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View Full Version : Metric Lead Screws Vs. Metric Transposing Gears



Lima9
04-21-2014, 10:34 AM
Hello all. I'm new here. Look forward to learning from all of you. I just had a "new to me" Polish made Toolmex (FAMOT) TUM35D1 lathe delivered. It's a 14x42. The seller advertised it as cutting both metric and imperial threads. I was informed that metric transposing gears are required for cutting metric threads. I was also told by another individual that a metric lead screw is required for cutting metric threads. Which is it? Or are both required? I believe it is set up for imperial thread cutting. All the dials are graduated in thousandts of an inch. The thread cutting placard lists both metric and imperial threads. Thanks for any advice and clarification.

John Stevenson
04-21-2014, 10:56 AM
Sounds like you have an imperial machine but to be sure you need to check the pitch of the leadscrew to see if it's imperial or metric.
Short answer is transposition work well and no need for an extra screw.

Normally the thread chart, what you call a placard should list the setups needed.

Any chance of a picture of this chart and someone will walk you thru it.

TGTool
04-21-2014, 11:09 AM
I had a query asking if I could make a dial for his lathe so it would read metric with an imperial crossfeed screw. Yeah, well, sort of. I could do a dial with graduations every .02 mm but there would be one extra half a division at the end. Any measurement requiring less than a full turn wouldn't be bad. However doing the addition for multiple turns will put the vise grips on your brain.

Toolguy
04-21-2014, 11:23 AM
Time for a DRO in that situation.

Old Hat
04-21-2014, 11:29 AM
You may need to re-orient the gears on the "Banjo" for all your threading posibillities.
Is there a plaquard like this on your lathe.
http://s19.postimg.org/90x5l5usj/Gear_Threading_Ratios.jpghttp://s19.postimg.org/imqq1glyb/Egyptian_Hieroglyphs.jpg
For some threads on some lathes you may need to leave the half-nut engaged untill
the thread is complete. You have to back-up after each pass.
http://www.lathes.co.uk/southbendmodelfive9inch/img16.gif

Paul Alciatore
04-21-2014, 03:37 PM
To answer your question as to what is needed to cut metric threads, assuming that you have an imperial pitch lead screw, you do not need both a transposing gear and a metric lead screw. The idea of a pair of transposing gears is that they allow metric threads to be cut with an imperial lead screw. So, to cut metric threads, you need either a metric screw OR a pair of transposing gears, but not both.

You may wonder about precision. Are the metric threads exact or is there an error? Well, the international body that determines standards of measure has defined the inch as EXACTLY 25.4mm. There are no more decimal places in that factor, just 25.4, PERIOD. Not at the tenth decimal place, not at the 100th decimal place, not even at a million decimal places. It is exact, BY DEFINITION of the inch. The result of this is that you can convert from an imperial lead screw to metric threads (and vice-verse) with a fairly simple gearing combination. The smallest size for one of those gears for an exact conversion will be 127 teeth. The other gear will have some round number of teeth, like 120, 100, or 50. Notice the 120:127 gear pair in the chart that Old Hat posted above. These two gears will work in one direction to convert an imperial screw for metric threads and in the other direction to convert a metric screw for cutting imperial threads.

There are other, smaller gears that come close to the exact 1.27 ratio have been used for this conversion. One such pair is 47:37 teeth. Their ratio is 1.27027027027..., a close, but not exact transformation factor (about 0.024% error and good enough for most work). There are others that are used, but only with the 127 teeth or a multiple of it will the conversion be exact.

When I say the conversion ratio is precise, I mean just that, and only that. Thus, if there is an error in the original, imperial screw, then the same percentage of error will be present in the metric threads that you cut using it. The transposing gears do not improve the error percentage. In fact, any error in the gears will be added on top of the lead screw error. The gear errors will average out, but there can be cyclic errors as each gear in the gear train makes it's own revolution.

So, you will need the gears shown on the threading chart on your lathe. You do not need another lead screw.

This is the general case for all lathes that use a lead screw to cut threads. The exact gears used may differ somewhat, but the 127 factor is the common element in all of them and for an exact conversion, it is achieved with a 127 tooth gear somewhere in the chain.

Georgineer
04-21-2014, 06:12 PM
What Paul said.

