Some joints beg to be silverbrazed

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I'm not lecturing here, just sharing what I'm learning as I go. Nothing new here for the accomplished and experienced. No politics either, just metal.

Reply to
Don Foreman
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Extremely cool, Don. That brass slug clamping collar is the cat's pajamas.

I also like all the other projects on your home page!

--Winston

Reply to
Winston

On the pro ones, they just drill and tap the thin section of the wall. You can figure out how long *that* lasts!

Seeing the lovely fine job of keeping the silver solder under control at the joint, that you did, reminds me of a story that the torch brazer at work told me one time.

I had been admiring the similar thin ring of braze on a part he had recently done, and mentioned that my stuff aways finished up like it had been dipped in braze. The EZ-flow-45 goes everywhere.

He replied that his stuff looked like that too, but then he always put it in the electropolisher and zapped it for few minutes. Turns out the braze has a much faster etch rate than the steel!

Jim

Reply to
jim rozen

While I agree that an increase in are obviates denting and marring, friction does not = pressure x area. It is totally independent of area. Friction is dependent only on the "coefficient of friction" and the "normal force" (force perpendicular to the two surfaces). Anyway, beautiful project and well executed. I will certainly steal your ideas and have learned a lot. Thank you!!!

Ivan Vegvary

Reply to
Ivan Vegvary

Couldn't find the home page. Do you have a link?

Thanks, Ivan Vegvary

Reply to
Ivan Vegvary

Hey Don,

Looks good. As I'm getting older, I find parking the buns for short periods really helps my sore legs.

I bought two wooden stools when I was in the old shop which had lots and lots of room, but they get in the way now with the narrower aisles I have in the new place. I don't want to have to drag the stools around behind me, or move them out of the way every time I cross the shop, or buy more, to restrict aisle movement. So, I'm thinking I'd like to make some of the ones I used to see that hooked under the bench-top and swung out for use. Has anybody got any "plans" or suggestions for these? Hate to re-invent the wheel.

Take care.

Brian Laws>

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Reply to
Brian Lawson

Ivan, I don't believe this is true. Consider: if you have a wagon rolling slowly along, and you put down one 2x2" piece of wood skidding along the sidewalk, it will exert a dragging force, right? Common sense says if you put down 2 of them, you will get double the dragging force, right? Well, I say you'd get double if you just put the sticks closer together, or even if you made them one stick with double the area.

Just my 2¢ worth .. - GWE

Reply to
Grant Erwin

If you put two of them down, or if you double the width of the stick, you've doubled the mass you're dragging, which doubles the force normal to the friction plane...and you revert to Ivan's formula. You've doubled the friction because you doubled the force.

Ed Huntress

Reply to
Ed Huntress

This may be a situation where the area comes out in the wash.

Lets say that you have a 100kg block on a 1m square skid. You end up with a pressure of 100Kg/m^2 between the ground and the bottom of the skid. This gets you some arbitrary frictional force.

If we double the area of the skid we now have only 50kg/m^2 between the skid and the ground, so presumably we should have half the frictional force, but we also doubled the area, so we have twice the reduced force to account for.

2*0.5=1 just like the first case.

In the nitty gritty physics of it, area is in there, but since it falls out of the equations every time, we never see it and it doesn't matter.

In the real world geometry may make a difference for other reasons like the angle your rope makes with the test rig, flex in the skid, etc.

-- Joe

-- Joseph M. Krzeszewski Mechanical Engineering and stuff snipped-for-privacy@wpi.edu Jack of All Trades, Master of None... Yet

Reply to
jski

Force = weight/area Friction = Force * coefficient of friction

coefficient * weight Friction = -------------------- Area

So, if your area goes up, but your weight remains the same, friction actually drops. Or, if you increase weight and keep area the same, friction will go up. With your example you'd get no increase in friction because while doubling weight you're also doubling area. Like multiplying the last equation by 2/2. However, you would increase friction if you started stacking the boards on one another. Then it's like multiplying the last equation by 2/1. This all assumes a flat, level surface so that "normal force" and "weight" are interchangeable. Gets slightly more complex on a hill.

Reply to
B.B.

