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A different way of doing metric threads on an inch lathe?

CalG

Diamond
Joined
Dec 30, 2008
Location
Vt USA
In a lot of ways, sure.

Levers differ from gears with how a lever doesn't make a full rotation and the ratio of a lever can be easily adjusted- No fixed center distance to deal with.

You could do the same thing as the lever arm with compound gears moving a rack attached to the toolholder, but then you'd have to change gears for every pitch.

I think the lever on the bed is a simple solution to the problem and it could be superior to changing gearing as the half nut function is retained.

To prototype the concept I think I will replace the toolpost with a short piece of linear guide rail fit to the compound T-slot and a threading tool mounted to the guide truck.

In counter, a "lever system will not accommodate a long thread, such as a lead screw.

Such is life.
 

EPAIII

Diamond
Joined
Nov 23, 2003
Location
Beaumont, TX, USA
As I have said, I have played with the problem and the math. There is just no way of getting around the need to synchronize the movement of your lever arm or whatever, WITH THE ROTATION OF THE SPINDLE.

When you disengage the half nuts, your lever, with it's "mechanical linkage" may still be engaged to something, but the spindle will be going round and round. And you are not counting those revolutions or making any account for them. So, when you engage the half nuts again, your lever may start at a given point with reference to (with a linear distance to) some feature of the lathe, like the carriage or the headstock casting. But unless you engage the half nuts at the proper point, it is going to start cutting the new pass at a more or less random point around the work piece. And that means that you will, in all probability, not be synchronized. You will have better odds at a blackjack table in Vegas.

Lets look at it another way, suggested by your "...locked to the carriage motion..." statement below. I am envisioning a tool tip that is moving along the same path relative to the lead screw each time the half nuts are engaged. In other words, along the ways of the lathe or in it's distance from the head stock casting (which is firmly fixed). Lets use some real numbers:

Lead screw: 10 TPI
Engagement of half nuts: on any 1/10"
Thread being cut: 2.5 mm (only slightly different from 10 TPI)
Difference: 0.04mm
The gearing between spindle and lead screw is 1::1 as it would be for a 10 TPI thread.

So you engage the half nuts and your tool's tip starts at, say 100mm from the face of the head stock casting (a machined flat there). And you keep the half nuts engaged for a travel of 1" or 10 turns. But your linkage moves the tip of the tool for 25mm or 0.04mm LESS than the half nuts travel. It is now at 125mm from that machined flat on the head stock casting. This will give you a metric pitch of 2.5mm. So far, so good.

But now you disengage the half nuts and move the carriage back to the starting point where the tool's tip is 100mm from the head stock. The spindle continues to turn by an unknown number of revolutions. You make no account for this. You are ready for the next pass on your thread. You are not accounting for spindle revolutions so lets say that you engage the half nuts after 128 spindle revolutions. In this case, the lead screw has also made that same number of revolutions, 128. The tool's tip will start cutting at the exact same point from the head stock as before. No problem there. BUT your spindle has rotated 128 rotations. AND the thread that you are cutting, which also has been rotated by 128 rotations, is METRIC, so it will not be in the same rotational position as it was when the first cut was made. It will be 1/127 of a rotation past the point where it would have been in that same rotational position. It will be off by about 1/127 mm.

While your tool's tip traces exactly the same path in terms of the distance from the head stock, but the work piece will not be synchronized to that path and you will be cutting a second path down it.

In short, you will not be able to open and reclose the half nuts unless you have a method of accounting for the number of spindle rotations. Any even multiple of 127 will work and other numbers may also work, depending on the exact English and metric pitches and gear ratios used.

I specified closing the half nuts on any 1/10" division, but using the 1, 2, or even the 4 inch marks on a standard English threading dial would be no better because those distances of 1", 2", or even 4" are all not an even number of mm. (1" = 25.4mm, 2" = 50.8mm and 4" = 101.6mm). You would need a point where both the English and metric measure come out to whole numbers.

It would be a lot easier to discuss this if you would post a drawing or sketch of what you are proposing. If you plan to actually build this, I am only trying to save you a lot of wasted effort. It CAN work. It is just that you will need to keep the half nuts closed and back up or somehow account for the number of spindle revolutions before the half nuts are reclosed.



I think you're implying the pantograph idea won't work with the half nut? It's a good video, but I didn't catch anything relevant to this discussion in there.

Think about it like this-

With this, lets call it "Pitch offset device" you have an offset distance that is linear and locked to the carriage motion. Has zilch to do with any rotating gearing in the machine.

