Practical Sheet Metal Design Part 2
Tolerances for Sheet Metal parts:
A bit of history first: When I started working in sheet metal in 1975 we generally worked to +/- .063 inches. In the shop I worked at initially there wasn’t a pair of calipers to be found. Everything was checked with a tape measure. When I interviewed at Byers Precision it was stressed to me that they worked to much closer tolerances, .010 - .015 typically. This led to a funny story…On my first day on the job I was given a print, told to figure the flat pattern, shear, punch & form the part. With the interview still fresh in my mind I determined to accomplish making this part to the +/- .010 tolerance. I sheared the part using a tape measure to size it, punched the holes on a manual one station punch and then proceeded to form the part on a hand brake. I worked and worked on this part trying to get it within the goal tolerance. An hour or two passed and even though I had a nice looking part it was still not within tolerance. I was starting on my second attempt when the foreman came over and asked me what the h**l I was doing. When I explained that I was having a hard time holding the part to +/- .010 he laughed and said that the part was not critical and as long as it was within an 1/8” of an inch it was good enough! If there is a lesson in that story it is this; know what the tolerance is before you waste a lot of time.
The factors that come into play in sheet metal are numerous but let’s just look at one now.
Material thickness: Take a look at the mill tolerances of sheet stock and you will see that the range of thickness for a given gauges bottom and top range actually overlap the gauges on either side. In other words a thin 11 gauge could be sold as a thick 12 gauge while a thick 11 gauge could be a thin 10 gauge. Engineers should allow for 5% thickness variation on thickness which would equate to .006 on 11 gauge. And consider that this amount doubles for each bend in the part as the difference will be seen on each side of the bend. Can tighter tolerances be held? Yes but a premium must be realized in order to achieve them. Hole location from the edge of a part can be held to a tighter tolerance but from a bend to a hole is again affected by the material thickness. Today’s modern numeric control punches and lasers have a positional tolerance of .003 to .005 inches so this must be figured in along with the sheet thickness variation. The only way around this would be to place the hole after the forming operation which is costly. So the in effect the tolerance for a hole from a bend is 10% (2 x 5%) or .012 + .005 or .017 inches for 11 gauge material. Some sheet metal shops will also want additional tolerance allowed for material elasticity variations that affect the form dimensions. At Byers we feel that we overcome these variations to some extent by use of custom K-Factors for the different materials. We also try to overcome these variations at the forming operation by varying the bend radius of the part to get the desired result. Engineers, check a sheet metal gauge chart before designing a part. You might be surprised how many times we get designs calling for 11ga but the part is actually designed with a .125 thickness. 11ga is never .125 thick, it is more commonly .120. That’s .005 difference before we even start modeling the part. This difference shows up most when the engineer has some dimensions on the inside of the material and others on the outside of the material. Be consistent, dimension to the inside or the outside but not both.
Since I’m discussing sheet metal gauges I must comment on gauge callouts for non-ferrous materials. Don’t call out for 11ga Aluminum. This is an obsolete and leads to confusion. Don’t be surprised if you get a call about this. Most designers are not even aware that gauges differ between ferrous and non-ferrous material which is why I will always call you and ask you what thickness in inches you actually want. A much better way to spec non-ferrous thickness is to give the decimal equivalent such as .090 or .060. Look at this chart to see the difference between ferrous and non-ferrous gauges
For more information on reasonable sheet metal tolerances see this page:
Everything I’ve said in this posting should be taken with a grain of salt. If you can design you part within these boundaries then you will get a better price on the production. But if tighter tolerances are a must then they can be done but you need to expect to pay a premium for them.
Friday, November 21, 2008
Friday, November 14, 2008
November WNC-Aheville SWUG Meeting - minutes
More SolidWorks 2009 Goodies
At the November WNC-Asheville Users group meeting we enjoyed good food, conversation and presentation. Tom Wilson cooked up his excellent barbecue chicken and ribs with all the fixings. Wes caught us up on what is going on with WNC-SWUG. Several People brought up issues and questions that they had about SolidWorks. This is what is so nice about these meetings; you will either find an answer or at least find out that you’re not the only person experiencing the issue.
After the meal Wes gave us a short run down on what’s going on now and in the next few months. SolidWorks World was the 1st topic: February 8-11 in Orlando Florida. As of now we only have one ‘user’ that will be attending. As a person who went last year I strongly recommend you going. If it was at all possible (i.e. I won the lottery) I would be going again this year.
