Dimensioning for dummies (me)

I’m trying I swear I’m trying! I’m new with AutoCAD, my background is with Esprit, but that’s ok because the more I use AutoCAD the more I’m liking it

 

I’m just now dangerous enough to know how to draw some stuff, now need to dimension it out “accurately”. Accurately enough to send to a customer and I don’t need to be embarrassed by having anything wrong so I’m coming here for help once again!

 

Please have a look at the attached and give guidance PLEASE!

 

Thanks in advance!

G20-0050_JIM.dwg

You don't want to send that to anyone.

First of all, what are your desired units?

 

Your part is modeled if feet, yet it is very small (about 3 inches).

Why would you do that?

 

Your dimensions go to 4 decimal places (which is very expensive to manufacture), and the dimensions don't really make logical sense.

What dimensions do you really really really want?

There really isn't all that much to dimension. Basically you have three radii to call out and you should probably indicate an angle, which would be typical, between two of the lobes.

 

I would recommend drawing your object at full size in model space and then switch to a layout. Create a viewport in your layout, assign a scale to the viewport (dependent on the size of the sheet of paper you will be plotting to) then dimension and label everything in your layout.

 

The other option would be to label and dimension everything in model space.

 

BTW...I have a couple more suggestions.

 

Add some centerlines.

 

Set up separate layers for your geometry, dimensions, centerlines and text. Assign each a different layer color too.

 

This is for demonstration purposes only. Your results may vary.

My object, centerlines and dimensions are all on separate layers.

 

I'd be interested in knowing what the actual size of this object is supposed to be. Currently it appears this object has a diameter slightly under 1/4".

 

To test my theory about what dimensions, etc. are needed to create the object shown I used the information in red, per the image above, and successfully reproduced the geometry. I would strongly suggest you abandon the dimensioning technique that you used.

Edited November 19, 2014 by ReMark

I have not done any mechanical drafting in a while, so I too would probably put a few too many dimensions on this object.

 

Depending on the manufacturing process, I'd also like to see three diameters, the lobe center circle, the outermost diameter, and the diameter of the inner "shaft?".

 

The way an object is dimensioned is completely dependent on what the person making the part needs to know in order to make it. One thing they don't need to have is repetitive dimensions of TYPICAL features, and distances. It is important to show the overall size, (outer diameter) so the craftsman knows how much material he needs. In this case, it appears the craftsman can make this item from the low "E" string from a stand up bass.

 

Consider what is irrelevant. In the case of this particular object, a straight line dimension between any two of those lobes, or their center points is irrelevant because it is impossible to locate a start or end of that dimension from any other meaningful feature point on the object, other than the primary center point. The particular distance between the primary center and the center of each lobe is important, thus we show the diameter (or radius) of the circle that passes through the center points of the lobes.

 

The only features of this object that are directly relevant to all other features are the primary center point of the whole object, and the angle of separation between all the repetitive features around the circumference. Let's assume this is the cross section of some item made of steel bar or rod, even though that may not be what it is at all. The way this profile probably will be made is to have the stock material fixed into a cutting tool, like an end mill machine after the outer diameter is established, maybe on a lathe. One space between the lobes will be cut out, then the material will be rotated 72 deg. to the next space until all five are cut away. If this object were not so small, it would be possible to use a very expensive custom end mill that will establish the profile of one facing side of two lobes, and the radius of the inner surface between them at once.

 

In order to make a feature of an object, one needs to know how far away that feature is from another permanent feature on the object.

 

Now on to accuracy and efficiency. The original drawing is constructed of non-polyline arcs, and there appears to be a gross error concerning units and the size of this object.

 

It appears that you did pretty well getting all the snaps lined up and tangent. Unless there is to be discernible edges on an otherwise round feature, the point where a line and arc, or two arcs join, must be tangent.

 

Assuming the Lobe center circle radius = the inner circle radius + the lobe radius. an accurate method would be to draw a circle using the inner radius, then draw another circle using a lobe radius, and place it's 6:00 position quadrant snap on the 12:00 position quadrant snap on the outside of the large circle. If this is not the way the lobes are placed, the small circle can now be moved in or out relative to the big circle, to get it in the right place.

 

Then draw two circles using the "fillet" radius on either side of the "lobe", using the Tangent, Tangent, Radius option.

 

Then Polar array these three small circles (Items = 5), then explode the array so all the circles can be trimmed.

 

Then trim off the parts of the circles not needed.

 

(Whether or not you used that method to draw the object, you should go on to the next steps.)

 

Then PEDIT (multiple), and convert them all to polylines, join them all, close the resulting polyline set.

 

In well under 2 minutes I had the image done, in a closed polyline, which is useful for CNC machine language conversion, if needed.

 

I rounded off the precision a bit and invented my own size specifications.

 

It took another minute to place the center lines, and dimensions.

 

I added a few new layers, and a dimension, and multileader style.

 

Here is s layout plotted at 10:1 (extremely small object) on 8 1/2" x 11" to pdf.

G20-0050_JIM-Layout1.pdf

Edited November 19, 2014 by Dana W

In my reconstruction of the object I drew one lobe in much the same way Dana did but before doing a polar array I trimmed away what wasn't needed.

In my reconstruction of the object I drew one lobe in much the same way Dana did but before doing a polar array I trimmed away what wasn't needed.

Exactly. I was just thinking of a way to make it faster, and that would be to trim up a 1/5th section of the whole thing and array that, as in one complete lobe, and one adjacent inner surface arc. Naaah, that would require arraying a bunch of trim fences, so Go with What Remark did.

Before the OP addresses the use of polar array he needs to work on the accuracy of his geometry and then move on to such things as setting up layers, text styles and dimension styles.