2016 POTD Thread Archive

savarin said:
This is a pretty good explanation for metric threading
http://www.conradhoffman.com/metricthreading.htm
down the bottom he gives a method of using the thread dial so you can release the half nuts.
I haven't tried it yet as I said I just keep it engaged. So far I haven't had to thread to a shoulder and the small amount of run out I cut has been sufficient for the lathe to come to a stop.
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I read this and it makes sense. It also occurs to me, that if you have a DRO and engage the nut at the same DRO position AND the same thread dial number that would work as well.
R
 
mark_f said:
Well ..... I did something stupid today. I tried out my new transposing gears for metric threading on my South bend lathe.....BUT.....I tried them out on the almost finished collet chuck I made. The metric threads were the last step. I SCREWED IT UP! ( I should have experimented on a sample part)... I learned the hard way, the gears work great, but when metric threading, you MUST ALWAYS ENGAGE THE HALF NUTS ON THE SAME MARK ON THE THREAD DIAL. If you start on number one, you must start every pass on that number. Using any other line or number will split the threads. I finally figured that out and got a good thread but it was too small by this time and the nut is a little too loose for my taste.. So... I made another collet chuck and will finish it up tomorrow.
By the way..... it says NOWHERE in the write up for this metric threading about that detail. I think it was kind of important to be left out of the write up.
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I don't believe that you can get by with engaging the half nuts on the same mark when cutting metric threads on a lathe with an inch lead screw. The half nut works on the fact that there is an simple integral relationship between the half nut thread and the thread you are cutting so the pattern repeats periodically on the lead screw. That pattern is determined by the pitch of the lead screw and the number of teeth on the indicator gear. For example, my Atlas has 16 tpi on the lead screw and the 32 teeth on the thread dial gear so the thread dial repeats every 2". My Grizzly has 12 tpi on the lead screw and 48 teeth on the thread dial gear so it repeats every 4". Metric thread repeat is measured in mm so it would be highly unlikely that you would match up properly. You should never disengage the half nuts when cutting metric threads with a lathe with an inch lead screw.

However

Tom Lipton on OXTool did an excellent video on how to use the half nut when cutting metric threads. You can can disengage the half nuts provided you can ensure that you are engagin the same gear tooth with the same thread on the lead screw. He does it like this: Disengage the lead screw as you would for inch threading. reverse the lathe and renege the lead screw when the mark on the indicator is aligned again. Continue running in reverse until you are clear of your thread. Reposition your cutter and start the next pass.


While this doesn't eliminate the need to run the lathe in reverse, it does permit you to thread up to a shoulder without having to worry about the spindle coasting to a stop or hand turning to thread the last few threads.

CAUTION: If you have a threaded mounting arrangement without a locking mechanism on your spindle, you could unscrew the chuck.
 
rwm said:
I read this and it makes sense. It also occurs to me, that if you have a DRO and engage the nut at the same DRO position AND the same thread dial number that would work as well.
R
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Actually, it won't as the relationship depends upon how many revolutions the lead screw has made. The smallest integral ratio between metric and inch is 127:5 You could move to a position, note the thread indicator mark, disengage the half nuts and run the lathe until the mark aligns again and reengage the half nuts. A 8 tpi lead screw will have had to rotate 127 x 5/8 or almost 80 revolutions to bring the lead screw/half nuts back into the same alignment.
 
RJSakowski said:
Actually, it won't as the relationship depends upon how many revolutions the lead screw has made. The smallest integral ratio between metric and inch is 127:5 You could move to a position, note the thread indicator mark, disengage the half nuts and run the lathe until the mark aligns again and reengage the half nuts. A 8 tpi lead screw will have had to rotate 127 x 5/8 or almost 80 revolutions to bring the lead screw/half nuts back into the same alignment.
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RJ- I am not following your logic. If the DRO says you are at position 1.00 and the thread dial reads 1 won't you be at the exact same physical location on the leadscrew always at those settings? How could you be elsewhere? The DRO tells you the distance down the leadscrew and the dial tells you the rotation of the screw. Is there another variable?
R

Edit:
Um...RJ is right. The other variable is the spindle rotation of course. Yes, in my plan you would be at the same place on the leadscrew but you would not know the rotation of the spindle anymore. Hence, the 80 possible spindle positions. Kudos to the man with the fish!
R
 
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rwm said:
RJ- I am not following your logic. If the DRO says you are at position 1.00 and the thread dial reads 1 won't you be at the exact same physical location on the leadscrew always at those settings? How could you be elsewhere? The DRO tells you the distance down the leadscrew and the dial tells you the rotation of the screw. Is there another variable?
R
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The problem is the lead screw is turning so the position that you are on the lead screw is moving in or out. As an illustration, you could position the half nuts on the lead screw at a certain thread indicator position and z axis position. Now disengage the half nuts and move the carriage until the thread indicator reads the same and reengage the half nuts. Now run the lathe to bring the carriage back to the same DRO reading. The DRO reads the same, as does the thread indicator but you are clearly not engaged with the same thread on the lead screw.

It works for inch threads on an inch lead screw or metric threads on a metric lead screw because of the relationship between the indicator gear and the lead screw thread but it doesn't work for metric threads on an inch lead screw and vice versa. It also wouldn't work if you wanted to cut some oddball thread like 15.28 tpi with an inch lead screw even though you might find a gear combination to do so.
 
Found my 16 oz ballpien hammer in two pieces after the Christmas holidays, so bought a replacement hickory handle and finally got around to "hammer day" yesterday. Installed the new handle - used a wood rasp for the fit-up. replaced a lost wedge in my 12 oz ballpien hammer and tightened the wedge in the 4 oz hammer.

After the hammers came some lathe work; making a cone shaped point to try and break a couple of rocks in the drive area in front of the hay shed. The cone was started from a piece of 3.5" shafting. Threaded one end to fit 2.5" X 8 TPI drill pipe then started the taper. Work is mounted between centers and the point end is 1.375" in diameter now and I have to remount the work to finish the point. The threads will be protected by a collar from another piece of drill pipe but nothing comes easy, have to break the rusted joint loose.
Have a good day!
Ray
 
Don't you hate it when you buy something only to find that they haven't "upgraded" ALL the fuses?

David
 
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