Securing threaded backplate

[Martin KyteParticipant @martinkyte99762]

Usually when adjusting a 3 screw setup you use 2 keys in 2 screws turned in equal but opposite directions. This rotates the item along an axis bisecting the screws and running through the 3rd. Once that plane is correct you just adjust the 3rd screw and you are there. Simples. Standard leveling technique for tribrackets in serveying.

regards Martin

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Stevenson on 24/02/2017 09:19:39:

So a 4 jaw chuck with all the jaws tightened is unstable ?

I think all the books that advocate using a 4 jaw for greater gripping power need re-writing then ?

.

NO, John

That's NOT what I wrote

MichaelG.

.

Edit: This calls upon what is [should be] pretty basic engineering knowledge; which is why I used the word reasonable [in its meaning of 'capable of being reasoned'].

Perhaps Jason's mystery correspondent [who found my use of references so irritating] could oblige by doing a little research and posting some helpful links.

Edited By Michael Gilligan on 24/02/2017 10:02:25

[John Stevenson 1Participant @johnstevenson1]

Posted by Michael Gilligan on 24/02/2017 09:26:34:

For the sake of completeness:

If a semi-permanent solution was required for "the Myford problem", it should be perfectly possible to secure a good backplate to the threaded spindle-nose [using e.g. Loctite 638] and machine it to replicate the popular flange-fixing arrangement.

MichaelG.

[obviously, the viability of this idea would depend upon individual circumstances]

.

And the influx of owners lining up to throw themselves off Beachy Head although it would be a very good alternative and a better fixing method.

Fortunately it being a 'working' part it will be left in steel or cast iron otherwise there would be another argument whether to paint it grey or green.

[KWILParticipant @kwil]

Alternatively you could interpose a plate between the chuck and backplate, with the necessary location steps. Insert a suitably machined 2MT arbor, which locates in this intermediate plate (radial pin). Drawbar locks whole assembly.

Yes it does work and was exhibited at Sandown MEX one year.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Stevenson on 24/02/2017 09:34:07:

And the influx of owners lining up to throw themselves off Beachy Head although it would be a very good alternative and a better fixing method.

Fortunately it being a 'working' part it will be left in steel or cast iron otherwise there would be another argument whether to paint it grey or green.

.

That's better, John

… but don't forget the ghastly Cyan [or whatever they called it] option on the final Beeston models.

**LINK**

MichaelG.

[JasonBModerator @jasonb]

Posted by Michael Gilligan on 24/02/2017 09:31:04:

Perhaps Jason's mystery correspondent [who found my use of references so irritating] could oblige by doing a little research and posting some helpful links.

Probably too busy in his workshop and for some reason he mostly chooses to post his very high quality work and offer advice on another forum now

[JasonBModerator @jasonb]

I'm inclined to agree with JS on this one that in this particular use having 3 screws will not make things anymore stable than 4.

Taking Michael's 3 legged stool as an example if we were to shorten one leg then the stool will just tilt until all three legs touch the ground due to the downwards direction of the forces on the stool. So the stool is once again stable.

Now take our Gript-tru or Hemmingway item, if one of the screws was slackened off the chuck will then only be resting on two of the screws or even one depending on what position the screws are in , fail to retighten the third screw or even not tighten it with the same torque as the other two and the part is unstable, same as if you had one jaw of your 4-jaw looser than the others.

Infact the stool can reach a point wher even with three legs it will start to become unstable if one leg were a lot shorter than the others therfore all the downward forces would be acting on the short leg until you get to the point where the stall will fall over.

 

J

Edited By JasonB on 24/02/2017 10:31:02

Edited By JasonB on 24/02/2017 10:35:09

[Michael GilliganParticipant @michaelgilligan61133]

Oh dear … ^^^

I might try to explain, later.

MichaelG.

[Martin KyteParticipant @martinkyte99762]

Posted by JasonB on 24/02/2017 10:30:15:

I'm inclined t

oooh you don't want to be inclined it's all supposed to be level!

:0)

Martin

[not done it yetParticipant @notdoneityet]

Jason B,

 

IFthe milking stool was perfectly symmetrical (and built with the legs sufficiently spaced, as it should be), shortening one leg would never cause it to fall over (centre of mass would remain within the effective base). But if two legs were shortened by different amounts …. it would no longer remain stable in all circumstances – unless the legs were vertical and at the extreme edge. I think!

 

Silly analogy in the first place – as the stool only has three legs so all legs will be in contact with uneven ground. whilst in stable equilibrium.

Edited By not done it yet on 24/02/2017 11:39:58

[JasonBModerator @jasonb]

NDIY

As I said if one leg were shortened a lot then the stool would lean to such an extent that the mass would start to move beyond the base and then it will become unstable.

