Shifting from CAM to CAD

      Scaling drawings for machining. (CAD Forum) May 17, 2003

Question
I am going from CAM to CAD and wondering how drawings are made and how scaling works. I think all drawings should be made at full size (1"=1") because then I can machine from them (makes my job easier).

Our shop drawings are 22x34. I have done some in AlphaCAM. I drew all at full size. When all scales on one page were the same (say 1/2"=1'), I just scaled the 22"x34" by the factor (24x) then placed my full size drawings where I wanted and printed them out at the inverse of the factor (1/24).

When I had different scales (say 1.5"=1' on that 1/2"=1' sheet) on that page I would insert that into my drawing and blow it up (scale 3x in this case) and still print at 1/24. So I would still have individual files for each item on the sheet and one file for the page with those inserted files.

Now we have quick CAD. Is it standard to draw at full size, and how do you get different scales on the same page? Was I close in the longhand non-CAD way of doing it?

I have gotten emails of arch drawings and they seem to be tiled, with each tile able to have its own scale. When sent to me they were full size (and overlapping other tiles) but if I scaled them down to the printed size, they looked right. So that leads me to believe they are drawn full-size and merged together somehow, just to print out looking right. Any help?

Forum Responses
(CAD Forum)
From contributor J:
It is very normal for drawings to be done at full scale, 1"=1", and then scaled down to fit a particular sized paper.

What you have described with the architectural drawings you get in email sounds to me like a function of AutoCAD. AutoCAD used two different areas, Model space and Paper space. You draw at full scale in model space, then change to paper space for title blocks. It's sort of confusing until you figure it out.

Think of it as looking through a window at the world outside. The whole world is out there, at full scale, but you can only see a small portion of it through the window.

AutoCAD allows the user to change the scale of a window. This changes how an object appears in that window. Depending on what software is used to create the original drawings, your software could produce some strange results on the screen, but should print it out reasonably alright.

As for your question about different scales on the same sheet... If I'm working only in model space, and want different scales, I draw it at full scale first, and then scale it either up or down as I need. Inserting drawings into your sheet that have been done at different scales is a little more tricky. You'll have to know the original scale, and the scale you want it to be, then figure out the scale factor for insertion.



From the original questioner:
If you drew it in model space (or just full size) and put all the dims and text notes in there all at a certain size, but you wanted to print out at a different scale than planned, could you get the text to always output at a certain size, like 1/8"?

Little less confusing:
1/2"=1' drawn text = 3 output text= 1/8"
1.5"=1' drawn text = 1 output text= 1/8"

I want 1/8" text size output (always).



From contributor J:
Yes, getting the dimensions to come out at the same size is easy.

This only works in AutoCAD, but the trick is to draw in model space, switch to paper space, and tell the program to dimension the model space object using the paper space dimscale. Also, if you know the scale factor of the displayed object (ie: the paper space scale, not the full size) you can set a program variable called the dimlfac to the correct scale to get accurate dimensions.

There is even another method that I've not talked about called XREF. That is when you tell one drawing to display another drawing. It's used mainly when you have several drawings that have the same part as a portion of an assembly. Any changes to the referenced part are automatically made in all the drawings where that part is used. Saves a lot of redrawing time.

Here's an example. Suppose that you use #10-32 bolts in several projects. You've drawn the bolt, and attached information about cost, vendor, manufacturer, etc. for easy access on a BOM. Now, along comes The Music Man, and he makes a deal with your purchasing department to sell them the same bolt for 10 cents less than your present supplier.

Your job is now to change every drawing to show the new price and vendor. Just for fun... how long would it take you to change, say, 500 drawings with, maybe, 5 of these bolts in each drawing? If you were using AutoCAd and had made the bolt an XREF, it would take about 2 minutes to open the drawing of the bolt, make the changes and save it. Now each time a drawing is pulled with that bolt, the change shows up all by itself.

There are many ways to make things look exactly the way you want them to look. The ones we've talked about are just a few. I've been working with AutoCAD for 15 or more years, and there are so many that I still don't know them all, and the program changes every couple of years as they upgrade and "improve" it.



From contributor M:
But how do you use XREF to create the dimensions?


From contributor J:
You size the XREF to the proper scale when you insert it into a drawing and then dimension it in the normal manner.

Remember that we have already decided that the only way to draw something is in *full* size. So, once the drawing has been set up to be plotted at scale on paper, it's just a matter of dimensioning the drawing normally. I always set the text style and dimstyle to plot at whatever size I decide is best for the drawing before I dimension it or add text notes.

In most cases where a drawing is to be used as an XREF there are no dimensions attached to it, just the full sized drawing of the part.

