Travelling Steady.

[mgjParticipant @mgj]

Well fair enough – Since the steady is there to resist forces, and you say you understand them, I don’t quite see your point. We know what the forces are, and what they can do. So clearly the “right” answer is the one that best resists them directly, in so far as is practical

 

If one understands the problem yet chooses to ignore it – thats someting I find…. most unusual in an engineer.

 

So yes, as someone with an MSc and a year or two in engineering, I am quite happy to differ – because it doesn’t make engineering sense. Why start introducing a moment, and the potential for a long slender workpiece to lift , when you don’t need to?

 

One can achieve perfection in this case, and yet one shies away from it?

 

 

 

[KMPParticipant @kmp]

Sorry, too many words so willhave to start again

Keith 

Edited By KMP on 12/03/2011 10:56:46

[KMPParticipant @kmp]

I do not mean to argue with anybody, all posts in this thread I have read offer good practical advice and will work in a relevant situation. The discussion also identifies the two main viewpoints in such situations, the practical (worked for me), or the theoretical (detailed scientific analysis of the process). Both have their application of course but I would argue that in this instance too much “discussion” of the theoretical may serve to confuse the OP. Encouragingly, in this thread both camps appear to understand what the other is trying to say; sadly that is not always the case on engineering forums.
 

My main gripe is that without a full specification of the job or actually seeing it we all have to assume and therein lies confusion for the less experienced. To understand and make relevant all the advice so far offered one would need to be an experienced engineer. So firstly, a plea to Geoff, for clear advice we need at least the length, diameter, pitch, thread form, material and the degree of accuracy you have to achieve. Along with the relevant tooling you have or are willing to make or buy. Any machining problem has many solutions and we need to find yours.

 

The main point of contention appears to be where to support the work, in my limited knowledge, I believe the best position is directly opposite the cutting point. This is where the difficulty starts and some of the potential problems have already been identified. This position is more critical the longer and thinner the thread is. In my simple understanding this is because the forces involved in the cutting process have to be resolved by the rest, headstock or tailstock (and of course the tool post but I guess that is of sufficient rigidity in this case). If the latter two the forces are transmitted through the workpiece and can cause distortion. However, practicalities can affect what is happening and such things as soft fingers being mangled and reducing the support available during the cut or hard fingers rolling the burrs into the work can both affect the best practical supporting position as of course can machine access and rest design. So given that we have not got the tooling for the perfect solution (in my limited practical experience we would have used a boxtool or roller box where support straddled the tool point), we have to compromise. The position of the top finger is also important as the work will bend up and away from the tool and try to go between the two fingers. Here again a number of factors will affect our success but, if we are cutting a long thin fine pitch thread of a full form with soft fingers it will go there if we ignore the burrs and do not re-set the fingers.

 

Assuming a typical leadscrew of not less than 6mm or so and using the rest in the pictures I would suggest a position where the rear edge of the support point is directly opposite the tool point. The pitch of the thread will take the burr slightly to the rear of the rest and so will neither wear the rest or be rolled in. For threads smaller than this I personally would cut the thread with the rest directly opposite the tool and check the damage to the rest and reset if required after a couple of passes if cuts are small as they should be. Where possible I would make new finger tips that straddled at least 6 thread crests and more if possible.

 

Keith

PS Still to crack the formatting when cutting and pasting from Word

Edited By KMP on 12/03/2011 12:33:44

[KWILParticipant @kwil]

I cannot agree about not allowing for the helix angle, try cutting a 2.5mm trapezoidal without it.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Martin Cleeve many years ago described a bushing steady which he had made himself to overcome a problem with screwcutting a long small diameter feedscrew . This consisted of a plain plate of mild steel bolted to the chuck end of the carriage and having a hole bored in place and fitted with a previously turned bush to carry the work piece . Since it was a DIY job all dimensions could be adjusted to best suit the work that he was doing . For general purpose use this set up would be a bit inconvenient but there is no doubt that where the set up is possible a relatively long complete bush gives far better support than the two fingers on the normal travelling steady .

