imperial screwcutting on metric lathe

On Sun, 28 Dec 2008 12:41:43 -0000, "Amateur Machinist"


    ....sigh...here we go again. When do the schools go back...? If you guys have a peeve..take it to e-mail or the playground please. --
Chris Edwards (in deepest Dorset) "....there *must* be an easier way!"
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
>

No "peeve" here Chris. The poster asked a question and got an answer.
Regards
Brian
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 28 Dec 2008 12:41:43 +0000, Amateur Machinist wrote:

Is that near the sewage-works?
--
sʇǝʌǝǝ

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Given Derek the night off, Beanie?
--
;-)
.
73 de Frank Turner-Smith G3VKI - mine's a pint.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Amateur Machinist wrote:

It could be both, maybe making people feel good about themselves IS the only practical application :)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Well, I've a couple of school maths texts here and the chapters on group theory are the only ones with no seeming practical application.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Amateur machinist wrote:

I'm a cryptologist, and many ciphers use groups, particularly but not exclusively public key ciphers.
For instance RSA uses the multiplicative group of invertible integers modulo PQ, where P and Q are primes, and Diffie-Hellman key agreement uses the group of integers modulo a prime.
Groups are less common in symmetric ciphers, in fact there are good reasons to ensure they are not groups under composition, but groups are not unknown - eg Pohlig-Hellman is a group, so (in a sense) is the one-time-pad and stream cipher, and there is some effort being made to create a secure cipher which is a group, though not much progress has been made as yet.
A detailed discussion of group theory is out of place here, but - a group is a set of objects, often numbers, combined with an associated binary operation which can be performed on any two members of the set, which also follows four rules:
there is an inverse for every element of the set, there is an identity element, the operation is associative and the group is closed.
In the Diffie-Hellman group for instance, the set is the integers less than a prime, and the binary operation is multiplying two of them together to get a result modulo the prime.
Groups have some interesting properties, which is why we study and use them. They lead on to the study of rings and fields etc, and provide a sideways entrance to the study of arithmetics, more usually approached from the axiomatic perspective.
But that's mostly pure math, rather than more-useable stuff - though it's surprising how often "pure" math turns out to be useful and used.
-- Peter Fairbrother
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Peter Fairbrother wrote:

Oh, and as to me personally using groups and group theory - the method in this 2004 paper is I think is unique in cryptography in using a nested set of four groups, each a proper subgroup of all the higher groups.
http://www.springerlink.com/content/q07439n27u1egx0w/?p dee7597a5b4ab2b9c5797c35b22044&pi=4
It's what I do.
-- Peter Fairbrother

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
<snipped for brevity>
An interesting and informative post Peter, which I enjoyed reading. Thank you.

That reminds me of a something I was told re Object Oriented Programming.
>> In the Diffie-Hellman group for instance, the set is the integers less

Used, if I recall, to establish a secure code between to essential strangers. They pass (in the clear) some basic numbers (inc a Prime) and using index laws and modulo maths can establish a secure code. Not looked at it in some time but I'm pretty sure that is the basics.
Thank you again.
Brian www.g8osn.org.uk
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Brian Reay wrote:

