WTB - 48" Straight Edge - Illinois

I'm looking at the Shars 2"x8"x48" granite straight edge which is $530 plus shipping. Seeing as Shars is local to me I save on shipping. This seems like a decent deal but figured I should check for used deals first. I found a thread on PM that suggested that granite SE's are a dime a dozen.

I'm intending to cast epoxy against the edge and perhaps against 2 edges which is why I think the granite might be a good idea. The weight is actually a plus for the technique I'm hoping to use. Anyway, does anyone have a 48" or so straight edge for sale or suggestions?

chmedly said:

I'm looking at the Shars 2"x8"x48" granite straight edge which is $530 plus shipping. Seeing as Shars is local to me I save on shipping. This seems like a decent deal but figured I should check for used deals first. I found a thread on PM that suggested that granite SE's are a dime a dozen.

I'm intending to cast epoxy against the edge and perhaps against 2 edges which is why I think the granite might be a good idea. The weight is actually a plus for the technique I'm hoping to use. Anyway, does anyone have a 48" or so straight edge for sale or suggestions?

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I'm curious about the project you're doing. If casting against a straight surface because you need the cast face to be straight, a granite straightedge might be a good bet because of the surface finish. A cast iron straightedge, while flat at its contact surface, would probably have a scraped finish. All the high points are guaranteed to be at the same level (within a small range) but the low spots in between could be anything. Casting against it will reverse the surface texture so you're casting will have low spots that are a very consistent depth (ignoring shrink changes) but the cast high points might be all over the place.

I wonder is a separating media you intend to use and curing deformation of the epoxy itself will throw away all the precision provided by a straight edge.

TGTool said:

...If casting against a straight surface because you need the cast face to be straight, a granite straightedge might be a good bet because of the surface finish. A cast iron straightedge, while flat at its contact surface, would probably have a scraped finish. All the high points are guaranteed to be at the same level (within a small range) but the low spots in between could be anything. Casting against it will reverse the surface texture so you're casting will have low spots that are a very consistent depth (ignoring shrink changes) but the cast high points might be all over the place.

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MichaelP said:

I wonder is a separating media you intend to use and curing deformation of the epoxy itself will throw away all the precision provided by a straight edge.

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Good points, both.

Keeping in mind that A) Epoxy is known first and foremost as a BINDER, and that B) granite will ALSO have inter-grain pockets of "some" depth, it comes down to need of a separating medium/very level and consistent release agent in either case.

And then-also.. at least the possibilty of need for planing, scraping, stoning, lapping... "finishing" of some sort.... of a (filled?) Epoxy surface.

Less of a "free lunch" in this opration than planned and expected?

Very probably so.

Believe I'd be looking to use a SE - better-yet, a surface plate - to "back" a strip of "float" glass as used for large closet-door mirrors.

Continuous-cast over a very long trough of molten Tin or the like, it has astonishingly good flatness and surface finish at low cost, just isn't as rigid in bending as "glass" implies.

2CW

Addressing both comments:
Yes, the intention is to cast a flat surface to mount linear guide rail. I figure I can put on a flat surface and a raised edge (to line the rail up to) at the same time. Theoretical in my mind at this point. Separating media will likely just be mold release wax (Maguire's). This seems like a better idea than self leveling epoxy. I don't think West system will deform much at all while curing. Are you thinking that the exotherm temperature rise will cause issues?

Any suggestions from anyone about this would be great. But also maybe an answer about whether I'm likely to find a better deal on a granite straight edge used than buying new Chinese from Shars.

chmedly said:

Addressing both comments:
Yes, the intention is to cast a flat surface to mount linear guide rail. I figure I can put on a flat surface and a raised edge (to line the rail up to) at the same time. Theoretical in my mind at this point. Separating media will likely just be mold release wax (Maguire's). This seems like a better idea than self leveling epoxy. I don't think West system will deform much at all while curing. Are you thinking that the exotherm temperature rise will cause issues?

Any suggestions from anyone about this would be great. But also maybe an answer about whether I'm likely to find a better deal on a granite straight edge used than buying new Chinese from Shars.

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SE? PLENTY of cheap surface plates out there if you can make the space and deal with delivery costs. Illinois ain't exactly Antarctica, either, so they'll not all be far away.

Small Tools, up in Cleveland put a used 30" x 48" Herman one-time Grade A that looks as if it might still pass calibration into my place at a good price, highly professional packing and delivery included.

That said, I'm not sure I'd want epoxy even involved in support for a "good" linear rail.

Real metal (eg: NOT shiney-wood with its thermals and stresses) and shims or scraping to very, very precise alignment, rather.

These linear guide rails are for a wood cutting router if that gives this some context. Further, there are metal fillers that can be added to epoxy to affect it's "capabilities". I think that any "reverse mold" issues, like were mentioned earlier, will either be within a reasonable tolerance for this application or might be remedied with some light sanding. I think I'll find out after some test molds. I recognize that rigidity is rigidity and this may be either a futile adventure or the tail will wag the dog but we'll see.
As far as Surface Plates go, I was planning to go that route but after more thought I think a straight edge will be a lot more convenient for shop space and maneuvering during these epoxy operations. I don't need a big flat table, I just just need a 2" or so edge.

chmedly said:

These linear guide rails are for a wood cutting router if that gives this some context.

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Well yes. It does. Loominum extrusions, Mark One Eyball, mebbe a length of fine wire and adjustable screws rather than shims. EG: mounted precisely? yah, but.... to just WHAT as the immovable-object "backbone" FOR those mounts?

Wouldn't even uncrate one of my Davidson D600's for sawdust-making nor shiney-wood working gadgetry.

The materials move more in any given day than the "machine" error is likely to be all year.

Synthetic granite countertop backsplashs are typically 1.25" thick, 4 inches wide, 3 rounded corners with 3 polished surfaces, as cast. They are cast in a mold. I have several of them and they are flat to .001" per foot. (for example, one of mine has a difference in width of like 3.970 to 3.974" across 6 feet of its length) The 1.125" surface isn't flat across the 1.125" dimension though (warpage i suspect), so you would want to lap them with a cheap 150grit 2x6" diamond lap until the diamond lap starts scratching at least 50% of the surface before bolting the linear rails to them, otherwise the rails will be twisted as the bolts suck them into the block. the edges aren't perfectly 90 degrees to the face either.

for mounting the bolts to the rail i would cut dovetails in the granite and drill and tap dovetail shaped metal wedges, then epoxy them in after aligning the bolt holes, rather than epoxy them in first and then drill and tap (i assume you don't have a milling machine?) because you typically only have .010" of clearance between the bolt hole in the rail and the bolts.

I would not pay money for them, they are incredibly expensive, instead check the dumpster at the local granite countertop outfits. some of them only have 2 finished surfaces because they cut the 4 inch wide ones in half to make the 2 inch wide ones.

only problem with them is they are typically polyester resin rather than epoxy.


another option is mounting the rails to the 4" wide surface (which is flatter across the 4" surface but not globally flat), then shimming the granite, not the rail. this gives you better local stability for the rail and a wider area to fill the gap between the foundation and the shims with epoxy.


the problem with bolting the rail to epoxy alone is the epoxy creeps and you end up with a wave in the rail between each bolt. fillers help avoid this problem but don't eliminate it completely.