If someone has dimensioned the drawing to be brought in, you would turn the dimension layer *off* before bringing it into your drawing. That way the dimensions on the XREF do not show in your production drawing.

Again, we are talking about how it's done in AutoCAD. Other programs have different methods of doing the same things. You'll have to find them in the program you are using.



From contributor M:
Maybe I'm missing something here because I still don't see how this accomplishes what the poster wants.

You wrote "You size the XREF to the proper scale when you insert it into a drawing and then dimension it in the normal manner." What "XREF" are you referring to?

Also, how do you dimension a scaled drawing in the normal manner? If a drawing is scaled, the dimensions will be scaled, too. The drawing shouldn't be scaled anyway because it was drawn at full scale.

I don't use ACAD, but to create a scale drawing it is simply plotted at the desired scale. Therein lies the problem with dimension height - it is plotted at scale, too.

I still don't follow how you use XREF to do what the poster wants - "Get the text to always output at a certain size, like 1/8"." That is, regardless of the plot scale, he wants a dimensioned, full-scale drawing to always have dimensions that are 1/8" tall.



From the original questioner:
Yes, that is the question at hand. We don't have ACAD either - we have QuickCAD. It is like a $100 CAD program. But we want to upgrade later to ACAD. If I have a general idea, I can apply it to what we do.


From contributor M:
I use FastCAD 6 - it is a high-end CAD program. It has XREF - "An XREF placed within a host drawing “points” to another physical drawing file. The XREF then appears in the host. Because the XREF pointer is embedded in the drawing, the XREF files automatically load with the host drawing."


If you aren't using AutoCAD with model and paper space, but are simply plotting out your drawing to a specific scale, you might want to try this.

Let's say you have your drawing all set up for plotting at 1/8" = 1' with dimensions at 1/8" tall. If you need to plot out at 1/16" = 1' your dimensions will shrink to 1/16" tall and you will need a magnifying glass to read them.

Do you have a dimension scale variable in your CAD package? In AutoCAD it is called Dimscale. You would have to double this variable and update all your dimensions to get the proper size printout. This is very easy to do in most CAD packages. The problem that you will have to deal with is when you double your dimension and arrowhead sizes, often they will overlap, look cluttered, or be totally unreadable. You then have to spend time moving them around to get them to look good.

A better way may be to set the drawing up for the smallest possible plot scale and then grow the dimensions as needed. This way at least they will always be readable.

Once, I created script files and attached them to custom buttons to simplify the updates. If your package allows this it may help.

I dislike paper/model space. Now I draw everything full scale. I have a custom VBA script set up that reads the extents of the drawing size and sets my dimension size for 1/8" when printed out on 8-1/2" x 11" paper. That's all we use here.



From contributor M:
Apparently using XREF doesn't accomplish the task of making the dimension plot at the desired height regardless of scale. I never could determine how that would work.

Also, I don't understand how determining drawing extents accomplishes anything. Setting dimension height is not a function of drawing extents or paper size. It is a function of the scale factor, nothing else.



From contributor J:
What I meant by sizing the XREF correctly wasn't some excuse for XREFs not being drawn at full size.

Even if you have drawn everything at full size, the program still asks you to specify an insertion scale factor. So, the insertion size would be 1:1. If you have drawings that are not drawn at full size, then you would have to provide a scale factor to bring them back to full size on insertion.

Also, if your library is like mine, you might have both Standard and Metric drawings in it that you use every day.

Anything that was drawn using the scale factors to produce full size drawings in Metric format is really huge compared to what happens when you draw in US Standard. It's one of those pain in the butt functions of ACAD. Different unit settings produce different full size scales.



From contributor M:
I don't use ACAD. Based on what you are saying, I'm glad I don't. Since I draw all drawings at full-scale, "scale factors" don't enter in to it. In FastCAD, "different unit settings" don't have any effect whatsoever on "full size scales." Unit settings only determine what unit of measure is displayed. I don't see how unit settings should effect scale. In other words, the scale doesn't know that the units are inches or millimeters. The only difference is that it either displays 1" or 25.4mm. That has nothing to do with scale.


From contributor J:
The difference in units means a lot in ACAD. Since it's designed as a general purpose drafting software for use by many different disciplines, it has a wide variety of settings.

Also, the units control how dimensions are displayed. However, where this all comes into play with scale factors between Standard and Metric components is really simple to explain.

Drawing using Standard inch - feet measurements, I tell the system to draw a line 1 unit long. It sees that as being an inch. My counterpart in Europe draws a line 1 unit long. His system sees that as being a millimeter. To make a 1 inch line, he needs 25.4 units.

