Need help spring back question

[Shotout]

We have been getting an influx of formed parts at work lately but not the tooling to make them. We've been having to build tooling and this is somewhat outside of our area of expertise. I currently have 2 part sets to build and haven't been able to figure out spring back for one part. I'd really appreciate it if someone could help me out with the formula for how to figure spring back on a full radius bend; and what the factors are and how to determine them. Attached is a jpg of the part. Material is .125 normalized 8630 steel.
Thanks
Scott

[juergenwt]

Shotout - A lot depends on your material - grain, temper etc..
For a simple part like yours we would take a piece of CRS about 1 1/4" thick and bore a 2.250 dia hole. Cut it in half and provide an adjustable stop on top. Now make a first blank with the dim.s available. Keep a record! Locate the blank on top against the stop and press down with a 2" dia bar (punch). Now see what you have and adjust from there by making a new blank and moving your stop. Mounting all in a die set would be one option.
If you need to bring in the sides - bore a smaller than 2.25" radius on the bottom of the die, kind of like a small dip in the center and you may also cut down on your 2" dia punch. It is actually easier than it sounds. A few blanks - try and adjust. Keep records of your blank dim's. Case harden.
One alternative would be to leave the blank longer and trim in a secondary operation after forming.
If this is a die for a large production run you would want to invest in a book on forming dies. Here you will find a formula that will get you close, but you will still be making some blanks to get to the right size for a developed length. You could be looking at a draw die with pinch trim action or even a forming die using a urethane rubber cushion and just a forming punch. You will need a good size hydraulic press for this option.

[Shotout]

I had planned on doing something like what you suggested to test starting with a true dimension and then working it in but when the planner agreed to expedite it kind of killed that. We normally hydrofrom on a rubber press or break press and all that is O condition alum on average .060 to .090 thick so you can see where this is a departure from our normal tooling requirements. This one was a little more involved (omitted flange and material) so I was hoping someone could offer me a short cut. I did pick up the Die Design Handbook by David Smith but it was kind of closing the barn door after the horses got out. Hopefully I'll have digested it before the next job.

You suggested making the bottom of the die tighter than the outside radius of the part. Is that to coin the material to alleviate spring back? If so with a typical part is that a normal practice and is it OK just so long as you don't reduce the material out of the specified tolerance range? Does the typical drawing or part spec normally say when it isn't allowed? I just haven't had to make may form tools before and like I said they are in material that is normally very forgiving so this is pretty new to me.

Thanks for taking the time to answer and the advice.
Scott

Originally Posted by juergenwt Shotout - A lot depends on your material - grain, temper etc.. For a simple part like yours we would take a piece of CRS about 1 1/4" thick and bore a 2.250 dia hole. Cut it in half and provide an adjustable stop on top. Now make a first blank with the dim.s available. Keep a record! Locate the blank on top against the stop and press down with a 2" dia bar (punch). Now see what you have and adjust from there by making a new blank and moving your stop. Mounting all in a die set would be one option. If you need to bring in the sides - bore a smaller than 2.25" radius on the bottom of the die, kind of like a small dip in the center and you may also cut down on your 2" dia punch. It is actually easier than it sounds. A few blanks - try and adjust. Keep records of your blank dim's. Case harden. One alternative would be to leave the blank longer and trim in a secondary operation after forming. If this is a die for a large production run you would want to invest in a book on forming dies. Here you will find a formula that will get you close, but you will still be making some blanks to get to the right size for a developed length. You could be looking at a draw die with pinch trim action or even a forming die using a urethane rubber cushion and just a forming punch. You will need a good size hydraulic press for this option.

[bwprice100]

Hi Scott

With spring back you can do a test bend with your material. Just bend a test strip a set amount say 10mm and then measure how much it did actually bed when released an you will no how much to allow as a percentage to over press.

With regard to the blank size the 'Chordal method is the most accurate.

Brian

[Shotout]

It just occurred to me, but can this part be treated as 2 90deg bends?

Originally Posted by bwprice100 Hi Scott With spring back you can do a test bend with your material. Just bend a test strip a set amount say 10mm and then measure how much it did actually bed when released an you will no how much to allow as a percentage to over press. With regard to the blank size the 'Chordal method is the most accurate. Brian

[bwprice100]

Originally Posted by Shotout It just occurred to me, but can this part be treated as 2 90deg bends?

No Not really, there will be an increase in material thickness the nearer to the top edge you get; the materila has to go somewhere.

Brian

[KilrB]

What are your tolerances?

Using both the arc length method and standard bend formula I get a blank length of 3.237".

However, this does not address your question about spring-back.

Are you experiencing spring-back with the 8630 on this or in other apps?

Try it in an existing form and see what it does, there are few substitutes for that.

As juergenwt says, there are simply too many variables.

Temper, grain direction, rigidity of your set-up, press tonnage, etc. will all effect your outcome.

I've not worked with 8630 but depending on the variables I've had over-form to allow for anything from zero to 15% spring-back.

[Shotout]

I didn't have time on it. My first ever "real" die set and it was expedited by the customer and for some reason the planner accepted despite my warning during quoting about development time as this was outside of what we have experience in. It did turn out well however.

I had a tolerance of .030 and ended up with .051. From there I able to use the tolerance of the leg length and reduced my length by part of my tolerance to get the whole thing in. I did have too much over bend in the flange that was omitted from the attached (OP) file. I used a generic formula from another press and it was 1.74 deg, turned out I needed less than that but it was easy hand work to get it into the .5 deg tolerance.

Grain direction did matter a lot so I added a drawing with a grain direction call out for water jet. I now have a few points of data I've charted for our press so I hope to be able to get a little closer next time.

Originally Posted by KilrB What are your tolerances? Using both the arc length method and standard bend formula I get a blank length of 3.237". However, this does not address your question about spring-back. Are you experiencing spring-back with the 8630 on this or in other apps? Try it in an existing form and see what it does, there are few substitutes for that. As juergenwt says, there are simply too many variables. Temper, grain direction, rigidity of your set-up, press tonnage, etc. will all effect your outcome. I've not worked with 8630 but depending on the variables I've had over-form to allow for anything from zero to 15% spring-back.