metric thread on my imperial lathe

hello folks i have a harrison l2 imperial lathe and i need to cut a M18x1.5 left handed threads, i have a box of change gears.

What would you suggest for cutting this thread please.

chris

Chris,

There will almost certainly be a gear combination which will basically convert your lathe from tpi to metric pitch. It may well involve a 127 tooth gear, but may be other very close conversion ratios. You will then need to thread without the use of the thread dial as that will be inappropriate. The usual is to reverse the lathe for returning the carriage, after a cut, in order to keep the cuts superimposed.

Whatever you do, do not open the lead-screw half-nuts under any circumstances between passes.

Whatever you do, do not open the lead-screw half-nuts under any circumstances between passes.

While good advice for a beginner, that is not entirely true. There are several vids on utoob which clearly demonstrate that the half buts can be opened - but must be closed again before reversing beyond the end of the cut.

Unnecessary for braked lathes or where plenty of undercut is available after the thread, but useful for instances where things are 'limited' in some way.

Whatever you do, do not open the lead-screw half-nuts under any circumstances between passes.

Well I would heed the advice above, as from bitter experience cutting metric threads on imperial machines. I ended up with a multi start thread, especially on my Myford . The cuts did not superimpose over one another

Leave the half nuts engaged at all times once you start, withdraw the tool out at the end of the thread and reverse the machine to start position and then take next cut, simple and completely risk free.

not done it yet I'm not going to flame you as I was simply giving "beginner" advice as chris mcnicoll's original post suggests he is just that. Most reasonably experienced machinists are likely to know how to calculate the threads available from the selection of change-wheels at their disposal. The alternative is to download one of the many free PC applications that take the repetitive maths out of running through the various combinations.

If Chris McNicoll is an experienced machinist I offer my apologies.

Edited By Brian Oldford on 25/04/2017 17:31:43

im not experienced in any way shape or form, i just wanted some help, ive had a look in screwcutting in the lathe book and from what i can see i need a 50 tooth and a 127 tooth, both of which i have in my box of goodies.

Chris, if you have those gears then you can do metric threading, in principle, and hopefully the threading tables will give you the gear combination for 1.5mm pitch. I assume the L2 has a tumbler reverse so you can cut LH threads. There is an excellent little book on screwcutting in the lathe by Martin Cleeve which gives all the gen. As pointed out above you can't easily use the thread indicator so have to wind the lathe backwards after each cut - being a Harrison it may have a chuck mounting that doesn't mind being run in reverse. And avoid disengaging the half nuts between cuts as it is difficult (though not impossible) to get the leadscrew back in sync with the thread - this is because the LS is (presumably) imperial but the thread metric.

Good luck!

Well I never, there are some closed minds around.

I did say it was good advice for a beginner to keep the half nuts closed? Yes, I do think I did!

Can you open the half nuts during the threading operation? Yes you can.

Did I explain that they need closing before returning the carriage to the start of the next cut? I think I did.

Before you get all hot and bothered about it, why not try it? It works. It can certainly help in certain circumstances. I now do it that way on occasions. So it must be simple enough.

Very sorry if some cannot manage it. Perhaps they did not have a thread dial by which to find the correct position to re-close the half nuts before returning the carriage for the next cut.

I think abom79 and doubleboost have both demonstrated how simple it is by way of demonstration on their vids, among others probably.

Martin Cleeve's book Screwcutting in the Lathe has charts on pages 54-55 for cutting metric threads using standard changegears on an Imperial lathe without needing the 127T gear. Not sure what tpi your leadscrew is but he suggests for 1.5mm pitch with 8tpi leadscrew to use gears 40- 60/50 - 65/35 - 38. This gives an accuracy of 1 in 4,446 which should be close enough for the average home shop!

He does tell you how to calculate it yourself, but don't ask me to explain it.

Further to the "don't open the half nuts" instruction.

