By Dennis Collin
I frequently get calls where AutoCAD users experience strange things happening with blocks, xrefs etc. where they are coming in either too big or too small. Obviously, this should never happen as all AutoCAD users no matter the discipline should always draw at 1:1 size, but plot to a scale. Should adverse scaling occur then the first thing I check is that things are drawn to a 1:1 ratio. Open each drawing and either measure a dimension or measure something that is known. A doorway, wall thickness or another familiar element.
Let’s take a typical internal door as an example. If the distance command reads in the hundreds of units (typically 800-1100) then we can assume the working units are millimetres. If a measurement of less than 1 unit is obtain (typically 0.8-1.1) then the working units are likely to be metres. As a check I will look at the title sheet and refer to any notes with regards to units as a confirmation.
Bearing this information in mind, open each drawing and check the drawing units. The scale units setting should be set the same as our assumption from the first check. Typically, in the UK the units setting should typically read millimetres for engineering, electrical and architectural drawings and metres for large scale site plans, urban planning drawings and highways. Unless you are working with the Americans the units should not be inches and seldom unitless.
Once we have validated the drawing units, it is advisable to run the DWGUNITS command, this controls unit, type, precision and whether and how blocks scale in the current drawing. All these settings should tally with the UNITS command and I generally recommend that blocks should scale to the current drawing units.
With regards to blocks and external reference files they also have a unit to work to, they should reflect upon how they are drawn, and again typically will be set to a metric unit of either millimetres or metres dependent on discipline and industry. It does not matter what the units are, provided they reflect the way they are drawn. A block set to millimetres will scale correctly, for example, in either a metre or inches-based drawing if everything has been set correctly in terms of units and the variables set out in the DWGUNITS command.
Checking a furniture block in a typical building plan. Note the block unit which is set to the same unit type as the main drawing units.
The block unit is established as the block is created. Typically, this unit should reflect the units it was created in. If the block is inserted into a different drawing with different units, then the block should scale correctly.
An external reference is in effect like a block dynamically linked to another drawing file which will update in the host drawing when updates are made. However, it too has units which relate to the units of the incoming linked file. If this is all set correctly, a metres-based survey drawing will scale correctly to a millimetres-based floorplan. This aspect can be seen below:
The current drawing units are millimetres, but the External Reference units are metres. This is perfectly acceptable and AutoCAD, provided everything has been configured correctly, will scale a reference file by a factor of a thousand. If the reference drawing was measured in inches the factor would be 25.4 which is the number of millimetres to the inch.
NB. It is worth noting that incorrect units also have an adverse effect on annotations such as linetype patterns too.
Referenced in a post here:
These concepts and settings are outlined with our AutoCAD Essentials course and expanded upon in more depth with the AutoCAD Intermediate course. Our groups are no more than six delegates and provide a good environment to get the best out of your software. For more details on our courses and bespoke training days visit https://training.cadline.co.uk/ or contact one of our team on 01784 419922 where we will be more than happy to help.