Gears, Fears & Tears... if I don't get it right!

[Ian Bee]

Okay... I have owned an Australian Hercus 9A (copy of a Southbend) lathe for maybe twenty years now.

At first, it only ever got a little rudimentary use, but as time has passed, it plays a larger role in my work.

Great little lathe, and when I bought it all those years ago, it came with every conceivable Hercus accessory you would ever need, including a STACK of change gears.

Now, here's the thing... I am wanting to learn threading, and I have ALL of the equipment, and a little knowledge, so it should be fun, but... I am still not sure if the gear train I am running (which I have NEVER changed), is in actual fact, the set factory correct gear train.

I have spoken to Hercus here in Adelaide, South Australia, but they are of no help, and don't really want to help.
I have scoured the net, and believe I have found what is purported to be the STANDARD gear train for the Southbend, which is as follows...

The typical set-up for the quick-change gearbox is
20 tooth stud gear,
80 tooth idler gear,
56 tooth gear on the input shaft of the gearbox,
40 tooth stud gear used for very course threads
(This gear is used in place of the 20 tooth gear and stored as a spacer on the input shaft of the gearbox when not in use).

This makes sense, sort of, but WHY, do I have all of these OTHER gears, as well as a 127 tooth for metric?

Can someone perhaps demystify the correct FACTORY set-up, for the imperial lathe I have, as well as somehow, explaining to me WHICH gears I need to change, to cut metric threads, and how?

Even a picture would help.

I have the second edition copy of the Hercus manual, but it doesn't really elaborate that much I'm afraid...

I think I'm missing something here, or over analysing.

Cheers

Ian

 

[Ebel440]

You may want to try and get a copy of the book How to Run a Lathe by southbend it explains a lot i may be off on the title a little bit but its always recommended. Also you have many gears to cut different thread pitches it uses a different gear ratio for different threads the gears will be changed for each pitch.

 

[SE18]

I have a 9A and all the gears are inside the gearbox, not outside. Maybe the Herc is different. I only have one spare gear used for course threading

 

[Ebel440]

Sorry didnt see you had a quick change gear box i dont know how that works exactly with the different gears then

 

[Bill C.]

Okay... I have owned an Australian Hercus 9A (copy of a Southbend) lathe for maybe twenty years now.

At first, it only ever got a little rudimentary use, but as time has passed, it plays a larger role in my work.

Great little lathe, and when I bought it all those years ago, it came with every conceivable Hercus accessory you would ever need, including a STACK of change gears.

Now, here's the thing... I am wanting to learn threading, and I have ALL of the equipment, and a little knowledge, so it should be fun, but... I am still not sure if the gear train I am running (which I have NEVER changed), is in actual fact, the set factory correct gear train.

I have spoken to Hercus here in Adelaide, South Australia, but they are of no help, and don't really want to help.
I have scoured the net, and believe I have found what is purported to be the STANDARD gear train for the Southbend, which is as follows...

The typical set-up for the quick-change gearbox is
20 tooth stud gear,
80 tooth idler gear,
56 tooth gear on the input shaft of the gearbox,
40 tooth stud gear used for very course threads
(This gear is used in place of the 20 tooth gear and stored as a spacer on the input shaft of the gearbox when not in use).

This makes sense, sort of, but WHY, do I have all of these OTHER gears, as well as a 127 tooth for metric?

Can someone perhaps demystify the correct FACTORY set-up, for the imperial lathe I have, as well as somehow, explaining to me WHICH gears I need to change, to cut metric threads, and how?

Even a picture would help.

I have the second edition copy of the Hercus manual, but it doesn't really elaborate that much I'm afraid...

I think I'm missing something here, or over analysing.

Cheers

Ian

You only need to change two gears to achieve the correct gear ratio. I know you change the one on the lead screw and the other is on the drive shaft. The others are called idler(s) gears that transfer the power between the other two gears. (It has been over 40 years since high-school shop since I done it.) Be sure to leave a little play, very little gap, between the gears so oil can lubricate their teeth. Put a couple drops of oil on the main gear so the oil will spread to the other gear teeth. Be sure to close the cover door after oiling the gears because oil will sling off the gear teeth once you start the lathe. Good luck.

 

[Redirish]

Okay... I have owned an Australian Hercus 9A (copy of a Southbend) lathe for maybe twenty years now.

At first, it only ever got a little rudimentary use, but as time has passed, it plays a larger role in my work.

Great little lathe, and when I bought it all those years ago, it came with every conceivable Hercus accessory you would ever need, including a STACK of change gears.

Now, here's the thing... I am wanting to learn threading, and I have ALL of the equipment, and a little knowledge, so it should be fun, but... I am still not sure if the gear train I am running (which I have NEVER changed), is in actual fact, the set factory correct gear train.

I have spoken to Hercus here in Adelaide, South Australia, but they are of no help, and don't really want to help.
I have scoured the net, and believe I have found what is purported to be the STANDARD gear train for the Southbend, which is as follows...

The typical set-up for the quick-change gearbox is
20 tooth stud gear,
80 tooth idler gear,
56 tooth gear on the input shaft of the gearbox,
40 tooth stud gear used for very course threads
(This gear is used in place of the 20 tooth gear and stored as a spacer on the input shaft of the gearbox when not in use).

This makes sense, sort of, but WHY, do I have all of these OTHER gears, as well as a 127 tooth for metric?

