Surface Plates: Granite vs Cast Iron 

That's kind of what he did. He tried from teaching a&m and runs an educational website and RU-vid channel. He occasionally works at the Georgia museum of agriculture doing machine work and such. I would guess he considers this kind of thing his day job though.

For lapping components like carb mounting faces, oil pump faces, small heads, etc I just use a plate of glass.

A friend of mine went to a company that made tombstones. He got one where they misspelled the name of the deceased, it was really cheap. He then lapped it in.

Tempered glass works in a pinch too.

@@transmaster perfect scrap material for this purpose a mistake in a headstone cannot be fixed it must be replaced, theyd sell it to you for a song lol!

I'd be careful with glass plates, they are more flexible than you might think. Also the flatness isn't that great, I built a simple device for measuring flatness and tested a pane of glass, the indicator jumped all over the place even if it seemed to be flat. Anyway, I built a simple surface plate out of a broken granite tile (actually two, both halves were usable). The surface was somewhat ground already, but just to be sure I got some abrasive powder (silicon carbide) and lapped both plates onto eachother. The result turned out pretty good. It's not perfect, but I'd definitely trust it way more than a piece of glass.

Heavy enough to influence the orbit of the earth based on where it is sitting, I reckon. That's just plain huge.

C = (F-32)*5/8 F=8/5*C +32 Straightforward conversion every schoolboy knows

@David Martin actually it's 9/5 and 5/9, but you're right otherwise.

@@davidmam I think the conversion factor is actually 5/9 not 5/8. But therein lies the difficulty because it’s hard to remember! Otherwise I’d agree with you that the math isn’t too scary.

quick and dirty for C to F double the c and add 30 ,not completely accurate but near enough plus its a easy in the head one works the other way also - 30 / 2

Negative 40 is negative 40, in both. steve

Very well stated !!

Yeah no doubt. Armor plating is just that, armor. Hardened tool steel essentially. Ceramic will cut or abrade most anything. Even considering the plate was designed to stop ordnance, it's still impressive it stood up to 3 years of what amounts to the worst possible conditions outside of what it was made for. Pretty awesome.

if I was doing a scrape job that had to really be close (say a P&W jig bore), I would always try to use a cast iron plate. I have always felt that cast iron rubs better than granite. Probably just me, but granite tends to smear the blue rather than simply hit the high spots. Still you can do quite well with granite if you know what you're after. Most people use way too much blue on the granite surface. Still for something really big there's usually no other way. I most always rubbed my strait edges off a granite plate that was cut to a certain contour (extremely important) but would be out of spec when measured. Most folks start off on a bad footing when they start a scrape job (or lap a granite plate). Really big ones are never moved, so you have no real choice in the matter. Anything smaller than a 6' x 9' was brought to my shop, and I'd let it set for two or three days. Cast iron is easier and you can start on it in 24 hours. A machine must be leveled and then tweaked in till the bottom axis is as strait as you can get it. I might add here that I usually did this with a Federal Electronic level, or at least a laser (the electronic level is much easier). Sometimes it takes the better part of a week to get this done as the machine will change everyday. I once had a Devlieg jig mill that took a solid three weeks on the x-axis alone. Still the numbers were a bitch before you started (.000020" in 12 feet). After you level the base everyday, you can start on the next slide and rough it in. Once it quits moving you can go to work. In the shop, I always had three or four strait edges to scrape, and several cast iron parallels. Angle plates can be nasty to do if done right. You need three angle plates to master off of, and this alone is rare. Yet they numbers are easy. Small ones (6"x 6") I had, but a big one I didn't. My master angle plates (a wooden box of five) were kept locked up and never used. My final check for squareness was with a 12" callibrated cylinder square.

I watch Indians and Pakistanis do some ummm “interesting” repairs with welders and lathes….i would LOVE to see Keith comment objectively on those videos!!!

@@RASAllusion, Some of those videos are fun to watch, it’s almost unbelievable some of the stuff that they rebuild. They’re not afraid to work, that’s for sure.

absolutely!! use it for typical shop needs those are really good surface plates for us hobby shop machinists/mechanics. Ive used one for years and it works great!

Rough castings right out of the sand are FULL of stresses. Either you "season", as Frankford did, or you employ sophisticated thermal cycling, like Clarke Easterling does.

Quite so. Even moving a granite to a different location on your shop floor, or jacks under the plate require calibration to assure plate is still to desired flatness after relaxing into new location. We're talking millionths here. Wouldn't matter for what I was doing in my shop; mattered to the aerospace shops I've worked in.

Nice! Though the misses might not agree

@@zHxIxPxPxIxEz I’ll let her put some flowers on it as well.