 

 

Seems a good place to mention a couple of points arising from Harold Hall’s recent article on steadies .

 

(1) A plain round bar can be set to run perfectly happily in a fixed steady when the bit in the chuck is a few thou eccentric . The only requirement is that the lathe spindle and bar axes intersect at a point inside the steady – ie that the bar is running true where it passes through the steady – it doesn’t matter what the rest of it is doing .

 

(2) There are completely standard methods of setting up both plain bars and more complicated work to run between chuck and steady just using one or better two dial gauges .

Edited By MICHAEL WILLIAMS on 12/03/2011 13:11:57

[mgjParticipant @mgj]

Graham – I agree entirely about burrs- good sharp tool. Perhaps I am fortunate, having a Quorn, but for others, possiblyly a tipped tool for screwcutting is the way to go. And if, as you say, there are no burrs, then the carriage doesn’t get dragged about.

 

With the harder work, perhaps using differnt tips is a counsel of perfection – as long as one doesn’t pressthe fingers in too hard and chucks a bit lof lubricant about, and I think in this case grease of often better than oil. The soft materials are/can be a different story but hte average ME doesn’t machine long slender length ver often. In the samller sizes the little combination tool is pretty handy.

[KMPParticipant @kmp]

Graham, mgj, Hi,

Sorry no subterfuge intended, I am neither a “Rocket Scientist” or a “50 years on the dials” man. My background is in Aircraft but includes large machine shops repairing worn or damaged parts.

I have no issue at all with what you say but feel that we need to put it in context for the typical “home workshop”. As you say, only luck will see you with a “round” bar off the shelf but it will not be straight either. My concern would be if Geoff, on his new machine with an unfamiliar rest can better the errors one would typically find on the OD over a long (?) length. If the leadscrew is less than 13” (shows my age) long I would suggest Silver Steel would be a reasonable starting point but again not perfect. I would have suggested EN8 (BS 970 080M40 Carbon Steel) or even better 226M44 (BS970) listed as “USACUT 45”. Now I am wandering and suspect those type of details will not bother Geoff at the moment.

I totally agree with your and mgj’s comments on burrs but, once again, we must put it in context of someone who is developing his skills. I have found in helping others into this wonderful world that “sharp tools” is something they either learn instantly or after a period of struggling. For some reason many see tool grinding as a “black art” or something that needs a Quorn and the ability to use it (I have yet to build one myself but it is on the “to do” list). The tipped tool path is fine but one needs to select the insert carefully as the cheaper variety for general use will not be that sharp, unless one has found the newer generation of “aluminium” positive rake tips that are sharp and superb on most of our materials. I can’t really recommend as I do not know what the thread angle is being produced, 60 deg would be easier.

Again, I agree with your comments on finger tip wear, but the newer engineer also has other factors to contend with. Chip removal is often neglected particularly where no coolant flow is available or is inadequate. A problem on subsequent cuts but can cause havoc when they get between fingers and work. I find cheap paint brushes invaluable particularly if I am using cutting oil. One other major issue is with using any old “mild steel” whatever that is; I have seen some round bar that would not cut cleanly with the sharpest tool.

Sorry Geoff if I have confused the issue for you and profuse apologies if in my assumptions I have put you on the wrong part of the learning curve. I am guilty of always starting with the very simplistic but in this hobby without someone looking over your shoulder, the basic can be the most difficult to pick up. Most writers on the subject are at the other end of the curve.

Keith

[mgjParticipant @mgj]

Keith dead right.

 

I just wanted to get away from this notion that you put the fingers anywhere but (if possible) directly in line with the axis of the force, to stop slender objects lifting. We have all had that problem and we all know it leads to wrecked work.

 

I managed for years without steel fingers – I just did my feed/leadscrews with bronze tips and didn’t worry. I still wouldn’t worry on the Myford, because I don’t have steel tipped fingers for it, and so low on my list of priorities are they that I doubt I ever will – ain’t that important for us amateurs.

 

I only went on about sharp tools because of the burr issue. That’s why I suggested tips – because if one is starting thats one of the tool grinding cops outs in an area where perhaps its a touch more critical than others. But frankly I could live with the burrs – as a non grinder of tools, I had to build the Quorn of course!

