Assume v. Presume

I heard the 'one pence' that prompted my ire on the BBC 18:00 news last night and surprisingly often elsewhere. 'One pee is fine, albeit a little lazy for my money, as pee is the pronunciation of 'P'
BH has found that the thruppeny bit had 12 sides. So having to have an odd number of sides is blx.
 
Telegraph Magazine today:

"The mines buried round the school required much less weight to activate them. The only reason I can think of is to target pupils."

Appalling.
 
prpr said:
A three-sided coin isn't going to roll, so it's not just odd-ness.
Click to expand...
You wanna bet (click the GIF to animate)?

RollingOvoid.gif

This is the design principle used for the 20p and 50p, and I don't see why they would want to change it now.

Scotland v Wales calls...
 
Black Hole said:
This is the design principle used for the 20p and 50p, and I don't see why they would want to change it now.
Click to expand...
Yes, they are all constant diameter coins so they'll work in machines. The new pound clearly isn't as the mint themselves specify its maximum diameter. How machines will handle them I have no idea as it's one less characteristic that can be checked for validity.
 
Trev said:
That's how you drill a square hole as well.
Click to expand...
An urban myth. You get a square hole with rounded corners. Plus, you need a square guide for it to revolve in, I assume.


Rather better is this.


Anyway, it was not rolling that was the key point but being of uniform width. The Rouleaux triangle was the simplest example of that. Perhaps the name gave the impression it was something to do with rolling?
 
Last edited by a moderator:
Mike0001 said:
An urban myth. You get a square hole with rounded corners. Plus, you need a square guide for it to revolve in, I assume.
Click to expand...
Very slightly rounded corners, yes, but it doesn't need a guide and it isn't an urban myth.

 
Last edited:
If it has rounded corners, it is not a square.

If you rotate a Rouleaux triangle about its centre, it will cut a circular hole. You need a guide to reciprocate it to drill anything like a square hole.
 
It guides itself, all you need is a floating drive. The Rouleaux produces rounded corners, sure, but there are better designs that achieve very-close-to-sharp corners.
 
The centre describes an orbit consisting of four sections of an ellipse. Something has to make it do that.

I have no idea what a floating drive is. I understood these drill bits to be just the bare skeleton of a Rouleaux, three cutting edges. Maybe we mean the same thing, but it is no ordinary drill bit. Are you implying that the guide is built into it? A mechanism to steer the centre would be enough.
 
Did you look a the video in post 2790? For one thing those corners look far sharper than the perimeter of the Rouleaux curve, and I'm not certain but I think that as long as the bit is forced to rotate, the forces acting on the bit by the workpiece cause it to move in that locus naturally.
 
Last edited:
The illustrations Mike provided in post 2789 (I've only just noticed them because they were added in while I was composing 2790) show (in the first) a cutter being guided by a cam in the form of a Rouleaux. The geometry constrains the sharpness of the corner available by this method. The second is completely irrelevant to this discussion.

It's all irrelevant to the original point: that prpr claimed a three-sided coin couldn't be rolled in the manner I described.
 
Black Hole said:
Did you look a the video in post 2790? For one thing those corners look far sharper than the perimeter of the Rouleaux curve, and I'm not certain but I think that as long as the bit is forced to rotate, the forces acting on the bit by the workpiece cause it to move in that locus naturally.
Click to expand...
I did watch, but was confused whether the bit included the cylinder above it or not.

My own experience of wonky bits is that they don't drill in a controlled way.
 
I believe that is a floating drive stock - which provides torque to the bit and constrains z but allows play in x and y. However, you're right, it might include some kind of epicyclic to control the x and y relative to the rotation of the bit. But it can't be the Rouleaux cam because of the tightness of the corners.
 
I don't get it. Those corners still look too sharp. I think that blogger is putting two and two together to make five.
 
You must log in or register to reply here.
Back
Top