Metric threds

[chipenter]

I have a 9" South Bend the with a modified Clausing gear box fitted , I would like to cut metric theads the fly in the ointment it the banjo has a fixed spindle for the idler gear , 24 teeth on the tumbler 96 teeth on the idler and 24 teeth on the 8 tpi lead screw , the banjo pivots to the tumbler I can change the idler gear but nothing works , I can cut new gears  but they have to add up to 120 at 18 DP , help please .

[chipenter]

took the gears off to mesure properly its 16dp not 18 as I first said , if I use 20dp gears I can fit a 127/100 with a 40 tooth gear on the lead screw , just have to make a spindle and buy a 127 gear , still don't know what gear on the tumbler gears to fit or the formular to work it out .

[chipenter]

Cracked it the internet provided the answer , 32 gear on tumbler 127/100 and 40 on the lead screw gives 12.7 tpi , that gives me 2 mm pitch lead screw , from the set of Myford gears I am missing 32 and 127 so this mod will cost less that 50 quid .

[chipenter]

Just orded 127 and 32 change gears from RDG special offer both gears came to a total of £20  .

[AdeV]

TBH, this is exactly why I bought  lathe with a gearbox.... the thought of having to deal with change wheels put me right off, maths having never been my strongest subject. When I discovered that my 'box would do metric AND imperial, well, I was a very happy bunny indeed...

I've never really got to grips with how changewheels work exactly. OK, I understand the basic principle, but how do you get the different sized gears to mesh properly, and why is a "tumble reverse" so called? Is that the same as the "tumbler" you mention in one of your posts above?

[Pete.]

Even with a gearbox, many lathes need changewheels especially the smaller ones and even more especially if you want to cut metric and imperial threads. The 127 tooth gear is required for perfect transposing though there are a couple of very close smaller alternatives. The Atlas uses 64 tooth gears (half of 128) which is very close.

[DMIOM]

..... how do you get the different sized gears to mesh properly, and why is a "tumble reverse" so called? Is that the same as the "tumbler" you mention in one of your posts above?

Ade - although the input (from motor) and output to leadscrew are fixed in space, the other pivot(s) aren't permanently fixed - typically there's a swinging, slotted arm (which can be known as the 'change gear bracket' or 'banjo') which allows you to position intermediate gear(s) to mesh with varying size drive- and driven- gears. Tumbler reverse is just inserting another pinion in between two existing gears so as reverse the drive direction.

Dave

[chipenter]

Ade the tumbler revese is the three gears in a triangle that pivot around the bottom gear , moving the second gear into contact with the lathe spindle reveses the drive to the lead screw , and leaving the left hand gear free , adjusting gears is just a thicnes of cartridge paper or untill they are quiet .

[Jonny]

I haven't any change wheels with mine and regularly cut metric and imperial on same bar, just dial it in job done.
Spindle 40T feeding 94T
94T feeds 44T
44T feeds 88T on Lead screw gives 56TPI to 3 1/4TPI or 0.2mm to 3.5mm pitch plus modular.
Every things done with gearbox. Only have to swap round 44T to 88T and vice versa to cut from 4mm pitch to 14mm or 14TPI to 2TPI plus modular stuff.

[chipenter]

I haven't any change wheels with mine and regularly cut metric and imperial on same bar, just dial it in job done.
Spindle 40T feeding 94T
94T feeds 44T
44T feeds 88T on Lead screw gives 56TPI to 3 1/4TPI or 0.2mm to 3.5mm pitch plus modular.
Every things done with gearbox. Only have to swap round 44T to 88T and vice versa to cut from 4mm pitch to 14mm or 14TPI to 2TPI plus modular stuff.

My problem is the banjo spindle is fixed no groove to adjust for the gears .

[vintageandclassicrepairs]

Hi Chipenter,
It may be worthwhile fabricating a new slotted "banjo to accomodate the different gears needed?
On a couple of lathes I worked on the wheels were 127 and 125 teeth to provide the metric threads
the pitches would then be "near" the imperial settings (ie. 20 tpi = 1.25mm so to speak)
This seemed easier to convert in my scrambled brain  rather than your 127/100 combination (5:4 ish)

I always checked the pitch with a dial gauge against the carriage before cutting metal

HTH
John

[chipenter]

Hi Chipenter,
It may be worthwhile fabricating a new slotted "banjo to accomodate the different gears needed?
On a couple of lathes I worked on the wheels were 127 and 125 teeth to provide the metric threads
the pitches would then be "near" the imperial settings (ie. 20 tpi = 1.25mm so to speak)
This seemed easier to convert in my scrambled brain  rather than your 127/100 combination (5:4 ish)

I always checked the pitch with a dial gauge against the carriage before cutting metal

HTH
John

That was my first thought to , I alredy have 100 and 40 gears and the 32 127 should be delivered tomorow , all I have to do is bore them to fit and bolt the 100/127 together , and should be a simple change over involving two bolts .

[Pete.]

Hi Chipenter,
It may be worthwhile fabricating a new slotted "banjo to accomodate the different gears needed?
On a couple of lathes I worked on the wheels were 127 and 125 teeth to provide the metric threads
the pitches would then be "near" the imperial settings (ie. 20 tpi = 1.25mm so to speak)
This seemed easier to convert in my scrambled brain  rather than your 127/100 combination (5:4 ish)

I always checked the pitch with a dial gauge against the carriage before cutting metal

HTH
John

127 teeth provides perfect transposition. If you only wanted 'near enough' there are more conveniently-sized combinations that produce good enough threads to make fasteners etc.

[chipenter]

Gears arived today got them bored out and bolted together , the fixed shaft is 3/4" x 2 1/4" and I will make  spacers one 1 1/2 x 3/4 bore and one 3/8 x 11/16 bore for the lead screw end .

[chipenter]

Got it finished the gears are nice and quiet and cut a 2mm pitch thread , using the instructions from Matt Tinkers Eggwick thread on screw cutting up to a shoulder , I have learnt something today thanks to Madmodder .