Take a look at almost any metric drawing in the world. A simple thing the size of a pocket calculator has numbers in the 100's for the length and width dimensions, whereas the same thing drawn by American standards would have numbers like 2.5 x 3.25 for the length and width.

Both of us are drawing at full size. I tell my system to draw a line that is 2.5 units long. He tells his to draw a line that is 250 units long (just numbers here, not real measurements). The difference between 250 units and 2.5 units is 247.5 units. Can you see the reason why metric drawings have to be scaled down to show up at full size in American Standard drawings now?

If I were to insert a metric drawing into a standard drawing, a simple metric nut and bolt assembly would be larger than a Mack truck next to a mail box.

If you never have to deal with this kind of stuff, you're lucky. I have to contend with it every day and have set up special routines in ACAD to handle the process.

Let me make a suggestion. I'll draw you a full size component using the metric system and if your system can read DXF files, I'll send it to you. You place it in one of your drawings and you'll see right away exactly what I'm talking about. I promise, no tricks... just the difference from what the computer shows as full size when drawing in metrics as opposed to drawing in American standard units. If you'll take the challenge, here's a hint on how to make it work… 03970079.

To answer the second half of your question…

Setting the drawing extents (ACAD calls this LIMITS) has nothing to do with how the scale factors work. It just tells the computer how much "screen area" you want to see when you tell it to zoom out to the entire drawing area. It's most helpful when you make a mistake and have some element of the drawing end up way out in left field away from the main part of the work. Usually this causes the element to disappear from the screen until you tell the system to zoom extents.

The computer finds all the drawing elements and expands the screen to show them all. Now you either have the drawing nice and neat, exactly where you expect it to be, or you find something stuck far away from the drawing off by itself. You can either move that offending element back to the drawing area, or erase it.

In ACAD, you can not only set the drawing limits, but lock them in place. This creates a virtual fence around the drawing area you have set up and prevents anything from being placed outside that area by mistake, or on purpose.



From contributor M:
Okay, I understand that. When you wrote "unit settings," since we were discussing dimensions I thought you were referring to dimension units, not drawing units. However, in FastCAD, regardless of the drawing unit (inches, feet, meters, etc.), when I enter a length of 1", 1', or 1mm, it draws it at that length. But inserting a drawing that has different drawing units presents no problem. For example, I draw with my drawing units in mm - .03937 inches per unit. If I insert a drawing of a line 1 unit long that has units in inches - 1.0 inch per unit, that line will insert into my mm drawing at a length of 304.8 units.


From contributor J:
Sorry about that. I should have been more clear in what I wrote.

I can do the same as you when I'm making the drawing. I can define what a drawing unit is just like you described. It's when I have to insert drawings from other people that I have to start jumping through the scale factor hoop.

In any case, to get the dimensions to plot at the correct height (which was the original question) all I have to do is set up what paper size and scale I want the finished drawing to be plotted at, and then set a dimscale factor to match the right sized dimension text to the plotted drawing.

Let's say I want to plot the drawing at a factor that produces a finished product on paper at 1/8" = 1' (it doesn't matter what size paper, the factors stay the same). I first set the dimscale to be 96. That sets up the size of my arrow heads, and how the final drawing will fit on the paper.

Then I multiply the text size by a factor of .125 and set that figure (12) as the overall text height. This produces dimensions and text notes on the finished plotted drawing that have a text height of 1/8".



Back to the original quest. When you get into CAD, everything should be drawn full scale, 1:1, or whatever you want to call it. In other words, real world. The only objects scaled are annotation (dims and text) and hatch patterns (maybe). You have to realize that this is exactly the reverse of the old way of thinking in pencil drafting. In AutoCAD, scaling the annotation is handled basically in one of two ways: (1) setup various dimstyles for different scales which factor the size of dimension elements against your target output size (say 1/8" tall), and dimension your object in model space, or (2) dimension your objects in paper space using associative dimensions and one dimstyle. I think the former works much better than the latter, although you should have a good layer naming system. Dimensions of different scales will want to be on their own layers. For manufacturing, using model space with dimensions and saved layer states is the way to go. Paper space is great for the printed output. We rely on both heavily. The best tip is to draw as if you are building the assembly. Forget about lines on a page --- you left that behind with the pencil.


The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor S:
I program CNC for a very custom commercial millwork shop machining many jobs for universities and Casinos in Canada (metric) and also do many jobs in imperial for export to the US. I do all standard cabinet work using parametric manual programming. For reception desks and other curve intense items I lay out using ACAD and dxf out. The easiest way I found is just to make copy of the shop drawing in model space and insert a CNC template that contains dimension style for your imperial parts using the secondary dimsyle showing. With the shop drawings in the shop you don't need to plot your copy unless you need to label parts to be put back together.



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