Dimension Question

[bSnacks10]

Our company uses ordinate dimensioning. We have standards established but when new guys come in there's usually discussions as to what was taught in school etc.

Question:
1. How do you approach the 0,0 position, should it be of the same edge "a" or how its shown?

a. How would the fabricator prefer it?​

2. How to correctly dimension a slot, "b" with the overall length or "c" ordinate length?

I appreciate the help!

 

[ctopher]

That depends on your design. What edge do you need to dim from? The fabricator will make per dwg.

Chris, CSWP
SolidWorks '20
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[bSnacks10]

For this plain mounting pate I would dimension from the outside(s) (as shown). Then there are 4 cutouts at the corners.

Regarding that 0 horiz. dim at the bottom, I wasn't sure how the industry views having the 0,0 at "a" and then having to show positive dim's in both the up and down. Hopefully that makes sense.

 

[TheTick]

Edges are better than corners. Edges can be bumped up to stops. Also easier to find with nearly all measuring equipment. Virtual corners suck.

I machined bunches of flat ordinate-dimensioned parts. The origins you have are perfectly fine.

Your slot dimensions are awful for fab. (Slots are generally awful, anyhow.) Dimension the total length and width on their own, and locate the center with ordinates.

Function always should trump form, but there's value to be found in making parts easier to make and measure.

 

[3DDave]

I feel like the class teaching how to make graphs is missing some students.

Depending on who the target audience for this particular drawing is, it is more typical to either locate the slot ends, which is what a CNC nibbler software is looking for - they automatically subtract the slot half-width as the radius of the punch.

Alternatively if there is a punch that is made for that exact slot then dimension to the center of the slot and call out the length and width directly or by note or by tabulation.

There is a slight chance the CNC will be used to nibble in a way that one would dimension the start and stop locations of the straight sides of the slot, but this changes the slot length when the slot wide changes, a typically unwanted, but not impossible requirement. Not sure why a longer slot should happen if the slot is made wider.

In manual machining locating the ends of the slots is preferred. The metal worker just scribes them and plunges at some place along the slot and advances the cut to one end and then reverses to the other end, coming just tangent to the marks. This way no matter what the radius of the cutter is the travel allowed by the slot is the same, particularly if the cutter is at the small end of the width tolerance.

It would be untypical to show the delta location (the "8", in this case.) The point of ordinate dimensioning is that any number in the series is the exact value of the distance from the origin with no further computation. This would be an exception. Not illegal but it increases the chance of not getting what was desired.

 

[Wuzhee]

Most of the time it's up to the manufacturer where he/she will put the origin, they can have preferences too.

You should ask them in the first place. I'm sure they'll answer the rest of your questions also.

If you were to set the 0,0 on the intersection of left and bottom edge they can use a dial gauge and dial up the origin. Or use precision blocks, dial them up and make a jig for precise repeatable work.
I've seen precision laser cutting/punching machines which doesn't require a fixture and can set an origin wherever you want.
Don't worry about bidirectional coordinate dimensioning. It's pretty common here in Europe (afaik). But 99% of the time coordinate dimensioning just can't put through the design intent/function. It's also very awful regarding gd&t.

About the slot:
Center point for punching, outer dimensions for laser cutting/CNC.
Either way, modern machines utilize a bunch of digital format so it should be less of concern where to put dimensions. The focus should be on functionality.

 

[ctopher]

Dim slot C is better than slot B. You have to look at how it's tolerance stacks.

Chris, CSWP
SolidWorks '20
ctophers home
SolidWorks Legion

 

[looslib]

Are the parts being cut on a laser/plasma or punched?
If punched, it is best to dimension the center point of holes and slots. This is what they will program to with the proper punch size.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[drawoh]

bSnacks10,

What is the end requirement for the part? Consider the possibility that your dimensioning is correct!

--
JHG