I bought the book "Machine Tool Reconditioning" with the intent of learning some limited scraping capabilities. With the latest isue of HSM and the lathe reconditioning article I am fired up to give it a try. I have assembled everything to attempt some small jobs except a straight edge and have found them to very expensive. I read somewhere, which I can't locate again, that steel does not make a good straight edge so I would suppose that would leave out aluminum. I have two related questions. Why is cast iron so much better as a straight edge for scraping machines and why is cast iron so expensive realtive to steel? Please excuse me if this is an obvious question but I am new to this and can't find an answer elsewhere.
Cast Iron Stock is largely sawn from large cast boules. This is an expensive process. There is continuously cast product called Dura-Bar that is a little more reasonable since it is not saw cut. The main point is that cast iron is just that - cast. It can't be formed by rolling and be the same material. That is why most iron straight edges are made from a purpose built casting.
Micheal Morgan used to sell a raw casting for a scraped straightedge. I don't know if he still does.
One of my short straightedges is made from a piece sawed from the column of a dead B&S #13 tool and cutter grinder. If you scrounge persistantly in the industrial junkyards and foundry scrap in your area you might find the starting material for your own scraped straight edge.
Fetch along a kit consisting of a Sawzall, a collection of bi-metal blades, a way to power it (long estension cord, generator etc,) a laundry spraybottle, and a two gallon container of water with a little washing soda (keeps the rust down). Oh and a helpt to share the tasks and run the squirt bottle.
With this kit and helper you can hack out a piece of cast iron from the arm containing in its section the dovetail shape and enough wall to form a stiffener. It may not be pretty or light by the time you get it cleaned up and detailed but it will serve the purpose.
You might want to consider the following idea. I came up with it when faced with the dilemma of needing a very long and very accurate straight edge, on a very limited budget. It is far from perfect as it is only useable on surfaces that exhibit concavity or are flat. The good part is, it might cost about $5.00 to make, and is truly dead straight. One other benefit is that it can be used to calibrate or check the accuracy of another straightedge.
A single .010" thick guitar string.
1 length of material, wood, aluminum, etc., of small dimensions (e.g. 1 x 2 wood) and about 30" long.
Two thin posts of some kind, about 2" long, with a small groove (less than .005" deep) cut at one end of each post.
A single guitar machine head. (available at any music shop that does repairs).
1. Mount the posts at either end of the beam.
2. Mount the machine head at one end.
3. Fix the guitar string at one end, pass it over top of the two posts and take up the slack with the machine head.
What you now have is a small simple truss that is dead straight (in fact I would argue that it is straighter than any manufactured straightedge). It has its limitations, but I have found it to be very beneficial in my shop. Actually it can be used on some convex surfaces as well, if you use a little ingenuity, such as lifting the ends pressed against a given surface minutely with blocks, and then using a feeler gauge.
Cast Iron, due to it's crystaline structure, does not warp (after it is stabilised).
But casting, is also more expensive than rolling, so that is the reason ..partially.
Steel can make a good straight edge, but cast iron will always be better for the above reason.
My Father-in-Law was a T&D maker and ground straight edges for his company.
first, you should know that whenever you grind a piece of steel, it warps ...due to stress,heat,grain etc
It can be a tedious job....as follows;
First, you want a very accurate surface grinder..with hydralic ways if possible.
The "edges" were supported vertically, without creating any (!) location stresses, and the top edge ground. then it was removed, and measured (they did it optically) and flipped over and the bottom ground, then repeat the above.
Flip grind, flip grind, and so on..
Each time less material was removed..say .005 first pass, then .003, then .0015, then .0010..
Now a good man could come out straight AND to size !..less skilled and they blew the size for sure..took a lot of skill, patience, and ability to evaluate the amount to take off OR leave for the next pass..
sometimes, it would be .002 top, .0005 bottom,.00175 top...you get the picture
so now you know why its $$$
you can always buy a cheap edge...but those are "straightened" ( waiting for a chance to release that stress !)