Dummy Leg drafting and development

Dummy leg drafting and development is used in the piping industry. A dummy leg is a stand that is used to support a section of piping but it could also be a branch pipe added to an elbow, straight pipe or transistion

It gets a little tricky when you have to layout and weld one of these supports onto an elbow or other circular shaped pipe fitting. It also gets more difficult when the support has to be offset from the pipe centerline, perhaps because of building structural steel that might be getting in the way.

In the past, in order to figure out how to cut a stanchion or support pipe that intersected an elbow meant you to do parallel line development.

That meant hand drawing a section through the fitting where the stanchion intersected and then using your drafting machine to project horizontal and vertical guidelines onto a detail of the stanchion.

With a full size drawing made by parallel line development a welder/fitter can wrap the the drawing around the stanchion as a template and mark exactly where he or she has to torch cut or hand grind

Download a sample pdf full size drawing here

Needless to say the development template has to be dead nuts otherwise the stanchion wont fit and is just becomes scrap. (The welder fitter will have some choice words for you too !)

3d drafting software makes things a little easier because once you have the conditions modeled you can just dial in the necessary pipe diameters and cut the elbow through the stanchion to get the profile just right.

After that is done, you can generate the flat development using the CAD software. The only thing that needs to be done after that is to add guidelines on the drawing to aid the welder / fitter in locating the drawing once he or she uses it as a template.

This kind of template creation is not limited to the piping industry. It can be used with brackets for traffic signals for example, where one support tube has to join to a lamp post or support at an angle.

Do you need pipe templates ?

If you are joining pipes and need full size pipe developments printed out for pipe cutting, I can generate them and have the drawing couriered to you at the job site. I need some basic information about what you are joining

1. The main pipe or fitting you are joining to and its size and specifications
2. The size and specs for the branch pipe you need to cut
3. The angle the branch pipe meets with the main pipe
4. The offset if the branch pipe does not pass through the centerline of the main pipe

Pipe development drawings are printed full size so they can be wrapped around the branch pipe for marking and layout of the pipe. 

Contact me at 905-467-0233 if you need more information

Scanning drawings and large format scanner facts

Scanning drawings is worth considering if you have a lot of old legacy drawings. Pencil or ink drawings, sepias or blueprints, its often a good idea to get at least some of them scanned.

Scanning the drawings to tiff or pdf will preserve the images and allow the drawings to be used over and over again without worry about damage to the originals. Scanning also gives you the opportunity to add cataloging to your drawing files, so that its easier to find details and assemblies when you need them in a hurry.

Drawings are usually scanned to tiff format or pdf format or both. Tiff files have the advantage that they can be imported and overlaid on top of autocad drawings.

New geometry can be created without having to redraw old reference lines. So long as the old drawings are accurate and to scale, this process will work fine. The tiff files will have to accompany the cad drawing wherever it goes. Tiff files also dont offer any opportunity for catalogueing beyond their filename.

PDF files can be made from the Tiff files. PDF files are often smaller than the tiff files and so allow many drawings to be kept for reference purposes. It is also possible to perform OCR or optical character recognition on the tiff file prior to conversion into PDF format.

This has the advantage of allowing thousands of drawings to be searchable by name, drawing number, drawing title and even by notes within the drawing. That is so long as the OCR does not turn handwritten text into nonsense and goobledygook. You need to manually monitor this process to make sure it works, and edit the mistakes it makes. But when it works, OCR combined in a PDF file gives you powerful drawing searching capabilities.

There are some problems you can run into with scanning drawings. For one, any dirt or smudges will get transferred to the tiff file just like it was part of the drawing. Often you have to manually erase these artifacts later.

Handwritten text will often not be translated by OCR and have to be redone manually. Old blueprints wil have their blue background transferred to the tiff file. You will have to turn the image into a "negative" to get a white background with blue lines so that the tiff file is useable for CAD.

With old sepias, the tiff file will pick up all the old revisions made with erasing fluid. These messy areas will have to be manually edited later. All of this involves extra time and work, but depending on the importance of the old drawings, it may be worthwhile and quicker than redoing the whole drawing over in your CAD system.

Scan2Cad is a great software package if you want to do your own drawing scanning and raster to vector conversion. I have always liked their responsiveness to customer questions. You often get one of the software designers replying to your questions.

Scan2Cad works with your scanner to produce tiff files and many other formats. It also does raster to vector conversion which means it converts image (raster)files to CAD (vector) files. Scan2Cad converts hand drawn linework to linework that can be manipulated in your CAD system. This can be a huge timesaver.

For day to day scanning work, I have found that the Mustek 11" x 17" format scanner below is a good workhorse. It is not an expensive unit and if you need only a portion of a large drawing, larger "D" and "E" size sheets can be laid flat on its table since its top cover is easily removeable. This scanner can save you the cost of going to your local cad service bureau to get the odd scanning job done.