Can someone perhaps demystify the correct FACTORY set-up, for the imperial lathe I have, as well as somehow, explaining to me WHICH gears I need to change, to cut metric threads, and how?

Even a picture would help.

I have the second edition copy of the Hercus manual, but it doesn't really elaborate that much I'm afraid...

I think I'm missing something here, or over analysing.

Cheers

Ian

The other gears you have are used with the 127 tooth gear to cut metric threads. You can find the proper chart for that on ebay for $25.

 

[Hawkeye]

Check page 60 in the Hercus manual.

View attachment Hercus Lathe Manual.pdf

 

[12bolts]

Ian,
The guy at Hercus who used to be quite knowledgable and helpful is unfortunately no longer there and as they are not in the business of making/selling lathes anymore they have no interest.
I do not deserve any credit for the following.
"The most common lead screw is 8 TPI, a pitch of 0.125". That's 3.175 mm, not a terribly convenient number from which to derive the standard metric pitches. The purpose of metric transposing gears is simply to scale that pitch into a more useful value that can be divided or multiplied, using common gear ratios, to get the standard metric pitches.

The traditional pair of transposing gears have 127 and 100 teeth. The lead screw pitch, 3.175 mm, divided by the ratio of those two gears, 1.27, gives us 2.5 mm, a much easier number to further divide or multiply into standard metric pitches. The important thing to remember is that using metric transposing gears allows you to treat your inch lead screw as a metric lead screw. The rest of the gear train simply multiplies or divides that new pitch value to get to the desired pitch. If you think of the transposing gears and lead screw together it avoids having to include the specifics of the conversion in every gear calculation.

A simple example will reinforce the concept. You've installed the metric transposing gears and now think in terms of having a 2.5 mm pitch lead screw. You want to cut a 1.0 mm pitch thread so you need some gears in the path between the transposing gears and the lead screw (or spindle, depending on where the transposing gears are installed) with a ratio of 1:2.5. A 16 and 40 tooth pair would do the job (as would various others having the same ratio). If you have a lathe with a quick change gearbox, the same rules apply. You still need an overall 1:2.5 ratio, using some combination of external gears and the ratios available within the gear box. I'll cover QC gearboxes in more detail later, but for the moment we'll assume a simple change gear machine with no gear box."

Cheers Phil

 

[Ian Bee]

Phil, thank you!

Ian,
The guy at Hercus who used to be quite knowledgable and helpful is unfortunately no longer there and as they are not in the business of making/selling lathes anymore they have no interest.
I do not deserve any credit for the following.
"The most common lead screw is 8 TPI, a pitch of 0.125". That's 3.175 mm, not a terribly convenient number from which to derive the standard metric pitches. The purpose of metric transposing gears is simply to scale that pitch into a more useful value that can be divided or multiplied, using common gear ratios, to get the standard metric pitches.

The traditional pair of transposing gears have 127 and 100 teeth. The lead screw pitch, 3.175 mm, divided by the ratio of those two gears, 1.27, gives us 2.5 mm, a much easier number to further divide or multiply into standard metric pitches. The important thing to remember is that using metric transposing gears allows you to treat your inch lead screw as a metric lead screw. The rest of the gear train simply multiplies or divides that new pitch value to get to the desired pitch. If you think of the transposing gears and lead screw together it avoids having to include the specifics of the conversion in every gear calculation.

A simple example will reinforce the concept. You've installed the metric transposing gears and now think in terms of having a 2.5 mm pitch lead screw. You want to cut a 1.0 mm pitch thread so you need some gears in the path between the transposing gears and the lead screw (or spindle, depending on where the transposing gears are installed) with a ratio of 1:2.5. A 16 and 40 tooth pair would do the job (as would various others having the same ratio). If you have a lathe with a quick change gearbox, the same rules apply. You still need an overall 1:2.5 ratio, using some combination of external gears and the ratios available within the gear box. I'll cover QC gearboxes in more detail later, but for the moment we'll assume a simple change gear machine with no gear box."

Cheers Phil

 

[BobC]

Just a note for those who know that a 127 tooth gear is used to transpose imperial/metric but don't know why.

It's the lowest tooth count where inch meets mm exactly, all whole numbers.

There are 25.4mm in an inch, (easy/good one to commit to memory) exactly, but since gears cannot have just a part of a tooth, (well... unless you lose count on your dividing head...) the common ground must be found.

The lowest whole number that coincides with both is found by multiplying the fractional number, 25.4 until it comes out even. (Solving for X works too, but... well... that's hieroglyphics or algebra or somethin'.....)

This occurs at five first, kinda' easy 'cuz you already knew and can do it at a glance, .4 X 5 = 2, a whole number, which again in your head, multiplying by 2, 3 & 4 weren't. So 5 X 25.4 = 127, all nice and whole, even if it's not....well... even.... but that's also a proof-at-a-glance, 7 cannot be evenly divided, proof that 127 is the lowest even tooth count.

I'll bet that there's not a guy on this forum that can't do, 4 X 25 = 100 + 25 = 125 + 2 = 127, in his head and most can skip the 4 x part and just jump right in at 5 X 25 and then add that easily found 2.

With appologies to those that find this tidbit too elementary but the Admin. keeps shaming me, "Bob you haven't posted.......":whiteflag: and also because I hope that this will be useful for some. Sometimes a little mental victory can open a door to the more complex....like gearing.)

Bob