Our next meeting will be on January 8th, 2009. Wes encouraged us to all bring a product that we designed in SolidWorks to this meeting. He also shared that from the recent poll he conducted that the most sought after topic for meetings was hints & tips. So next year expect more member presentations of the base SolidWorks products and less 3rd party add-on presentations.
Wes then opened the floor for problems, questions and comments. I brought up an issue I’ve been having with the visibility of sketches changing on its own. No one had an answer for that but other people were having the same issue which in a strange way made me feel a bit better. Wes is having a problem with the “Dynamic highlight from graphics view” option toggling off by itself. A bunch of us have experienced this, I remember it being an issue in SW2007. Walid is having an issue with sketch visibility between different configurations. No one else had seen this but it might be related to the problem I mentioned.
Keith Dacus took over and showed us his favorite things that are new in 2009. I present them below in list form.
1. Triad is now active…change view orientation by clicking on the triad axis.
2. Magnifier…’G’ turns it on & off. With the cursor in the magnifier, roll the mouse wheel to zoom in and out. Hold the ‘alt’ key down and roll the wheel to section what is in the magnifier. Move the magnifier by moving your cursor to its edge or by ‘ctrl’ middle button. And yes you can ‘ctrl’ select within and out of the magnifier. Really slick.
3. Double clicking the middle button is the same as typing ‘f’ for zoom all.
4. In the equation editor there is now a button on the bottom that pulls up a selector of all the properties.
5. Drop your bill of material into the assembly. This will allow you to manage all aspects of it while working on your assembly. It might have been at this point where we chased a rabbit and talked about Phil’s need to display a quantity as “as required”. We discussed virtual components and what they are used for. I asked about printing the BOM and Keith informed that while it can’t be done within the assembly file you can now do it in the drawing.
6. Instant 3D now works within assemblies allowing you to have dynamic feedback to change distance and angle mates.
7. Error reporting has been improved by the addition of phantom lines showing missing edges. For example when an edge is removed that a subsequent fillet modified you actually see a ghostly dash line showing the edge that is no longer there.
8. Rib tool – In previous versions you had no choice where the draft originated from, now when you apply draft you pick which face maintains the rib width. (not a very good explanation)
9. Measure tool now shows dual dimensions...I like this!
10. Multiple handles to pull and push on when in Instant 3D. Again we chased another rabbit here and talked about how Instant 3D should only be used for conception…hard dimensions should be applied at some point and Instant 3D turned off to prevent mishaps.
11. Weldments – several new and cool improvements here. Add chamfers and weld preps to gussets. Include weld gaps as required and some really neat trim/extend features. I wondered if the Weld Gap functionality would allow for negative values since we typically have a problem with shrinkage from the welding. In other word I need the members to be a little bit longer to compensate for the shrinkage. Keith checked it out and we found that you are limited to a positive value for the weld gap. But Michael Jolley verified that you can perform a ‘move face’ on the end of the weldment member and the cutlist updates accordingly.
12. Sheet metal – Convert to sheet metal and cross breaks...sheet metal is boring so that’s all I’m saying about that. Just kidding, once again we chased a rabbit and talked about how the convert to sheet metal is useful but will make a part that might not be manufacturable.
We wrapped up with some more questions and comments. Walid wanted to know how to do a ‘tabulated’ drawing. We received a short course on design tables from Rodney Hall. Wes closed the meeting by throwing out (literally) backpacks and briefcases to those who wanted them. He then drew names for the three $25 gas cards provide by our sponsor Rapid Sheet Metal.
Great meeting, thanks to everyone involved. Here are some pics taken by Rodney and me.
At the November WNC-Asheville Users group meeting we enjoyed good food, conversation and presentation. Tom Wilson cooked up his excellent barbecue chicken and ribs with all the fixings. Wes caught us up on what is going on with WNC-SWUG. Several People brought up issues and questions that they had about SolidWorks. This is what is so nice about these meetings; you will either find an answer or at least find out that you’re not the only person experiencing the issue.
After the meal Wes gave us a short run down on what’s going on now and in the next few months. SolidWorks World was the 1st topic: February 8-11 in Orlando Florida. As of now we only have one ‘user’ that will be attending. As a person who went last year I strongly recommend you going. If it was at all possible (i.e. I won the lottery) I would be going again this year.
