Thread cutting on a mellor

Hi Guys.

This is my first post so i hope it is in the right place.

I have bought an old Mellor lathe that I am very happy with as I only needed it for one quick job, but.... now I'm looking at learning/teaching myself to cut some threads on this.

It came with an assortment of gears but I think there may be a few missing.

The main problem I am having is working out which gear goes where. there is a thread cutting list on the side of the machineI believe this is the backgear and bullwheel.

This is the Lead screw but I am not sure how to measure TPI. if it is from the top or bottom of the thread, the verniers are set at 1" so i don't know if this is 4 or 5 TPIAnd this is whats on the back of the thing.The gear on the back of the main spindle is 32 teeth and the one after the tumbler gear is 32 but the first in the chain here is 20 teeth. I'm not sure if this is wrong and this first one should be 32 teeth also.

Any help would but gratefully appreciated.

If anyone needs any more info. pictures or gear tooth counts please let me know and i will add to the post..

Thanks

Mat

There is not enough information here unfortunately.

It looks as though it is a traditional set-up so I'll assume it is.

Assuming the first gear is at spindle speed and the last is at leadscrew speed;

The first line of the table says 4 tpi is at first gear 40 teeth and last 40 teeth. The gap in between indicates one or two neutral idlers (i.e the same size as each other.)

40 teeth in and 40 teeth out plus idlers (which change nothing) means no overall ratio change. Therefore the leadscrew is probably 4 tpi.

There are web sites that calculate lathe gear trains but without a manual you''ll need to study the subject.

The small gear as you have it is in position A on the label. The next gear is just an idler as set up so fills in the B and C positions. The final gear D is the one on the leadscrew. The leadscrew is obviously 4 turns per inch as shown on your vernier.
I'd guess that the D gear is an 80 at the moment so you have the set up for 22 tpi which is 5/16th whitworth.

Thanks for the replys guys.

Starting to get my head round this a it now. still confused with a few things. on the table 6tpi for instance has no A gear and it seems the B and D are idles that only leaves a D gear???

how can it work with only one gear?

Mat

Lead screw is 4 TPI. Measure it by using a ruler across 4 threads either left hand side of thread to left hand side or right side to right side. Difference between 4 and 5 TPI, or similarly coarse threads, is so large that its easily seen against a ruler.

Had to sort out same issue for a friend who got one around 30 years back. Lathe long gone so I can't go and check. Sorry. As I recall it gears A, B, C, D go on the banjo, the gear driving the train was correct on his machine and he never changed it. Dunno the tooth count tho'. Easy way to check is to set up as Capstan Speaking suggests with a 40 tooth gear at both A and D with another of suitable size at B to span the gap. Tooth count on the middle (idler) gear doesn't matter as it merely fills the gap without changing the ratio. You need a pair as shown for B & C to alter the ratio.

With that set-up put a mark on the bed turn the spindle 4 times and make another mark. Measure the distance between them if its 1 inch then the drive gear is correct. If its different the error will be in proportion to difference in the number of teeth between the gear fitted and the right gear. Time for a little maths to work out the right one.

If you don't have two 40 teeth gears set up the lowest number TPI you have gears for and turn the spindle the requisite number of times to get 1 inch of movement. Various ways of marking the bed, anything you can see but is easily removable will do. I've used chalk, pencil, tape and spirit based felt tip various times always managing well enough. Best to turn the spindle backwards so the carriage runs towards the chuck as there is more room for the ruler. If you aren't quite sure turn twice as many times to get 2 inches of movement so any discrepancy will be doubled.

The plate is confusing, " stands for ditto i.e same again not no gear. The A gear is 40 from 4 to 10 TPI and 20 from 11 to 22.  As I recall matters my friends machine only had one 40 engraved on the top line so it wasn't quite so confusing.  Guess it made engraving easier and faster.  My friend also had a 127 tooth metric conversion gear with his which I refused point blank to figure out unless he needed to do a metric thread.  Far as I know the only thread-cutting he did was when I taught him how! 

Clive

Edited By Clive Foster on 03/04/2015 22:31:28

Hi Mat,
If your vernier is set to 1.00" then the leadscrew will be 4 TPI which agrees with "CapstanSpeaking's" deductions. Further to the above deductions I believe the small gear (Which you say has 20 teeth) will rotate exactly 1 revolution for 1 revolution of the spindle. The gear that is fitted where the 20 tooth gear is fitted depends on the pitch of the thread you want to cut. If you have the gears show in the chart to cut the thread you want then just use the setup shown on the chart. If you do not then you will have to see is you can achive the same ratio between the spindle where the 20 tooth gear is fitted at the moment and the leadscrew. If you do not want to do the calculation yourself then there is a program called nthreadp.exe that is available to download from here that will do it for you. Using the example above to cut a 4 TPI thread you would replace the 20 tooth with a 40 tooth and put another 40 tooth on the end of the leadscrew. You would put 1 or 2 idler gears (The number of teeth does not matter) on the banjo to fill the space between the two 40 tooth gears. If instead of fitting a 40 tooth in place of the 20 tooth you left the 20 tooth where it is you would cut an 8 TPI thread.

Les.

If you look closer, the other lines have ditto marks so it's A = 40 -idler- D = 60.

40/60=0.66 which fits with the 4 to 6 tpi proportion.

I suggest you get one of those pocket books on screwcutting. It's not practical to learn gearing through a forum.

ok. so if all the gears go on the banjo should i have to places to fit them?

as it its there is only one spindle to fit gears on, i think it would all make sense if there was an A position a B position and D was on the leadscrew.

thanks again for all your help guys i will give this a go in the morn and let you know how it goes!!

Mat

The chart shows three gear positions although B and C are ganged on the same shaft and run at the same speed. There are only one or two ratio changes on this lathe. It all seems correct but I can't tell if you have all the gears on the chart.

Edited By Capstan Speaking on 03/04/2015 23:08:32

Matthew

After overnight reflection I think I may have been wrong in stating that all the gears go on the banjo. Especially as you have only one stud. Try putting the A gear on the stud, setting up a coarse TPI train and measuring the travel as in my previous post. If it works out you are home and dry.

Apologies if I gave you a bum steer. After 30 odd years it can get difficult to keep everything straight.

Clive