|Neil Wyatt||02/09/2021 17:56:18|
18899 forum posts
|Neil Wyatt||02/09/2021 17:59:44|
18899 forum posts
The answer is simple, if your lathe doesn't struggle, then no need to do anything fancy.
If it does, then the over by half for every one in is simple and no trickery needed to calculate the depth of cut. And has exactly the same effect as angling the top slide.
174 forum posts
If you have three axis DRO on your lathe (x, y and topslide) you can set the topslide to 30 degrees and engage 'vectoring' such that the radial movement is displayed when you move the topslide. This gives the depth of cut. Works very well and is what I do....
|Alan Charleston||04/09/2021 08:26:38|
|126 forum posts|
One thing which hasn't come up yet in this discussion is the question of top rake on the cutting tool. While cutting a thread into brass doesn't require any top rake on the tool, if you are cutting steel a zero top rake tool gives a poor surface finish and some top rake needs to be provided. The way this is ground results in a positive rake on the left side of the tool and a negative rake when cutting with the right side of the tool. If the tool is fed straight into the work, one flank will be cut with a positive rake cutting edge while the other flank will be cut with a negative rake cutting edge. They can't both be the optimum configuration. While not completely eliminating cutting with a negative rake, feeding the tool in at half the thread angle will greatly reduce it, particularly as the thread deepens. Could this be the reason why the practice is so widespread?
I have a vague recollection that when I was taught to angle the feed when cutting a thread that the reason given was that it reduced chattering.
The reason I know that zero rake tools give poor surface finishes in steel is that I never got the hang of grinding top rake onto threading tools and used zero rake tools and ran a die nut down the finished threads to tidy them up. The problem went away when I bought a tangential cutting tool system from Eccentric Engineering. Using this system both sides of the tool have positive rake. Once I figured out how to grind the tools using the jig, my steel threads came out with great surface finishes and didn't need tidying up with a die nut.
|Martin Kyte||04/09/2021 11:11:05|
2638 forum posts
Couple of observations (and I use both methods, half angle and sraight in)
Firstly there are times when cutting a thread is desireable on a fairly slender workpiece in order to ensure that the thread is concentric before using a die to clean up. Top slide set over puts less pressure on the work so you get less deflection. In addition to this the tool does not have to be exactly ground to the precise thread angle (hard to acheive if the tool has side rake). If it is slightly less than 55 or 60 degrees (according to thread form) the flank angles will be generated by the toslide angle provided the leading flank of the tool has been set accurately.
Secondly for tools that have only a small radius on the tip (such as inserts) it is neccessary to take extra cuts once depth has been acheived by moving the tool axially to generate the flat bottom of the thread and ensure that the flanks are correct (that the threads are not too wide). This is not easy with a set over top slide.
Far from being a 'Myth' it's a case of horses for courses. Choose the methodology that suits the thread you want to cut and the equipment at your disposal.
|Chris Mate||04/09/2021 18:45:57|
|83 forum posts|
Interesting, I am curious to know if the thread strenght, torgue that can be applied, had been tested for each method-?
|Chris Crew||05/09/2021 09:52:44|
179 forum posts
"Ahh, but what you're forgetting Gray is that most on this forum work to NASA standards and have limited time for what used to be a HOBBY. Also from an angled luddite."
Ian, I loved your comment, it really amused me!
I had a bit of a light-hearted 'run-in' a few months back with someone over the correct setting of clearances of lathe change-wheels. Personally, at the time, I thought the suggestion that was being made was pedantic nonsense and tried to say we are mostly amateurs pottering around in back-shed workshops, not working in the tool-room at Rolls-Royce. Naturally, I was sentenced to being in the wrong!
Your comment encapsulates the point I was trying to make wonderfully. Well Done!
|Graham Meek||05/09/2021 12:00:10|
|431 forum posts|
A Cautionary warning,
One problem I have with advancing the topslide in unison with cross-slide is this could cause a collision if working to a shoulder or a face when using a lathe equipped with a screw-cutting clutch.
Using such lathes the screwcutting finishing point is initially set before screwcutting commences, sometimes with thou's spare as regards clearance. Thus for anyone who has made any of my screw-cutting clutches I would urge them to follow the method outlined in the articles.
Henry Ford's Company once said, when asked to build the Rolls-Royce Merlin Engine during the war, that they could not work to the Rolls Royce drawings as the tolerances were too slack.
Just because we work in sheds, or sometimes centrally heated workshops, there is no reason in my book why we cannot aspire to make better things. Those who look at their work, and think, I could have done that better are to be commended. This is how we were taught to hone our skills as apprentices, and as far as I am concerned this ethos has paid me huge dividends.