You've doubled the area, but you've also doubled the normal force - for the case where you are dragging two boards side by side. The force per unit area is the same.

IF the boards are stacked on on top of each other, then the force per unit area is indeed doubled. The top board is just an extra mass riding on the lower one.

The problem with using the relation that says that fricional force is proportional to some constant (mu) times the normal force is, it's only a very simple approximation.

Static vs sliding frictiona coefficients comes right to mind first off. The value of (mu) is different for an object that is moving along, vs one that is stationary and one is trying to get it moving from the dead stop.

Then there's an entire class of surfaces and materials that

*don't* obey the simple rule at all: one important one that springs to mind has to do with rubber tires on pavement.

One would think that the stickyness of a tire on pavement would decrease, as the contact patch area increases, right? The vehicle mass is the same, the contact patch area goes up, so the force per unit area goes down - so the friction decreases, right? Wrong. In the real world, the frictional forces increase as the contact patch area increases. The reason for this is that the tire rubber locks in to irregularities in the pavement surface. The system does not obey the physics 101 equation.

Anyone who's ridden a motorbike tire at the limits of traction understands that smaller contact patches mean watch out!

The example of dragging a board behind a car is another case where the simplest mathematical formula for a phenomenon does not agree with the real-world results.

JIm

Reply to
jim rozen

I used Easy-Flow 45 too. I photographed the "good side", Jim! There is a little bit of color spread on the other side.

I like the electr>Seeing the lovely fine job of keeping the silver solder under

Reply to
Don Foreman

I may have missed the entire problem (I didn't read it from the top), but the force per unit area doesn't come into play, Jim. Or, if you want to consider it, consider that doubling the area but halving the force per

*unit* area leaves friction the same.

In fact, when you're talking about dragging wood over asphalt or concrete, cogging effects are going to be big, and may dominate the problem. As a comparison, without cogging effects, drag racers couldn't reach more than around 155 mph in the 1/4-mile.

I thought the problem concerned just the wood. If it does, and if you double the mass of the wood that gravity is pulling down, normal to the direction of travel, then you double the friction. That's true, theoretically, regardless of the area of wood being dragged. As Ivan said, the basic formula considers the coefficient of friction and the force normal to the direction of travel. Area doesn't come into play at all.

In practice, it does indeed effect the resistance you'll encounter. But that's mostly non-frictional resistance: cogging, cleavage of wood fibers, chemical adhesion, etc.

Static and sliding friction are another issue, but static friction, in practical problems like this, typically pales in comparison with cogging.

No. The friction would remain exactly the same, regardless of tire area, in that example. The force per unit area goes down, but the area goes up. In the end, the two cancel each other out, and it boils down to coefficient of friction and normal force, as Ivan said.

Right. Cogging. That's why Big Daddy Garlitts broke the theorectical speed in the 1/4 mile in the mid-'50s.

Reply to
Ed Huntress

You are right. Thanks for catching that. Same force * same coeff of friction = same drag, but less pressure with greater area.

I should have said that greater area permit greater clamping force (hence more friction) without damage to the shaft.

On Sun, 31 Oct 2004 15:17:47 GMT, "Ivan Vegvary" wrote

Reply to
Don Foreman

Don is right as are you. Friction equals (approximately) force x coefficient of friction BUT force equals pressure times area.

Ted

Reply to
Ted Edwards

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Check out the milling machine lamp.

Nifty!

--Winston

Reply to
Winston

Grant, What Ivan is saying is that if you have say 200 lbs of steel in the form of a 4 by 8 inch square tube, it will take the same amount of pull to pull it over your shop floor, whether the 4 inch side or the 8 inch side is against the floor. Independent of area, not independent of weight. This is generally true. Not true on ice where high pressure melts the ice, not true where high pressure deforms the surface ( like a ton of steel with a small area breaking the concrete ).

Dan

Reply to
Dan Caster

Reply to
Robert Galloway

And you _will_ let us know the details, please. Also what electrolyte, etc.

Ted

Reply to
Ted Edwards

It is indeed but I'd bet that an LED based replacement is on the todo list. :-)

Ted

Reply to
Ted Edwards

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