Educate Me On Thread Dials

I have an old Shepard Lathe that I am working on. It did not come with a thread dial and never had one. I found a gear from a treadmill height adjustment that meshes with the lead screw perfectly. I intend to use it to make the dial. Fabbing it up should not be an issue.
The problem I have will be setting the graduation marks on the dial. The gear has 10 teeth. Every dial I've used is divisible by 4. I have no idea how I would go about setting this up.
What procedure should I use to mark off the dial to ensure it is accurate.

Once you get it set up on the carriage, engage the half nut and make witness marks on the dial and pointer with a sharpie. Then start cutting threads on a piece of scrap and see what happens at different points. You will probably find the lines will be across from each other. Don't be afraid to experiment.

The number of marks on the thread dial depends upon the lead screw pitch and the number of teeth on the engaging gear. My 6" Atlas has a pitch of 16tpi on the lead screw, 32 teeth on the gear, and 4 marks. My Grizzly G0602 has a pitch of 12 tpi, 48 teeth on the gear, and 12 marks on the dial.

Thread dials are generally set up so that engaging on the same mark will place the cutter in the same position on the thread being cut for all threads that are offered on the lathe. The Grizzly will cut all threads divisible by 3 when engaged at any mark on the dial, any integral number of tpi at positions 1, 4, 7, or 10 (or 2, 5, 8, or 11 or 3, 6, 9, 12) and any odd multiple of 1/2 tpi at positions 1 or 7 (2, or 8 etc.).

I suspect that you will want a tooth count greater than 10. It is late and the brain won't engage enough to figure out the relationship at the moment but I also suspect that your gear has to have a tooth count that is a multiple of your lead screw pitch.

I'm a newbie, so take what I say with a grain of salt. The numbered marks on the dial should correspond with the travel of the carriage in inches with the half nuts disengaged. You could use a dial indicator to measure the carriage travel and then mark the dial accordingly. The tooth count of the gear will need to be a multiple of the TPI of the lead screw.

This video is very helpful:

A good explanation but it doesn't hold for My Grizzly lathe. The Grizzly moves 4" for a full rotation of the dial or six numbered positions. This works out to 2/3" travel for each numbered position.

Which model Grizzly, RJ?

planeflyer21 said:

Which model Grizzly, RJ?

Click to expand...

G0602

my lathe has a dial that looks like the dial in the video, only it has a single mark. I had assumed it was missing a glued on plate with more markings like other lathes [I've looked through the manuals for them], but it MIGHT just have one.

When I actually get working with it, I will likely stick with just reversing the lathe, rather than "if it's this kind of thread, I can do it this way, but all the rest I have to do it this other way".

Pics of the dial and lathe here:

http://www.hobby-machinist.com/threads/info-about-force-international-ml1440gh-1-lathe.42874/

The dial tells you one thing, at which point around the circumference of the lead screw the 1/2 nuts are engaged.

Therefore the marks would be dependent on the lead of said screw plus the gearing, as an example threading an 8 TPI thread on a lathe with an 8 TPI lead screw anywhere on the screw will work, any multiple of 8 will not need a thread dial such as 16,24, 32 TPI.

With the gearing between the spindle and screw creating leads that are not evenly divided by the lead screw pitch may require a divided dial.

If you are a hobbyist that does not envision ever manually threading 50 parts per day for days on end then put 1 reference mark and use it every time, in this way you can not go wrong.

The thread dial will repeat every (teeth) leadscrew pitches, so yours is likely only to work for multiples of 2&1/2 tpi for a typical 4 tpi leadscrew, 1&1/4 for 8 tpi, not a great deal of use... What pitch is your leadscrew?

Assuming an 8tpi leadscrew, a 16 tooth will repeat every two inches of travel, so will work for multiples of 1/2 a tpi (useful for pipe threads etc.) if you always select the same line on the dial, 1 tpi for either of the opposite lines, 2 tpi for any of the 4 points of the compass, 4 tpi if you add in NE, NW etc. for 8 lines - which should prove enough!

If you have an oddball leadscrew (mine's 6 tpi) the gear wants to have enough teeth for a whole rotation to be a sensible number of inches travelled - e.g. mine has a 24 tooth gear which allows for 1/4-tpi pitches. The disadvantage is that you can wait quite a while for the mark to come around on some (very odd, 4&3/4 tpi f'rinstance) pitches!

My lathe's handbook recommends leaving the half-nuts engaged and reversing the motor to return the carriage on all but the longest of threads, that has the advantage that metric threads on an Imperial lathe are simple (and vice-versa), it helps if you can reverse at a higher speed though.

For metric threading, it's necessary to have multiple gears on the dial to get one that works for the different pitches - a lot more complicated than Proper Imperial threading!

Dave H. (the other one)