Will adding a dart in a formed part decrease spring back?
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    Default Will adding a dart in a formed part decrease spring back?

    I'm working on a design for a progressive tool for one of our products and my soft try-out tool leaves a couple bends shy of the ideal 90 degrees. I know there's ways to overbend the material, but this is new stuff for me so I'm trying to keep it simple.

    I have used darts and ribs in simple press brake parts before and they do wonders for stiffness and seem to help consistency. I'm thinking adding darts in these features, the darts would be formed after the bends are made in the same hit, would help push material around in the corner and contribute to the part coming out nearer a true 90 than the 85 or so I'm getting now.

    Not sure it matters, but this is 11 gauge mild steel. .125 inside radius. length of bend is about 1.25" times two. Thinking .250 pins for darts?

    Any input?

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    I'm assuming two darts?

    Soft forms to determine bends?

    I would start with .125 pins with a well rounded lead in.

    The reason is, you can always make the darts bigger. Really hard to go smaller.

    I've always used the 5 degree rule. Make the tooling to overbend 5 degrees, You can always back off of the bend.

    Lots harder to go more

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    Many times a good die maker can "set" the bend by adjusting the radius of the forming punch and/or die section. Talk to your die maker.
    John

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    If you've made the radii of punch/die by offset of the material thickness then springback is to be expected and may change in the middle of the coil, let alone from one coil to another. Die design has to accommodate that variable by being able to adjust for this. Darts may help, but that's still a static solution for a potentially variable material condition. I know you want to keep it simple, I strive for that too, but material that's variable is not simple. A bead set and/or die shut height control is the simplest solution I can think of without resorting to cams, collapsing sections, or ready-benders. All of which are good (for larger runs) but add to complexity/cost.

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    The use of the word "darts" left me wondering. Somehow I did know what you were talking about or at least I strongly suspected. But I tried to look up that meaning of the word and ran into a real problem. Several dictionaries made no mention of it. I searched with several combinations of search terms with no luck. I looked for images and got so many dart boards that I could not count them. DART stands for Dallas Area Rapid Transit.

    I even found a Dart Container Company right here in my own town of Beaumont, TX. Who knew?

    But if you want to find the meaning of a dart in relation to a bend in sheet metal, GOOD LUCK. It will take some searching. I probably would not have found it if I did not have a half way good guess to start with. I finally found this:

    Ready Benders Bender READY rocker

    They at least show an illustration of darts in a bend. I guess there must be references to this in text books on the subject. But I do not think that there is any formal definition of this kind of a dart out there. At least not one that would invoke an image in the mind of a person who has never seen one.

    No question here, just talking about my search to satisfy my curiosity.

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    Huh, got it on the first or second search. I’ll admit I wasn’t familiar with the term “Dart” for a gusset in the bendy breaky world, but I think it was “sheet metal die dart” or something like that.

    the trick is to hit image on the search engine. Then takes all of another second to get it. The bandwidth on an image is vastly superior and allows you to narrow down wether you’d in an English pub, or the metal fab world pretty damn quick mate! Cheers

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    I think the precision sheetmetal world has it's own language.

    I figured anybody who could answer the question would probably know what a dart is.

    I'm trying to learn as much as I can about progressive tools and it's a new world for me.

    I'm making single station tools to make the parts by hand in my press brake, but I think I will have to step up to progressive hard tools before long.

    Toolmaking is a whole different level. Can make a guy feel pretty dumb.

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    No reason to feel dumb, it's just another area you're not familiar with yet. I'm just getting acquainted with Master Cam after decades of being away from tape fed machines, MDI, and back when a bubble memory was new. Should I feel dumb or just of touch? Progressive dies have rules/guidelines you've not had to work with, but perhaps do now. You'll get it, there's plenty more reading available now then back in the dark years. Good luck.

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    Die Makers Handbook Jerry Arnold Hardcover 9780831131326

    This will help you with the basics.

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    In my experience darts usually set a bend pretty well, but you may still have 1 or 2 degrees springback. They also require a lot of tonnage to set correctly and must be machined accurately to get them to hit correctlly. If the darts are holding the rest of the die open it could give you lots of trouble.

    Darts are typically "crash" formed. Forming the darts after the bend is formed will likely make the underbend worse, as the darts will have to pull in material, opening up the bend.

    Since this is a progressive tool, I typically will design critical bends to be done in 2 hits. This allows you to form bends less than 90 degrees without any complex tooling such as ready benders. You simply slightly adjust the centerline from hit to hit to add or subtract overbend.

    part_beads.jpg

    2_station_overbend.jpg

    img_0614.jpg

    img_0632.jpg

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    A coworker of mine used to refer to these as "karate chops" and that's what I have always called them. Nice to know the actual term over a decade later. Recently I have found fabricators reluctant to incorporate these (along with any type of form really) but they can be very useful for the stability of a flange. That said I suspect adding one to address spring back might just make the spring back permanent. I'm not a toolmaker though.

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    Quote Originally Posted by TheBigLebowski View Post
    In my experience darts usually set a bend pretty well, but you may still have 1 or 2 degrees springback. They also require a lot of tonnage to set correctly and must be machined accurately to get them to hit correctlly. If the darts are holding the rest of the die open it could give you lots of trouble.

