8 Replies Latest reply on Aug 18, 2017 10:32 AM by Melvin Donahoo

    Flat Pattern sheet metal in drawing, best method for using angle dimensions

    Melvin Donahoo

      I have a sheet metal radial skin .032" that has a series of radial holes for attaching to a structure. I have created the part using sheet metal and unfolding the part and creating a sketch of the hole pattern then extrude cut and folding the part again. I used a radial sketch of the angles that the holes were to be created and dimensioned the arc length from the end of the part. I then linked that arc length to place the holes in the flat pattern section of the sketch. So if I need to update the part all I need do is update the angle in the sketch and it will flow thru. I used angles as the corresponding radial ribs that the skin attaches to are angularly placed.

       

      I've decided that the best method of showing the hole patterns in the drawing are in a flat pattern using angular ordinate measurements however I have not found a method of actually doing this without overriding the linear ordinate dimensions.

       

      If anyone has a suggestion as to how to accomplish this or an alternate method to show angular dimensions I'd appreciate it. 

        • Re: Flat Pattern sheet metal in drawing, best method for using angle dimensions
          John Stoltzfus

          The way I would have designed a part like that would be to create the base Rib Assembly first.  This assembly would have the holes in there proper place and controlled either in the Part or the Assembly file. Then I would have added the Skin by going to - Insert/New Component, then modeled the Skin part in place and adding the hole center points to the Skin Part, then added the Sheet Metal, insert Un-Fold bends, find the hole point locations, use the hole wizard to create the holes in the flat, Re-Fold.  

            • Re: Flat Pattern sheet metal in drawing, best method for using angle dimensions
              Melvin Donahoo

              Thanks John, that method certainly would be a faster method of building the skin part. I generally shy away from linked top down modeling. It's more a personal preference that has some weight in past experiences with various CAD systems. It always seems to be a big load on the system when dealing with large assemblies. I usually keep the default settings for no linking between parts when I transfer hole locations between parts and my OCD always makes me fully dimension the sketch anyway.

               

              What's your take on dimensioning the drawing in flat pattern and trying to use the angles for dimensions. I'm Leary of using linear dimensions in the flat pattern given the shrink or stretch for various methods of manufacturing the curved part.

                • Re: Flat Pattern sheet metal in drawing, best method for using angle dimensions
                  John Stoltzfus

                  Melvin Donahoo wrote:

                   

                  I generally shy away from linked top down modeling. - (is this correct) - You only shy away because of the unknown? Top down design is what I have done for years, with the exception of converting parts from one CAD package to another..

                   

                  What's your take on dimensioning the drawing in flat pattern and trying to use the angles for dimensions. I'm Leary of using linear dimensions in the flat pattern given the shrink or stretch for various methods of manufacturing the curved part.

                   

                  If you create a point in the sheet metal, flatten and then drill your hole, you'll be fine to dimension it using linear dimensions.  If I were the guy in the shop I would want the sheet dimensioned in the flat, impossible after it's rolled... It'll fit fine.

              • Re: Flat Pattern sheet metal in drawing, best method for using angle dimensions
                Bernie Daraz

                Back a bunch of years we received drawings from a well known postal machine manufacturer. The formed part was fully dimensioned but the flat had only overalls for the length and width. I don't remember ever accepting a drawing that instructed us to use the flat pattern. I did use the flat pattern after I corrected the flat pattern using AutoCAD and the stretch command. My bend allowances not yours.

                 

                If the part was more complex I would 'attach' the various drawing views to determine a flat for quoting.

                 

                Most of the guys here would suggest that the flat pattern is for reference only.

                • Re: Flat Pattern sheet metal in drawing, best method for using angle dimensions
                  Steven Dod

                  I would add the holes to the "rolled" part using a 3D sketch in hole wizard.  In the 3D sketch you can use centerlines with dimensions to a related plane for the angles.  If the base part angle changes then you just make the same angle change in the skin.  Once designed you can show the angles of the holes using centerlines on a side view.  I would also show the hole locations in the flat pattern as reference dimensions.

                   

                  Steve

                    • Re: Flat Pattern sheet metal in drawing, best method for using angle dimensions
                      Melvin Donahoo

                      Steve I like this method however every method i try seems to have some drawback. I made a test part with your method and created a drawing. In the drawing flat pattern can not place center marks on the holes. I assume because they are actually polygons. this is why I originally settled on placing the holes after unfolding the part. If the shop wants to use the flat pattern to place the holes your method might be a bit painful for the CAM end of things. I've done a bit of searching the forum and this center mark issue has been discussed a few times, doesn't seem to be an answer for it.

                       

                      Thanks

                    • Re: Flat Pattern sheet metal in drawing, best method for using angle dimensions
                      Bernie Daraz

                      I had a little time to think about this and offer my suggestions for your consideration. Since you previously stated you add the holes in the flat I was thinking you could use my old method adapted for SW.

                       

                      First thing I do is create an equation for a bend allowance. The one shown in the picture will give you a .5 K Factor, judging by the scale of the PDF I believe correct. The part used for this example just for illustration.

                       

                      So, to place your holes you simply sketch a hole and to dimension it to the angle as you require just by adding the ‘position angle’ * the equation value and the proper dimension for the hole will be determined.

                       

                      Be aware that your ‘zero’ must be at the proper location for this to work.

                       

                      I’m wondering how displaying the angles on the drawing helps your guys on the floor.

                       

                      Part1 Snip.JPG

                      Part1 Snip 15.JPG

                      Part1 Snip 2.JPG

                        • Re: Flat Pattern sheet metal in drawing, best method for using angle dimensions
                          Melvin Donahoo

                          Thanks Bernie,

                          I essentially did graphically what you wrote an equation for.

                          In my company I have no control of whether the parts are made in house or outsourced. My rationale for dimensioning the part with angles is that is how the part will interface with with the angled ribs when assembled. I spoke with our sheet metal guy and if he did the part he would not roll but rather use the CNC brake to place a series of very small bends to simulate the circular shape. He stated its how he does all parts and almost never uses the slip rolls anymore. Another shop might do it differently. I don't want to get delivered parts that don't fit and have a vendor state that "you gave me the flat pattern linear dimension". an old checker once told me that a drawing is a contract with the vendor, it's what you are supposed to receive and if you don't dimension it properly you will eventually end up paying for bad parts. The flat with angles stated on it seems to be an effective way of stating what the end product should be. I eventually settled on the flat linear dimensions as reference dimensions and showing section views perpendicular to the cylinder that cut thru the radial holes and dimensioned the angles. It only took 4 sections to dimension all the radial holes.

                           

                          I very much appreciate the responses everyone has given.