Yep.
I don't often talk about crypto on non-crypto fora, 'cos I think it's fantastically interesting but most people just go uh?, but - Diffie-Hellman really is astounding. That two people can establish a secret, openly, and an observer can't deduce the secret is just amazing.
Some other amazing things you can do with crypto: You can query a database and get an exact number of bits from the database - but the database operator can't tell which bits of the database you got.
So you can look up something in a database and no-one can tell what you looked up. It's deniable too, delete the numbers behind the query once it's answered, and even you can't tell what it was.
You can also get a database to count the number of times a word or phrase occurs in the database, without the database knowing what the word is!
Digital signatures and certificates you probably know about, but they were only discovered in the 1970's, and they are pretty amazing too.
Then there's steganographic file systems, which hide the number and sizes of the files they may contain, and, relying on distributed trust rather than mathematics, there are mixnets which can defeat traffic analysis, hidden servers (where no-one can tell where the server is, but you can still get a page from it) and .. I'll stop here.
The problem today is that most of the actual implementations are cr*p.
Robert Morris's (ex-NSA, author of the Unix "crypt" library) rule one of cryptanalysis, "First look for plaintext", holds now more than ever - it's usually a lot easier to find plaintext than to break the crypto.
People just don't encrypt for whatever reason, even when they should. Operating systems generate copious temporary copies of many files, which are seldom if ever securely deleted.
Red/black separation (separating encrypted and plaintext signals) is hard, and seldom done correctly - people pWn machines all the time.
Keys are too short and subject to various attacks, including brute force (trying all possible keys), rubber hose ("Give me the key and I'll stop beating you") and the nice truncheon (aka RIPA) ("Give me your keys or I'll send you to jail for x years" - which doesn't need a Warrant or a Court Order, a Policeman issues the demand) attacks.
Some effort is made to prevent man-in-the-middle and other protocol-based attacks, but almost all present systems can be beaten using this type of attack - it's just that people don't bother.
Good modern ciphers are probably unbreakable by man today, and it's not too hard to implement one which will be almost certainly unbreakable ever, except maybe by God, though again few people bother.
And why should they? It's so much easier to get the data by breaking the system than breaking the crypto.
Though we shouldn't forget Robert Morris's other maxim: "Never underestimate the attention, risk, money and time that an opponent will put into reading traffic".
I'll stop here, really this time.
-- Peter Fairbrother
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

http://www.springerlink.com/content/q07439n27u1egx0w/?p dee7597a5b4ab2b9c5797c35b22044&pi=4

But that is a lousy approximation for pi and the end of the link :P
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On or around Tue, 23 Dec 2008 19:53:51 +0000, Cliff Ray

it is, but one radian is about 57.summat degrees, since the 3rd side is curved, not straight.
the thing about dimensions is right though.
--
Austin Shackles. www.ddol-las.net my opinions are just that
Travel The Galaxy! Meet Fascinating Life Forms...
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

So miles per gallon being length per length cubed should be <blank> per square metre?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 24 Dec 2008 07:18:52 -0000, "Amateur machinist"

Quite so, allowing for the fact that neither miles nor gallons are in m^3, but the principle is correct. The 'error' as such would be a simple ratio (dim-less number) of miles/m per gallons/m^3. It can be useful in checking that you've got relationships in equartions correct. As noted earlier, both sides of an equals sign must have the same dimensions.
When working with any of the heap of dimensionless numbers (eg Reynolds, Mach, Froude, Nusselt etc etc) or some of the more obscure units (viscoscity gives some of the most strange, being pressure-seconds) totting up the string of dimensions and checking that they do cancel to the desired result is a quick easy means to see if you've made a basic clanger. More subtle clangers are still entirely possible;-)
Richard
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 24 Dec 2008 10:26:02 +0000, Richard Shute

There are some unobscure derived quantitiess which when expressed in SI base units can look incromprehensible. Try these two.
1. m^2 kg s^-3 A^-2 2. m^-2 kg^-1 s^4 A^2
--
brightside s9

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Well I can read a screwcutting chart but I can't understand that.............
John S.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
John S wrote:

Resistance and capacitance, at least I am good at electricity :)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

That would be dimensionally correct. Of course, you could equally well quote sfc in litres per kilometre (hence having dimensions L^2) or kilograms per kilometre (ML^-1) or even joules per kilometre (MLT^-2). Such conversion factors necessarily take their dimensions from the units chosen for their definition; any attempt to relate them to other meaningful parameters will likely drive you nuts. Richard's recent post is entirely in point here.
David
--
David Littlewood

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
uk.rec.models.engineering wrote:

Yes, it resolves to 1/length^2, and so can be expressed in reciprocal acres or reciprocal ares. Alternatively, following the convention established for conductivity, in ercas or eras.
Regards,
David P.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 25 Dec 2008 20:00:29 +0000, David Powell

I hope you are aware of the old, but emerging contender to replace the ISO (MKS) system towit the FFF system ie Furlong, Firkin, Fortnight. With a distance, a mass and a time, ALL other units can be derived. Gives rise to some delightful but concise units eg the earlier mpg becomes furlongs/firkin, - I doubt I could manage the reciprocal firkin/furlong, but I'd be willing to try as long as it was Speckled Hen or HSB <G>
Richard
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.