Edited By Graham Meek on 05/09/2021 12:01:13
7921 forum posts
Horses for courses is my take too, though maybe a bit of a myth has developed due to conservative Model Engineers ignoring tool improvements since the angled method was first developed in the 19th century. We have better horses now!
The angle method dates back to treadle lathes and plain carbon steel cutters. These need all the help they can get, giving angled threading a strong advantage.
Time marches on. The value of angled cutting became questionable after lathes beefed up to drive HSS cutters. HSS revolutionised metal turning because the alloy's improved heat-hardness can remove metal 5 or 6 times faster than a Carbon Steel cutter. After about 1900 lathes became much more rigid to handle the extra strain. However, many small workshops were - and still are - equipped with relatively light machines, ½HP being typical fdr many years. And even with HSS and a hefty lathe, there are times when angled threading is worth it.
Time marched on again. Carbide can remove metal 10 to 20 times faster than HSS, and - provided the machine has sufficient power and rigidity and the work can take the strain - cutting threads straight-in is fast and efficient. VFDs have made it possible for hobbyists to own seriously chunky ex-industrial 3-phase machines so many have the wherewithal for brutal cutting.
I don't think the border between angled and straight threading is clear cut; it depends on what you have My 800W mini-lathe could thread straight-in with a carbide insert but I felt more comfortable using the angled method. Mini-lathes aren't tough guys. Since upgrading to a WM280, about 3 times bigger than a mini-lathe, I almost always thread straight-in with carbide inserts because the bigger machine handles it well. But I still angle cut occasionally - mostly slender work, exactly as Martin says.
Horses for courses.
Edited By SillyOldDuffer on 05/09/2021 12:25:55
|Speedy Builder5||05/09/2021 13:12:03|
|2501 forum posts|
For the last year or so, my threading has not been up to scratch, single point screw cutting most often cleaned up with a chaser. I needed to change some gears the other day to get 19tpi yesterday and noticed that the 56 t gear (standard) was actually 58 t !!
Boxford with screw cutting G/box
|Martin Kyte||05/09/2021 15:06:32|
2638 forum posts
In my opinion all this 'blame it on the treadle lathe' is a red herring.
How many treadly lathes had screw cutting. Even when they did the amount of power available was far greater than that required to do single point thread cutting with any method straight or angled.
More important is rigidity and as I said with small workpieces the limiting factor is often the resilience of the piece being threaded. As to accuracy the angled method essentially generates the flank angle by the topslide offset and is less reliant on the accurate grinding of the tool.
The downside is, as I said, the difficulty in thinning the threads to obtain the correct fit so you can use the same tool for various TPI's
22017 forum posts
Martin how does the angled method produce the profile and not need a correctly ground tool? General advice is to set the topslide to less than half the thread angle, to get the right flank correct would need it to be exacyly half and even then you could get a stepped flank. Also if tool angle is in excess of the thread angle. it will again cut the flank wrongly.
Partial form inserts also allow you to cut a range of tpi and will go deeper on the lower end of their range so no need to move them lengthwise to get the valley flat
|2402 forum posts|
Re your question to Martin - I think there is at least a partial answer to this in GHT's book.
|Martin Kyte||05/09/2021 18:07:48|
2638 forum posts
That assumes you can create a perfect tool geometry. Almost nothing about screwcutting is going to be exact.
Lets assume that you can grind a tool tip to just less than 55 degrees (whitworth thread). We shall grind some side rake so that will reduce the angle anyway. Lets assume its a degree less. Setting the topslide to 27.5 degrees and the leading flank of the tool bang on to a guage. The tip of the tool will travel to the angle set by the slide which will be correct. You are perfectly right about the steps on the trailing flank but how big are they. Lets take 4 thou cuts. The 'steps' will then be 4 X Tan 1 = 4x0.0174 or 0.07 thou which is to my mind an acceptable surface finish especially if you tidy up with a die. You can always get a tool to be less than the angle you want with a simple guage so tools with bigger thread angle can be avoided.
You are again correct that inserts will allow threads to be cut deeper but you may not always want to lose core diameter.
It does make sense to set to not quite the thread angle with inserts ensuring that the bulk of the cutting is done on the front flank giving plenty of space for the chip to be cleared and to ensure a scraping cut on the trailing edge to tidy up.
I would say this was setting to slightly greater than half the angle though.
In general I would use the set over method for my own home ground tooling (and I possess a cutter grinder so angles are generally close) and straight in with inserts.
I shall have to dig out GHT and remind myself what he says.
Screwcutting is one of those things that most of us don't do enough of to really get good at it, me included. Mostly taps and dies suffice for our needs, and I suppose I am really thinking of the need to produce a precision thread without additional chasers, dies or files. Getting something that will do up tight and not fall off after 5 minutes is easy. Threads to induce accurate smoothe motion or ensure perfect concentricity and pitch well thats something else.
