|Ian Johnson 1||16/09/2019 21:59:06|
|222 forum posts|
Although the Newnes book says to 'advance the tool slightly several times during the operation' I tend to move the top slide in by a third of the depth of cut. I don't have any scientific or mathematical evidence to back this up, only that in my brain a screw thread, particularly metric or unified, is an equilateral triangle, so a third of the depth of cut sounds about right. And the important cuts are the last two or three where the tool is cutting on both flanks of the thread, so the approach angle is irrelevant, as long as it doesn't rub too much.
Another thought. It will actually be half the equilateral triangle, a right angled triangle, with a stepped hypotenuse when the tool is moved down and along at each cut, if that makes sense? I'll have to do a bit more research and if I was any good at CAD I'd do an impressive drawing, but I'm not, so I won't!
|duncan webster||17/09/2019 00:36:56|
2345 forum posts
OK you want trigonometry, here we go. Moving the topslide by half the cross slide generates and angle of atan(2) which is 63.4 degrees. The theoretical angle for Whit form threads is (90-55/2) = 62.5 degrees, so the trailing flank will just scrape a bit, which is ideal. I reckon this is where Mr Whitworth got his 55 degrees from, otherwise it's a bit of an odd angle. It does work for metric (60 degree) threads, just scrapes a bit more
If cutting a big thread on a small lathe, you could try doing this for half the thread depth, then just advancing the topslide until full width reached, then go back and make it deeper, if you get my meaning. I've never tried this, and you'd need to make sure that SWMBO was out so you didn't get disturbed and lose count, but it should work.
Note this is not the same as 'plunge' cutting where the topslide is left alone. This cuts on both flanks at the same time, so the chips collide with each other. Yes it works with big lathes and full profile tools, but I am assured by a man who owns several that industrial CNC thread cutting programs arrange for the tool to be advanced down one flank, presumably by altering the angular relationship between lead screw and spindle. They also reduce the depth of feed to keep the chip volume constant. Watching CNC thread cutting is awesome, it's all done a lot faster than manual.
3988 forum posts
Thanks. Been to Bonneville salt flats watching motorbikes go really really fast.
621 forum posts
Lucky bar steward.
Envious? Just a bit....
|Michael Gilligan||17/09/2019 08:14:25|
14783 forum posts
An interesting hypothesis, Duncan
There may be some truth in it ... but the evidence suggests that Whitworth's choice was more statistically based:
In 1841 Sir Joseph Whitworth produced a paper on a universal system of screw threads. He then collected a variety of screws and proposed a universal thread using their average pitch and depth. The result was the 'Whitworth thread' with the depth and pitch of constant proportion, giving the 'V' thread a mean angle of 55 degrees and the number of threads per inch was specified for various diameters. The thread was first introduced in his own workshop and was in universal use by 1858. It was not until 1880, when his standard gauges and screw threads were in common use, that they were officially adopted by the Board of Trade.
|Andrew Johnston||17/09/2019 09:17:38|
5115 forum posts
Screwcutting is simple, but it takes a modeller to over-complicate it.
For normal threads I screwcut using full form inserts and plunge cutting, so that the cross slide reads total DOC directly. A lot of my threads are screwcut to a shoulder and using the topslide at an angle would make it more involved to set the axial finish point. With the plunge method it is simple to set the clearance to a couple of thou once before cutting the thread.
Screwcutting on a CNC lathe often uses the G76 macro. It is untrue to say it cuts down the flank as a default. Annoyingly each manufacturer has a slightly different syntax, but all contain a parameter that controls the infeed angle, a value of zero usually represents radial infeed, ie, plunge cutting. So the angle at which the tool infeeds is set by the operator. I understand that newer controls have an option for infeeding on both flanks, alternately one side and then the other.
I should add a caveat for manual screwcutting imperial threads on my imperial lathe. I have an Ainjest high speed threading unit which automatically trips at the end of each pass. The trip point is pretty consistent, so it makes it easy to screwcut rignt up to a shoulder, internal or external.
|vintage engineer||17/09/2019 09:52:03|
220 forum posts
Setting over the topslide in a production workshop would get you the sack!
|323 forum posts|
Vintage engineer, you beat me to it, but in a toolroom not setting the topslide to half angle would get you either sacked or demoted to production work!
|Mick B1||17/09/2019 10:16:19|
|1363 forum posts|
I think both of you are being unfair - I don't recall coming across either production or toolroom chargehands that were so hidebound in attitude.
If you're cutting a heavy precision thread, and shop availability makes you use a lathe that's rather small for the job, of course it makes sense to use half-angle to get the best chance of making a decent job, production or not.
In a busy tooroom, there's time-pressure on all sorts of jobs, so if the thread's within easy capability of the machine and tooling, and not especially critical for precision, use of plunge cutting would be well understood.
I've seen both methods in use in both environments. You might, of course, be required to justify your choice, so you better have yer ducks in a row...
Edited By Mick B1 on 17/09/2019 10:17:53
|323 forum posts|
Mick B1, you haven’t worked in the places I have.
|Mark Rand||17/09/2019 15:49:54|
|849 forum posts|
Ok, my turn:- I tend to set the top slide over, but to 63° from normal. That's because the tool post has got 40 positions, which works out at 9° per step. I don't want to have to adjust two dials per cut. Hardinge recommend not exceeding 1,000 rpm for threading, but I rarely exceed 600. Even at that low speed the process of left feed/quick retract/right feed/quick un-retract/increase DOC/left feed, etc. goes pretty darned fast. Five seconds per cycle is taking it easy.
|Jeff Dayman||17/09/2019 18:06:14|
|1723 forum posts|
HA HA HA HA HA etc.
I've worked for a few right tartars - Hitler didn't have a patch on 'em - every bit of nastiness they could muster, every minute every day. Not very good toolmakers / machinists either. Still don't know why they acted that way, except maybe they were always treated like that.One guy couldn't hit a thou tol if his life depended on it.
(But I've also worked for some really personable and fair ones - and they were very capable ones too.)
|Tony Pratt 1||17/09/2019 18:19:01|
|988 forum posts|
|Ian Johnson 1||17/09/2019 18:58:54|
|222 forum posts|
Impressive bit of Trig Duncan thanks. I think I'll stick to a third of depth of cut, that way the trailing edge won't scrape at all. And yes CNC thread cutting is a great thing to watch, not as fast as thread rolling though, it is fast!
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