# Create 0.75 and/or 1.00 mm threads



## Manderioli (Dec 12, 2017)

I have read a few threads about transposing gear sets to convert a 9A to cut metric threads. I don't need every metric but would like to prototype a product I am designing that consists of 0.75mm threads. I was considering cutting 32tpi so see if the error would be sufficient since I only need 3mm depth of threading.

What do I need to create 0.75mm threads only?


----------



## Holescreek (Dec 12, 2017)

What's the diameter of the part being threaded?


----------



## Manderioli (Dec 12, 2017)

The diameter would be 72mm x 0.75mm for an outside thread and also for an internal thread.


----------



## derf (Dec 12, 2017)

Use the feed instead of the half nut, and don't disengage it. I'm not real familiar with SB 9, but on my heavy 10", the setting for 11-1/2 tpi gives a feed rate of .0291". A .75 mm pitch is .0295, so for only  3 mm of thread, it should suffice.


----------



## Holescreek (Dec 12, 2017)

I ran your numbers through a thread calculator and it wouldn't accept a 72mmx.75, but I was able to fool it by inputing an inch diameter measurement and 32 tpi then switch it back to metric.  I don't think you're going to get the depth you want with that fine pitch.  The thread height will only be .42296 mm.


----------



## jocat54 (Dec 12, 2017)

I have some shop made aluminum gears for a SB9A for turning metric threads (37/47 compound gear) gets pretty darn close, using the QCGB, if you might be interested in them.
Here is post showing them  and some pictures with them mounted on the lathe http://www.hobby-machinist.com/threads/metric-change-gears-for-sb-9a.24348/#post-256454


----------



## Manderioli (Dec 13, 2017)

Thank you for all the replies. I do have the 11.5 tpi to feed the carriage at 0.0291". I will try the feed rate first but I'm assuming I will have one shot to cut the threads since I cannot relocate the carriage at the exact position. Roughly 0.0255" depth of cut. 

I did notice that I would not be able to meet the 33.8tpi if I use the half nut. 32tpi is closer error than 36tpi.

Holescreek, what do you mean I would not be able to get the depth? Wouldn't the depth of cut depend on the cutter (even though all threads are 60 deg)? Are you saying due to the pitch, if I try to get 0.6499mm depth, the threads would roll over each other?


----------



## derf (Dec 13, 2017)

You don't have to cut the thread in one pass. At the end of the threads, you simply shut the spindle off, then back out the cross feed, then reverse the spindle....all with the feed still engaged. Once you get back to the starting point, re-zero the cross feed and take the next cut.
 Depending on how much "coast" your machine has, you might have to practice rapidly backing off the cross feed the same time you stop the spindle. With that shallow of thread depth, it won't amount to a half of turn on the cross feed.
Even if you had the correct change gears for metric threads, you would still have to use this method because of the imperial lead screw.


----------



## Holescreek (Dec 13, 2017)

Look at the chart, specifically height and flat at crest. At finished depth the flat at crest is pretty small going deeper will remove it and reduce the OD of the part.


----------



## jocat54 (Dec 13, 2017)

derf said:


> You don't have to cut the thread in one pass. At the end of the threads, you simply shut the spindle off, then back out the cross feed, then reverse the spindle....all with the feed still engaged. Once you get back to the starting point, re-zero the cross feed and take the next cut.
> Depending on how much "coast" your machine has, you might have to practice rapidly backing off the cross feed the same time you stop the spindle. With that shallow of thread depth, it won't amount to a half of turn on the cross feed.
> Even if you had the correct change gears for metric threads, you would still have to use this method because of the imperial lead screw.




With the right gears and compound gear you would cut threads as you normally would with the half nuts---you just leave the half nuts engaged and reverse the spindle.


----------

