[HELP] 6" Fortis/Clausing lathe

[Seanlogie]

And back gear selected ?

[Mayhem]

Hi Sean,

The pitch of your leadscrew doesn't matter when you are threading or feeding, as the gearbox has been set up with these parameters already.  All you need to know is what pitch thread you wish to cut, or the feed rate that you wish to use.  I'm actually going to dumb it down a little and define both of these.  I apologise if you already know this.

Threading is the process of cutting a thread into the work (either external or internal) using (most commonly) a single point tool.  The tool is ground to give the thread type (60° US or Metric, 55° Whitworth, 29° ACME etc).  Threading is done using the leadscrew, which is the long ACME threaded bar that runs from the gearbox to the end of the lathe bed.  You engage the half nuts to move the tool at the right speed to cut the selected thread.  

Feed is when you use the gearbox to move the tool across the work when turning, boring, facing or parting.  The saves you from having to manually wind the handles and gives a much better finish.  You lathe may use the leadscrew to do this (in which case there will be a slot running the length of the leadscrew) or it may have a separate shaft.  I'm guessing your lathe uses the leadscrew. You will have a different lever or knob on the apron to set the feed and another to set whether the feed is longitudinal (z axis) or transverse (x axis).

There are lots of videos on YouTube that will show this in action and I recommend Tom's Techniques (even though he does talk funny ).

I've taken the liberty of taking your original picture and showing the settings to thread 10tip and 32tpi threads, as well as to set a .005" longitudinal feed rate.  Some of us are visual learners and can read a book until we are blue in the face whilst seeing it once makes absolute sense, so I hope this is helpful.

Setting for 10tpi
   

Setting for 32tpi
   

Setting for .005" feed
   

(12-05-2016, 01:22 PM)Seanlogie Wrote: ...now i feel like a dork ...

Don't - we all had to start somewhere and it is best to ask than jump in and do some damage!

[Seanlogie]

Thanks Mayhem very much appreciated.

[Seanlogie]

Well I gave it a go the other night .

[Dr Stan]

(12-09-2016, 09:54 AM)Seanlogie Wrote: Well I gave it a go the other night .

Did you get the TPI you wanted?

[Seanlogie]

I've read somewhere ,can't remember where that there are some imperial threads that are near as not the same as some metric . Did i imagine reading this or is there some truth in it ?.

Sean

[EdK]

(12-09-2016, 02:21 PM)Seanlogie Wrote: I've read somewhere ,can't remember where that there are some imperial threads that are near as not the same as some metric . Did i imagine reading this or is there some truth in it ?.

Sean

Here are a couple I can think of off the top of my head.

10TPI ~ M2.5
20TPI ~ M1.25

They're not exact but pretty close.

Ed

[Vinny]

This is from the Logan lathe website FAQ. It should provide some insight:

4.1. How can I cut metric threads on a Logan lathe?

4.1.. Logan lathes came with gears for inch threads and an inch leadscrew. The stock Logan lathe can’t cut precisely accurate metric threads without replacing gears.

Some metric threads are close to inch threads. Slight inaccuracy can be accepted in some cases. In those cases, try the nearest inch thread. For example, if you need a metric pitch of 1.0 (1.0 thread per millimeter) you want 25.4 threads per inch. The Logan gearbox can cut 26 threads per inch. That may be accurate enough for a short bolt.

To cut precise metric threads, you need to have a 127 (or multiple of 127) tooth gear in the geartrain. Logan sells a 127 tooth gear for this purpose, but it is expensive. You can cut metric threads that are within 0.02% accuracy using more economical 37 and 47 tooth gears. These are also available from Logan and much more economical.

Here's a chart comparing TPI to Metric thread pitches:
http://www.newmantools.com/tech/pitchconversions.htm

[Mayhem]

The problem with trying to get a metric/UN fit is that the you need to ensure that the threads are short. As Ed pointed out, 20tpi and 1.25 pitch are extremely close and one of the short thread gauges will fit both nicely. The only way to distinguish between the two is with a ruler. You need to measure the crest over a one inch length. Only the 20tpi thread will have a crest that lines up with the zero and the one.

If you are making mating parts then I wouldn't worry about trying to make a metric thread. Just pick a thread that works for your application (i.e. coarse or fine) and have at it. If you are making something to fit existing parts, then that is where you will have to take Vinny's advice. Of course, you can make (or get made locally) any gear you need. You don't have to send off to the U.S. for one.

The thread you cut looks very nice although somewhat shallow. Measure the number of crests over one inch and check that number corresponds to the settings on your gearbox.

[Vinny]

I don't recommend buying gears from Logan. If it's not in ready stock, you'll wait a month or more. May as well buy from China and have it shipped to the US. Actually that might be quicker! They're expensive too. In the US, I've been using American Metric (ametric.com) for gears. Inexpensive, they accept paypal, and they're fast.