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Metric Thread On Imperial Lathe

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RichardShute: --- Quote from: andyf on December 19, 2010, 07:28:02 PM ---I think that I was misled by the video. The camera angle was such that I mistakenly thought the topslide was set at a much greater angle than 29.5 deg to the cross-slide axis. Andy --- End quote --- That is the nub of this whole discussion. In the video John gave a clear description of the well documented rotated-compound-slide method and says the cut is taken on the left of the tool with the right of the tool just scuffing the rear face to keep it clean. BUT... when the top slide was rotated in the video, it was indeed rotated by 29.5 deg from the spindle axis as John agreed in a later part of the discussion. The inevitable consequence of this is that the top of the thread crest is gradually removed with each successive cut as the thread is formed. In the end, although the root diameter is correct, the crest of the thread is undersize by roughly half the thread height as shown in my diagram and Pete's and implied by the last of Bill's. It does, surely, matter a lot what the approach angle is, if the result is a significantly truncated thread. Using a low angle to get a fine feed is a well established technique, but in plain turning there is no concern about preserving a particular tool path, in thread cutting, by definition, there is - it's the thread. Ned, I was wrong to say the crest is removed by the heel of the tool, that is not the case, the top of the thread is taken away by the tip on successive passes. The tool could only cut on the right as well as the left if the compound angle were less than half a thread angle from the cross slide, but not at the 30deg from spindle axis we are discussing. Sorry for mis-leading you. doubleboost, I would be interested to know what is the OD of your finished thread. If you started with 40.00 and cut a 1.5 pitch thread, with the compound set as shown in the video I would expect you to have lost about 0.6mm per side so the finished OD would be about 38.8, depending on the tool tip radius. Richard

Pete.: --- Quote from: Ned Ludd on December 19, 2010, 07:23:26 PM ---Hi Pete, As I said "once the cut is under way" how can the tool cut anything on its right hand edge when it is moving sideways to the left. If I am wrong, wouldn't be the first time, the only cut on the right of the tool would be the small incremental in-feed, while the left hand edge cuts full depth, I would not describe this as "crowding" would you? --- End quote --- Yes I would. Let me start by saying that I am a novice machinist and as such have been and am happy to be corrected if something I say is not right. When you directly in-feed, you take the first pass and the tool cuts both sides. The leadscrew moves the tool along it's path continually cutting both sides up to the end of the thread. When you set up for the next pass, the leadscrew moves the tool along exactly the same path as controlled by the leadscrew, but the tool is fed in extra depth, so the tool is still cutting both sides but deeper. Look at it another way, imagine you were cutting a dead-square thread with a full-width square tool. After the first pass, you return and feed the tool directly in using the cross-slide and take another pass. Will the tool now cut the thread deeper by cutting on the end of the tool, or wider by cutting on the side of the direction it's travelling? --- Quote ---As Bogs very wisely says, nothing is set in stone about thread cutting. What matters is that you can produce a decent thread, first and foremost. With more experience you can vary your methods to suit your different needs or tooling. There are Pros and Cons for all the various different methods of single point thread cutting. I have never seen a fully comprehensive treatise on single point threading, but I am sure it would run to hundreds of pages and even then some chap working quietly by himself in a shed, would have yet another different way of doing things. One of the best books for the home machinist is the third in the Workshop Practice Series, "screw cutting in the lathe" but it is now sadly a little outdated, even so it still makes a good primer for those new to the subject. If it were to be up-dated it would have to include some modern developments like Bogs' own type of self lifting tooling. What is important is that instructional books, or other teaching methods, are not open to misinterpretation by an averagely mechanically minded person. Ned --- End quote --- Wise words indeed but there should always be room for someone to disagree with anything that's said. With the very greatest of respect, I actually don't agree with Bogs that it's acceptable for the compound angle to be greater than half the included thread angle especially when it's double because I think it will produce the saw-tooth form as shown in my sketch. I don't see how a few thou cleanup cut is going to be enough to turn a 90-degree included angle thread into a 60-degree one especially on the OP's 12TPI thread, but as I said I'm a novice machinist with no more than a handful of single-point threads to my name so there might be something I'm missing there. I certainly won't stamp my feet and get all silly if I'm proved wrong, but I'd like to BE proved wrong rather than be told not to argue when I think I'm not. EDIT: It was Bill Todd who cut a 12TPI thread, the OP cut a 1.5mm pitch thread for his collet chuck.

Ned Ludd: Hi Pete, Argue away, without discussion how is anyone going to get out of the rut of narrow thought? I, too, have to be convinced about cutting threads at shallow angles, as I wrote in an earlier post I would have thought that the last third of the cut would have to be in-feed, to correct the thread form. OOPS, I just checked back to see exactly what I posted, no post. I wrote a post, I know I did,  but it seems it did not get through and I did not check, silly me.  Ned

Ned Ludd: Hi Guys, My apologies, I must have saved my post and not sent it. The following was written as a reply to Bogs and should have appeared on page one. Hi Bogs, I do have a little experience in thread cutting, and the only way I can see the method in the video working would be if the last third of the cutting was of the "straight-in" method. By doing it that way, after the bulk has been taken away, the correct thread form could be produced by the insert. Though having said that, as he is using "proper" inserts, why not just go straight in and use the insert as it is designed to be used? While on the subject of threading, I have been experimenting with the swing up tool. I think yours is a stiffer and superior  design than the German chap's. To be fair to him, he was only cutting very fine threads not 1/2" Whit!  I have tried JohnS's with die head chasers, but on some metals the loading is, I find, too much. I am currently trying one using triangular carbide threading inserts, which shows promise. The next experiment will be with the Oz design of tangential threading tool in your style swing up holder. So far I have found the best method of getting a clean cut thread is to use the Oz cutting tool shape in my own design holder, using the offset top slide method but with a cross slide stop for quick action. Ned

doubleboost: Hi I made another chuck (for the rotary table) I used the same insert but went straight in this time. The diamiter before machining the threads on both pices was39.8mm. The finished thread measures 39.7 on both jobs and the threads look identical. I proberly lost the 0.1 as i polished the finished thread with emery tape. Regards John

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