If you disengaged the half nut and ran back to the start you haven't lost your place. The mechanical linkage is what is keeping your timing. Anywhere you lock that half nut onto the leadscrew it is going to be 100 percent in time with the spindle to cut the inch pitch that it's set for.

In this way, YES, you can use the half nut for metric threading because instead of the metric pitch you are cutting being some prime number ratio division of the leadscrew it is a perfectly even leadscrew division PLUS an offset percentage. The offset percent is ALWAYS connected. The connection is never broken. The linkage maintains the timing that leaving the half nut engaged provides for cutting a metric pitch with gearing.

Keep in mind here we are talking pretty small numbers. The difference between 12.7 and 13 TPI is .00181" per thread. That's about 47 thousandths over 2" of thread length.

So you would in effect be cutting a 13 TPI thread with a linear 2.3 offset percentage.
 

Garwood

Diamond
Joined
Oct 10, 2009
Location
Oregon
But now you disengage the half nuts and move the carriage back to the starting point where the tool's tip is 100mm from the head stock. The spindle continues to turn by an unknown number of revolutions. You make no account for this. You are ready for the next pass on your thread. You are not accounting for spindle revolutions so lets say that you engage the half nuts after 128 spindle revolutions. In this case, the lead screw has also made that same number of revolutions, 128. The tool's tip will start cutting at the exact same point from the head stock as before. No problem there. BUT your spindle has rotated 128 rotations. AND the thread that you are cutting, which also has been rotated by 128 rotations, is METRIC, so it will not be in the same rotational position as it was when the first cut was made. It will be 1/127 of a rotation past the point where it would have been in that same rotational position. It will be off by about 1/127 mm.

Princess Bride - "Truly, you have a dizzying intellect." - YouTube

Read what you wrote. Think on that for a minute.

What if the thread is inch or metric? The starting point of the threads doesn't change. It's still in the same spot in relation to the spindle.

If you open the half nut and reclose it in a different place the lathe still cuts the same exact inch pitch it did whether the spindle and leadscrew turned 128 or 128,000 turns.

Here's a visual for you-

If you cut a metric thread using the mechanism I proposed, then you disabled the mechanism, locked the threading tool and cut another thread right over the top of the metric thread you just cut, what would the shaft look like?

ANSWER: It would start out looking pretty good for the first few threads then you would see the separate pitches divest from one another. If you could measure the difference in pitch from inch to metric at any point along the cut thread that value would be the same as the ratio of the lever arm applied from where the two pitches were in time.
 

EPAIII

Diamond
Joined
Nov 23, 2003
Location
Beaumont, TX, USA
Well, knock yourself out. But don't say I didn't warn you. And you still have not provided even a pencil sketch so I may be barking up the wrong tree.

Not sure I understand your Princess Bride reference.
 

FredC

Titanium
Joined
Oct 29, 2010
Location
Dewees Texas
The Home Shop Machinist magazine (gasp!) had an article about a device like this a couple of years ago. By using an angled bar, like on a taper attachment, you end up multiplying the leadscrew speed by the cosine of the bar angle, or something like that.

I was cleaning up my desk this morning, did not get very far thew away a few old catalogs and found the HSM article. It starts with the Nov/Dec 2017 magazine. Not sure how many articles. He does show some wood mockups. Cover photo looks interesting and it might be worth researching if you have the need. I have metric change gears for both manual lathes and I have 3 CNC lathes so it was not something I would pursue, others may look just to see his approach.
 
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john.k

Diamond
Joined
Dec 21, 2012
Location
Brisbane Qld Australia
Using a taper attachment set for the calculated angle,the tailstock is offset a suitable amount,so that the tool follows a path parallel to the axis of the work........this method was commonly used to counteract hardening shrinkage.
 

john.k

Diamond
Joined
Dec 21, 2012
Location
Brisbane Qld Australia
Incidentally ,cutting ISO metric is childs play compared to BA (which is metric)..........Ive seen a massive list of calculations to cut the full range of BA (British Association) threads on inch lathes ......the progression of BA threads is such that the largest "0BA" is 1mm pitch ,the next smallest "1BA" is .9mm pitch,the next smallest again (the common 2BA) is .81mm pitch....and so on.....below 4mm dia ,metric threads are inadequate,and the BA range is far superior.
 

gustafson

Diamond
Joined
Sep 4, 2002
Location
People's Republic
I always thought the Hardinge overarm threading deal for the chucker was the hot setup
Seems it would be straightforward to devise one that allowed any metric bolt and its associated nut to be loaded and off you go.

Think I would threadmill in the cnc first tho
 








 
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