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Metric Thread On Imperial Lathe

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BillTodd:
Here's a 1" 12tpi thread I cut the other day (@ ~400rpm with a full depth insert in BMS )

You can see the striations caused by the each pass. However, although it is cut at 31° (59° on my top slide), there are no obvious steps on the trailing edge (right-hand side).

[edit] Actually, now that I can see the tips clearly, it looks like I've over done the in-feed (or picked up the wrong insert??) . As it happens I'm going to have to slice off and re-cut the thing as a dual start thread (i.e. 6tpi) as soon as I get the appropriate change gears for the lathe.

Bill

RichardShute:
Bill,
as you say, it looks fine. A 1 degree error is a near miss, but we were initially talking about a 31deg error and that would make a significant difference.

The saw toothing may be less obvious depending on the tip radius of the tool, but the reduced height of the thread profile would still be the same as each successive cut removes a little more of the crest with the trailing edge of the tool.

Richard

Bogstandard:
I think you are all missing the major point here.

It really matters not one iota at what sort of angle you are coming in at. Half the thread form angle minus 1/2 degree from the cross feed motion is observed to be the optimum, as at that figure, you are keeping the trailing edge fairly clean cut.

But you should not be worried about what is happening on the trailing edge, purely because you should always finish your threading cut with a minute feed of the cross slide, which then cleans up perfectly both faces of the cut thread, as long as you have set the cutting tip up correctly with the gauge in the first place.

I stated that I use the compound at a shallower angle for cutting very fine threads, which is what I do.

When you set your compound over, you can control the depth of cut much better than by using the cross feed, and by setting it at say 5 degs from parallel to the spindle, you can use the feed of the compound to take parts of a tenth off the diameter of a bar during normal lathe turning operations.

I use that method, but using a slightly larger angle, so that I have much better control of the depth of thread when cutting fine ones.

Maybe not in the books, but it was shown to me many years ago by an old timer as a method to get superior results, and I adopted it. The final tiny cross feed takes care of any 'sharks tooth' effect on the trailing edge cut.

Everyone has their own methods of cutting single point threads, but there is no need to argue over it or shove it down everyone elses' throats because you think their method is incorrect.
Make your methods known by all means, and if someone wants to take it on, then they will do, but for other people who have their own methods of doing it, and if it serves them well, and are quite happy to do it that way, then certainly let them carry on using it.

Nothing is written in stone, in fact for thread cutting, I think there are more conflicting words of text than for any other machining operation, so it has to be a personal decision as to which commandment you follow.


Bogs

Ned Ludd:
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?

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

andyf:
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

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