evaluating used straightedges

I eventually want to learn a little about machine scraping, but haven't located the Connnelly book yet. Now I have a chance to buy some scrapers and some master straightedges from the widow of a machine rebuilder. Is there any way that I - in my ignorance - can judge the quality and value of the straightedges? One is said to "have rust" (so I assume they are cast iron), and another "has one good edge left". Others are presumed to be in good condition. I haven't seen them yet, but I need to know what to look for when I do see them. Can anyone suggest a reasonable price range for used cast iron 18" and 36" straightedges?

Brad

If there are at least three, of similar enough lengths, you can check them against one another, and if necessary true them up to as good as you want....theoretically.

One good edge? Personally I would assume they are anything but straight and flat, just due to storage conditions, never mind rust.

If I were you, I'd still buy them if possible, unless they are pretzels or in two pieces (suitable discounts for condition). I'm still looking for straightedges myself.

Send them here, I'll look them over and tell you which one is OK and obtain EPA disposal of the others.

I smell something stinky! (Ren & Stimpy voice)









[This message has been edited by morsetaper2 (edited 12-08-2003).]

Forrest, you're too kind. But since I can't afford the EPA disposal fee I guess I'll just have to store them on the premises.

How many straightedges does one really need? And what would be a reasonable price for a 36" straightedge in fair condition? At least tell me how many digits to expect to the left of the decimal point! (I haven't seen prices from the seller yet.)

Brad


[This message has been edited by NewMiller (edited 12-08-2003).]

Brad, your not likely to have many people looking for scraping stuff in your area. Tell her a hundred bucks for the lot. Your gonna have to put a lot of sweat equity into this stuff to make it worth a damn again. Your buying well seasoned castings, machined and of questionable accuracy, whats that worth to you? I have several straight edges and a 4 foot granite master to check them against just taking up room in my shop. Are they for sale?. NO! Maybe someday they will come into play. But the castings are worth a lot to me. They are hard to find. So I hold onto them and if I get the time I will do something with it all. It's much easier to start with an old straight edge, than to make one from scratch. The commercial ones are designed to be thermally balanced as well so they don't distort from a few degree temparature change. Up to you, feel her out, but pay as little as possible, your buying machined castings. Bill

Thanks, Bill; that gives me the perspective I was looking for. I don't want to cheat the lady but I don't want to cheat myself either.

...really do need to find that book.

Brad

I remember the metrology class where we learnt how to check a straightedge - for all the wrong reasons - I can't forget the clang of the straight edge tipping off its slip gauges onto the surface table, and the slips bouncing onto the floor....

Anyway, if you want to check for straightness, this method was to start with a Grade 'A' flat surface, sit the straightedge on equal size slip gauges, the straightedge should be resting on its "Airey" points (the distance between the Airey points is 0.554 of the straightedge length). Then check the gap between the straightedge and the flat surface at 7 equal spaces between the Airey points, and 2 places between the Airey points and the end of the straight edge. This gives a total of 13 readings, counting the Airey points.
Record the measurements and make yourself a little graph to show the differences along the straightedge.
According to my dusty old notes, our test straightedge had a maximum error of .003mm, (.00012") hopefully it was still OK after its fall.

Not really much help to you I guess, but may be of interest.


[This message has been edited by Peter S (edited 12-09-2003).]

There is a reversal method for checking straight edges. Mount the edge to be inspected on a machine slide such as a milling machine. Set an indicator fixed to the machine and move the slide so the indicator reads the error. This is the sum of the machine travel straightness error and the straight edge error. Now reverse the edge end for end and carefully replace it so that the edge is in the same place on the slide. Without moving the attachment point of the indicator extend the indicator holder to allow the indicator to now trace the edge which is looking the opposite direction. The straightness error now indicated is again the sum of the machine straightness and the edge straightness but the edge straightness error is now of opposite sign so you can subtract it out of the total error which leaves you with the slide travel straightness error only and thus the edge straightness error only which is what you want to know. The bonus is that you now know the slide travel straightness error also. This is a routine test for ultra precision machine tools such as measuring machines. It works best if you use an electronic indicator and a strip chart so that you get a hard record of the error motions but you can do it by recording measurements every inch or so and making a graph by hand. Probably not practical to do this at a garage sale though. A quick and dirty check might be to mark the edge with a felt tip pin to color the surface and then slide it around on a good surface plate a little and look for the shine of high spots. A perfectly straight edge and a perfectly flat surface plate will remove all the marker color very quickly. Probably should go over it with a slip stone to take off nicks before rubbing it on a surface plate. It won't hurt either the edge or the plate unless you do it all day.

Thanks to everyone for the feedback. Peter S and Cass, that's really good information that I hope to use in the future, but it probably won't help in buying the straightedges. I think J Tiers and Mr Bridgeport have told me what I need to know for now - if they don't have obvious physical damage, buy them cheap enough that I can spend time truing them up myself. Got to find that book...!

I was just wondering if you have any idea what a 4" thick granite plate somewhere around the 4' x 4' square might be worth? The one in question has several holes drilled and tapped in it, for mounting other things on it. (It is part of a piece of machinery) I am just wondering if it has enough value to warrant me purchasing it and making room in the garage for it. The color of the plate is dark grey.
Thanks for the help. Sandman2234

A piece of granite 4'x4' that is only 4inches thick is not going to be very stiff and is therefore not going to make a very good surface plate. For comparison a 3ft. x 4ft. grade B Starrett pink granite plate is 6" thick and a 3 ft. x 5 ft. is 8" thick. A grade A or AA plate is 8" and 10" in those two sizes. It will make a pretty nice layout table but it is likely not very flat. I would support it on a piece of rubber backed carpet rather than the typical 3 points because it is a little thin. Look up the price of a comparable size plate from Enco or some other import peddler and don't pay more than about half that. The holes and threaded holes are handy and most of the time they don't effect flatness any noticeable amount because metal inserts are glued in.

Thanks Cass.
The granite is sitting on a table, made from angle iron and formed sheetmetal. It has a couple of camera's and a TV screen for looking at the stuff up close.Also has those roller type movements that CNC guys are so fond of. Plus a 3 or 4 nice micrometer heads. Kind of neat, but I am not sure if I would use it enough to warrant the purchase.

[This message has been edited by sandman2234 (edited 12-14-2003).]

Good Evening,

NewMiller, I have been looking for the same book and believe I have found it. "BOOK ON "MACHINE TOOL RECONDITIONING," published in 1955. An "Oldie," but one of a kind. They do not list the author but I hope it's Edward F Connelly. Link below and is listed at bottom of page.
http://www.dapra.com/biax/scrapers/

-Paul

It's Connelly

Guys,
Powells books in Portland, Or. www.powells.com has it too, as well as J&L industrial.
It seems to run around 90 bucks.
Steve


[This message has been edited by andromeda (edited 12-15-2003).]

Yes, I knew it was the Connelly book, and in fact I just recieved my copy from Drapa last Monday. Wish I had known about some of these other sources, as I could have saved a few bucks from the $104 + S&H that I paid. (But that was better than the $125 e-mail offer that I got).

This is a very interesting book. I won't say it's worth $100 to everyone, but I think I'm going to find a lot of useful information there even if I never touch a scraper.

Brad