Here is a list of all the postings Kiwi Bloke has made in our forums. Click on a thread name to jump to the thread.
|Thread: Setting bearing preload.|
Neither can I find a concise set of instructions for preload setting. I suppose it's not surprising, given the huge range of different geometries and applications.
The ages-old (official) way of setting automotive front wheel taper roller sets is to apply considerable preload whilst rotating the wheel, by tightening up the stub-axle nut to a given torque (by which time it takes a bit of effort to turn the wheel), and then back off the nut one or two flats. This results in a few thou axial play. So that's not what we want, is it?
Land Rover transmissions were full of opposed taper roller bearings. You set preload by shimming the outer tracks and measuring the torque required to rotate the supported gear-shaft (by wrapping string around the gear and pulling it with a spring balance). It could take several trial assemblies to discover which shims to install, so it wasn't a quick job. I got to know the L R parts man quite well. He had large boxes of shims, all untouched. "Oh no, we never open 'boxes here, we just put in reconditioned ones". Having had my fill of L R unreliability and rebuilding, I now have a Toyota...
The vertical spindle of my Maho milling machine has opposed taper roller bearings, the rollers bearing directly on the hardened spindle. I assume this is for reasons of compactness and to minimise possible sources of eccentricity and 'slack'. The manual instructs one to grease-pack the bearings and assemble with a bit more preload than necessary to just remove any axial play. Then run at high speed and monitor the temperature of the housing. >50C = too hot = too tight. Sorry this is a bit vague - manual is packed away: I'm between workshops...
Seems a good stratagem, in the absence of other info. However, for a small bearing set, and oil-lubricated, perhaps raising the temperature of a relatively massive housing to this sort of temperature would only result from massive over-tightness. So I suppose the message is to tighten up until end-play is certainly abolished, and then a bit more, but if the spindle feels tight or runs hot, slacken off a bit. If you look at the manufacturers' data sheets, the permitted axial loading is surprisingly high, so we can afford to experiment.
Sorry, not much help. Might be comforting to know that you're not alone though...
|Thread: Parting Off MEW225|
Cabeng: It's pretty obvious that the arrows on GHT's fig 5.1 are the directions that the tool tip will tend to be deflected, assuming that it pivots about the pivot points shown: it's not a force diagram. The arrows are tangents to the radius of rotation (as drawn).
Thank you for your force diagram, about which I'd hope there will be no fundamental disagreement. Perhaps you would give us another diagram showing the direction of expected tool deflection for that system. GHT's pivot points are gross simplifications, as he admits, but will bear some similarity to reality.
If anyone remains doubtful about how a tool deflects under load, look at its deflection with a sensitive dial gauge - it's frightening! In 'conventional' turning, with sharp tools and 'normal' overhang, a front-mounted tool is forced down and therefore swings into cut; a rear-mounted, inverted tool is forced up and out of cut. If there's disagreement about this, there shouldn't be. The front-mounted system is inherently unstable (positive feedback), the rear-mounted system is stable (negative feedback). 'Normal' turning is (usually!) successful because the system is able to resist the cutting forces well enough so that deflection is small and the positive feedback doesn't run away. The ancient spring tools' geometry was such that the tool tip deflection was closer to a tangent to the work surface than the 'normal' set-up: it moved from a positive-feedback system to a stable, negative-feedback system when loaded.
Neil: The world is not flat. I accept that this, and all that has ever been said and written is merely opinion and that one can argue philosophically that there is no truth or even reality. However, when observation leads to hypothesis, and when hypothesis is tested by attempted refutation, and when repeated observation accords with the hypothesis and when the hypothesis allows predictions which do not get refuted, we have what is known as the scientific method. It is a rational system and is the basis of science and technological progress. I do not act as GHT's advocate because I subscribe to the 'it is written, therefore it is the Truth' religious dogma, but because he was knowledgeable and experienced and actually understood what he wrote about. That understanding seems not to be as widespread within these fora as one might hope. I had hoped to help, by pointing people to a useful and reliable source of enlightenment, but I can see that I was wasting my time. I'll shut up now.
|Thread: Unimat 3 Vertical Head Quill|
Thanks. Strange way to assemble, don't you think? I wonder how bearing pre-load was supposed to be controlled. So, supporting the quill's nose end and pressing out the spindle might be the way to go (munting the bearings on the way)? Sounds a bit too close to the well-known Rip, Sh*t or Bust technique for comfort...
|Thread: Parting Off MEW225|
Our Editor has written:
Let's be fair, the diversity of opinion on the physics of parting off is enormous, and anyone offering an explanation is putting their head above the parapet.
I don't personally agree with the explanation, but then again I think I can see flaws in any explanation I have seen presented.
