How big would a reference surface be for a 120 inch straight edge.

[Peyton Price 17]

I am thinking of getting a 120 inch brown and sharpe camel back straightedge. How long would another surface have to be to have it rescrape it?

 

[benmychree]

Somewhere nearly as long ---

 

[graham-xrf]

I think, with care, there are some methods that allow to confidently scrape in something that is longer than the test surface. From memory the surface needs to be a bit more than half the length. This surely must be a thing so common, it be well known by now.

In theory, if you flattened 1/3, then moved attention to the next third, and so on, the tolerance extrapolated would be worse than if you had a huge surface to begin with, but still good enough for you. Maybe it is mentioned in Connelly's book.

[Edit: In Connelly's book "Machine Tool Reconditioning" 8.15 on Page 48, there is reference on what to do about having a surface plate that is too small, though the stress is how to use it in combination with a straight edge to overcome the problem when working a a big piece. This is not the same situation as how to make the straight edge straight in the first place.]

 

[graham-xrf]

There may be some mileage in what this guy did with .. hand tools only!
It is longer than his surface plate - but not by much. Surface plate looks at least 3/4 or more

--> Hints on making Straight Edges out of Steel

 

[Dabbler]

Q to @Peyton Price 17 : Do you really need a 120 inch straight edge? Will it make 2 good 58" ones? Is that overkill for what you need?

-- you can easily scrape in a 58" straight edge on a 48" surface plate. As mentioned by @graham-xrf , the basics are in Connely's book.

 

[MontanaLon]

Q to @Peyton Price 17 : Do you really need a 120 inch straight edge? Will it make 2 good 58" ones? Is that overkill for what you need?

-- you can easily scrape in a 58" straight edge on a 48" surface plate. As mentioned by @graham-xrf , the basics are in Connely's book.

Well, a 48" surface plate is usually 36" wide so the diagonal would be 60". You could scrape in a 58" straight edge easily on that.

But a 10 foot straight edge? What are you working on? Most of the stuff in my shop would have trouble supporting the weight.

 

[matthewsx]

Theory vs. practice.

Guys who need a 120" straightedge probably aren't in the "hobby" machinist category.

@Richard King 2 knows this kinda stuff and if you really want to learn I strongly suggest listening to him. If it's just an academic question then simple arithmetic should answer the question.


John

 

[Richard King 2]

There is an easy way to first test it to be sure it needs scraping. Flip it over and brace it with the originally scraped surface up. It must be solid. Place a precision level .0005/12" in the middle and level it. Then using a tape measure and sharpie marker measure out 1/2 the length of the level, stop align the edge, wait 10 seconds, light pressure on each end. It should be .00005" to .0001 per 12" .

 

[Richard King 2]

If it is bad, then you locate the lowest spot or area and scrape that area to your short master not moving it more then a 1" or 2 when rubbing. Once that area then move out 1/2 the distance of your short master and scrape the high area until it starts to blue in the finished area., etc. etc. This called step scraping. Also use the level to test as I described before.

 

[Peyton Price 17]

Q to @Peyton Price 17 : Do you really need a 120 inch straight edge? Will it make 2 good 58" ones? Is that overkill for what you need?

-- you can easily scrape in a 58" straight edge on a 48" surface plate. As mentioned by @graham-xrf , the basics are in Connely's book.

because it is huge and who doesn't want one. I think i might be getting a big metal planer so it would be good for testing the wear.