Paul Sellers' panel saw

[Jim Koepke]

He doesn't seem to get the simple concept of power density...

What is power density? Just wondering.. ... ......

Not an expert, but boiled linseed oil produces heat when it polymerizes. If a rag is spread out on a nonflammable surface the heat will not be able to reach a point of combustion in most cases. If the rag is wadded up tightly, the internal heat build up may rise to the point of combustion.

To be specific, power equals the energy created by the polymerization of boiled linseed oil. The density of this energy causes it to increase into a high enough temperature causing combustion.

Please realize that this is my understanding of the term and I may be totally wrong. It has helped me in preventing flaming rags in my shop and work spaces.

Like George says, best to not leave them laying around.

jtk

[Ray Bohn]

There are a few out there. I picked this up for under $50 including shipping from used tool seller.
No. 7, 20" 11pt xcut. It took a hour of cleaning.

[lowell holmes]

Thanks Jim, I get the gist of it. I had never heard the term before.

I'm terrified about oil finishes and application rags. If I can't do it any other way, I'll put the spent rags in a bucket of water outside in the middle of the yard.

I will otherwise lay a rag outside on a hard surface and let it dry. Metal garbage can lids make a good place to lay them.

[george wilson]

I prefer to not take chances with oily rags. If my shop burned down,I'd never be able to replace the many years of tools I have made and accumulated. I put them outside till they get dry,then throw them out.

Nice saw find,Ray.

[Patrick Chase]

Not an expert, but boiled linseed oil produces heat when it polymerizes. If a rag is spread out on a nonflammable surface the heat will not be able to reach a point of combustion in most cases. If the rag is wadded up tightly, the internal heat build up may rise to the point of combustion.

To be specific, power equals the energy created by the polymerization of boiled linseed oil. The density of this energy causes it to increase into a high enough temperature causing combustion.

Please realize that this is my understanding of the term and I may be totally wrong. It has helped me in preventing flaming rags in my shop and work spaces.

Like George says, best to not leave them laying around.

jtk

Yep, that's almost exactly right. It's actually areal power density that matters.

Rags spontaneously combust when they reach a certain (autoignition) temperature. Oily cotton autoignites at ~250F (BLO itself autoignites at 640F though, so I suspect that the standard "oily cotton" number may be assuming a different oil).

Polymerizing oils release a certain amount of power (as heat) per unit mass while they polymerize, and that heat must be dissipated from the surface of the rag. The rag's temperature will be roughly proportional to the amount of oil it contains (more volume -> more heating power -> higher temperature) divided by its exposed surface area (more area -> better power dissipation -> lower temperature). That's the areal power density.

When you wad or fold a rag up, or stack a bunch of rags on top of teach other, you hide most of their surface area, and that's a Very Bad Thing (tm). That's why the standard advice is to hang used rags such that both sides of the rag (front and back) are exposed to air. If you do that you'll notice that they stay cool to touch during polymerization. In contrast a wadded rag will easily reach ignition, particularly if airflow is poor.

It doesn't hurt to play it safe of course, though I'd note that laying it face down on a surface is worse than hanging unless that surface is conductive and acts as a radiator (George's metal lid is a good choice).

EDIT: The crazy thing about the Sellars post is that he was replying to somebody who was soaking a rag in BLO. Soaking -> no oxidation -> no polymerization -> no heat -> no problem. Not something that I'd personally do though - too much risk of forgetting about it while pouring the oil out or something like that.

[Glen Canaday]

There are a few out there. I picked this up for under $50 including shipping from used tool seller.
No. 7, 20" 11pt xcut. It took a hour of cleaning.

I picked up a 1900s 20" 10ppi xcut D8 for $1.50 at a thrift store. It's the saw that got me started on them.

I understand that size was really common, but I personally haven't seen them. There are worthwhile saws still out there.