I know Michael's kinematic theory ignores forces but in practical terms we have the force of a milk maid on the stool and the force applied to teh screws to make the chuck move both against gravity and the friction from the main chuck retaining screws which are just eased off to adjust the chuck. So as soon as a force is applied to one of those adjusting screws things become unstable unless balancing forces are applied via the other two screws

[Jon GibbsParticipant @jongibbs59756]

The kinematics arguments may, or may not, apply to the Grip-tru chucks with 3 screws but in the case of the Hemingway version (and my knock-off) the adjusting grub screws are merely aids for adjustment of the alignment and don't provide the retaining force since once adjusted the backplate is held by the main screws.

So, I don't think the arguments can really apply or am I missing something?

Jon

[Martin KyteParticipant @martinkyte99762]

I know you often don't get a choice on how many screws hold a chuck but when the Grip-Tru type adjustable backplate is made why do they insist on still using 3 screws for adjustment.

Adjust three screws at 120 degrees is a right pain working out what is going to move what way.

Why don't they use 4 and mimic a 4 jaw chuck where you work in two sets of 2 jaws for adjustment.

Maybe it's because with 4 adjusters on a 3 jaw chuck you cannot fit the adjusting screws in without running into the pinion holes. Even with radial screw adustment rather than tapers you still wind up having to position your adustement screws within 15 degrees on the chuck jaw centres on 2 of the . I suppose you could make the chuck longer and duck underneath the pinion holes but that would increase overhang.

Comments please.

regards Martin

[peak4Participant @peak4]

Posted by Martin Kyte on 24/02/2017 09:29:07:

Usually when adjusting a 3 screw setup you use 2 keys in 2 screws turned in equal but opposite directions. This rotates the item along an axis bisecting the screws and running through the 3rd. Once that plane is correct you just adjust the 3rd screw and you are there. Simples. Standard leveling technique for tribrackets in serveying.

regards Martin

The official instructions for a Griptru differ though and say to move one screw at a time.

See Page 5 of this pdf.

I don't believe it's anything to do with stability, and feel that 3 legged stools should remain in the milking parlour; I've been known to fall off them due to instability, but that could have been due to the whisky.

Camera tripods get quite a lot of use though, and since they don't normally have micro adjusters, unlike surveying heads, I use the one leg at a time method of leveling rather than having several grand's worth of optics come tumbling down.

I still think 4 screws would be easier to adjust a chuck but I guess it's down to the space available to add the conical micro screws on the Griptru. On a 3 jaw chuck, there's obviously 3 spare spaces between the jaws.

Maybe to come up with a 4 screw adjustment, would result in a longer chuck and hence more overhang from the spindle nose.

 

All the Best

Bill

p.s. Looks like we were typing at the same time, but you beat me to it .

Edited By peak4 on 24/02/2017 12:16:07

Edited By peak4 on 24/02/2017 12:25:08

[JasonBModerator @jasonb]

Posted by not done it yet on 24/02/2017 11:36:23:

IFthe milking stool was perfectly symmetrical (and built with the legs sufficiently spaced, as it should be), shortening one leg would never cause it to fall over (centre of mass would remain within the effective base). But if two legs were shortened by different amounts …. it would no longer remain stable in all circumstances – unless the legs were vertical and at the extreme edge. I think!

Well while waiting for another coat of lacquer to dry I thought I would test the theory.

Nice stable stool with 3 equal legs on a slight splay.

Shorten one leg ( thepointed one) and so far the stool is still standing but you would want to be a bit carful how you sat on it as more likely to tip to the right.

Now as I said if the leg was shortened a lot then you will reach a point where it decomes UNSTABLE

J

[Martin KyteParticipant @martinkyte99762]

Hi Bill

You are perfectly correct re the griptru as the tapered cone system only pushes and you need to pull as well to do two at the same time. The same applies to tripods. Where 3 bidirectional threaded adjustments are concerned it is however possible to obtain orthogonal adjustment planes by contra-rotation of two screws for one plane and the 3rd for the other.

Setting tripods level over a pin always baffled me. I could do it no problem but could never quite figure why it worked. You basically went side to side with one leg for one plane and up and down for the other to get the head level. Then adjusted each leg to move in three planes to get vertically over the pin. The top would no longer be level but usually withing the fine adustement of the tribracket mount for the theodolite.

regards Martin

[Martin KyteParticipant @martinkyte99762]

Yes Jason but if you bolt the stool to the floor it is unconditionally stable.

regards Martin

PS waiting for my board to etch

[not done it yetParticipant @notdoneityet]

JB,

That is more like a BAR stool than a milking stool. A good milking stool would bump on the rim before falling over.

Made by someone who has clearly never milked cow from a stool. I have, btw, on very numerous occasions. One side was always made, or worn, shorter so that one could 'lean' in towards the cow. You also need to be fairly low down to reach her teats. Small pails were used for two good reasons too!

[Neil WyattModerator @neilwyatt]

I've given the example of a telescope finder before.

Have two adjustment screws at 90-degrees to each other for getting the chuck in the right place, and a third screw for securing once the other two are set.