 

I’m actually a very pragmatic downmarket/mostly good enough sort of model engineeer, but I don’t get over impressed when people post stuff that is clearly unwise or can lead to disaster. The less experienced pick it up and dive into trouble, and the older ought to know better and none of us are so ancient or good that we cannot learn or change. It’s about open minds.

 

Here we just got dragged into lunacy, because there was a lack of thought, in engineering terms about what was going on in a machine tool – and that was perhaps why I got more technical – but then its a technical subject, and those that wish to progress need to learn, and those that don’t learn continue to get it wrong. (Still I never got the maths out to prove my point algebraically )

 

My background – I was a soldier, and not a nut strangler either. But my degrees were in AVF design and guided weapons engineering. (pluys racing cars – they were important)

I have no beef with a word you said.

 

 

 

[mgjParticipant @mgj]

Thaks Gray – but now I’m going to disagree with the idea of silver steel for leadscrews. Its only half an apology, because one of ther other recommendations was IMO the best selection. (Maybe SS wasn’t your recommendation?  )

 

anyway …. wherever it came from

 

Let me have a go at SS first. Often its recommended because its tough, but for me that is only half the story. Its very difficult, except on exposed surfaces like say loco crankpins or the like, to get a really high finish. So when you add that to a bronze feed nut, unless one is very lucky, one will be through the feed nut a bit sharpish.

 

So what one needs is a material which above all can be finished well, because in general on leadscrews there is plenty of bearing area. Surface hardness/toughness for us, is not an issue. (normally), So EN8 for my money is a much much better choice. (Or even 220M07. For most modelling applications – because you get such a fine finish)

 

If wear really is a problem (and even a Myfords feedscrews are not hard!) then one has to surface harden. Well casehardening will almost guarantee to creat distortion, so one is left with nitriding – if its absolutely necessary. Which it probably isn’t

 

At one stage I used to help a friend knocking out milling machine feedscrews – the originals weren’t hard at all. You could file them easily. Nor were the replacements – no we didn’t file them, but I know what they were like to machine. And I know darned well where the steady fingers were set and why!!!

 

Quite nifty at Acme threads me!

 

Going back to loco crankpins – I did make mine out of SS and what a pain it was, getting all 6 to exactly the same dimension (+/-.0001″). For no gain in wear properties. Next time it will be 220M07, and case harden, for a better, harder – where it matters – and more easily made product.

 

 

 

 

Edited By mgj on 13/03/2011 16:41:12

Edited By mgj on 13/03/2011 16:44:56

Edited By mgj on 13/03/2011 16:49:14

Edited By mgj on 13/03/2011 16:58:01

[TerrydParticipant @terryd72465]

Posted by Graham Meek on 12/03/2011 16:10:58:

Hi KMP,

………………….

 

The reason I ask is that drawn bar will seldom be round and dependent on the diameter will have a helical wave running down the outside due to the rolling process. The travelling steady should always follow the tool on initial cuts so that these imperfections are not transmitted to the work via the steady fingers.

 

…………

 

Graham

Hi Graham,

 

you have me really confused on this one, You say ‘drawn’ bar is ‘rolled’. I was always under the illusion that ‘drawn’ bar was actually drawn after rolling and pickling which, given accurate dies, produces quite an accurate material. The drawing process should remove any imperfections such as you mention due to rolling. That is the whole purpose of drawing surely.

 

Best regards

 

Terry

[KMPParticipant @kmp]

Gray, mgj, Terry, Hi

Gentlemen, and I thought we were getting on so well. In Graham’s defense twas I, I suggested that Silver Steel could be an alternative. In my defense, I didn’t recommend it and suggested En8 or 226M44 both of which I have found superb for this application. The suggestion was nothing to do with its toughness or hardness but was in relation to having an accurate, consistent outer diameter to work with rather than to have to surface a long slender bar. My own experience of Silver Steel is that it certainly needs more careful selection of speeds and feeds but, in its normal (soft) state, it machines better than many of the alternatives. As mgj says there is no need to harden it in fact life is made much more difficult if you do. I have found that with a, sharp tool, SFM at the lower end of the range, and with a positive depth and feed all will be OK. For small tasks I tend not to use lubricant and have found “suds” better than oil if necessary. It certainly doesn’t like any rubbing or “dust” cuts with a blunt tool, hard spots will quickly appear. So if you tend to creep up on a finished size, leave it alone and pick one of the other suggestions. Do though make sure you actually have En1a, En8 et al, as many suppliers are apt to take liberties when supplying our small needs.