As I said before it helps to realise what a particular method solves for you before choosing it. Normally we do like I did and use the way we were shown until the "hang on a minute" moment comes or you start reading 'Thread' threads if you excuse the pun.
22017 forum posts
Another assumption is that I may just buy an insert and holder which will have the correct angles handy for those without a T&C grinder. The insert may even have the right radii which helps if it's a non standard size and a die is not available or simple because the thread was screwcut to save buying a die
Also being that the insert is held at right angles to a nice long holder it is far easier to set it correctly than trying to line things up by eye with a fishtail thread gauge as you have a 100mm plus length of holder to line up.to.
Lastly far easier to get the angle of feed correct using the straight in cross slide method* than risking the topslide scale being right or having to faff about setting with say a sine plate if you have one. ( * Bit of sideways movement if it's a deep thread or light machine to ease the load)
Given all those factors the beginner may get a more accurate result with inserts fed straight in. Infact don't really even need to buy a threading insert and holder if you have a DCMT or DCGT tool and want to do 55deg threads Or TCMT/TCGT for 60deg unless you want to get close to a shoulder. Well that's my way of looking at it anyway.
Edited By JasonB on 05/09/2021 19:30:43
|Michael Gilligan||05/09/2021 19:33:55|
19601 forum posts
The round-bed Drummond for one …
”… the inexpensive Type A "Round Bed" was announced in the Model Engineer Magazine of May 21st, 1908. Publicised, with some fanfare, as the first £5 screwcutting lathe …”
That said, I do agree regarding the power
Edited By Michael Gilligan on 05/09/2021 19:38:26
|Martin Kyte||05/09/2021 22:43:30|
2638 forum posts
I was just supposing a case amongst many. I don't disagree with you Jason. As I said i would go straight in with inserts. My point was if you undersand what each method does then you realise when to switch. You obviously use inserts so it makes sense to go straight in. I'm not advocating one thing over another.
|Ian Johnson 1||05/09/2021 23:06:28|
|370 forum posts|
I forgot these photos were in my album, must have used them when this topic came up some time ago.
This is the method in 'Newnes Engineers Pocket Book' explains it all quite nicely. The technique is mentioned in bottom of the first page and carries over to the top of the second page.
'the tool is advanced slightly several times during the operations, but allowing the tool to cut all over on the last one or two cuts to obtain a thread to correct form'
That'll do me because that is the way I have always cut threads on a manual lathe, nice and simple!
|Graham Meek||06/09/2021 12:02:13|
|431 forum posts|
I think the whole matter of the "set-over method" was spoiled when some one published tables for various "in-feeds" on the topslide for each thread.
This put people off big time, as many have told me so.
Those who have visited my workshop have seen how easy and quickly I can set-up and produce threads which involve no calculations, no setting with vernier protractor and the topslide was even set to 45 degrees in one instance to prove a point. As long as the tool is square to the work every thing will be fine.
It was even clouded further, when another set of tables was produced to cater for the 1 degree less than half the thread angle.
If the originator of the tables had read GHT's original article on Screw-cutting, the need for such tables would have not been necessary.
Lastly it would pay to observe the manufacturers tolerance on the angle of a commercial tap, before worrying about slight variations in tool geometry due to side rake etc.
|Nigel McBurney 1||06/09/2021 14:24:32|
965 forum posts
According to early workshop practice,threads were cut with a tool with side rake and the cross slide set over,the thead was cut and then finished with a thread chaser,either hand held or clamped in the tool holder,These old chasers were carbon steel,so were all the cutting tools,and most dims were measured using fixed joint calipers and a rule,my neighbour over 50 years ago still owned the workshop where he and his father before him had a large shed which held a lot of machine tools plus capacity to keep four traction engines under cover and space to maintain others. ,the smallest lathe was 6 inch capacity with screw cutting by change wheels,and driven by belt from the overhead shafting or by a big treadle,the owner explained that the treadle was often used for small jobs as it saved all the hassle getting the big big electric motor going with the extensive line shafting. the lathes were really old ,the crosslide v ways were set below the level of the top of the saddle,possibly making it easier to mount jobs on the saddle for boring. the various features indicated build around 1880s .I have a carbon steel chaser with the name Joseph Whitworth stamped on the shank,chasers went back a long way. I myself being trained as an instrument maker where chasing direct without single point tools was the usual way for cutting threads, and I now have a good range of HSS machine chasers for screw cutting direct into brass and steel, though I have hss and carbide single point tools . Though you have to be quick and coordinated to withdraw the chaser and disengage the half nuts when working up to a shoulder.All the various methods of screwcutting have there use,it all depends on material,equipment available,and skill of the operator.
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