What is the flatness of a BS 36" steel straght edge Marked "Grade A"

BuffDan · Dec 29, 2013

I picked up a BS 36" steel straight edge marked B.S. 863 Grade A
Its 3/8 thick, one edge beveled, 2 1/2 " wide.

I cant seem to find a reference for straight edge flatness/accuracies/grades.
Would this be the same as a surface plate grade A ?
.000050” per 12” ?

Thanks,
Dan

TDegenhart · Dec 29, 2013

Do a Google search for "grades for straight edges and surface plates" Among others you will find this from Starrett.

Standard metrology products, accessories, surface plate calibration products, covers, cleaners, cases

They say, " inspection grade A accuracy is .000050” per 12”"

Tom

The real Leigh · Dec 29, 2013

BuffDan said:
I cant seem to find a reference for straight edge flatness/accuracies/grades.
Would this be the same as a surface plate grade A ?
.000050” per 12” ?

Definitely not the same as a surface plate.

You can flex a 3/8" thick steel bar 2-1/2" x 36" feet long with your hands.

The source of error here is the cross-section area of the bar, being too small to maintain rigidity.

- Leigh

The real Leigh · Dec 29, 2013

TDegenhart said:
Do a Google search for "grades for straight edges and surface plates" Among others you will find this from Starrett.
They say, " inspection grade A accuracy is .000050” per 12”"

That accuracy is for granite parallels. The 1" x 2" cross-section are only 12" long. The 36" long parallels are 2" x 4" cross-section.
Product page here: Parallels

The Starrett straightedge comparable to the OP's has an accuracy spec of 0.0002"/12". Its flatness is not spec'd at all.
This is basically a ruler, so accuracy refers to its calibration marks.
Product page here: 385-36 Steel Straight Edge with Bevel Edge

- Leigh

BuffDan · Dec 29, 2013

Ok, This makes sense.

It has marks 20" apart which indicate "support here".
Similar to what Starrett indicates: The 380 and 385 Series in sizes 36" and longer is marked with arrows at two
suspension points. If the straight edges are brought to the work and used on
edge, they should be suspended at these two points to minimize deflection.
So I'm sure this straight edge doesn't have near the rigidity of granite version, hence not as flat.

What I couldn't find was a reference to steel straight edge "grade" specs.
I'll assume this spec for the Starrett is the same as the B&S
Thanks for the help!
So, ~ .0006 over 36"
Dan

The real Leigh said:
That accuracy is for granite parallels. The 1" x 2" cross-section are only 12" long. The 36" long parallels are 2" x 4" cross-section.
Product page here: Parallels

The Starrett straightedge comparable to the OP's has an accuracy spec of 0.0002"/12". Its flatness is not spec'd at all.
This is basically a ruler, so accuracy refers to its calibration marks.
Product page here: 385-36 Steel Straight Edge with Bevel Edge

- Leigh

johnoder · Dec 31, 2013

May be a special, or just an oldie. In my pre zip code The Shop Tool Manual by B&S the beveled 36" is 599-526-36 and is neither hardened nor makes any reference to its "grade".

Its stated use in in drawing lines and checking things for relative straightness - not a flatness checker

BuffDan · Dec 31, 2013

I found out that BS 863 = "1939 Steel straightedges of rectangular section"
A BSI, British standard.
Dummy me I assumed B.S. was Browne & Sharp...

I am searching to find out what this standard is, but, all the references so far require $$$ to download..

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What is the flatness of a BS 36" steel straght edge Marked "Grade A"

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