Cutting Metric Threads

[Anorak Bob]

Bob is there any particular reason why you need to thread to a shoulder,or why you udercut maybe narrow.
Just for my understanding could you give a shoulder size that you are threading upto.

Peter,

The photos below are of the dividing head spindle nose and a part of the travel stop for the 13. The later is metric and because I turned the thread by hand I could stop it at a fixed point, no need for a runout groove. The nose thread, well I haven't got big enough goolies to do that under power.

I've been caught out a number of times being frugal with runout grooves. Sometimes the attention wanders and the half nuts aren't opened quick enough. The groove becomes wider, if I'm lucky.

BT

[Pete F]

Very nicely done, as per usual, Bob. I try to taper the end of threads like this, for no good reason other than I just think it looks a wee bit better.

Pete

[pipeclay]

Looking at the threaded areas it possably would of been fine to have an undercut at the end of the thread.

[pipeclay]

Pete,if you use the safety cutout on the Hercus as a feed stop/spindle stop, the spindle will stop under load,you will run the risk of damaging the cutting tool.

[Pete F]

Ok, is that switch a safety cut out only? I haven't got that lathe yet, and don't expect to for some time, so not familiar with it beyond just seeing it there.

Are you talking about damaging HSS or carbide tools? I have stopped HSS bits under load many times and I haven't thought it was doing any damage to them, however am happy to be corrected on this. Carbide however you're quite right, maybe not so much to simply stop the lathe, but when I've turned the chuck backward in a thread I've chipped carbide blades, and in one case totally destroyed one edge of an insert. They definitely don't like that so I treat them with a bit more respect these days. I've actually gone back to HSS for many operations now as it has a number of advantages on these little lathes.

Pete

[pipeclay]

All of the turning tools can be damaged when stopped whilst cutting,a lot depends on the load on the tool,profile of the tool,how quick the spindle stops,etc.

The switch at the rear is only a safety cutout,designed I think more so for training instutions.

[Pete F]

Hmm, ok thanks for that. I must admit it's not something I make a habit of doing, but that's precisely the way I do threads without a runout groove and I haven't noticed any damage. What is the correct way to run a thread to the end without a groove to run into? I just slow the lathe right down until it finally stops exactly where I want it to.

Yes I can see the value of that switch even as a safety feature. My current lathe has he scars of students who drove the tool and various other components of the lathe into the jaws. Many times I work right up close to the jaws and it would be nice to have a cutout set in case I went a poofteenth too far. Not that I ever mistakes amke

Pete

[pipeclay]

If the diameter of the shoulder is reasonably larger than the threaded section and you have a lathe that you cant dissengage the leadscrew you would have to stop the thread a safe distance back from the shoulder.

If you were making a matching female thread you cut a larger diameter at the start of the female thread the length that you stopped short on the male thread,this would allow the two parts to mate flush.

If the shoulder was a similar size to the thread you could just retract the threading tool at or near the same spot each time.

I cant recall ever seeing a thread that runs all the way to a shoulder,if it did it would make it a fracture point that if it could be done,as you would need to allow for the width of the cutting tool.

Some of the better manual lathes do have a threading function where you can set the retraction point and the start point of the thread,when the tool gets to the desired location the tool retracts,returns to start,opperator sets depth and the threading process begins again.

In general its allways good practice to give an undercut if possable.

[cba_melbourne]

.....
Whilst searching for a suitable set of transposing gears for cutting metric on my imperial machine, I read up on my old tech notes & searched many books, only to find that the book I have on the lathe, namely "Text Book of Turning" by P F hercus1990 edition on page 61 simplifies things.
If you happen to have a copy please check it out as I am getting a bit confused.
The chart ( top right hand corner of page, inch to metric) for my imperial model A lathe shows a compound gear of 60 to 63 tooth.
The same applies for the model B & C... Inch to metric chart (bottom right hand corner of page) 60 to 63 tooth.
Now we go to your lathe a metric lathe & we look at the chart in the top left hand corner Model A metric & it denotes a compound gear of 63 to 64 tooth. for metric to inch.
The same applies in the chart in the bottom left hand corner of the same page for model B & C metric to inch the use of a compound gear of 63 to 64.
So in summary there is some discrepancy here with compound gears of 60 to 63 (inch to metric) vs 63 to 64.(metric to inch)
regards
Bruce

Hi Bruce, yes I have the text book that came with my 260. You are right, the original transposing gear set for the 260 lathe is 63/64 teeth, regardless if the leadscrew itself is metric or imperial. However, this is only a close approximation. The mathematically correct gears would be 126/127.

This is so, because 1 inch is exactly 25.4mm. 127 is the lowest exact multiple of 25.4 (5x 25.4 = 127). Any lower multiple is not an exact number. But an exact number is needed to make a gear. A gear 63.5 teeth would work too, but nobody has yet succeeded making such a beast

63/64 gears (and many other "close" combinations) are used for two reasons only: snaller gears are cheaper to make, and they need only a smaller and cheaper to make cover.