This is engineering. It is science, and therefore obeys immutable physical laws. It is not a matter of personal belief - however fervently held - nor magic or superstition. It's not mysterious, and it's not worth trying to re-invent the wheel. Disagree with science at your peril. The phenomena are understood. The misinformation contained in the original article and in this thread does no-one a service. If you want to know the truth, just read George Thomas' articles, starting in Model Engineer 5 March 1976, V 142, p 228 (was it really that long ago?) and re-printed in his book, The Model Engineer's Workshop Manual (TEE Publishing, 1992).
Chris, my criticism is not driven by zeal, but by a desire not to see misleading material published. I take no pleasure in criticizing the efforts of contributors to MEW or its editor, but, sometimes, things go wrong. I'm glad you agree with my explanation of tool deflection. It's worth emphasizing that a rear-mounted tool behaves as you describe only when it is inverted and the lathe runs 'forwards'.
Merryweather's holder, as shown in the photos, will behave as you describe only if the lathe is run in reverse, which is inappropriate for lathes with screw-on chucks. The article fails to make it clear in which direction the lathe was supposed to be run. The holder could, of course, be reversed and the lathe run 'forwards' and it would behave just the same, with the same apparent advantages. However, it and the topslide would be competing for the same space, hence, I suspect, for its rear mounting.
The problem of sideways forces resulting from angled tool front faces are pretty obvious. There is a possibly more worrying sideways force and deflection problem, however. If the tool is supported by a structure to one side of its tip, it will be deflected sideways, in much the same way we have discussed. This can be a concern, particularly in quick-change toolholders, because of their 'sideways overhang'. Merryweather's holder is good in this respect: the tool tip is almost in the same vertical plane as the holder's mounting to the cross-slide. It should therefore be free from this sideways deflection.
I don't for a minute dispute that Merryweather got good results fom his set-up. Clearly, it works. The holder is quite a nice idea and I commend his originality. My criticism is of his explanation of the physics and the lack of clarity in the description of the use of the thing. I think newcomers could be misled.
|Thread: Unimat 3 Vertical Head Quill|
I've recently acquired a very-little-used Unimat 3, with vertical milling head. Not sure why, exactly, but there you go...
Bizzarely, there is a few thou axial free movement of the spindle within the vertical head's quill, rendering it more-or-less useless for milling. The manual and spare parts diagrams show the quill as an assembled unit, with no internal spares available. I can't see how the quill unit is assembled. There's a collar on the spindle shaft, at the pulley end, but no evidence of an expected thread, nor pin-spanner holes, nor any obvious means of removing the collar to get at the innards - assuming that removing the collar is what is needed.
Anyone got any ideas?
|Thread: Parting Off MEW225|
I can't believe I'm reading the confusion in this thread... The late, lamented George Thomas wrote pretty much all anyone could ever need to know about parting off (and many other topics) in his many comprehensive Model Engineer articles and his books. Perhaps it's time for a re-run?
Merryweather's article is BAD and MISLEADING (yes, I'm shouting!). It may not be his fault - perhaps it has been mangled by editing or production errors, but it doesn't belong in anything with aspirations to be an authoritative journal. As printed, his explanation is faulty. What determines in which direction a tool is deflected under cutting loads is the relationship of the direction of the resultant force on the tool to a line joining the cutting tip to the centre about which the tool and its holder, toolpost, etc. can rotate. (By rotate, I mean move by flexing. It will certainly flex - nothing is infinitely rigid, and Myfords certainly ain't!). A rear parting tool, inverted, with the lathe running in the 'normal' direction is successful at avoiding dig-ins because, as it is loaded, it tends to be deflected up and away from its point of contact, thereby relieving the forces upon it. Just think how it would move if its toolpost were hinged to the cross-slide. Similarly, but the opposite, for parting-off with a tool in the 'normal' toolpost.
The article is not clear. Merryweather's Fig. 5 bears no relationship to the photos of his toolholder set-up. I THINK he must part off with his lathe running in reverse: there seems to be chip build-up on top of the tool (photo 3). This, of course, isn't a good idea for lathes with screw-on chucks... If this is correct, photo 3 suggests that the tool and holder will be deflected clockwise, pivoting about its mounting to the cross-slide, and therefore relieving the cut, as load is applied. If this is the case, the set-up could be reversed, with the tool-holder mounted 'in front' of the spindle, and the lathe run in the 'normal' direction. It is, in fact, more-or-less a re-invention of the 'spring tool' (look it up), although rather less springy....
Another reason given for the success of rear parting-off tool-holders, especially on non-rigid Myfords, is because the tool loads lift the cross-slide, minimizing dovetail clearance, and tighten-up the whole shebang, reducing chatter likelihood. This doesn't apply to inverted-vee beds, of course.
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