So screws at 0, 90 and 225 degrees.

Best of both worlds

Neil

<edit> Bear in mind the adjusting screws are only used for alignment and you lock the whole thing up afterwards, so you could probably get away with just two screws and care.

Edited By Neil Wyatt on 24/02/2017 16:32:42

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 24/02/2017 10:41:31:

Oh dear … ^^^

I might try to explain, later.

.

Here goes:

First: the 'three-legged milking stool' is not "mine", it is simply a commonly used point of reference in discussions of kinematics. … It is an excellent example of intuitive engineering [or perhaps evolution by natural selection] getting the right answer before the theoreticians explained why.

Most people will appreciate that a three-legged stool is inherently stable on any firm surface … so, reference to it puts everyone 'on the same page' for the discussion.

Unless they are perfectly matched [which has a probability closely approaching zero]; Two rigid bodies will only make stable contact at three points. … One or two point contact is unstable, and anything more than three would require some deflection.

The example of the stool assumes a 'floor' and the force of gravity, but it is easy to translate to other configurations.

Now: In the example of the GripTru chuck, the three screws are adjusters [not clamps] and their purpose is to accurately centre the axis of the chuck on the axis of the lathe. Three point contact is stable, and that is why only three screws are used. … If the 'more convenient' four screw adjuster was used, then the components would first settle on three points and then [potentially] be disturbed by the action of the fourth screw. Admittedly, hese effects/risks are small; but surely the GripTru's raison d'être is to avoid them so far as is practically possible.

We use a very similar arrangement for optical alignment on microscope stands … the difference being that they are simple radial screws, and one of the three is typically replaced by a spring-loaded ball  … centring is then a simple matter of manipulating two screws.

MichaelG.

 

Edited By Michael Gilligan on 24/02/2017 18:04:38

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Jon Gibbs on 24/02/2017 11:55:02:

The kinematics arguments may, or may not, apply to the Grip-tru chucks with 3 screws but in the case of the Hemingway version (and my knock-off) the adjusting grub screws are merely aids for adjustment of the alignment and don't provide the retaining force since once adjusted the backplate is held by the main screws.

So, I don't think the arguments can really apply or am I missing something?

Jon

.

Jon,

I hope my last post clarified that ^^^

Yes … The adjusters are exclusively for positioning [not clamping] and the reason for using three is that it's 'closer to perfection' because three-point contact is in the natural order of things.

… It's about risk avoidance.

MichaelG.

[John Stevenson 1Participant @johnstevenson1]

Another practical post gone pear shaped……………………………………

[Ian PParticipant @ianp]

Posted by John Stevenson on 24/02/2017 19:02:37:

Another practical post gone pear shaped……………………………………

I wonder why we use the expression 'pear shaped'?

Do we know what shape the object was before it changed to a pear shape.

Nothing wrong with a pear shape if you are a pear either.

Ian P

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Stevenson on 23/02/2017 23:28:30:

I know you often don't get a choice on how many screws hold a chuck but when the Grip-Tru type adjustable backplate is made why do they insist on still using 3 screws for adjustment.

.

Strangely enough, John … I think it was you that pushed the thread into digression.

• You asked a [probably rhetorical] question
• I answered it simply … which should have been sufficient.
• You responded with a sarky remark
• Jason aligned himself with you
• Others joined in the discussion.
• I expanded-upon my original brief answer.

…. What did you expect ??

MichaelG.

[John Stevenson 1Participant @johnstevenson1]

Posted by Michael Gilligan on 24/02/2017 19:29:41:

Posted by John Stevenson on 23/02/2017 23:28:30:

I know you often don't get a choice on how many screws hold a chuck but when the Grip-Tru type adjustable backplate is made why do they insist on still using 3 screws for adjustment.

.

Strangely enough, John … I think it was you that pushed the thread into digression.

• You asked a [probably rhetorical] question
• I answered it simply … which should have been sufficient.
• You responded with a sarky remark
• Jason aligned himself with you
• Others joined in the discussion.
• I expanded-upon my original brief answer.

…. What did you expect ??

MichaelG.

{quote my me ]

So a 4 jaw chuck with all the jaws tightened is unstable ?

I think all the books that advocate using a 4 jaw for greater gripping power need re-writing then ?

[unquote ]

I presume that is the sarky bit but if you read it it makes perfect sense on an engineering forum unlike a milking stool.

For anyone who has actually used a grip tru or adjustable back plate you slightly slacken the holding bolts, push and pull with the adjusting screws until minimum run out occurs and then tighten the holding bolts.

It matters not one fig if it has 1, 2 3, 4 or 27 [ although with 27 you need friends ] adjusting screws as once the holding bolts are tightened it's no longer instable, unstable because the stable door has been closed when the last holding bolt was tightened.

All I was pointing out in reply to whoever posted the picture of the Hemingway kit back plate, was that 4 is easier than 3.

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