Calculating flat stock needed to roll circle?

Date 03-29-2010 15:59

Theoretically the length of you material will determine the I.D of the rolled section. Note this is true when doing a large radius bend in relation to the thickness of the metal. If doing small diameters. I would use a sheet metal bend allowance calculator.

Many can be found online here's a link to one.
http://www.engineersedge.com/bend_allow_calc.htm
http://www.engineersedge.com/calculators/bend_allowance_pop.htm

but I must ask what type of machine are you using to do the bend, for instance a 3 ring roller will leave a flat, unless the part is tacked together and even then it won't produce a perfectly round ring.

By bozaktwo1 (***)

Date 03-29-2010 17:01

If you're using a sliproll former, it will leave a flat, the length of which will vary in proportion to the roller sizes. A large machine may leave 2-3" of flat at the front end. There are a few ways of overcoming this, mostly depending on material type and thickness. You can try to turn the piece and roll it from the end to the beginning...be careful to get it aligned true, or your cylinder will now be a frustum. My preferred method was to add the length of the flat to the stretchout and roll it, then cut the flat "tongue" off the cylinder prior to joining.

If you need to be exact, you can figure your bend allowance on the neutral axis of the material. However, most small gauge sliproll formers are not wholly accurate across their stated working width, and the larger rolls will leave a larger tongue. For my money, I would make the blank oversize and cut the excess off when done.

Either way, good luck!

By louis2

Date 03-29-2010 19:53

By neutral axis, do you mean the circle size of the middle of your material thickness? In other words if the material was 3/8" thick and the circle diameter was 2' O.D. I would back down the diameter used in my equation to 23 5/8".

Yes we do use a 4' sliproll former. We usually add some to the length of the flat and cut off the excess. We also trade ends as we are rolling so as to try and help with the flat spot.

By bozaktwo1 (***)

Date 03-30-2010 17:28

There are probably as many different methods for calculating BA as I have fingers and toes! I always use the neutral axis (theoretical centerline of the material thickness), because that's how I learned. Material thickness counts for a lot, though. For example, I used to calculate sheet metal (under 1/8") using a completely different formula and never taking the NA into consideration (the number is just too small to make a difference, especially for plain old sheet steels). I don't have any of my old fab notes here, but I used to have a formula for everything; every type of material, thickness, and forming method. The thicker the material, the more difficult it is to get the exact size. Remember the inside surface is being compressed and the outside surface is being stretched, so your material type plays a big role in the success of a given calculation. Higher yield steels are uncooperative at best!

With 3/8" at that diameter low carbon steel I think I would do the same as what you are doing. Cut extra and take off the flat. I might not even turn the piece to roll it again, depending on how true the roll is in one direction. Are you welding it together on the seam? Is your roller handling the 3/8" thickness okay? a 48" roll set sounds sort of light for that thickness for any kind of length. Are you doing rings?

By louis2

Date 03-30-2010 18:17

Man you are good. Yes when we are going that thick we are doing rings. Your right the 4' slip roll is not much on thick stuff. If we stay close to the end where the rollers connect we can roll a 1/8" x 4" wide piece of flat and a 1/2" x1/2" solid square bar or a 1/2" solid round. We don't do a lot of this just for a few projects in class. We made some snow men one year. Three rings graduating in size. Add some 1/2" round for arms, fingers, legs, and a top hat. 1/4" round in the rod bender you make the cornkob pipe. A strip of 1/4" in back down the front and across his head add some 1/2" flat washers for buttons and eyes and you have a stick snow man. What I was wanting the formula for is more for teaching purposes than anything else. I would teach it in the classroom then have the students cut and roll a piece to show that the math works. I'm going to use your middle of the thickness formula for the 1/2" stock. It seems like that was the way I was taught so many years ago also.

As far as building something to work with I think I will do what you said about adding the extra and cutting off the flat.