If you have a long section of thread to cut ( I recently had a leadscrew that was 200mm of 2mm pitch to cut) you can open the half nuts and move the tool back by hand but with a big BUT in the method. You need to move a distance that has an exact number of inches and an exact number of pitches as well. For a 1mm pitch this is 5" or 127 pitches. For 1.5mm pitch 7.5" moves 127 pitches. If you have a DRO fitted then this is reasonably easy to do but I would not recommend it for a beginner.

The choice of to do this or not will depend on a number of factors such as how long the threaded section is and how fast reversing the motor is driving the saddle. If you can't reverse the motor then you need to hand crank the spindle backwards because you should not touch the tumbler reverse (has anyone mentioned this fact earlier?) because it can have the same effect as opening the half nuts and losing position in the thread.

The best instruction I can think of is photograph or sketch the change gears before you start changing the gear train setup to ensure you can go back to where it was before. Then practice what you need to do with some scrap until you are happy that you have got everything sorted out and you know what you need to do for a successful outcome.

Martin C

Why don't we stick with answering the OP and not throw in something he's not even asked for, probably don't know as he's a complete beginner ??

OK so it is possible to use the threading dial. has been since Victorian times. I was taught it as an apprentice before Double boost was even born. Would I try to teach a beginner that method ? No, he needs to learn to walk before he can run.

As an aside has anyone on here ACTUALLY tried this method or are they just regurgitating what's been posted on You tube ?

For myself the threading dial on by TOS lathe has been hanging down, out of mesh for over 15 years and I reverse under power.

This lathe, in that time has probably done more threading that all the posters lathes in this thread.

So back to the OP, what gears does he need to use on his Harrison L2 to cut a 1.5mm pitch LH thread.

He says in a subsequent post that he has a 50T and 127T gears in his box of stuff.

Anyone?

Does someone have a threading chart for that lathe? Know if there is a tumbler reversing mechanism on that lathe?

Barring that, if the OP shares with us the TPI of the Harrison's lead screw and posts a pic of the change gear stud area of his lathe, the resident experts should be able to calculate the gear train required.

Ok John, you have a method that works for you with your lathe. It will also work for anyone else who can reverse the motor on their lathe. Do you know if the OP can reverse the motor on his lathe? Everyone is saying reverse the lathe and don't open the half nuts but a beginner may assume that means using the tumbler gears. A lot of people have lathes that do not have a reversible motor, a lot of posts I have seen in the past are from people asking "why would they need to reverse their lathe motor" as they have never done it or needed to.

The OP was asking what was needed in the way of gears, that has been answered within reason from the information from the OP. The second part was how to do it. For a beginner the instructions need to be don't open the half nuts but also don't move the tumbler gears, If you don't have tumbler gears you need to put an extra idler in the gear train. If you can't reverse the motor you need to hand crank the lathe backwards and if you have a long thread to produce then you maybe should consider moving the carriage back 7.5" to save hand cranking as it could save some time and effort. Everyone is trying to be helpful but we have not been given what we could consider full details of the equipment available and the part to be produced because as the OP said he is a beginner. If we submit what is considered to be useful information the OP can read it, digest it and either comprehend what is being said or ask further questions and possibly submit more information.

Are you trying to discourage people from trying to help?

Martin C

Chris,

I suggest you ignore the silly squabbling and look at this:

http://metal.duncanamps.com/software.php

I have only used the ML7 version (because I have an ML7) but the Hobber Gear Calculator looks promising. You tell it what changewheels you have, and it works out what you can achieve with them.

I have been most impressed with the version I use, and it saves a lot of head-scratching and pencil-and-paper work.

George

i have got hold of a copy of the screw cutting book to read.

My lathe does not have a reverse gear though.

chris

That's the application I had in mind in my second post on this thread. I use it from time to time.

You normally run the motor backwards, not engage a reverse gear. Unusual to have a lathe without a reverse direction on the spindle. Are you sure?

Muzzer, the original question was about a left hand thread hence the requirement for a tumbler reverse or an additional idler gear in the gear train (which is what the tumbler reverse is).

Martin C

Edited By chris mcnicoll on 27/04/2017 18:14:11