as the number of marks used divide into the total distance of one full turn of the indicator

Ian