    Darts are typically "crash" formed. Forming the darts after the bend is formed will likely make the underbend worse, as the darts will have to pull in material, opening up the bend.

    Since this is a progressive tool, I typically will design critical bends to be done in 2 hits. This allows you to form bends less than 90 degrees without any complex tooling such as ready benders. You simply slightly adjust the centerline from hit to hit to add or subtract overbend.

    part_beads.jpg

    2_station_overbend.jpg

    img_0614.jpg

    img_0632.jpg
    Those are beautiful tools! My sheetmetal experience with darts is machining the die to take hard dowel pins pressed in.

    Here's an example of one of the prototype parts in a tool I made. I need to add some formed features to this part and re-make this tool to hopefully do it all in one hit.

    I have a 250ish ton old straight side Bliss press that runs a mellow 22 strokes a minute. I have access to a bunch of die parts that look about the right size to make the tools I envision in my mind. Ultimate goal to make 22 parts a minute from coils...
    Attached Thumbnails Attached Thumbnails bridge-tool-blank.jpg   bridge-formed.jpg   blank-hand.jpg  

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    Quote Originally Posted by Garwood View Post
    Those are beautiful tools! My sheetmetal experience with darts is machining the die to take hard dowel pins pressed in.

    Here's an example of one of the prototype parts in a tool I made. I need to add some formed features to this part and re-make this tool to hopefully do it all in one hit.

    I have a 250ish ton old straight side Bliss press that runs a mellow 22 strokes a minute. I have access to a bunch of die parts that look about the right size to make the tools I envision in my mind. Ultimate goal to make 22 parts a minute from coils...
    Nice, that is a great part to learn on if you are interested in tool and die.

    Before you begin building new forms, there are a couple of quick things you might be able to try.

    It looks to me like your forms may not quite be hitting home, if you look closely the corners of the part you can see that the edges are raised. That radius should look pretty even across the length of the part.

    The best way to check this is to "blue up" the entire part before hitting it (light coating of Prussian Blue, the same stuff scrapers use, across the part) When you hit the part, it should reveal bare metal on both sides of the formed radius as well as the wiped edge. If its not, you often just need to hit it a little harder.

    Putting a strip of lead in the forms and hitting it will also roughly tell you if your hitting home, it should measure the same as the part thickness when you check it. the extra force required to form sheet metal often requires the ram to be brought down more even though it leads out ok.

    blank-hand_li.jpg

    bridge-formed_li.jpg

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    Yea, I am here trying to learn too. I did have a vague idea of what it was, but wanted a better description or definition. That's why I did a search. And I shared it for others who may not have wanted to say they didn't know.

    I know I am dumb but I am working on it. I figure by the time I know everything I will be 105 yo and have two feet on banana peels.



    Quote Originally Posted by Garwood View Post
    I think the precision sheetmetal world has it's own language.

    I figured anybody who could answer the question would probably know what a dart is.

    I'm trying to learn as much as I can about progressive tools and it's a new world for me.

    I'm making single station tools to make the parts by hand in my press brake, but I think I will have to step up to progressive hard tools before long.

    Toolmaking is a whole different level. Can make a guy feel pretty dumb.

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    Not to threadjack, but I have been contemplating the redesign of a dieset for one of my products and this thread really peaked my interest.

    Garwood, if you dont mind I would like to post a drawing of my part tomorrow. Hopefully to keep this constructive feedback rolling.

    I form a U shaped part out of 3/16 HRPO steel. approximately 1.5 leg length and 1.5” overall width, roughly 1.25” long. My current tooling is a gooseneck die and I coin the bends in two separate hits.

    I would like to make a new die set that bends the complete part in one hit.

    First question: How did you calculate the springs rate putting pressure on the Pad? (part keeping center portion of part flat)

    Thanks, Teddy


    Sent from my iPhone using Tapatalk

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    Quote Originally Posted by kineticmx View Post
    Not to threadjack, but I have been contemplating the redesign of a dieset for one of my products and this thread really peaked my interest.

    Garwood, if you dont mind I would like to post a drawing of my part tomorrow. Hopefully to keep this constructive feedback rolling.

    I form a U shaped part out of 3/16 HRPO steel. approximately 1.5 leg length and 1.5” overall width, roughly 1.25” long. My current tooling is a gooseneck die and I coin the bends in two separate hits.

    I would like to make a new die set that bends the complete part in one hit.

    First question: How did you calculate the springs rate putting pressure on the Pad? (part keeping center portion of part flat)

    Thanks, Teddy


    Sent from my iPhone using Tapatalk
    I found a vague reference to using 1.5 times the forming force for a pressure pad. I made a wild guess that I was between 5 and 10 tons forming force. I dug through my spring box and found I had 4 springs with enough stroke and they felt stiff enough in my arbor press. I designed the tool so the pressure pad hits .05" before the forming starts.

    I screwed up a few things in the tool like I intended to have about .003" per side over material thickness, but I ended up at zero. It still works fine.

    I made the tool from scrap 2in plate and $20 in stripper bolts.


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