CAD - What Co-Ordinates to use?

[Davek0974]

Hi all

I'm drawing some machining layouts for a project I'm working on, they are only for my use on manual milling etc.

Is there a prefferred coordinate system used to locate the features on each part ?

I have drawn up with absolute dimensions - relative to two reference faces but is it better to use a polar system - centre the DRO on the work and work outwards???


Dave

[J Tiers]

Since the machine adjusts in Cartesian co-ordinates, it is probably best to use them for the drawings. No need for conversions.

[Stepside]

Dave

My preference is to dimension with Ordinate dimensions. To avoid the the plus/minus issue, I tend to place the X0.0Y0.0 at the lower left corner of the part. This placement gives all positive values to the details/hole locations ect.

For slots, edges of parts and the like, I draw the center line of the tool and dimension it with the same system. With everything in the First Quadrant it helps eliminate the errors that creep in due to visitors, phones, restroom breaks and mind wandering.

I also dimension the drawing with feature sizes which gives me something to measure with calipers ect. I put the two dimension systems on different layers and print one copy of each style. By turning off one or the other layer and printing I avoid the confusion or too many dimensions.

Pete

[Davek0974]

Dave

My preference is to dimension with Ordinate dimensions. To avoid the the plus/minus issue, I tend to place the X0.0Y0.0 at the lower left corner of the part. This placement gives all positive values to the details/hole locations ect.

For slots, edges of parts and the like, I draw the center line of the tool and dimension it with the same system. With everything in the First Quadrant it helps eliminate the errors that creep in due to visitors, phones, restroom breaks and mind wandering.

I also dimension the drawing with feature sizes which gives me something to measure with calipers ect. I put the two dimension systems on different layers and print one copy of each style. By turning off one or the other layer and printing I avoid the confusion or too many dimensions.

Pete

Thanks all.

This is pretty much where I was going so will carry on. I like the idea of two drawings, one with the feature dimensions on.

[Jimmer12]

I prefer to dimensions from the back left corner, so your Y datum is against the fixed jaw of the vise and doesn't need to be picked up for each part.

Unless I'm designing an injection mold component, then I always go center out.

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[Doozer]

If the drawing is for you and you are the machinist,
then anything goes.
If you are an engineer or draughtsman, it is most preferred
to make drawings pertinent to the function of the part.
If the machinist has to do some math, be that as it may.
But the job of the drawing to communicate features and
tolerances of the part.

-Doozer

[Jaakko Fagerlund]

If the machinist has to do some math, be that as it may.

If the machinist has to do math to make the part from a drawing, the person who drew the thing hasn't given even a second of thought on how the part is machined or even worse, has never been/seen machining. This is very easy to see when you get a drawing for a turned part and the dimensions are all over the place with the part zero somewhere else.

[Jaakko Fagerlund]

I prefer to dimensions from the back left corner, so your Y datum is against the fixed jaw of the vise and doesn't need to be picked up for each part.

That's a good tecnique, but I would suggest getting a vise (like Quad-I) that has the fixed jaw near you, that way it makes more sense and the parts are always closer to the operator

[Jimmer12]

That's a good tecnique, but I would suggest getting a vise (like Quad-I) that has the fixed jaw near you, that way it makes more sense and the parts are always closer to the operator

Of we didn't already have 10 vises that put the fixed jaw at the back, I'd suggest that. But our shop is switching to an FCS fixturing system so it all goes out the window soon anyway.

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[garyhlucas]

There is no absolute right way and much of what we were taught 40 years ago on paper no longer applies. I always dimension for the process. If it's vise work in a CNC I'll likely work from the back left corner, unless the part is round then it is likely to be from center. If the layout is done with a tape measure and pencil then ordinate measures from one end of a part will rule. If parts are to length and you have hole to hole alignments to maintain then never dimension from both ends. CAD means you aren't calculating anything so give the machinist or fabricator all he needs without doing calculations. It is nice when the machinist can catch YOUR mistakes before the part is.

[Doozer]

Jaakko- The intention of the drawing is not to cater to the machinist,
it is to properly communicate the design intent of the part. Ordinate
dimensioning may look nice, but it can also communicate tolerance
stackup between similar groups of features. Best to point dimension
between like features, and locating datums, than just throw ordinates
at it because it is, "easier". Dimensioning for the process is wrong.
You might not get the part you wanted.

-Doozer

[George Bulliss]

give the machinist or fabricator all he needs without doing calculations.

I had a boss who liked to prove his superior intellect by making the guys on the floor work for the info on the prints. I was berated one time when I asked for a dimension. Turns out all I had to do was take a dimension from sheet 1 and another from sheet 6, trig them out, and arrive at the dimension for the area in question on sheet 4. He paid me by the hour, so after that I simply took my time to solve the puzzles.

[Paul Alciatore]

I see much wisdom in the answers here. First, Doozer hits on the place where I start when drawing a part: the requirements of that part. If the part is not designed to work, then nothing else matters. So, when designing a part, when doing the layout either on paper on in CAD, I think about what the part requires and work from that.

If the drawing is for you and you are the machinist,
then anything goes.
If you are an engineer or draughtsman, it is most preferred
to make drawings pertinent to the function of the part.
If the machinist has to do some math, be that as it may.
But the job of the drawing to communicate features and
tolerances of the part.

-Doozer

But then, Jaakko has a point about the machinist.

If the machinist has to do math to make the part from a drawing, the person who drew the thing hasn't given even a second of thought on how the part is machined or even worse, has never been/seen machining. This is very easy to see when you get a drawing for a turned part and the dimensions are all over the place with the part zero somewhere else.

This is specially true if you are going to be the machinist. It is a lot easier to do the math in the CAD program instead of doing it with a calculator, standing at the lathe or milling machine. Easier and MORE ACCURATE. You will make more mistakes at the machine than at the computer. At least that is what happens for me.

If you are making a drawing for professional use by others, then please do follow standard conventions for dimensioning and as for the machinist's concerns, let the chips fall where they may. You are trying to communicate the part, not the numbers needed by the machinist. Tolerances will be a critical part of this process and they should be specified from a point which makes the part work properly, not any particular convention.

But if you are going to be the machinist, then things change. You will know how it will be machined and what numbers you will need. I often dimension parts that will be machined in my mill using the upper left corner as the reference. That way I can just dial in the coordinates from the dimensions on the print. You can feel free to use any other corner, but my screws/dials work out well that way. On a lathe part I would start from one end on the central axis because that is how a lathe works.

[Jaakko Fagerlund]

Jaakko- The intention of the drawing is not to cater to the machinist,
it is to properly communicate the design intent of the part. Ordinate
dimensioning may look nice, but it can also communicate tolerance
stackup between similar groups of features. Best to point dimension
between like features, and locating datums, than just throw ordinates
at it because it is, "easier". Dimensioning for the process is wrong.
You might not get the part you wanted.

-Doozer

You are correct when you do designs for others, but the topic was about prints for own use. Thus there is no reason to not draw for a process.

[J Tiers]

I think this was hit upon earlier, but it bears repeating.

If you have a group of holes, that all fit a given part, and have a tight tolerance within the group.... What you do NOT WANT TO DO is dimension them all from the main datums individually. Dimension to one hole, and dimension the rest "internally" from that one.

Individual dimensions put two tolerances on each direction, group dimensioning puts ONE on each direction.. Plus it communicates the intent better, if that matters. I see people insist on the other way all the time, and I have started letting them have what they want. I'll do it my way thanks....they can do theirs. Been bitten by that one already, learned the lesson.