Cutting Metric Threads

[Abratool]

Spent this morning machining up a number of small components on the Hercus 260 model A Imperial lathe. The parts included some single point screwcutting of 14 mm OD by 1.25 mm Pitch threads.
Now 1.25mm times .03937 = .049 inches
The closest on my Imperial lathe is 20 threads per inch = .050 inches.
Not good enough !
So I while back I purchased from "Pipeclay" a full set of transposing gears for cuting metric threads on my imperial lathe.
These gears have been really useful, thank you Pipeclay, they work very well.
A simple set up of a 36 tooth stud gear driving a 60 tooth then 63 tooth compound gear driving a 40 tooth screw gear with the quick change box on the lathe set at 36 TPI & presto all done.
The use of a thread micrometer I purchased from Tools 4 Cheap also facilitates the whole job, & makes things easy as screwcutting progesses.It takes the guess work out of the job.
I have been cutting threads on lathes for 56 yrs since my first at 13 yrs of age.
So there has been a lot of learning, but must say enjoying cutting metal more that ever.
regards
Bruce
ps Please lets know if anyone on the forum uses a Thread Micrometer & any hints.

[pipeclay]

Bruce I have used them in the past for large threads or threads where there might not be a sample nut,they are also very handy when working to a set tollerance.

As too tips on use I can only think to say make sure the anvils are positioned correctly on the vees,with fine threads it can take a little to make sure they are centred on the crest and valley and not one thread off.

[Abratool]

Bruce I have used them in the past for large threads or threads where there might not be a sample nut,they are also very handy when working to a set tollerance.

As too tips on use I can only think to say make sure the anvils are positioned correctly on the vees,with fine threads it can take a little to make sure they are centred on the crest and valley and not one thread off.

Thanks Pipeclay
I have also found it a bit tricky to get the vee anvil located over the thread crest & the other anvil in the thread valley & for me anyway, requires the use of a magnifier. The Thread Micrometer is very handy where the nut is not available, & its got to be correct, when it comes off the lathe, as you outlined.
Anyway its all enjoyable work.
regards
Bruce

[pipeclay]

Just make sure that you have the correct anvils in for the thread.

[cba_melbourne]

Bruce, there are two sets of transposing gears for Hercus. 60/63 and 120/127 teeth.

The former are smaller diameter and fit well under the cover. But the conversion is not perfect, there is a small error. It will not matter if you cut fastener threads. But it is not suitable to cut a leadscrew, as the error is incremental and adds up to a massive error over a leadscrew length..

The latter is a mathematically perfect conversion, but the gears are rather large and may prevent you from closing the lid.

I have a metric 260 lathe. 95% of any threads that I cut are metric. The remaining 5% are only short fasteners where a small error is irrelevant. Maily NPT air fittings. That is why I only bought the 60/63T transposing gears. If I ever wanted to cut imperial leadscrews, I would have to buy or make a set of 120/127 gears



I do not own a dedicated thread micrometer. A thread micrometer is helpful if you need to make threads to a specific tolerance (but if you need them that accurate, you also want the 120/127 gear set to get the pitch as accurate too) . It is possible to use a normal micrometer instead of an expensive thread micrometer, with three special gauge wires of precise diameter, but it is rather fiddly. Threrad gauges are much faster to use, but you need two (male/female) for each pitch. I personally mostly fit outside threads to an existing nut, and vise versa.If I have taps /dies of the required diameter available, I do use them to impart the perfect final shape and dimension to screwcut threads. That is a lot quicker and cheaper than to use a thread micrometer. It also imparts the proper thread profile, without having to keep several thread cutting tools with different tip radiusses to suit the pitch. And it cleans up rough cuts (cat's tongue finish) that certain non-freecutting materials leave behind when screw cut no matter if one uses lard or oil or whatever to lube. Another method is to use a tap in a fashion like a chaser to get the profile of a male thread right.

I think if you fit threads together for one off jobs (like a steam engine), you do not need a thread micrometer - but if you are making parts that must be interchangeable or conform to a tolerance standard (like car spare parts) then it is another story. Chris

[pipeclay]

Chris not sure if you made a typing mistake,or have incorrect information,but the 2 gearsets that you mention will not interchange and work.
They probably will both fit on the Banjo (not sure if the End cover will close and allow the safety interlock to function) but they will not be suitable if interchanged.

