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+--- Thread: Help with drawing (/thread-330.html)
Help with drawing - Turbinedoctor - 04-19-2012
I was wondering if someone could help me put this drawing into usable diamentions. I am trying to figure out how far in and down each 45 should be cut and how much flat between the 45's on the sides.
http://en.wikipedia.org/wiki/File:M1913A_Rail_CrossSection.svg
Thanks Turbo
RE: Help with drawing - stevec - 04-19-2012
I, for one, don't understand the question . Is it the hypontenuse (longest or 45° side) of the triangles formed by the 45° sides?
or the 2 equal short ones that you seek?
If so Pythagora's theory states that the square of the Hypotenuse is equal to the sum of the squares of the other 2 sides in a right triangle.
So .164 x .164 = .026986 and .026986/2=.013493 sq.rt. of which is .11615937 or .116 so the triangles each have 2 equal .116 sides and the 3rd (hyp.) is .164
I also have no idea what the dim. .748 is, represented by the -C- in a box under it.
RE: Help with drawing - EdK - 04-19-2012
I don't know if this is right but here's my attempt at your request.
Ed
Edit: removed drawing since it likely was incorrect.
RE: Help with drawing - Hopefuldave - 04-19-2012
Hi Turbo,
Have a look at http://en.wikipedia.org/wiki/Geometric_dimensioning_and_tolerancing - it gives a brief rundown on what the dimensional tolerance symbols mean!
Starting with the parallelogram in a box, that's telling you there's a 0.005" limit on flatness;
The (+) crosshair shows positional tolerance - 6 or 10 thou";
(M) tells you that dimensions are "maximum material condition" - or "as big as it's allowed to be"!;
© and -©- tells you that those dimensions are symmetric about a centre-line.
It *looks* like the .748" dimension is between the mid-points of the two upper bevels, so it should be triggable from that assumption?
Just my ha'pennorth,
Dave H. (the other one)
RE: Help with drawing - EdK - 04-19-2012
(04-19-2012, 05:53 PM)Hopefuldave Wrote: © and -©- tells you that those dimensions are symmetric about a centre-line.
It *looks* like the .748" dimension is between the mid-points of the two upper bevels, so it should be triggable from that assumption?
Dave H. (the other one)
That's what I assumed and based my drawing on that so maybe I got lucky and did it right.
Ed
RE: Help with drawing - Tony Wells - 04-19-2012
The 0.748 is given as a gaging dimension. In other words, not to be measured, but can be used to determine the other dimensions based on the assumptions that the theoretically perfect 0.748 is built into the gage used to inspect the part.
I'll go to the shop and sketch it up also to confirm Ed's work. GDT can be a bit hard to interpret if you're not used to it. Dave covered most of it.
Edit:
That drawing is misleading. The 45 degree angles do not come to a sharp edge. There is a flat that will be 0.0235 wide. Let's see if I can do the attachment correctly. I only added the "missing" dimensions. You will still need to refer to the other sketch.
Too large as an attachment, so inserted drawing should be here:
![[Image: test1.jpg]](http://i790.photobucket.com/albums/yy181/GrayLox_album/test1.jpg)
RE: Help with drawing - PixMan - 04-19-2012
Under the current Geometric Dimensioning and Tolerance System in use in North America today, (ANSI 14.5 1982?) I've never seen the -C- used as it is within the dialogue box in the upper right. It presumably references datum -C-, but should only be used as it is shown in the dialogue box in the lower right area of the print (without the - - on either side of the C). The current system has no symbols or convention that I am aware of to denote "symmetry".
Note that the features within a box are known as "basic" dimensions, which have no tolerance of their own. They are known as "gage" dimensions from which other dimensions or features are derived.
RE: Help with drawing - Tony Wells - 04-19-2012
The latest standard I am aware of for us is (ASME) Y14.5-2009, but I believe the Symmetry callout was there in the previous version. It is still in use.
Ken is correct about the -C- being incorrectly drawn. If it is specifying Datum C it should simply be a capital C in a rectangular box.
Basic dimensions do have a tolerance, but not the same tolerance as would be given in the title block of the drawing, such as +/- 0.005 for 3 place decimals. The tolerance is according to the feature definition block referenced back to the datum controlling the basic dimension. In this case, the basic dimensions are given with modifiers on actual size of the feature and Datum C as manufactured. This time we see a positional tolerance zone of 0.006 on the outside edges (the 0.835 -0.005 dimension), modified to enlarge the tolerance zone up to 0.015, by allowing the feature to shrink within its limit. That comes from the -0.005, (the tolerance of the feature), plus 0.006 (True Position tolerance), and 0.004 (datum C @ it's LMC). The way it works is basically that the smaller each control feature is, the more error is allowed in location of the basic dimensioned feature. The intent is to actually give more overall tolerance than could be given with hard dimensions and the standard tolerance conventions. The smaller the features get, the sloppier you can work, in other words.
RE: Help with drawing - DanH - 04-19-2012
These are the picitany rail dimensions. In my work I have measured many different rails on guns and have found a lot of variation in the dimensions. The clamping systems for most mounts allow for this variation so I wouldn't get too hung up on the exact dimensions shown on the spec's. I have always used the layout dimensions at nominal for my cad layouts and design with no issues.
RE: Help with drawing - Turbinedoctor - 04-19-2012
Guys thanks for all the replies, I am going to need some time to desipher it all means to see if myquestions have been answered. I have read drawing before but nothing like these.
One of the main things I was looking for was how far to come in from the sides withe the bevels and how much of a flat to leave between them from top to bottom. Hope that makes sense.