Graham,

Welcome to the Forum.

Yes, ask questions and answers will soon appear. IF folk understand what you are asking.

What do you mean by the "Primary gear"?

If the lathe has back gear, under normal circumstances the drive pulley is secured to the Bull wheel in some way.e (On the big sister, ML 7, IF I remember from 20 years ago, there is a "key" which locks the pulley to the Bull wheel (The big gear not far behind the chuck ). The capscew is slackened and the "key" can then slide to and fro, radially to lock the Pulley to the Bull wheel, or to disengage it..

If back gear is to be used, the "key" needs to be moved inwards, to disengage the pulley from the Bull wheel.so that when Back gear is engaged you are not trying to drive the spindle at two different speeds mat the same time. If you do, everything locks up!

(Normally, to engage back Gear, the shaft carrying two gears,can be moved so that they mesh with a gear on the Spindle and the Bull Wheel, to provide a two stage reduction in speed.

My present lathe has a spring loaded dowel which locks the Bull wheel to the pulley so that when the Back Gear cluster is brought into mesh, the dowel is disengaged, so the drive is via the two stage reduction, This runs the Spindle at a lower speed and provides more torque. (Often about a 5 or 6 times speed reduction ).

The drive comes in via the pulley, the small gear drives the large gear on the Back Gear shaft, and the small ,gear on the shaft drives the Bull wheel which is fixed to the Spindle. .

It would be a good idea if you bought a book, such as L H Sparery, "The Amateurs Lathe", or Ian Bradley's "The Amateurs Workshop", or one of the other books on lathework.

These books answer a lot of your questions, and give a lot of other useful information, to enable you to make better use of your lathe, and avoid difficulties.

FWIW my advice is to get to know then lathe and how to use its features by making small tools, before trying to make anything complicated.

Making simple tools will allow you to gain experience, and confidence, and leave you with a few tools which will be useful for along time. come.

For a start, I would suggest making a Centre Height Gauge. This involves just simple turning, some drilling and tapping, and you will have a tool which simplifies setting tools to correct height so that they cut properly.

Then you can progress to things like Tap Wrenches, or a Tailstock Sliding Holder for Dies (This will ease cutting external threads )

There are designs for these sorts of things in books, by such people as G H Thomas, or from time to time in magazines such as Model Engineers' Workshop. If you look through Albums, you will see pictures of small tools such as these, made by various posters on this Forum.

Far better to make mistakes, you will, we all do, on a bit of mild steel, rather than an expensive casting from a kit!

Eventually, having gained more experience, confidence and skill, you might then want to make a Tailstock Sliding Tap Holder…

HTH

Howard

Edited By Howard Lewis on 10/07/2022 02:19:19

We cannot see pictures on your computer. You need to place them in a forum album 1st.

regards

You appear to have put a photo link to your own PC which we cannot see,

see the top of the page ALBUMS click on it and click create an album, give it a title and then upload your photo's to it,

Then you can add it to this page by click the camera sign which says ( insert image from album) above your message

To me, that looks like the gear on the Leadscrew.

That is driven by change wheels (Or change gears) Their function is provide the correct ratio between the Spindle and the Leadscrew, so that the Saddle moves the correct distance for every revolution of the chuck )

Buy the books so that you know the correct names for the parts of the lathe, and their function!

Then you can ask the right question and get the right answer.

Most often, the changewheel set up is is used for cutting screw threads. The ML7 has a Leadscrew with 8 t p i (Threads per inch)

Since the ML7 is an Imperial machine, I will deal in Imperial units.

Otherwise you will start asking about screws that are 6.354 mm in diameter, when the truth is that they are 1/4"

The threads on the lathe will be BSW and BSF with, probably gib strips BA. (This is where Tubal Cain's "Model Engineers Handbook" comes into its own to explain the mysteries!

So if you wanted to cut a thread with 8 tpi, the gears would be set give a ratio of 1:1. so the driving gear, beneath the tumbler reverse would be a 20T, driving an idler, to a 20T on the Leadscrew. The idler is merely there to transfer the drive from the Driving gear to the Driven, and does not affect the overall ratio.

If you wanted to cut a 16 tpi thread, the ratio would be 2:1, so that the Saddle moves a shorter distance for every revolution of the chuck.

If you wanted 12 tpi the ratio would need to 1.5:1 so the the 20T Driver would eventually transmit to a 30T Driven gear

For other ratios, you might need to use a Compound Idler, where two gears are combined.

Purely as an example, if you wanted to use the Leadscrew to provide a power feed for plain turning, you might set up a 20T driving a 60T on the Idler, which is coupled to another 20T which drives a 80T on the leadscrew.

This set up will provide an overall ratio of 12:1 which would mean that the Saddle would move the Saddle 0.0104" for every rotation of the chuck. This is fairly fine feed, but not the finest, which ideally would need to be of the order of 0.004" per rev, in my view..

Where are you located?

Try to find a local Model Engineering Society, so that you can find like minds, to whom, armed with the knowledge from the books, you can pose questions face to face and obtain more detailed explanations and answers.

Someone, hopefully. will tell / show you how to grind HSS tools for the lathe and to set them at centre height.

HTH

Howard,

Graham,

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