Mgj, one day we might explore the wear rates of two mirror surfaces when under load without a pressure oil film, but Geoff’s thread is not the place I feel. You will also have to promise that the maths are kept out of it.

 

Terry, the “accepted” standard for drawn steel rod between 6 – 18mm is an allowable deviation in size of 0.07mm (2 thou in my old money), in fairness the actual is normally much better but still worth checking if the diameter is important. Anyone had a tight die on nominal bar?

 

Keith

Edited By KMP on 13/03/2011 18:38:08

Edited By KMP on 13/03/2011 18:41:09

[Anonymous]

Hi Keith,

 

Oh dear you’ve been short changed; I make 0.07mm nearer 3 thou.

A tolerance of 0.07mm, up to a diameter of 18mm, comes under a h10 tolerance class. However, the ‘out of round’ condition is specified to be not more than half the OD tolerance.

Regards,

Andrew

[mgjParticipant @mgj]

I wasn’t attacking the SS person – with apologies to all the innocent. Just saying why I wasn’t so keen, and if I had the choice…..And hopefully putting sound reasons.

 

 

KMP – you are right about mirror surfaces and oil. Very often the smoother surface can generate more heat because of the increased area in contact, and sometimes the surface is so shiny the oil won’t stick.

 

One wants to have a nice open grain surface. (cast iron holds oil well at the molecular level) You pros I daresay can cut SS to a wonderful finish. Quorn or not, mine is a slightly more than open. More sort of like a badgers bum. Hence my reasoning.

 

220M07(Pb) is pretty good stuff. We get very familiar and tend to write it off – its weak, it cuts well, its soft etc. Well it ain’t that weak, and that lead in it makes it very slippery, and a very good choice for sliding surfaces, in absolute terms, never mind model engineering.

 

Talking of pros- Gray – I saw the Seagull you made in another mags advertising blurb. Beautiful job.You could win a gold medal with that, I should think. Really beautiful.

Edited By mgj on 13/03/2011 19:23:08

[KMPParticipant @kmp]

Hi Andrew,

 

An ex MOD man, Hmmmmmmm

 

You are right of course (2.7 sooooomething thou) but I would say understated rather than short changed

 

No defence, but it is Sunday and getting late for us old folks

 

Regards

 

Keith

 

 

[KMPParticipant @kmp]

mgj Hi,

 

Certainly didn’t see it as an attack at all, but will take issue with “you pros” if you were refering to me . Acceptable finish is what I would call my attempts with SS. I have seen “wonderful” though and on an old Drumond. When an apprentice my “mentor”, who had the use of one arm produced work that was more like “Graham’s than mine will ever be.

 

Regards

 

Keith

[KMPParticipant @kmp]

Graham hi,

 

I certainly haven’t taken any offence at all; I saw the discussion as a bit of “merry banter among the men” as well. I am new here and do not want to upset anyone particularly as I am enjoying the relaxed banter that sets this forum apart from the majority. If the humour in my “tongue in cheek” defence is likely to upset anyone I will stop immediately.

 

Graham, Meyrick, please don’t take my comments as anything other than good intentioned banter, I have enjoyed our discussion very much and am looking forward to learning much from you all.

 

Keith

[TerrydParticipant @terryd72465]

Hi Graham,

 

Companies will draw bright bar through dies up to at least 80mm diameter on modern drawbenches with very strict tolerances both as to diameter and roundness. I’m not sure where your helical wave came from but not from the drawing process I believe.