Our next meeting will be on January 8th, 2009. Wes encouraged us to all bring a product that we designed in SolidWorks to this meeting. He also shared that from the recent poll he conducted that the most sought after topic for meetings was hints & tips. So next year expect more member presentations of the base SolidWorks products and less 3rd party add-on presentations.
Wes then opened the floor for problems, questions and comments. I brought up an issue I’ve been having with the visibility of sketches changing on its own. No one had an answer for that but other people were having the same issue which in a strange way made me feel a bit better. Wes is having a problem with the “Dynamic highlight from graphics view” option toggling off by itself. A bunch of us have experienced this, I remember it being an issue in SW2007. Walid is having an issue with sketch visibility between different configurations. No one else had seen this but it might be related to the problem I mentioned.
Keith Dacus took over and showed us his favorite things that are new in 2009. I present them below in list form.
1. Triad is now active…change view orientation by clicking on the triad axis.
2. Magnifier…’G’ turns it on & off. With the cursor in the magnifier, roll the mouse wheel to zoom in and out. Hold the ‘alt’ key down and roll the wheel to section what is in the magnifier. Move the magnifier by moving your cursor to its edge or by ‘ctrl’ middle button. And yes you can ‘ctrl’ select within and out of the magnifier. Really slick.
3. Double clicking the middle button is the same as typing ‘f’ for zoom all.
4. In the equation editor there is now a button on the bottom that pulls up a selector of all the properties.
5. Drop your bill of material into the assembly. This will allow you to manage all aspects of it while working on your assembly. It might have been at this point where we chased a rabbit and talked about Phil’s need to display a quantity as “as required”. We discussed virtual components and what they are used for. I asked about printing the BOM and Keith informed that while it can’t be done within the assembly file you can now do it in the drawing.
6. Instant 3D now works within assemblies allowing you to have dynamic feedback to change distance and angle mates.
7. Error reporting has been improved by the addition of phantom lines showing missing edges. For example when an edge is removed that a subsequent fillet modified you actually see a ghostly dash line showing the edge that is no longer there.
8. Rib tool – In previous versions you had no choice where the draft originated from, now when you apply draft you pick which face maintains the rib width. (not a very good explanation)
9. Measure tool now shows dual dimensions...I like this!
10. Multiple handles to pull and push on when in Instant 3D. Again we chased another rabbit here and talked about how Instant 3D should only be used for conception…hard dimensions should be applied at some point and Instant 3D turned off to prevent mishaps.
11. Weldments – several new and cool improvements here. Add chamfers and weld preps to gussets. Include weld gaps as required and some really neat trim/extend features. I wondered if the Weld Gap functionality would allow for negative values since we typically have a problem with shrinkage from the welding. In other word I need the members to be a little bit longer to compensate for the shrinkage. Keith checked it out and we found that you are limited to a positive value for the weld gap. But Michael Jolley verified that you can perform a ‘move face’ on the end of the weldment member and the cutlist updates accordingly.
12. Sheet metal – Convert to sheet metal and cross breaks...sheet metal is boring so that’s all I’m saying about that. Just kidding, once again we chased a rabbit and talked about how the convert to sheet metal is useful but will make a part that might not be manufacturable.
We wrapped up with some more questions and comments. Walid wanted to know how to do a ‘tabulated’ drawing. We received a short course on design tables from Rodney Hall. Wes closed the meeting by throwing out (literally) backpacks and briefcases to those who wanted them. He then drew names for the three $25 gas cards provide by our sponsor Rapid Sheet Metal.
Great meeting, thanks to everyone involved. Here are some pics taken by Rodney and me.
Thursday, November 13, 2008
Practical Sheet Metal Design Part 1
Pracitcal Sheet Metal Design Part 1
CRASH!
The ability of SolidWorks to do sheet metal design is both a blessing and a curse to sheet metal job shops. Understand this is no fault of the software but of engineers who don't really understand how bends are made in sheet metal. The example shown in the following figures is typical of what some of our customers will design. They even provide us with SolidWorks part files that they have have modeled. Can this part be made as modeled? Yes it can but it would required special costly tooling and extended lead times for the tooling. It has been our experience that making this in two pieces and welding two seams instead of one is more cost effective.
Figure 1 - the part
A simple part until you start looking at the bending operation. In the following figures you will see that I added typical press brake tooling and a section of the press brake ram so that you can see the problem with bending this part as modeled.