How big is the error in using a 63/64 gear combination instead of the correct 126/127 gears? The 64 gear is usually multiplied by 2 (2*64=128). If the 127 gear is 100% correct, then (128*100)/127=100.7874. In other words, the error is 0.7874 or roughly 0.8%. In other words again, if you were to make a 1m long leadscrew of 1mm pitch with the 63/64 gear combination, it would be 8mm off (that is.... 8 full turns off). If on the other side you are making just fasteners, then you will never notice such a small error.

Chris

[Pete F]

I cant recall ever seeing a thread that runs all the way to a shoulder,if it did it would make it a fracture point that if it could be done,as you would need to allow for the width of the cutting tool.

In general its allways good practice to give an undercut if possable.

The spindle nose Bob posted above sure runs close! If I were to cut this (imperial) thread on my new (metric) lathe, a job I almost certainly will have to do now to make various adaptors, I would not be able to release the half nuts. Whether it's the body of the tool (as I guess it would have been in Bob's photo), or the tip of the tool that is the limiting factor, one way or the other one of them needs to stop before hitting the shoulder. I cant really see a way that can be done when using transposing gear on the sort of lathes we're talking about here without stopping the lathe while under load.

Pete

[cba_melbourne]

Bruce, I have seen such a screwcutting stop as an optional accessory for both South Bend and Hercus lathes. But from the pictures alone, I still cannot figure out how it works and how you use it. Is the knurled screw threaded into the dovetailed stop body, or does it actually thread into the cross slide? In other words, does it stop the cross slide when going towards the workpiece, or when retracting it? And can you use it for both internal and external threads? Thank you for explaining, once I know how it works and how it is used, I may consider making one for myself. I have seen other solutions to make threading easier, simple ones like a flip-up toolholder, and quite elaborate ones like retracting toolholders. Chris

[Pete F]

Chris my Textbook of Turning shows the transposing gears as 63/64 for the AM and 60/63 for the imperial A model. You're quite right however, the 63 is an approximation for a 127 which it should be, and both those gears should actually be double the number of teeth.

Yes the threading stop is a little confusing when you first see it, but I believe the dovetail part clamps to the machine's slide. The knurled head bolt screws into the cross-slide but is free to slide within the fixture clamped to the machine's slide. To use it you roughly set the depth by where you clamp the fixture and finely set it by how far into the cross slide you screw the knurled bolt. When as the cross slide advances the bolt slides through the fixture until the head finally hits the fixture and is prevented from sliding further. Since the bolt is attached to the cross-slide, the slide also can't go further. Hopefully that's right, I've never actually seen and that's just how I was going to make mine so I sure hope I'm right

Pete

[cba_melbourne]

Pete,

So 127 is smallest prime number multiplying 25.4.

But where comes the tooth count for the meshing gear from, you are asking?


Let's compare two commonly used leadscrews. One 8TPI, the other 3mm.

The pitch of an 8TPI leadscrew is 25.4mm / 8 = 3.175mm

3.175 / 3 = 127 /120

or easier to see:

40 * 3.175 = 127
40 * 3 = 120


Another common example, one leadscrew 8TPI and the other 2mm

3.175 / 2 = 127 / 80

Another common example, one leadscrew 12 TPI and the other 1.5mm

25.4 / 12 = 2.1166667

2.1166667 / 1.5 = 127 / 90


So the number of teeth in the gear meshing with the 127T gear depends on the pitch ratio between the two leadscrews that we are converting from and to. However this ratio additionally varies with the ratio of any other gears present between spindle and leadscrew. And in practice, every lathe design is slightly different here, that is why there are many different transposing gear combinations out there. The only thing in common is the 127T gear, as this is the one that makes the correct translation between the metric and imperial world.

I hope it makes sense, I am not too good at explaining things the simplest possible way...
Chris

[Anorak Bob]

I have a Hercus cross slide threading stop but I have never used it as such. It has proven invaluable for radial graduation. My understanding of it's purpose is that it provides a stop for the retraction of the slide when threadcutting. My understanding is at odds with the description South Bend have for their stop - "the thread cutting stop is clamped to the saddle cross slide dovetail and is used for regulating the depth of cut for each successive chip when cutting screw threads." I simply advance the cross slide the desired amount. I set the micrometer dial to zero at the commencement of the first cut and at the completion of each cut I retract the slide to about twenty thou before zero and return the cutter to the starting point. Having the stop clamped in position would simplify the process.

Bruce needs to describe how he uses his.

BT

[pipeclay]

Chris can you put a link up to the information that you have found in regards to the 63/60 and 64/63 compound gears.

I am confused as to why the 64/63 would be used on both Metric and Imperial machines.

Could you also link to where the 63/60 compound gear is used.

If this is the case it seems that Hercus has been giving customers a bum steer.