Crazy Threads...

When a company makes a lathe for both the metric and the imperial markets, it is set up with different labeling, gearing, lead screw, and threading dial. Do you have a manual for the lathe? If so, check that all the tooth counts are correct. Sometimes a mistake is made at the factory and both metric and imperial components are mixed during assembly. QC would normally catch that, but QC is sometimes poor or nonexistent, and sometimes mistakes are made. I would also carefully check the last gear in the gear train that drives the quick change box. That is a compound gear. Make sure both tooth counts on that gear are correct. Also, you made that one thread that looks pretty good. Could you check that thread with a thread gage or a ruler to see if it is the correct pitch to match the machine setup used when it was cut?

Good idea... I will check the thread pitch tonight.

No manual. and very difficult to find info on the internet. not much out there.

I am cautiously optimistic the gear on the thread dial is wrong. it would explain why my threads are consistent, just consistently wrong...
However... if the gear were wrong, wouldn't that cause the half nut to not line up with the thread dial at all? as it stands, I can engage perfectly on number 2 & 4. no where else... everywhere else the half nut hits the top of the threads and engages just past the line or out in left field. IF the dial gear was wrong, wouldn't there be no consistency with 2 & 4 at all? shouldn't it just be all over the place??
 
Theirs an interesting guy on you tube that has quite a good explanation of the threading dial, worth a watch for general interest and might be handy. :)


Stuart
 
Theirs an interesting guy on you tube that has quite a good explanation of the threading dial, worth a watch for general interest and might be handy. :)


Stuart
I did watch that yesterday. he gets 2 inches perfectly, so would that mean his machine is metric?
Then I did the same thing I got 1 3/4" per revolution.
 
i went #1 to #1 and it gave me aprox 1.75".... ?
Then something is wrong. Do you always get exactly the same measurement when you do that test? It should be an integral multiple of one inch and should repeat to within +- half a leadscrew tooth if this is an inch machine. Count the TPI on your leadscrew with an inch ruler. Also count the teeth on your leadscrew in 10 centimeters with a metric ruler and count the number of teeth on your threading dial gear.
 
It does sound like their is an issue with the threading dial as your threads cut perfectly when not disengagueing the half nuts.

Their is a cheet for re indexing the lead screw and chuck, i used to do it when i had a metric lathe with no thread dial.

You can put a sharpy (marker pen) mark on the lead screw, a mark on the chuck and a witnes mark on the sadle and ways. You can then rotate the spindle untill the are in the same position again and possition the sadle at its mark.

Strangely i didnt do a lot of threading on that lathe ;)

Stuart
 
i did try that. i wasnt sure of the result... one full revolution of the thread dial gave me about 1 3/4".
so he wrote if its metric, ill get a neat round number... so being 1 3/4" i should assume its imperial?

so whats that mean for me then? that should still be threads per inch, no? not millimeters?
Actually, I said that you will get a whole multiple of an inch if it's Imperial. Getting 1.75", it is not Imperial. Please note that this assumes that the proper thread dial gear is used. My Grizzly lathe has a lead screw pitch of 12 tpi and 48 teeth on the thread dial gear, requiring 4.00" of travel to make one complete revolution of the thread dial. My Atlas has a lead screw pitch of 16 tpi and 32 teeth on the thread dial screw so it requires a travel of 2.00" to make one complete revolution of the thread dial. This integral relationship is what permits engagement of the half nuts at any point on the lead screw, as long as the index is aligned the same. By the same token, the same distance measured in mm is not a nice round number which is why we can't cut metric threads using an Imperial setup and the thread dial.

Another possibility is that there is a mismatch between the thread dial gear and the lead screw. If an 16 tpi lead screw was replaced with a 1.5 mm pitch, the thread dial gear with 32 teeth would still track (sort of) but instead of 2" of travel for one thread dial revolution, you would get 48 mm or 1.89". If thread dial gear was replace with a gear to match the metric lead screw, you should get a nice round metric number like 50mm. Since the metric thread system pitches are even multiples of .05 mm, the thread dial would have to be set up differently as well to conveniently cut common metric threads. (as stated above, engaging on the same index mark should always work though).

Check the lead screw pitch. Use a magic marker to mark every fifth thread so you don't go crazy trying to keep track. An Imperial lead screw will most likely be a nice even 8, 12, or 16 tpi. The pattern will repeat every 1" A metric lead screw will repeat on a metric scale. If you count the threads in 10mm, you would be able to accurately determine the metric pitch.
Next, count the teeth on the thread dial gear. As stated by John, it will be a nice whole number multiple of the lead screw pitch.
Considering your measure of carriage travel for one revolution of the thread dial, something isn't right.

One final possibility is that the drive pin on the thread dial shaft is missing or broken. That, coupled with a sticky shaft, would also cause the problem that you are observing. My Atlas gear is a press fit but the Grizzly has a key. If yours is keyed and the key is missing, the gear may be revolving on the shaft which would cause the problem.
 
BIG update...

so that nice perfect thread i did with the half nut engaged the whole time is NOT what i had the machine set up for... i had it set for 16tpi, put a thread gauge to it and the 15tpi gauge fits reasonably nice... so theres that.
also, i counted the teeth on the thread dial, 14...
also, someone asked if i get the same 1.75" per one revolution of the thread dial everywhere on the bed, yes, i tried about 10" away from my first attempt and got the same 1.75" per rev.

so lets recap... the lead screw is 8tpi, the thread dial is 14 teeth, i carved a "16tpi" on that piece of stock, the result is 15tpi...... soooooo.... i have no freaking idea whats going on. im wondering if this lathe is a Frankenstein...
 
There has to be a gear in the set up that is incorrect. Recheck to confirm the tooth count on all the gears.
Pierre
 
What IF the lead screw is wrong? what if the machine is metric and that an imperial lead screw? how would I know?
 
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