Thanks for the reply,
Louis

By bozaktwo1 (***)

Date 04-01-2010 17:28

Like Metarinka said above, the basic formula is used to resolve the inside diameter. This would be simply pi*Dia=Circumference. When you have to weld the two ends together, however, you have a pair of problems to contend with. First, if your roll former is not true, the seam will not be uniform (this doesn't affect your particular situation because you're doing rings). Second, it goes without saying, the flat or "tongue" will never become exactly the same radius as the rest of the cylinder. This is a case where in theory, you can get a number down to the molecule, but in practice you'll just end up frustrated. With steel, you can get very close because it doesn't become work-hard like say aluminum. As a matter of fact, with your low carbon steels the more passes through the rolls, the better your form is going to be. I have found that, using the NA as a starting point, I can get just about anything under an inch thick to within 1/16". So Pi*(dia+T)+L (flat). Be sure to include the entire flat plus the beginning of the form, as this area will not be the same radius as the rest of the part (you'll have to eyeball it, there's really no way to figure that out mathematically). Also, try to work from the middle of the rolls. It may take a little longer, but it's good practice for keeping the rolls true. The top and drive rolls are slightly tapered to allow for equal forces across the face of the rolls, so the ends will close up faster than the middle. Those grooves on the right end, by the way, are not for rolling bar; they are for rolled seams in sheet metal duct work. :)

I'm glad you're giving the effort to teach our young ones, and I hope you keep it up.

By ssbn727 (*****)

Date 04-01-2010 23:21

I agree with everything Curt mentioned, and I only want to comment on the safety aspects of teaching the youngsters of how to work around your set of rollers...
You must teach them the importance of safety around this equipment because anyone of these youngster can easily get one of their hands caught in between the rollers and another on not paying attention to the controls which will more than likely create a potentially very dangerous and life changing condition to one of these youngsters! :(

I am hoping that you and your school have the appropriate safety guards to ensure that none of the students can get their hand caught in between the rollers as one operates this equipment, because if you don't then please do not continue to let the students operate this equipment until the appropriate safety guards have been installed!

In my short existence on this planet, I've seen 6 people destroy their hands in accidents that could have been prevented if the proper guards were installed not only on a set of rollers, but also on shears, brakes, punches, and all sorts of other equipment that were not properly protected with safety guards, so please make sure that you have a very robust safety guard installed for the youngster's sake! Thanks for your time to read this! ;)

Respectfully,
Henry

By louis2

Date 04-03-2010 01:16

Thanks for your reply ssbn727,

This is an old piece of equipment that was there when I hired on. It does not have any guards on it nor do I see where any would have been mounted. It's a hand crank roller though so it’s a lot safer than a power roller. With that said yes I do caution my students about its use as well as how to behave in a shop atmosphere. If I find them playing with any piece of equipment my typical initial response is to sternly tell them that piece of equipment doesn't have "Tonka" written on it anywhere. After they shoot me that puzzled look I tell them it’s not a toy. Don't play with it.
Yes you are right everything we have in the shop can burn you, mash something, cut something off, or blow up and kill most of us in the shop. I'm quick to point out to them that we are subject to some of the same hazards as you guys out there working in the industrial settings. Safety is a big part of the program and I'm proud to say that we have not had a severe injury in about 9 to 10 years. Almost all of the injuries we have had over the years not counting small cuts and scrapes could be contributed to horseplay. Of course they didn't tell me that at first, but the truth does eventually come out.

Thanks for your reply. Louis

By louis2

Date 04-03-2010 00:44

bozaktwo1,

I didn't know that about the grooves. Yes we have used them for rolling 1/4" and 3/8" round stock. I just assumed that was what they were for.

Thanks

By Metarinka (****)

Date 04-26-2010 22:55

just as a note, another way to get rid of the "tongue" is to pre bend the part in a brake with a custom die. We do that on production runs, or else you would never get it perfectly round.

Then again we are trying to hold 0.020" on the roundness and dimensional tolerance so it was critical that they were rolled uniformly.

And your right there are many different ways of calculating BA, also as a note a lot of it depends on the type of tolerance you're trying to hit and how "round" it actually has to be, the repeatability of rolling isn't nearly as straight forward as other machines so holding tight tolerances becomes a pain.