Screwcutting not picking up in the same place

[charkmandler]

I have an odd problem. My lathe is a metric Warco (similar to a Grizzy 36x12), since owning this lathe all of my screw cutting has been imperial threads leaving the leadscrew engaged with a 127 gear. Today I needed to cut a metric thread 12mm x 1.75, I did a scratch pass to test the thread pitch which was good. The first real cut did not follow the scratch pass marks. The leadscrew was engaged on number 1 on the indicator both times. Testing repeatedly using the same index number it seems that it either picks up the original pass or creates a new thread in the middle of the scratch line.

The gearing is direct unlike using the 127 gear for imperial threads so I can see no reason why it does not pick up in the same place when using the same index number. For the job I cut the thread leaving the leadscrew engaged but this should not be necessary.

There is no slop in any of the slides and after the first attempt I just used the cross slide to advance the cut to eliminate the angled top slide - just incase I was forgetting years of screw cutting.

Any ideas please?

[GlennW]

I believe you have to leave the leadscrew engaged as you were doing for imperial threads.

[mihit]

Certainly an odd one. My first instinct is to cry "China" and leave it at that.
What is the gear combo that gets you to 1.75? That would be something like 40:32:120:40 on my similar sized machine. Means aught to me but someone may be able to spot an issue from that.
How much slop is there in your Threading Dial? did you run right off the piece and take up all the play before engaging cut? Don't suppose you have two "1"s on your dial? Or forgot you glasses and got 1 and 7?
I normally use the compound to pick up/chase threads... but the dial is supposed to avoid that.
It's a new one on me.

[charkmandler]

Not sure what the ratios are in the gearbox and yes this could be a China anomaly but one I need to sort as I have a number of metric threads to cut and leaving the leadscrew engaged takes so much longer. No discernible slop in the dial and defiantly picking up on the same number.
Another day and a fresh mind I've checked the setup and another test has the same results.
Is it possible to have an odd gear combo in the gearbox that is working as a transposing gear - the thing that rules this out in my mind is that I have checked a number of the combinations for different thread pitches and they all come out correct.

[GlennW]

Does the machine have a metric thread dial (consists of four gears from what I understand) or an "imperial" style dial? (single gear)

I don't have a metric lathe, but from what I understand, metric threading on an all metric lathe does not use the same technique as "imperial" threading on an all "imperial" machine as far as threading dial and leadscrew engagement.

I believe you have to change the threading dial ratio to correspond to the pitch being cut, but again, I'm not sure, as I don't have a metric lathe.

Sounds like something that Marv Klotz could possibly resolve!

Edit:
I just did a bit of Googling and found this.

"It is quite possible and in some cases quite feasible to cut metric threads with a metric lead-screw but a different approach is required as would be the case if cutting "inch" threads on an "inch" lead-screw.

Inch threads are easy as all that is needed is the tpi of both the lead-screw and the job. The threading dial teeth number is usually 4"/lead of lead-screw.

With metric threads there are very few "numbers of threads per metre or mm" as metric thread are expressed as their pitch in mm.

Metric thread screw-cutting requires a number of gears with differing numbers of teeth.

I have no real problems with it as my single gear (36 teeth - need to check) and my 3mm pitch lead-screw work very well.

It is all too easy to make a mistake with working how and where/when to "drop in" with the half-nuts.

I think I recall Nick saying that even he finds it easier to just leave the half-nuts engaged and just reverse the lathe as he also said as I recall do most/many others in Germany - and by extension, I'd add Europe or any other metric countries.

I leave my half-nuts engaged for metric (easier) and as my lathe is metric I (have to) leave my half-nuts engaged when cutting inch threads."

[charkmandler]

Thanks Glen thats really interesting. I've only ever cut imperial threads, originally on an imperial lathe, easy, and now on a metric lathe, just a bit more time consuming. I will look in to needing different gears for different metric thread pitches.
Thanks

[mklotz]

... I will look in to needing different gears for different metric thread pitches.

If you have a 3 mm leadscrew, this chart may be of help...

http://bbs.homeshopmachinist.net/thread ... ost1258773

[Mr Ron]

It would seem you would have to remove the 127 tooth gear to restore the lathe to all metric. It would be very difficult to just reverse the lead screw, unless your lathe has instant stop.

[johnfreese]

I have no experience threading with metric leadscrew. I did look up some information in the Nardini lathe. I have the the Imperial version. I looked in the section that applied to the metric version and found there were 5 possible pinions that could be used on the thread chasing dial. One changed the pinion according to the pitch being cut. Not knowing how the chicom lathes handled the issue, I can't comment further.

[John Hasler]

If you are cutting threads with a pitch of 1.75 mm and your leadscrew has a pitch of 3 mm each turn of the leadscrew will cut 1.7143 threads. Thus your threading dial has to account for that ratio by moving one full interval for every .58333 turns of the leadscrew.

[Bill Shields]

I have been down this road....all the advise is good...but the practical solution is:

when doing metric threads..do NOT disengage the 1/2 nut at the end.

stop the lathe, run it in reverse to back the carriage up to the starting point.

[earlgo]

The instruction book for my 1958 ATLAS lathe lists 79 imperial threads and 16 metric threads it will cut. It specifically says that when cutting metric threads, DO NOT disengage the half nuts. I have a box full of change gears instead of a QC box, in case you wondered.
--earlgo