The Home Machinist!

Hello, I would like to help a fellow metalworker on another group with a sheetmetal question that he has. What is needed, if possible, is a formula for figuring out the springback of thin metal (brass in particular) when a 90° bend is required. Or how much to overbend it for repeatable results. I imagine there are a million variables (material, equipment, etc....) but there must be a few rules of thumb. I wonder if a dial indicator could be used to measure the amount of overbend with trial and error until you find what works? Or some sort of positive stop. Would this even be relevant with proper equipment (can you tell, I'm not a sheetmetal worker... yet [img]/ubb/images/graemlins/wink.gif"%20alt="[/img]). I did a quick search of the forum and could not find anything dealing with formulas. Appreciate any help. Scott.

It's mostly trial and error. Make a guess and bend. There are tables giving general guide lines for diffferent materials. Obviously materials with a spring temper will need more overbend as opposed to softer materials.

One thing to be aware of, you can't sneak up on a bend. Partial bending and checking won't work. Once the part is bent it's slightly work hardened, bending it some more may get to the correct bend angle, but that setting won't neccessarily repeat for a new piece. Each trial has to be done with a new piece.

You have to keep the dwell time at the bottom consistent. Longer dwells may result in more "set".

Several customers have CNC brakes which supposedly can be programmed with material type, thickness, etc and they'll compensate for springback. The operators tell me they still need at least one trial piece to verify.

Thanks DR and I will pass this information along. I have to ask about the dwell time at the bottom... are you talking about the time between down/in stroke and up/out stoke of the die? Would this apply to a manually operated machine? Scottt.

Regarding the dwell time....I was thinking in terms of a press brake when I wrote that. Dwell would be the time the ram was held in the down position to allow the bend to set. The speed of the bending motion also is an issue. Same things would apply with any type of bender. To get consistency from bend to bend they have to be done the same.

First time I really took notice of this dwell time issue was when we were straightening some large brass pieces. We made some plastic dies for the press brake to overbend the parts back straight after machining. A few trials and we got the correct amount of arc in the dies to do the job. Put a part in and cycle the machine and we had a straight part. One time the operator left a part in the dies in the over bent position while we went to lunch. When the part was taken out it had taken on the overbent condition. The long dwell time had allowed the material to permanently take the overbend.

Metal does funny things doesn't it? I worked for one day on a huge press brake as a "farm-in" for a operator's helper that was sick and we were bending 3/8" plate that was about 30' (huge press) with a taper, 5' on the big end and 3' on the small end and the bend lines were to be held to 1/16 tolerance (ha). Two sections go together as one to form a tapered structural member. It was a bumping operation and our pieces came out every which way [img]/ubb/images/graemlins/blush.gif"%20alt="[/img]! Also the bigger sizes of pipe will bend different though a roller depending on the mfg. heat number. Thanks for the information DR. Scott.

Here is a link to another discussion in the past that was a great topic I get a bit verbose on [img]/ubb/images/graemlins/blush.gif"%20alt="[/img]. Without going into all of what I have posted on the subject again, do a search on Press Brake(s) you should find a good deal of this already covered.

There would be nothing wrong with hard stops if you have control over the reversal point of the ram. On thin material of say 22 ga or less., the vee die width will be about 1/4". The vee die angle is not critical as long as it is to the closed side, unless you are coining to a sharp corner. The penetration becomes very critical when air bending on smaller vee dies. To the tune of .004/degree variance on a 1/4" wide die.

I hope this will get you started. DR has stated the case very well. Dwell is rarely used in production forming, but for springy materials. Even that is rare if the dies are deep enough to overbend before bottoming out to coining. Not really an option on eccentric driven rams. A fancy option on hydraulic machines to improve consistancy. Coining is so hard on the equipment with eccentric type of brakes too. A slight change in material thickness from one plate to the next, can increase the load on the machine exponentially.

Older press brake thread

DC

Doug, I sure didn't intend to kick a dead horse with my posting, after all I did try a search, but I suppose I used the wrong "criteria". Thanks for the link to the old thread, I will pass that information along as well. And if you ever want to get verbose, I have an ear for anything longwinded as long as I learn something (just you email me) ! Just what the *(%^& is coining? It's all a learning process, right [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]? Scott.

Scott,

For certain there are no dead horses here. I just didn't want to copy, paste or transfer the various posts that have been taking up space here already. That was out of efficiency for time and a bit of lazy-ness on my part.

The difference between bottoming the dies and coining the material is how much tonnage is applied. Bottoming can be done on say 87deg dies to still get a sharp bend with spring back.

Coining is usually done with 90deg dies. The material is actually compressed at the bend. There is a fine line between bottoming and coining. There will be a point where the bend opens as tonnage is increased as a result of hitting the material harder. Then the bend will begin to close upon coining the material into submission. Very sharp bends can be achieved and a lot less bend allowance is needed in the flat pattern because less material is eaten up by the bend radius.

Learning by reading is only half the lesson. Putting the practice into action gains an experience to unfold. Each time it is used, another level of crafsmanship is built upon. I like the saying where "A masterpiece is made where creativity, craftsmanship and love come together!" errr something like that anyways....... [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]

DC

Doug, thanks for the explanation of coining. I have just recieved a book today that goes into great detail on sheetmetal work, among other things. I will have to do my homework first and post a question when I get really stuck on something.

I agree! Scott.

D R,

Much of what the CNC brakes do, is still the result of math behind the GUI the operator puts in results that make adjustments to get closer to the desired angle. All the OEM's seem to have their ways of getting to the same point.

One feature that I really liked was called Adaptive Bending. This requires that the control take into account several variables included in each gage range. Normally this will only cover 3 different gages like 14-16-18 or 10-11-12 etc.

The machine must be put through a teach function using samples of the actual materials of average stock for each gage thickness. This teaching is done over consecutive pieces grouped by gage runs and actual thickness and all must be the same width. The actual bend results data of each piece is recorded to the control. This is paired with the load cells on the machine and feedback from the ram position to create strain curve tables.

In actual use, any of the 3 gage thickness can be thrown at the press and it will use the tables to verify the loads to calculate the rams reversal point with amazingly accurate and repeatable bends. I have even put in material outside the range of the teach tables with astounding success.

Not 100%, but it was very useful for odd stock leftovers from different lots or vendors. A lot less cumbersome than segregating the different thickness and making adjustments for each range. Just cool, high priced technology.


DC