Lathe screw cutting chart - Page 1
Forum section: electronics / projects

author: Ario
author_rank: Regular Contributor
timestamp: January 18, 2021, 08:33:28 am
content: Hi thereI am trying learn screw cutting on my grandfathers old lathe. I am managing, except I am struggling to make sense of the screw cutting chart provided. My mechanical knowledge is limited. In the following diagram I would like to know the meaning of 'first stud' and 'second stud' driven and driver. Any help or insight would be greatly appreciated. a diagram to identify the different parts mentioned on the chart will help me. Thanks in advance. I hope this is the right spot for this post, knowing it is a mostly electronic forum.regards

author: rs20
author_rank: Super Contributor
timestamp: January 18, 2021, 09:09:41 am
content: Firstly, I am very jealous you have a lathe to play with.I can't really tell you which stud is which from the diagram, because I'm not very familiar with lathes. But I know just enough to be dangerous.It should be easy to tell which gear is on the lead screw. Since you didn't ask about that, I'll assume you've figured this out. Similarly for the driver gear.The image above has three studs, your lathe only has two. But the same principles apply. So, assuming the "driven" on the right of the diagram is the lead screw, this would be annotated:Driver1st S driven1st S driver2nd S driven2nd S driver3rd S driven3rd S driverLead screw8248321624832So basically, you can infer it by looking at the mechanism. From left to right is all the little gears along the way; each stud has a compound gear with two different tooth counts on it, so those are labelled according to whether they are driven by another gear or driving the near gear along:Whereas the driven and lead screws only have a simple gear on them (or at least, as far as the aspects relevant to the lead screw mechanism are concerned.)Let me know if you want more detail.

author: Ario
author_rank: Regular Contributor
timestamp: January 18, 2021, 10:18:28 am
content: Thanks so much!!!!. Just figured it out, and you confirmed what I figured out.I know I am lucky. Although it is difficult to find information on such old machine. Hope to find a good book somewhere in the future. I am now attempting t0 cut a 2mm pitch screw. If it works we figured it out. I will give an update.Thanks again

author: m3vuv
author_rank: Super Contributor
timestamp: January 18, 2021, 10:30:41 am
content: dont forget to back the tool off the work on the reverse stroke,otherwise it breaks the tooling.

author: Ario
author_rank: Regular Contributor
timestamp: January 18, 2021, 10:35:34 am
content: Thanks, I am aware. I have done some lathe work on it before. I was just not sure about the cutting of threads.

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: January 18, 2021, 12:39:21 pm
content: Prior to starting cutting always put a piece of scrap in the chuck and take a very light cut to double check the pitch. I learned that in my first week as an electronics trainee (the mechanical trainee had just left so I was both for a while...)BT

author: jpanhalt
author_rank: Super Contributor
timestamp: January 18, 2021, 01:23:02 pm
content: An added note: On many precision lathes (e.g., Harding, Monarch, Smart & Brown (UK) ) that have an English threaded leadscrew, one needs a conversion gear set to get accurate metric threads and visa versa. That set is often 127 teeth meshed with 125 teeth (NB: 127 = 254/2). You can get an approximate conversion without that. Do you know whether your leadscrew is English or metric? My British lathe (Smart & Brown) made in 1965 is English.

author: rs20
author_rank: Super Contributor
timestamp: January 18, 2021, 01:30:20 pm
content: Quote from: jpanhalt on January 18, 2021, 01:23:02 pmHis leadscrew is imperial (1/8" TPI / LPI). The "2mm pitch" metric setting actually gives 1.995714286mm. Yes, I have too much spare time :-)

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 18, 2021, 02:22:58 pm
content: Is it a plain change gear lathe, or does it have a quick change gearbox too? I can't quite tell from the pictures. It looks almost identical to the arrangement on my small Logan. I've got a couple pages you might find useful here-http://www.conradhoffman.com/hsm01.htmOn the metric threading page, if you hold your mouse over the gears, it will tell you the names.

author: Ario
author_rank: Regular Contributor
timestamp: January 18, 2021, 03:29:35 pm
content: Hi thanks for the reply.Definitely imperial. I don't really mind the imperial system.BTW mine is a Hobson

author: Ario
author_rank: Regular Contributor
timestamp: January 18, 2021, 03:31:15 pm
content: Quote from: BurningTantalum on January 18, 2021, 12:39:21 pmThanks for this advice !!! It will come in handy. I will ask again after I tested it out.Thanks

author: Ario
author_rank: Regular Contributor
timestamp: January 18, 2021, 03:32:08 pm
content: Quote from: Conrad Hoffman on January 18, 2021, 02:22:58 pmThanks for this resource sure it will come in handy!!

author: Ario
author_rank: Regular Contributor
timestamp: January 18, 2021, 03:34:01 pm
content: Quote from: rs20 on January 18, 2021, 01:30:20 pmYes I believe you are right, sure it is close enough for me to learn something

author: Ario
author_rank: Regular Contributor
timestamp: January 18, 2021, 03:50:37 pm
content: Just confirmed that the pitch is indeed correct, thanks rs20. Sadly botched my first attempt. But I at least know I know how to set the threading pitch now.

author: Ian.M
author_rank: Super Contributor
timestamp: January 18, 2021, 04:27:43 pm
content: Additionally, if cutting metric threads with an imperial leadscrew (or visa-versa), except in an emergency, do *NOT* disengage the half-nuts driving the carriage, as the position increments at which they will re-engage aren't in metric. In most cases, you'll have to disengage the tool, stop and reverse the drive to run the carriage back to prepare to take another cut. Don't thread towards and up to a shoulder, it wont end well! You have to figure out how to cut such threads working away from the shoulder.Edit: See Conrad's comment below, and his article athttp://www.conradhoffman.com/metricthreading.htmon how to exploit the fact that there is always guaranteed to be *ONE* position at which you can reengage the half-nuts with no offset between imperial and metric to mess things up, namely the exact position you disengaged them at!

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 18, 2021, 04:56:29 pm
content: Actually, you can disengage the half-nuts and thread to a shoulder when doing metric on an imperial machine. I do it all the time. It's well worth learning to do it and I describe the technique on my web page. I first saw it on the Practical Machinist forum and can't believe it isn't more well known. Yes, it takes a bit of skill and a lathe that doesn't spin down forever, but that covers most Southbend, Logan and similar.

author: Ario
author_rank: Regular Contributor
timestamp: January 18, 2021, 05:12:33 pm
content: Quote from: Conrad Hoffman on January 18, 2021, 04:56:29 pmThanks looking at your website now!!

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 18, 2021, 06:47:00 pm
content: That double compound gear chart for metric is actually pretty good. You wouldn't want to make a metric leadscrew because of the slight error, but it's plenty good enough for most anything else, even though it doesn't use a 127 tooth gear.

author: langwadt
author_rank: Super Contributor
timestamp: January 18, 2021, 08:01:01 pm
content: Quote from: Conrad Hoffman on January 18, 2021, 04:56:29 pmTom Lipton demonstrates it here:https://youtu.be/HXt4TWa382Q

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 19, 2021, 01:30:44 am
content: Ha! That's great. I thought I should make a video to show it better, but he's done a far better job.