Southbend Change Gears

[Paul LousickParticipant @paullousick59116]

(This question has been asked hundreds of times on various sites and the answers are overwhelming)

I am looking for a chart for the manual change gears on a 9″ Soutthbend (Boxford) lathe to cut metric threads.

The lathe has an 8 tpi leadscrew and I have a 47/37 tooth transposing gear.

Thread pitch required = 1.0mm, 1.25mm, 1.5mm, 2.0mm

[Speedy Builder5Participant @speedybuilder5]

Not sure about your transposing gears, page 75 from Know Your Lathe:-

[Paul LousickParticipant @paullousick59116]

A 47/37 gear has a ratio of 1.27027 instead of 1.27000 which is good enough for most applications.

This table is not in my Southbend manual and the text is a bit unclear but I think it refers to a gearbox position. My lathe does not have a gearbox.

Thanks Speedy,

[DC31kParticipant @dc31k]

Two places to look, that would be as good as definitive, are the South Bend section of Practical Machinist forum and the South Bend literature available at vintagemachinery.org

You can locate a pdf copy of ‘Know your lathe’, which will have a clear chart in it. I think the answer above is intended to point you in the correct direction rather than hand you something on a plate.

[Gerard O’TooleParticipant @gerardotoole60348]

The left chart that ‘Speedy Builder5 posted,, labelled fig. 127 shows the gears for lathes without the gearbox.

Here it is again without the other chart or text

 

[BazyleParticipant @bazyle]

The main forum for SB/Boxford is on Groups.io
https://groups.io/g/BoxfordLathe-UserGroup/topics

47/37 is not in the mainstream of conversion gears that I have heard of but if you have one does it look like a home made custom one of more match the others. Which other gears do you have? Most lathes have sets that go in steps of 5 but the SB/Boxfords use even numbers.

[Brian WoodParticipant @brianwood45127]

Paul,

I don’t have a table for Southbend , but I have calculated a generic table applicable to ALL lathes with 8 tpi leadscrews. There are no transposing gears, but a 63T gear will be needed.

Gearing layout is as follows:-

A is the driver after the reversing cluster, B the first driven gear, linked to C the onward driver for D on the leadscrew

Required 1.00   A = 30, B = 60, C = 40, D = 63       Result= 1.008 mm

Required 1.25   A = 20, B = 55, C = 65, D = 60       Result= 1.251 mm

Required 1.50   A = 30, B = 50, C = 55, D = 70       Result= 1.497 mm

Required 1.75   A = 30, B = 35, C = 45, D = 70       Result= 1.749 mm

Required 2.00   A = 40, B = 50, C = 55, D = 70       Result= 1.998 mm

I hope this helps

Regards   Brian

[SillyOldDufferModerator @sillyoldduffer]

My brains are mildly frazzled by belly ache this morning, but unless I’m missing something, the Boxford table provided by Speedy and Gerard should both work simply for Paul by him substituting the 47/37 transposing gear for the conventional 127/100 pair shown on the Boxford plate, ringed in red below:

The advantage of a 127:100 pair is that is gives a perfect conversion to 25.4mm per inch.  The disadvantage is the need for two whacking big gears, and many lathes don’t have enough room on the banjo for them.  Their need to cut metric threads is often achieved, somewhat less accurately, with a 63 toothed gear, and a different table.

Paul has a 47:37 transposing gear, which is another way of approximating metric:  25.40540541mm to the inch.   Plenty good enough for cutting short threads, but think twice before making a long accurate thread.

Bottom line, what Paul needs is achieved simply by substituting his 47:37 transposing gear for the 127:100 transposing pair shown on the Boxford plate.  As both transpositions produce nearly the same ratio, the Stud, Idler and Screw gear set-ups tabulated on the left of the Boxford plate should produce the advertised pitches.

The easy way is for Paul to prove I’m right, or not, is to try it.  With a fine felt-tip pen in the tool post, write a helix on a suitably big diameter rod with the lathe, and confirm several pitches are drawn as per Boxford table by measuring them with a caliper.   If all looks OK, cut a real thread and check that.

Dave

 

 

[mark smith 20Participant @marksmith20]

I have a southbend 9A(with gearbox) , i have cut metric threads a few times  for short thread  lengths of around 1″,worked fine . I dont have the 127/100 gear  but just used other combinations to give the least error.

My question is for the B and C models without the gearbox do these combinations work the same or not at all ??

For instance ive just threaded M1.5 by simply using a 54 tooth gear as the stud gear and an 80 tooth on the end of the gearbox. Setting the gearbox to 32 TPI.

I have a spreadsheet program off someone  on one  of the American forums that calculates the gear combinations and  pitch errors, but forgot how it works.

 

[Michael GilliganParticipant @michaelgilligan61133]

Looks good to me, Dave

MichaelG.

[DC31kParticipant @dc31k]

I do not know if this is a helpful way to view the problem or not:

The 37/47 gear turns his 8 tpi (3.175mm) leadscrew into a 2.5mm pitch leadscrew.

25.4/8 = 254/80 = 127/40. 127/40 x 37/47 = 4699/1880. 4700/1880 = 5/2.

The error is 1 part in 4700, which puts it at no. 18 on Martin Cleeve’s list, so I am not sure which stream of conversion gears it is not a main part of.

Once you have that basic conversion, the rest is simple – you want 1mm pitch, you add gears making a 2:5 ratio.

You want 1.25mm pitch, you add gears making a 1:2 ratio.

Hence, a change gear table for a machine with a 2.5mm pitch leadscrew would furnish all the other ratios.

We should note that the Boxford gears proceed ‘by fours’. The procession of standard South Bend gears would be a good thing to establish before making any decision on what table to follow.

[Grindstone CowboyParticipant @grindstonecowboy]

As an alternative, or for conformation of the charts shown above, I suggest taking a look at the “Ride the Gear Train” program written by Evan Lewis, a very active member of the Southbend/Boxford forum. I would expect the results to be the same as Brian’s, however.

You will be able to enter which gears you have, including any compound gears, choose wwhat percentage error is acceptable (including no error, but with the 47:37 rather than 127:100 you may not be able to achieve perfection).

Evan is always happy to help if you contact him via the forum with any questions.

Rob

Links to https://ridethegeartrain.com/

[peak4Participant @peak4]

Alternative program here too
https://www.imagesalad.com/lathenovice/lathegears/lathegears.html

Myford gear charts should work too

Bill

[Russell EberhardtParticipant @russelleberhardt48058]

Another useful program is NthreadsP which can be downloaded from the lathes.co.uk website here:

http://www.lathes.co.uk/threading/nthreadp.zip

You just load your list of change gears, tell it what pitch you want and it will find the best combination for you and calculate the error. It is a Windows program but I find it runs well on Linux using Wine.

Russell

[DC31k]

On DC31k Said:

I do not know if this is a helpful way to view the problem or not:The 37/47 gear turns his 8 tpi (3.175mm) leadscrew into a 2.5mm pitch leadscrew.

[…]

May I just say … YES that is an extremely helpful way to view the problem; because it cuts-through all the mental juggling that typically accompanies working with an unfamiliar system.

MichaelG.

[Paul LousickParticipant @paullousick59116]

Thank you gentlemen (sorry, politically correct gentle persons) for your assistance.

[Author]

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