Mig wire feed problem

"SteveB" wrote: It that is true, why not just jump up to .045" wire? That way, too, one would only have to buy one size of tips. Just a hypothetical suggestion to encourage discussion ........ '-) ^^^^^^^^^^^^^^^ I'll assume you are joking. Just in case you are not, I will add: The hole has to be bigger than the wire. And, that's not hypothetical ;-)

I don't think this is right. The hole gets smaller because the metal expands in every direction possible, including toward the inner dia..

Just a thought - to remove a bearing from a shaft dont you heat the bearing? If so, then that means the 'hole' in the bearing gets bigger when you heat it.

David - not sure of the above but....

s-boulet wrote:

s-boulet wrote: ...

Actually, it depends. If the the hole is small relative to the surrounding metal, it does get smaller. If the hole is large relative to the metal (e.g., a donut shape), the hole gets larger. I like to think of the edge of the hole as a ring of molecules. As it gets hotter those molecules get further apart. This increases the circumference and hence the diameter. Unless they are constrained (small hole in big piece), then they are compressed.

I'm sure that someone who actually knows what he is talking about will have a better explanation 8-)

Bob

"Bob Engelhardt" wrote: (clip) Unless they are constrained (small hole in big piece), then they are compressed. (clip) ^^^^^^^^^^^^^^ This has been discussed up-down-and -sideways in various groups, and I think you are the first one to bring up this nuance. You are absolutely right. The hole expands with the material if the entire piece is at the same temperature. If the temperature is uneven, so thermal stresses are developed, it's anybodys guess what happens. But, in the edges of a very small hole become heated, while the surrounding metal remains cold, the hole MUST get smaller. What actually happens is that the circumference of the hole cannot increase as the metal "expands" with heat, so it goes into compression. There is a number called "Poisson's ratio," usually about 1/3, as I recall from the dim past, which tells how much "fatter" it gets in compression.

Coming back to the copper tip on a MIG gun, it is possible that the wire expands due to heat, while, at the same time, the surface of the hole is forced inward by the UNEVEN heating of the copper.

Bob, you actually did pretty well.

A bearing race, will expand. A fender washer will make the hole smaller.

Gunner

Rule #35 "That which does not kill you, has made a huge tactical error"

Greetings and Salutations...

On Tue, 05 Apr 2005 11:38:11 +1000, quietguy wrote:

Seems like this has come up before...but, let me pass along my reasoning on this. The fact of the matter is that the hole in the center of a circle of steel will get larger when that chunk of steel is heated. Let's consider a steel washer that has a 1" hole in the center and a total diameter of 3". Now, lets 'open up' the circle formed by the metal of the washer into a rectangular sheet by cutting a radial slice and straightening out the curve. What that actually gets us is a trapazoid-shaped piece of metal that is 2" wide, with a base of a bit under 9 1/2" and a top of about 6 1/4". to simplify things...lets square up the ends...leaving us with a rectangle 2" wide and 6" long. The coefficient of expansion of steel is, roughly, 6.5 parts per million per degree F, or 6.5x10**-4 percent per degree F. Now...lets take that rectangle of steel we have and raise its temperature by 100 degrees. The rectangle will expand across its width by (0.00065 * 100 * 2) or 0.13", leaving it 2 8/64" wide. However, its length will expand by (.00065 * 100 * 6) or 0.39", leaving it 6 24/64" long. Now...since the LENGTH of the rectangle expands by (in this case) 3x the width, there is no way that if we bend the rectangle back into a washer that the center hole will be smaller. In all cases, for a "washer-shaped" sheet of metal, the width of the metal outside the hole is going to be significantly smaller than the average circumference of the "washer". So...in all cases, the circumference of the hole will increase more than the hole might narrow as the metal outside it expands inwards. Clear as mud, right *smile*. Regards dave mundt