"Composite" straight edges?

[TroyO]

I was mulling over straight edges and just can't see spending too much on one to scrape in my hobby level machine tools. I have a small prism I made form some windows weights, and it turned out great and the price was perfect, LOL. (Free). It's about 1.25 x 11.

I will need a couple of larger ones for the G0704 though, so I set about pondering cheap alternatives.

(1) What about casting JB weld against the surface plate? Using a clay "mold" you could cast right on the plate (After several coats of wax) and it should be a mirror of the extremely flat reference. You could insert some "ribs" while it cures to give it some additional stability... granite perhaps? The cool thing about this one is you could still "scrape it in" using traditional methods.

2) Combining granite tile cuts to create a straight edge and joining them with epoxy..... you can test the flatness of it before continuing and if you assemble it on the surface plate it should maintain it's flatness pretty well I'd think. There would have to be some vetting of the tiles as you went to make sure the cuts didn't throw it out of wack.

In the end, as long as it's stable(ish) and flat.... does it matter how/what it's made of? (Well... at least in a hobby setting.)

It's certainly inexpensive to try it out..... maybe $40 for both to give it a whirl.

Am I completely nuts? I don't see any drawbacks IF it works as expected... one key fact I'm off to research is if I can find any thermal expansion rates for JB Weld (Or any other epoxies.... I keep mentioning them but I swear I'm not a shill, LOL!)

CoC Pics attached

 

[DMS]

I think that much JB-Weld is going to cost more than the rough straightedge

For a straightedge, the material does matter. You want something that is fairly stable dimensionally, as well as something that transfers the blue well. Cast Iron is typically the preferred material. So how cheap are you trying to get away with? Durabar is somewhat expensive IMO, but it's not that bad, especially if you can find a drop big enough.

FYI, I have seen people "re-purpose" old cast iron surface plates into usable straight edges. If you can find one at a scrap yard, or on CL, that may be an option. Old cast iron table saw table might also work too, if you can find one with few enough holes. Just add a handle and scrape flat.

 

[TroyO]

Surprisingly you can get JB pretty cheap, LOL. I filled the base of the X2 with it for ~20 from Ebay. I guess I was thinking cheap in comparison to $350-$500 for a 24" Camelback casting. It's a combo of cheap and time-saving compared to scraping in a rough camelback too.

I actually have a big chunk of durabar I can make a wider prism from.... but it won't be tons longer. I figure I can cut some granite tile I have already (18x26) down and make a long/narrow edge for the table ways. I suspect granite, even in that form will be pretty stable.... maybe not enough to last for seasons of use, but enough to get a set of ways done.

I'm just shooting ideas out..... I was also looking at optical glass prisms too.... but they seem to only go up to about a foot.

I hadn't thought about scrap tables etc. I'll have to ponder that one.....

 

[cjtoombs]

As for alternate materials for a straightedge, I would think aluminum would be the next choice down from cast iron. It is relatively cheap, cheaper to ship if you have to order the piece you needed, realtivliy easy to cast if you have that capability (and a good casting aluminum alloy would likely scrape easier than 6061). It's fairly easy to scrape, if all the Gingery builds out there can be counted as evidence. I think the big problem with alumium would be more rapid wear, which would be less of a problem for a hobby scraper than it would be for a pro machine rebuilding shop. As for composites, I think the wear problem with aluminum would be even worse. Also, on a piece that size, there would likely be some shrinkage deformation, so casting it on a surface plate may not buy you much. You may still have to machine it flat before you scrape it in. Scraping something soft like that is doable, but it takes a different touch, or you gouge too deep. Good luck.

 

[DMS]

That's a pretty good idea. MIC-6 is a cast plate, and comes pretty flat as is. Might be a good start.

 

[TroyO]

Good point on the.... errr..... points. The cast mirror may be slightly inferior but it would still be darn close. Luckily anything cast like that should be able to be given a quick pass with a scraper.

I think I may try the idea in the second pic..... at least just to cut a slice off a tile and checking it for flat by basically just spotting it like any other. I could even dress out minor issues with diamond hones maybe. If it passes that test the rest seems like it would work.

I have the tile and a small wet saw so it's free to give it a swirl.

The Mic-6 cast plate is interesting too... for some reason it never occurred to me that you could scrape AL. FYI it's about $35 shipped for a 1x2x32.... not too bad. I might need to find a slightly larger hunk but it's a start.

Lastly, some of the casting resins would also be $35ish for 3.something pounds of it, which should cast something that looks an awful lot like a camelback.

Will it save money? Probably not..... but it's starting to sound like fun. LOL.

 

[DMS]

A place near me sells drops of mic-6 by the pound. You may see if there is a similar place near you, the "drops" can be pretty large.

 

[astjp2]

As for alternate materials for a straightedge, I would think aluminum would be the next choice down from cast iron. It is relatively cheap, cheaper to ship if you have to order the piece you needed, realtivliy easy to cast if you have that capability (and a good casting aluminum alloy would likely scrape easier than 6061). It's fairly easy to scrape, if all the Gingery builds out there can be counted as evidence. I think the big problem with alumium would be more rapid wear, which would be less of a problem for a hobby scraper than it would be for a pro machine rebuilding shop. As for composites, I think the wear problem with aluminum would be even worse. Also, on a piece that size, there would likely be some shrinkage deformation, so casting it on a surface plate may not buy you much. You may still have to machine it flat before you scrape it in. Scraping something soft like that is doable, but it takes a different touch, or you gouge too deep. Good luck.

Nope, it expands and contracts too much to be an effective straight edge. I can get rough iron castings for around 100 including shipping each new....from Greg Dermer. Do a search on here for contact info.
Tim

 

[cjtoombs]

Nope, it expands and contracts too much to be an effective straight edge. I can get rough iron castings for around 100 including shipping each new....from Greg Dermer. Do a search on here for contact info.
Tim

The whole point is to maintain the work area and straight edge within a fairly narrow temperature range. That's why even cast iron straightedges have wooden handles, to keep your body heat from warping them from diferential heating of the area where you touch it. Also, if you do a symetrical parallel design heating/cooling should have very little effect on the flatness of the surfaces. Bush Precision makes a line of aluminum straightedges with cast iron wear surfaces bonded to them:


http://machining.buschprecision.com...a6100-series-aluminum-parallel-straight-edges?

 

[DMS]

The Busch Precision site is interesting. If you want a laugh (or fright?), take a look at the prices on their camel backs. The small one is about 3x what I paid for my first car

I also want to add that both the previous posters have good points. Aluminum will move more than cast iron (coefficent of thermal expansion for aluminum is about double that for CI), but cast iron will move too. The thing you need to avoid in use is differential heating (getting one side at a different temp than the other) as this will cause the item to warp. Camelbacks have additional structure to prevent this, so if you can integrate ribbing into your design, that would probably help keep things flat.

Personally I am a little skeptical of the Busch design with cast iron facing. It looks like the design is balanced (cast iron on both sides) which will help, but it seems like a bad idea in general to mix materials like that. Maybe that line of straightedges is not meant for machine restoration?