Geometric Dimensioning and Tolerancing

Geometric Dimensioning and Tolerancing - Thats something is something only engineers use ?

GDT&T attempts to pull together design, drafting, manufacturing and production so that they all work together toward a common goal - "Producing parts that fit together" first time and every time.

This goal is never more important now that the person who designs and drafts the part maybe thousands of miles away from the shop that ultimately makes the part. The maker of the part, may not even speak the same language as the designer making the drawing the only viable means of communicating how the part is supposed to be made.

Old fashioned coordinate toleranced drawings are simply not good enough in this sort of environment. While they may adequetely communicate the size and shape of the part, coordinate dimensions dont make it clear how the part is inspected. What are the most important dimensions.

Coordinate dimensioning and tolerancing may lead to parts being having faces machined unneccessarily and at extra cost. It may also result in ambiguous instructions for inspection where bad parts end up being accepted and good ones end up being rejected.

Perhaps the most important part of the GD&T dimensioning system is in ensuring parts go together by identifying the critical features. Take for example this railroad car disk brake assembly.

If we were going to be preparing the machining drawings for the base component for this disk brake, It is reasonably certain that the flat face on the base would be our primary datum. This is the face where the disk brake assembly attaches to the train.

Just by doing this we have not only identified the most important surface but also communicated to inspection that this ought to be the reference surface used to check the position and characteristics of all bolt holes, and machined features.

Using engineering scales

Most CAD systems allow the drafter to draw at whatever scale he or she feels like. However, if fabrication shops are going to be using your drawings you will help them far more by drawing views to commonly accepted engineering and architectural scales.

Many shops like to keep drawings to a consistent scale so fabricators get used to knowing how drawings are laid out. This is just as important in plant layout and architectural drawings where 1/8"=1'-0" and 1/4"=1'-0" are standards for construction and building permit work. In machine design 3"=1'-0" and 1-1/2"=1'-0" are very commonly used in weldments and small assemblies. Details often are full size or half size.

If estimators are going to be taking off material requirements from your drawings they will also need to have the drawings made to scale so they can calculate the number of  panels required, steels sheets, rod or bar, whatever it might be.

Drawing to scale is also important since the CAD system can make scale templates more accurately than ever before. Drawings can be plotted at full scale and parts can be checked against them simply by placing them on the drawing. Paper plots can be glued to wooden forms and templates can be sawn to shape with minimal layout time.

The following table shows typical engineering scales and useful data to help set up your sheet layout.

There is also a small engineering scales power point presentation with sound that I put together about this subject. It's worth a listen if you are just learning the process of setting up drawings to scale.

You can use this chart in many different ways. Most often you would be starting off with an autocad file that was drawn full size in model space and now needs to have a drawing border to a specific drawing scale placed around it.

How to Scale a drawing to make sure it is accurate

• Back in the day you could only buy scales individually, not all in one kit like this one from Amazon. Plus they are made from more durable aluminum not plastic. 3 12 inch scales with 3 sides each cover 1-1/2" = 1'-0", 1" = 1'-0", 3/4" = 1'-0", 3/8" = 1'-0", 3/16" = 1'-0", 3/32" = 1'-0", 1/2" = 1'-0", 1/4", 1/8" = 1'-0" and 3 = 1'-0"

You can use this technique to check if you have the scale set correctly for your printed drawings Check the drawing scale, in the example at the left, the scale is 1" = 1'-0" Use a engineering scale. The number 1 means 1 graduation of 1 inch represents 1 foot Find an even whole number dimension on the drawing and check it with the scale Read the graduations on the scale, you should get the same dimension Check several other dimensions on the drawing to be sure the drawing is to scale

This is how you cheque the size of an object that is not dimensioned First check the drawing scale is accurate as above Use a engineering scale. The number 1 means 1 graduation of 1 inch represents 1 foot The smallest division on the scale is 1/4" Here the plate thickness is not dimensioned, but the scale reads 2 divisions or 1/2" Check several other dimensions on the drawing to be sure the drawing is to scale

Simply draw your drawing border actual size and scale it up using the reciprocal numbers until your object fits nicely within the drawing border. Under drawing scale in the title block, label the scale from the chart whose reciprocal created the best fitting border

Also if you have a drawing border that is 3/4"=1'-0" and you want to make it 1/2"=1'-0" look up the reciprocals for both scales. Since you are making your drawing border smaller in this case you need to divide 16/24 = .6666667. Scale the original border down by this amount and your scale is changed.

The Ratio and Decimal numbers can be helpful if you are working in paper space and need to know what the scale factors are for window resizing in order to create the proper scale.

Calculated  Industires has this neat little gadget for scaling off large printed drawings. This is particularly useful for estimators who must quickly find linear feet of cladding or pipe or any number of items from a large drawing set. The device works by first setting the scale then rolling a small wheel along the length you want to measure and reading the distance.