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Metric Thread On Imperial Lathe
RichardShute:
--- Quote from: doubleboost on December 20, 2010, 10:11:37 AM ---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 was 39.8mm.
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
--- End quote ---
Thanks for taking the trouble John, I'm very curious to know how this comes about. One aspect I had not included in the sketch I did before was the shape of a real thread which has a clipped top and a radiused root, both of which reduce the effect, but I still can not see how your result comes about.
Here is a notional 60 deg thread with no flat top and no radius:
This is a single groove made with a 60 deg tool fed in at 30 deg from the spindle:
and this is two adjacent grooves on a 1.5mm pitch, showing the clipping at the top of the form:
That makes no allowance for the flat top or root radius and is the form I was using previously. Not strictly accurate I know, but it domonstrates the point of discussion.
This is a true thread form with a flat to a depth of H/8 and a root radius at the full quoted depth of 0.92 (both figures taken from a Zeus book):
This shows the top again clipped, but by a reduced amount of 0.33 because part of the top is already removed by the flat and the root radius also reduces the amount removed.
Can anyone shed light on why this appears not to be reflected in the thread actually cut.
Richard
andyf:
I’ve been puzzling over the remark which Bogs made before his data got lost, and have come up with these approaches to “going in at an angle”.
Method 1. The diagram below shows how I normally set up for threading, with the topslide set at 60º to the work and at 30º to the cross-slide (actually 60.5º or 29.5º, but I’ve rounded them off). Depth of cut is applied using the topslide.
Method 2. The diagram below shows the set up for threading when applying depth of cut with the cross-slide, but also shifting the topslide (and thus the tool) towards the headstock a little between each pass to produce the angular feed effect. The rule of thumb seems to be that the topslide is shifted left half the amount by which the toplide has been fed in.
Method 3. The final diagram below shows the situation which seems to have caused controversy. The topslide is at 30º to the work and at 60º to the cross-slide. This set-up could be used to simplify the “cross-slide parallel to the work” method. Trigonometry shows that advancing the topslide when it is set at this angle will advance the tool into the work by half the amount indicated by the topslide dial. So in this case the cross-slide and topslide can be advanced by the same amount as shown by their dials to produce the same result as Method 2 with its parallel topslide. This avoids the bother of remembering by how much the cross-slide has just been advanced and then advancing the topslide dial by half that amount. Each dial is advanced by the same amount in Method 3.
Am I on the right track. or have I got myself completely lost?
Andy
Ned Ludd:
Hi Andy,
What an interesting idea, and yet another method. :D
Going back to the original post, the following You-Tube was pointed out on the ME site today. Alright, it is yet another video on threading and perhaps it is a touch long, there are five videos for the one thread, but I always like to watch others working. Oh, and it does seem a bit like an advert for tips and oil.
Ned
Bogstandard:
I think you are going into too much theory, and I would suggest you read my very first statement on it.
--- Quote ---if doing super fine threads
--- End quote ---
If you are going a lot deeper, then you will run into all sorts of problems, as shown above.
There are reasons for cutting on the leading face, and that boils down to removing metal without leaving behind the raggedy a**sed threads you usually get when using the straight in technique. What I actually do for very fine threads is to rough it out, without going too deep (hence the shallower angle, which gives greater control of the depth) then use a final straight in approach to clean the back edge up, with maybe one or two tenths infeed. I hope all that reads correctly, I've only just got up.
I find it works for me, but in theory, it shouldn't work. But as with a lot of things, practical doesn't always match theoretical.
That is why I hate it when people start throwing formulae around, it is just too confusing for the laymen who read such posts as ours, it makes them feel so inferior, that they never even try to do something out of the ordinary.
As I stated before, that was what was shown to me, where that old chappie got it from, I have no idea, but somewhere in the past, a machinist must have tried it out and found that it worked for him as well.
But I will say one thing, my tiny threads are always as smooth as silk.
I think where a lot of people go wrong is that they are only looking at standard sizes when threading, say 1/4" diam x 40 TPI. But in fact, you can just as easily have 3ft diam x 40TPI, exactly the same thread profiles, but on a much larger diameter. The critical part is getting that very fine thread to work with it's female counterpart.
Anyone who has visited my shop will see that I can get up to all sorts of weird things, but unfortunately, at this time I cannot show you anything, maybe next year after the patents are completed, but that also includes some strange but very fine threads, all single point cut.
I hope Peter (HS93) doesn't mind, this is a job I am attempting to finish off for him, but here is the sort of thing I am on about. Making a few new blades for a model steam boat variable pitch propellor.
The hub is less than 1" diameter and is full of bevel gears, and the threads have to be a spot on match for the ones already cut in there. Because if they aren't, because of the very tight tolerances on the unit, the whole lot would sieze up. I have made five stubs, just in case I bugger up on the machining to follow.
So as I was saying in a previous post, if you find a method that works for you, use it. If you like what you see, try it, but don't always expect it to be right for you.
Bogs
andyf:
--- Quote ---Bogs: I think you are going into too much theory.....
--- End quote ---
You're right, of course. But it's so cold in the shop that sitting indoors and theorising is all I feel like doing just now. Of course, what really should be doing is sitting indoors finishing off my tax return, but that's an even less attractive proposition than a sub-zero garage!
My thanks to Doubleboost for his video. I wish I had a 127T gear to get exact metric threads out of my imperial leadscrew, but there isn't room for one in Mod 1, so I would need to use a smaller Mod for the 127T and whatever it meshed with. After a lot of calculations involving my standard gears with two rather than one sets of compounded gears in the train, I can get get down to 0.01% error or less on the common metric and BA pitches. That's 1mm out over 10 metres of thread, or 0.0001" over 1", and my Chinese leadscrew may be no more accurate than that in the first place.
Andy
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