George

Lima9
04-22-2014, 10:31 AM
I'm going to complicate (for myself) this even more.
I have a copy of the manual and under the section titled "Screw Cutting" it reads:
"For threading the stops must be shifted beyond the carriage travelling zone. The thread pitch values are given on the plate shown in Fig. 16 (the threading data placard).[B] FOR CUTTING STANDARD AND FINE METRIC AND ALSO INCH THREADS THE QUADRANT CHANGE GEARS DO NOT HAVE TO BE CHANGED. THE ONLY OPERATION THAT IS REQUIRED IS SHIFTING LEVER 44 INTO THE POSITION CORRESPONDING TO THE RESPECTIVE THREAD TYPE, i.e.:
A-metric and moule (fine) thread
B-metric and module thread
C-inch and DP thread
and lever 45 to the position determining the multiplication factor (1:1, 1:2 and 1:4)"
Does this sound right? I take this to mean swaping out the gears is not required. If thats the case then what are the transposing gears for? They are listed in the manual as optional parts.

Still trying to figure out how to post a photo of the threading placard. Once I do hopefully someone can walk me through it like Mr. Stevenson said..

janvanruth
04-22-2014, 11:01 AM
you will be able without changing gears to cut most threads.
the transposing gears are needed to create to create the threads not indicated on your placard.
dont worry you will probably never need them
lever 45 is quite common on eastern european lathes and had been for a long time

John Stevenson
04-22-2014, 11:47 AM
Sounds like the transposition gears are already in the box at the end of lever 44 and lever 45 gives you three ranges extra to whatever is on the other levers not mentioned.

There is a sticky at the top of the General forum page explaining how to post pictures. I feel without a pic we are all in the dark.

EVguru
04-22-2014, 12:23 PM
I had a query asking if I could make a dial for his lathe so it would read metric with an imperial crossfeed screw. Yeah, well, sort of. I could do a dial with graduations every .02 mm but there would be one extra half a division at the end. Any measurement requiring less than a full turn wouldn't be bad. However doing the addition for multiple turns will put the vise grips on your brain.

There are quite a few machines that have dual reading dials (push pull) and transposing gearing inside the hand wheel.

lakeside53
04-22-2014, 12:58 PM
The TUM35 isn't a SB or like the usual "USA OLD IRON" - it's has more gears in the transmission/feed QC box than an Audi! I have one, and have had it in pieces :(

The pictures are on PM and a similar thread : http://www.practicalmachinist.com/vb/general/toolmex-famot-made-poland-tum35d1-questions-283875/


Lima9 - Your instructions are a little different to mine - maybe because you have the D1 variant or maybe because mine suffers from Polish-English (manual is both hilarious and frustrating).


It looks like your feed/threading selector gearing is structured slightly different to mine and you can in fact select both metric and imperial with the installed 37, 57, 62,72 gears. The 80-66, 80-49 set look like just another range for DP (note the little "pi" symbol).

YOU MOST LIKELY have an imperial lead screw so you will need to leave the half nuts engages when threading metric. Not sure how your threading dial indicator is arranged. Picture?

So.. why not just try following the manual and cutting a few threads? heck, it it works I'll have to find a D1 FEED gearbox to swap mine out with!


BTW - to help with one of my mysteries, can you post a picture of your crossfeed dial? Mine has both fractional and metric engraving.

lakeside53
04-22-2014, 01:03 PM
There are quite a few machines that have dual reading dials (push pull) and transposing gearing inside the hand wheel.

MY TUM has a large cross-feed dial with gearing inside for that purpose, but one of the tiny "clock like" gears is either missing or an obscure factory option. I just use the dual scales but have to remember that unlike the imperial engraving, the metric doesn't "wrap evenly" - i.e only good for one revolution when reading.

Paul Alciatore
04-22-2014, 02:27 PM
This does complicate things. Are you sure you have the correct manual and that the section that you are quoting does apply to your lathe?

Some photos would help. If you are not familiar with posting them, see the Sticky threads.




I'm going to complicate (for myself) this even more.
I have a copy of the manual and under the section titled "Screw Cutting" it reads:
"For threading the stops must be shifted beyond the carriage travelling zone. The thread pitch values are given on the plate shown in Fig. 16 (the threading data placard).[B] FOR CUTTING STANDARD AND FINE METRIC AND ALSO INCH THREADS THE QUADRANT CHANGE GEARS DO NOT HAVE TO BE CHANGED. THE ONLY OPERATION THAT IS REQUIRED IS SHIFTING LEVER 44 INTO THE POSITION CORRESPONDING TO THE RESPECTIVE THREAD TYPE, i.e.:
A-metric and moule (fine) thread
B-metric and module thread
C-inch and DP thread
and lever 45 to the position determining the multiplication factor (1:1, 1:2 and 1:4)"
Does this sound right? I take this to mean swaping out the gears is not required. If thats the case then what are the transposing gears for? They are listed in the manual as optional parts.

Still trying to figure out how to post a photo of the threading placard. Once I do hopefully someone can walk me through it like Mr. Stevenson said..