Who Needs A Thread Chasing Dial?

[Mad Monty]

The DRO reports only the linear position of each axis. The gearing in the headstock, and hence the rotational position of the chuck, is not correlated.

In the world of CNC, rotary encoders are employed to do that, on lathes and milling machines with rotary axes.

I get it now, thanks. I overlooked that the chuck and lead screw lose their correlation if you disengage to traverse rapidly.

Your comment on rotary encoders has me wondering this: if you marked the circumference of the chuck in small enough units (say, either 100 or 360 divisions to keep it simple), then between the DRO and the marks, couldn't you hit the thread pretty accurately?

 

[Tony Wells]

Well, using the phrase "pretty accurately" shouldn't apply in the world of gear engagement. It either is in the proper gear, or not at all. The spindle is geared to the feed mechanism through either the quick change geabox, or the change gears in a fixed ratio. This is independent of the spindle/motor drive gears in every lathe I have run. All that means is you can change spindle speeds without disturbing the relationship between the leadscrew and the spindle. I remember when I was learning, I thought that I couldn't change spindle speeds because I would lose my thread lead, but that is not so.

If you did index mark your chuck or faceplate, you would know the rotational position of the spindle, and since there is a fixed relationship with the leadscrew, I suppose you could figure out a way to know when to engage the half-nuts. I'll have to give that some thought to see if there is a catch. I would imagine it would be possible to spot the index mark on the chuck to your reference in the wrong place for some threads, but you shouldn't be able to drop in the half-nuts. Not really sure. I'll think about it, and maybe someone else will chime in also.

 

[John Hasler]

Well, using the phrase "pretty accurately" shouldn't apply in the world of gear engagement. It either is in the proper gear, or not at all. The spindle is geared to the feed mechanism through either the quick change geabox, or the change gears in a fixed ratio. This is independent of the spindle/motor drive gears in every lathe I have run. All that means is you can change spindle speeds without disturbing the relationship between the leadscrew and the spindle. I remember when I was learning, I thought that I couldn't change spindle speeds because I would lose my thread lead, but that is not so.

If you did index mark your chuck or faceplate, you would know the rotational position of the spindle, and since there is a fixed relationship with the leadscrew, I suppose you could figure out a way to know when to engage the half-nuts. I'll have to give that some thought to see if there is a catch. I would imagine it would be possible to spot the index mark on the chuck to your reference in the wrong place for some threads, but you shouldn't be able to drop in the half-nuts. Not really sure. I'll think about it, and maybe someone else will chime in also.

That's what the circuit I described upthread does.

 

[Tony Wells]

Right, John. But as I understand Monty, he only wants to out a degree wheel on the chuck and index from that.

 

[John Hasler]

Right, John. But as I understand Monty, he only wants to out a degree wheel on the chuck and index from that.

A degree wheel and a DRO would tell him when his tool is exactly lined up with the thread. However, that is not sufficient. It is also necessary to know that the angular position of the leadscrew is such that the halfnuts will drop right in when you engage them. Thus he would need a degree wheel on the leadscrew as well. That's pretty much what a threading dial is.

But as the leadscrew and therefor the threading dial are synchonized with the spindle the degree wheel on the spindle becomes redundant and all the DRO does is lets you restart at an exact place, which you could also do with a carriage stop.

 

[Mad Monty]

Right, John. But as I understand Monty, he only wants to out a degree wheel on the chuck and index from that.

That's right, but I hadn't realized that John's scheme was that closely related. I'll re-read it. Thanks both.

 

[Tony Wells]

That is correct. There could be positions of rotation of the spindle that would not synch with the leadscrew using only the degree wheel on the chuck. That was what I was trying to express.

 

[Mad Monty]

Well, using the phrase "pretty accurately" shouldn't apply in the world of gear engagement. It either is in the proper gear, or not at all. The spindle is geared to the feed mechanism through either the quick change geabox, or the change gears in a fixed ratio. This is independent of the spindle/motor drive gears in every lathe I have run. All that means is you can change spindle speeds without disturbing the relationship between the leadscrew and the spindle. I remember when I was learning, I thought that I couldn't change spindle speeds because I would lose my thread lead, but that is not so.

If you did index mark your chuck or faceplate, you would know the rotational position of the spindle, and since there is a fixed relationship with the leadscrew, I suppose you could figure out a way to know when to engage the half-nuts. I'll have to give that some thought to see if there is a catch. I would imagine it would be possible to spot the index mark on the chuck to your reference in the wrong place for some threads, but you shouldn't be able to drop in the half-nuts. Not really sure. I'll think about it, and maybe someone else will chime in also.

Tony, thanks for the explanation. I'm with you on change gears keeping leadscrew and spindle in lockstep. As for index mark, here's my thinking: if you know the position of the carriage from the DRO, and the angle of rotation of the spindle, haven't you pinned down the spot on the work you want to hit repeatedly on each pass? "Pretty accurately" to me is close enough for practical purposes, like the half-nuts should engage easily.

Oops, while I was drafting this, Tony and John, you have been kindly offering more info. Please disregard this post while I catch up. Thanks both for your thoughtful comments.

 

[JR49]

I watched a u tube video at least a year ago, where the guy showed how to cut threads without a dial, by putting marks on the chuck and (I think) the bed. He cut perfect threads, so it is doable. Unfortunately, my old brain has no idea which u tube video it was, but it certainly would be worth a few minutes searching for it if you don't have a thread chasing dial. Happy machining, JR49

 

[Wreck™Wreck]

If one has many parts to thread the dial is much faster. I did a manual threading job last year, 2 1/2-4 thread 7" long each part took 20 or more passes X 30 parts or 600 + passes, running the carriage back on the lead screw would get old in a hurry.