Figure 2
The first bend is no problem. you could start on either end of the flat blank with the same results.
Figure 3
The same holds true for the next bend. you will note that there is no interference between the sheet metal and the press brake.
Now you can plainly see the problem. Even if the 4.00" dimension in the part sketch was increased the part would still crash. The only solution that would allow forming this part as modeled would be what we call a 'window punch'. This involves using a tool that would fit inside the part has modeled and extends far enough past both ends of the part so that 'extensions' could be placed between the press brake ram and the punch. When the last bend is made, one of the extensions would be removed so that the part could be removed. This only works for softer material of a lighter gauge because of the loading placed on the tooling.
CRASH!
The ability of SolidWorks to do sheet metal design is both a blessing and a curse to sheet metal job shops. Understand this is no fault of the software but of engineers who don't really understand how bends are made in sheet metal. The example shown in the following figures is typical of what some of our customers will design. They even provide us with SolidWorks part files that they have have modeled. Can this part be made as modeled? Yes it can but it would required special costly tooling and extended lead times for the tooling. It has been our experience that making this in two pieces and welding two seams instead of one is more cost effective.
Figure 1 - the part
A simple part until you start looking at the bending operation. In the following figures you will see that I added typical press brake tooling and a section of the press brake ram so that you can see the problem with bending this part as modeled.
Figure 2
The first bend is no problem. you could start on either end of the flat blank with the same results.
Figure 3
The same holds true for the next bend. you will note that there is no interference between the sheet metal and the press brake.
Figure 4 - CRASH!
Now you can plainly see the problem. Even if the 4.00" dimension in the part sketch was increased the part would still crash. The only solution that would allow forming this part as modeled would be what we call a 'window punch'. This involves using a tool that would fit inside the part has modeled and extends far enough past both ends of the part so that 'extensions' could be placed between the press brake ram and the punch. When the last bend is made, one of the extensions would be removed so that the part could be removed. This only works for softer material of a lighter gauge because of the loading placed on the tooling.
I hope this explanation is clear to all those who design sheet metal with limited experience of the manufacture of sheet metal parts. I will continue in the coming days to share more instances where a small changes can save you money in your sheet metal designs.
Wednesday, November 12, 2008
WNC/ASHEVILLE SWUG MEETING TOMORROW
This is your last chance this year to attend our user group meeting. Barbecue will be on the menu as well as Keith Dacus presenting "What was not shown at the SW2009 Roll Out". Keith is an excellent presenter that will keep you entertained while you learn some of the great new stuff in SolidWorks 2009. I wish to thank him and TPM for supporting our user group.
I hope he concentrates on features in the base SolidWorks package. I hate getting all excited about some new feature only to find out that I have to have the professional or premium to take advantage of it. I was kind of interested in who has what so I set up a poll here for you to respond to. If you're not sure what you have here is a product matrix line.
One other thing...I know it's a day late but call or email a veteran and thank him for serving our country so that me and you can enjoy the freedoms that we have. You will be surprised at the response you will get. I email my uncle who served during the VietNam war and I could tell that he was touched by my thinking of him. I did the same for my brother who was in the Gulf war (the 1st one) and even though he was thankful of me thinking of him he was somewhat upset that he doesn't have the day off. I remember him pointing out when he worked another job that it was a shame that he got MLK day off but not Veterans day. Something to think about. Maybe 'THE ONE" will do something about this and make Veterans Day a national holiday like July 4th.
See you tomorrow night.
I hope he concentrates on features in the base SolidWorks package. I hate getting all excited about some new feature only to find out that I have to have the professional or premium to take advantage of it. I was kind of interested in who has what so I set up a poll here for you to respond to. If you're not sure what you have here is a product matrix line.
One other thing...I know it's a day late but call or email a veteran and thank him for serving our country so that me and you can enjoy the freedoms that we have. You will be surprised at the response you will get. I email my uncle who served during the VietNam war and I could tell that he was touched by my thinking of him. I did the same for my brother who was in the Gulf war (the 1st one) and even though he was thankful of me thinking of him he was somewhat upset that he doesn't have the day off. I remember him pointing out when he worked another job that it was a shame that he got MLK day off but not Veterans day. Something to think about. Maybe 'THE ONE" will do something about this and make Veterans Day a national holiday like July 4th.