[Abratool]

Thanks for your reply Cba.It is most appreciated.
It was interesting to read also your post of 5th July [email protected].
I read with interest, your info, when I was in the throws of getting into cutting metric threads on my imperial lathe.
You are obviously comfortable measuring & working in metric, but for me I have always done small measurements in imperial & when people start talking in metric I am always converting to imperial as it more relative to me.
Anyway getting back to the change gear story.......
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

[gherringh]

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,

The difference arises from the difference in leadscrew pitch between the Imperial and Metric lathes.

An imperial lathe has an 8 tpi leadscrew (3.175mm pitch) and a metric lathe has a 3mm pitch so the difference in leadscrew pitches is taken care of in the conversion gear ratios.

You will also note a similar discrepancy with the matching gear to the 127 tooth gear. (100/127 Imperial, 120/127 Metric)

--
David

[Anorak Bob]

Hello Bruce,

A question regarding technique. Given that the half nuts are not disengaged when using the transposing gears, how would/do you go about threading to a shoulder or even a narrow runout groove? My method involves manual labour. I turn the spindle by hand.

BT

[Abratool]

Hi,

The difference arises from the difference in leadscrew pitch between the Imperial and Metric lathes.

An imperial lathe has an 8 tpi leadscrew (3.175mm pitch) and a metric lathe has a 3mm pitch so the difference in leadscrew pitches is taken care of in the conversion gear ratios.

You will also note a similar discrepancy with the matching gear to the 127 tooth gear. (100/127 Imperial, 120/127 Metric)

--
David

Thanks David
That explanation clarifies things, well done.
regards
Bruce

[pipeclay]

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.

[Abratool]

Hello Bruce,

A question regarding technique. Given that the half nuts are not disengaged when using the transposing gears, how would/do you go about threading to a shoulder or even a narrow runout groove? My method involves manual labour. I turn the spindle by hand.

BT

Bob
With the half nuts engaged through the whole process, does require some more than normal concentration.
With the lathe running in back gear on the lowest speed 50 rpm, and as the cutting tool gets near the end of the cut, near the shoulder or undercut groove, have the left hand on the motor reversing switch & the right hand on the cross slide handle.
In one swift movement of both hands similtaneously the lathe is reversed & the cutting tool retracted. I should mention that one has to hold the "mouth" the right way to avoid a catastrophic mess. The slightest loss of concentration & its over & out. I screw cut a coarse metric thread on a friends woodworking lathe headstock spindle recently & like youself manually fed the tool by hand rotating the countershaft so the tool finished up near the shoulder. This was important to ensure the chuck of his lathe mated tightly with the shoulder on the spindle.
The recent job of screwcutting were a number of Chain Saw "Piston Stops" these are used to screw into the spark plug tapped hole to act as a stop on the piston so that the chainsaw clutch can be unscrewed.
In this instance I used an undercut groove & all worked well, under power.
A photo is attached, the threads are 14mm 60 degree metric by 1.25mm pitch.
Hey I am starting to think more metric.
regards
Bruce
ps A photo of the recently made screwcuting stop on the cross slide is shown also. It makes things alot easier.

[Pete F]

Hello Bruce,

A question regarding technique. Given that the half nuts are not disengaged when using the transposing gears, how would/do you go about threading to a shoulder or even a narrow runout groove? My method involves manual labour. I turn the spindle by hand.

BT

Bob, when in that situation I slow the lathe right down on the VFD and also use backgear. I can sneak up on the end of a thread quite well if I don't have a runout groove, yet the backgear still provides plenty of torque.

I really like that stop Bruce. My workshop time is very restricted right now but I will be moving this project up the priority list. It will be useful not only for threading but many other tasks as well.

Pete

[Abratool]

Bob, when in that situation I slow the lathe right down on the VFD and also use backgear. I can sneak up on the end of a thread quite well if I don't have a runout groove, yet the backgear still provides plenty of torque.

I really like that stop Bruce. My workshop time is very restricted right now but I will be moving this project up the priority list. It will be useful not only for threading but many other tasks as well.

Pete

Pete
Thanks, I have made a few gadgets for my lathe, but this stop, is one that has proved its worth. When we get around to the casehardening job I would like to caseharden the screw in this stop.
Regarding the subject of casehardening I noticed on page 87 of the Hercus "Text Book of Turning" there is a good explanation of the process.
Bruce

[Pete F]

Bob, on further thought, the 260 I (hopefully, I'll believe it when it's in my workshop) bought has an adjustable motor cutout switch. I don't know if they are standard on 260s or an option(???) I don't think it would be at all difficult to rig up a microswitch to similarly cut the power to the motor at a pre-determined point. If using backgear and turning relatively slowly anyway, I would think the lathe would coast to a stop to a relatively consistent position each time.

Pete