 

Best regards

 

Terry

[TerrydParticipant @terryd72465]

Hi Graham,

 

I quite agree with your Golden rule of toolmaking, I also checked everything and anything when so employed.

 

The British and European standards are quite exacting as Andrew points out the tolerances equate to h10. Oversize is not allowed in steel made to the standards. In Keith’s example (6 – 18mmdia) the tolerance is +0 to -0.07mm, and as Andrew said out of round would be a max. of 0.035mm. From 18 – 30mm the tolerance is +0 to -0.085mm Obviously some will be out of round, some will be accurate depending on the wear in the die. I never said otherwise, I just cannot see how a helical form can be created in the process of drawing. In the process the material is held firmly in a fixed axis and any rotation is out of the question.

 

Best regards

 

Terry

 

[Anonymous]

Posted by KMP on 13/03/2011 19:19:53:

Hi Andrew,

 

An ex MOD man, Hmmmmmmm

 

 

Oeeeeer, may be I should be economical with the truth on my CV!

 

Keith; I’ve certainly seen nothing wrong with your sense of humour. There’s no harm in disagreeing with people, and is indeed part of what makes such fora as this interesting. It is only when disagreements become personal that problems arise. Fortunately that seems pretty rare on this forum.

 

Regards,

 

Andrew

[Anonymous]

Hmmmmm, slowly but surely the memory is grinding into action out here in the sticks. I’m sure I have bought round stock with a helical pattern on it in the past. I have in mind that it was stainless steel of some variety. It’s too late to ferret around in the workshop now. I’ll have a look tomorrow evening to see if I can find some.

 

Regards,

 

Andrew

[mgjParticipant @mgj]

Well use the blush icon then.

 

Helical wave – I have some bar in the larger sizes – 1.5″ or bigger that has a spiral pattern on it. I’ve never measured it to see if its acutally a change in dimension.

 

SS -acceptable finish. I can get an acceptable finish using carbide tips, or HSS without too much rake, but it remains a dull grey and needs polishing to get really smooth. Using emery and oil I got my locos crankpins like mirrors. But inside a thread, no I couldn’t get it smooth enough to be good in a bronze nut. . In industry I suspect a lot of these threads are finish ground, so we, lacking such kit, have to go a different route. Technically I could use the Quorn head as toolpost grinder -but no way is that kid of grit getting anywhere near my lathe.

 

Having said that, Chaddock in his book on the Quorn shows it being used to grind all sorts of exotic spirals. There is a spiralling head where you mount a former on the back of the head, and effectively you copy grind, and on that basis all you have to do is dress the wheel to shape and set the right helix angle – which is no worse than grinding HSS to shape. Do a camshaft 6″ long or so, as well as spirals. I’ve done helical endmill flutes very successfully as well, so you could thread grind up to 6-8″ IIRC Chaddock used the system to grind the 2 cams on a model of the immortal, but ill fated BRM 1500cc supercharged racing engine.

 

[mgjParticipant @mgj]

Terry – can the die not rotate in some processes.

[KMPParticipant @kmp]

Andrew,

 

Nothing to worry about I have some in my own background that’s all. They tell me counselling would help but I keep telling them “I am not a number”.

 

Good bottle of wine tonight

 

Keith

[Nicholas FarrParticipant @nicholasfarr14254]

Hi all, I haven,t had any reason to measure out of roundness to any great extent in bright bar stock in places I’ve worked, but yes I’ve seen a spiral pattern along the length shown up by the layer of protection oil which is usually on them.

 

I don’t know that much about the drawing process in the flesh as you might say but I’ve wondered like mgj suggests, if either the die is rotated or the bar itself is rotated while being drawn. This I think may help keep the bar as striaght as possible and even out any wear in the die maybe.

 

Regards Nick.

 

[KMPParticipant @kmp]

Graham, Terry, mgj

 

A final thought before retiring, drawing is not the final process in the manufacture of drawn bar. After and during drawing the bars are straightened firstly by a series of three roller stations and finally by a two roller station (convex and concave rollers). Perhaps the final two roller particularly could impart the helical patern?

 

Keith

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