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AL320G - Threading Dial

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bazmak

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When single point threading I power feed to within a few threads and then turn the chuck by hand.Avoids hairy scarie
 

goldstar31

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When single point threading I power feed to within a few threads and then turn the chuck by hand.Avoids hairy scarie
Apologies for the delay but the Victoria Cross Memorial combined with the Anzac Day in London sort of got in the way!

So my present thoughts are the realisation that many small and new Far East lathes have a slowest speed of 100 rpm. When push came to shove 'my' 918 actually ran at a mind boggling 130rpm which was too high to avoid nervous breakdowns.

OK, I tamed the beast by newer pulleys but the classic mandrel handle made life easier- and safer.

Just a constructive thought?

Norman
 

Tjimptjomp

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I've been looking at the threading dial on my AL320G and thinking it must be usable even though the "manual" that comes with the lathe doesn't tell you anything other than how to set up the change gears.

So first off some research into threading on the lathe by reading Workshop Practice Series #3: Screwcutting on the Lathe by Martin Cleeve.
Now this is pretty jolly complete and after reading things several times I felt I had enough of a handle on things to make the next steps.

So looking at the threading dial the gear is 20 teeth, the lead screw is a 3mm pitch and the dial has 4 "lines" or sectors on it.

So what does all this tell us ... well after some manual measurements the following ...
We get 60m travel of the saddle from a single rotation of the threading dial and the lead screw turns 20 times during that travel.
i.e. 20 teeth x 3mm pitch = 60mm travel.

So back to the book, specifically from pages 71 with specific attention to pages 81 and 82. So from reading this section it appears that the key to being able to use the threading dial is that the pitch of the thread needs to fit some rules with regard to:
[a] the thread pitch being a whole multiple of the leadscrew pitch
the thread pitch being able to divide evenly into the amount of saddle travel represented by sectors on the dial

So armed with this hopefully correct info I built a chart to see what pitched threads I could use the dial for on the lathe and which I could not. And that is the kicker with metric threads, not all of them will work with the threading dial without some changing in the threading dials gearing.

The calculated table of threads. Blue lines show thread pitches not possible with the stock threading dial gearing.
View attachment 88154

Now to do a proof. I picked a 1.5mm thread, setup the lathe gears, found some scrap and got it ready to do some light cuts.
View attachment 88155
View attachment 88156

So, set up the and ready to go I had the lathe running nice and slow (60rpm) and leadscrew turning and ready to engage the half-nut on a any whole sector line on the dial. Let it run a light scratch, disengaged the half-nut, pulled back the cross slide, brought the saddle back to start position, shutdown and checked with a gauge. Photo's not the best but you can see that the scratch is bang on the 1.5mm pitch gauge. So far so good. Now for the moment of truth, can we get repeated pickup?
View attachment 88157


Well repeated the same process and engaged the half-nut on one of the whole sector lines. Not the same as the last one, just to make sure there was some more variability in the mix. Re checked the slightly deeper cut, BINGO, bank on the same lines as last time.
View attachment 88158

Okay according to my understanding from the book learning I should be able to engage the half-nut at ANY point on the dial gauge as the 1.5mm is a whole multiple of the leadscrew pitch. So thats what I did, engaged the half-nut at a point between the sector lines on the threading dial.
View attachment 88159

And the result was bang on!

I ran out of time tonight to do some more variations on threads, also will need to dig out some more scrap round to trial on. But so far the math seems sounds and practical testing is lining up with the book learning.
View attachment 88160

Conclusion at this point is that while not all threads can be done using this method a good chunk of them can be so the threading dial is no longer looking like the useless appendage it seemed to be for metric threading.

I'll do some trials over the course of the week plus I want to look into what gearing would be needed on the thread dial to do the other threads and if that is even worth the hassle given the non-disengaged half-nut method will work just fine on those.

Cheers,
James.
Hi James!
I also have a lathe that has a 3mm pitch. Its an old weiler from 1952. It is missing the dial and i thought i’d build one. My question to you is if you know what module your 20 & 21 teeth gear are. Or if you coulde provide me with the diameter of the gears. That would help a lot!

cheers
Thomas
 

goldstar31

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I have a Myford Super7B with a power cross feed similar or better than James's. So that is not a 3mm leadscrew but an * Threads per inch and WE, to convert our lathes to a metric function have to have a transposing gear of 127 correctly but workable substitutes of 63, or 21 are needed. Again, Myford gears for such purposes are not model pitches but are 20Diametrical pitch.
So if you are Swedish and working in in Metric form the reference to 21 is quite irrelevant:)
So what I deduce also having a Metric lathe( A Sieg c$) with a 3mm leadscrew is a set of gears( the module is irrelevant- but will facrorize in multiples of 5. So a dial gauge to work in metric is also in multiples of 5.

Going from a Metric Metric lathe to cut imperial threads it is prudent to gang a 127 transposing gear with a 120 one but you must leave the leadscrew permanently engaged and ignore a dial gauge if you have one.

Again, with metric feed screws, it IS possible to approximate feeds without fitting a DRO.

'Cleeve' ie Kenneth C Hart is invaluable in such cases as he in Screwcutting in the Lathe book tries to cater for all possible problems but in it Professor Dennis H Chaddock explains the number of permautations into thousnds only usuning gears of multiples of 5.

So this is brief but heady stuff and I must remind you that Cleeve only had one lathe- but TWO leads-crews - or actually half a lathe- a ML7 driven by TWO motors. So I synpathise with you.
But- and it might cloud the issue- Cleeve incorporated the dog clutch which was officially incorporated in some Myford ML10's. I had one.
Might I wish you the best of luck in what can be a very complicated problem.
Norman
 

joco-nz

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Hi James!
I also have a lathe that has a 3mm pitch. Its an old weiler from 1952. It is missing the dial and i thought i’d build one. My question to you is if you know what module your 20 & 21 teeth gear are. Or if you coulde provide me with the diameter of the gears. That would help a lot!

cheers
Thomas
Thomas - I sold the AL320 a while ago and replaced it with a larger and more capable AL346V. Its still a metric leadscrew lathe but I have not had to work out what pitch etc as it has a thread dial chart already on the machine along with the quick change gear box.

After work today I will have a snoop and see if it is 3mm pitch and if so see what i can deduce about the gears for threading dial.

Having said that, the threading dial is just a convenience. If I’m feeling lazy or a little tired I just keep the half nut engaged. There can be NO screw ups that way. And on short length threads its not really a material loss in time.

cheers - J.
 

joco-nz

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Lead screw is 3mm pitch. had a look at the gears for the dial and they look to be Mod 1. That is based on a visual inspection against the "to scale" profile pics in the "little black book" and by checking the measurement of the pitch on the gear.

An imperial 32 pitch looks to be too be a little big.
 

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