See you tomorrow night.
Thursday, November 6, 2008
CSWP Training materials
CSWP Training materials…are they worth their cost?
I was able to successfully pass the CSWA exam earlier this year by doing the sample test available from SolidWorks and a lot of reading from Matt Lombard’s book. My intention was to go for the Professional exam soon afterwards but that didn’t happen. I guess I’m feeling the same trepidation as I did before I took the CSWA. In my day to day work I use such a small subset of the SolidWorks program (sheet metal) I fear that I just don’t have the experience to even think of taking the CSWP exam.
So here is my question to the SolidWorks masses out there: Are any of the training materials worth the $300 - $500 that they cost. I’m thinking of the myigetit.com course and the one from Solid Professor. I’m thinking of asking the boss to ante up for this since he’s not sending us to SolidWorks World this year. But he will expect results, successful passing of the test.
Come on guys, give me some feedback. Surely someone out there has purchased one of these and has some advice to offer.
Thanks in advance,
Wednesday, November 5, 2008
Copy With Mates - A Simple Tutorial
The Copy with Mates command has always been an underused feature in SolidWorks for me. I'm not really sure why but every time I went to use it I had to refer to the program help to figure it out. On a recent project that involved a lot of hardware placement into the assemblies I took another look at the command and through repeated use I finally have a handle on it's use.
Take a look at this simple assembly that has a single PEM fastener installed with a concentric and a coincident mate to the sheet metal part. I need to replicate this feature with the same mates on all the holes.
Image 1
In Image 2 you see that I've selected the PEM nut and clicked the 'Copy with Mates' button. (I added the button to the toolbar, its not there by default.) The dialogue box shows the selected components and the mates associated with the components. Click in the box associated with the 1st listed mate, 'Concentric3' in this case.
Image 2
Now all that is left is picking the surfaces that will satisfy these 2 mates. You see in image 3 that I've zoomed up on one of the holes. I have the selection filter set to 'Faces' and I've already picked the inside of the hole in the sheet metal. You see a preview of the PEM nut placement.
Image3
Notice that next to the mate boxes there is a tick box with the word 'Repeat' next to it. Since all the PEM nuts will meet this mate requirement I will check the box next to the Coincident3 mate.
As soon as I check the box the PEM nut snaps into place as shown in Image 4 below.
Image 4
Make sure you click the OK button or the PEM nut will not be placed, but don't click it twice because that will exit the command and we have more hardware to place. All you have to do now is continue picking the face of the holes and clicking OK (once) after each PEM is placed.
Image5 - Shows that the face of the 2nd hole is selected and the PEM will be placed here.
Now think about how many clicks you will save by using the 'Copy with Mates'. Also note that if there had been a screw and washer inserted into that single PEM nut we could have applied the 'Copy with Mates' to all three at the same time. Here is the part with all the PEM hardware installed:
Take a look at this simple assembly that has a single PEM fastener installed with a concentric and a coincident mate to the sheet metal part. I need to replicate this feature with the same mates on all the holes.
Image 1
In Image 2 you see that I've selected the PEM nut and clicked the 'Copy with Mates' button. (I added the button to the toolbar, its not there by default.) The dialogue box shows the selected components and the mates associated with the components. Click in the box associated with the 1st listed mate, 'Concentric3' in this case.
Image 2
Now all that is left is picking the surfaces that will satisfy these 2 mates. You see in image 3 that I've zoomed up on one of the holes. I have the selection filter set to 'Faces' and I've already picked the inside of the hole in the sheet metal. You see a preview of the PEM nut placement.
Image3
Notice that next to the mate boxes there is a tick box with the word 'Repeat' next to it. Since all the PEM nuts will meet this mate requirement I will check the box next to the Coincident3 mate.
As soon as I check the box the PEM nut snaps into place as shown in Image 4 below.
Image 4
Make sure you click the OK button or the PEM nut will not be placed, but don't click it twice because that will exit the command and we have more hardware to place. All you have to do now is continue picking the face of the holes and clicking OK (once) after each PEM is placed.
Image5 - Shows that the face of the 2nd hole is selected and the PEM will be placed here.
Now think about how many clicks you will save by using the 'Copy with Mates'. Also note that if there had been a screw and washer inserted into that single PEM nut we could have applied the 'Copy with Mates' to all three at the same time. Here is the part with all the PEM hardware installed:
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