Rolling Plate.

[Lizzyrocks]

I'm not an engineeer. I'm a CWI, API 510, Level III VT type of guy. My question is "when rolling plate, what the formula for figuring the cutting lenth". We fabricate weld-on sleeves for the water/wastewater and oil/gas industries. The metal can be from .250" to .750". The two halfs have to have a .10" gap inbetween them. For what ever reason our engineering dept has a hard time getting these to fit. Their almost always to long. I'm the one doing the fit checks on these and would like to know for myself what the forula is........any info would be great.

 

[MikeHalloran]

The phrase "rolling plate" by itself often means rolling it hot between two rolls to reduce its thickness and increase its length.

You seem to be talking about "rolling cylinders from plate", between three or four rolls, and apparently whoever is figuring the sheared size is doing it less than perfectly right.

You can figure it out yourself, by computing the length of the neutral surface of the rolled cylinder. It's not a surface at all, but a theoretical plane in the flat plate that doesn't change its length on being rolled into a circle. Here's where it gets tricky; the plane lies somewhere between 35 and 55 percent of the thickness from the inner surface, depending on hardness and some other properties. First approximation, often good enough; halfway through.



Mike Halloran
Pembroke Pines, FL, USA

 

[metengr]

Do you mean width of the plate? If so, the necessary width is based on knowing the finished circumference (C=2*3.14*R) of the pipe and making sure to crimp the edges to avoid a flat spot. Seen this operation myself during an audit of a pipe mill.

 

[Lizzyrocks]

Ok, let me clerify. Lets say we need to build a 30" fitting. One half will simply be a piece of metal, the other half will have a nozzle welded in it. But the two pieces will be cut in the flat from a larger piece of plate. The two pieces will then be put in a roll to form them to a 30" forming diameter. but when I fit check them on a piece of pipe before they ship I find they are uasally to long " the two halves will touch before laying flat on the pipe.

We need each half to lay flat on the pipe with a .10" gap between them for welding purposes. What would be the formula for figuring that out?

 

[metengr]

So, if I understand your clarification above you are fabricating a clam shell welded sleeve on an existing pipe circumference. You need to roll form the plate to a slightly larger circumference to sit on the existing pipe and have 0.010" clearance between the halves for seam welding.

Because the ends of the split sleeve touch and not allow it to sit on the pipe means you do not have the correct bend radius for roll forming. If you lay this out using a cardboard template as most old time shops used to do you can determine the larger bend radius for roll forming and from there determine the cut length for the flat plate so that the split sleeves can sit on the pipe and have the clearance for welding.

 

[TJOrlowski]

Is the .1" clearance a root gap for a butt-weld that will join the two halves? Or is the .1" clearance a radial clearance for the sleeve over whatever you're putting it over, as metengr said?

Does "fit" mean the clamshells are too long, too short, or the clamshells are not square (inconsisent gap)?

If the .1" clearance is for a root gap, here's what I would do:

The cut length of a 360 degree cylinder is 2*Pi*R, but R is to the centerline, so you have to add or subtract half the thickness of the plate, depending on whether you're calculating by the OD or the ID. If you need to roll a cylinder out of 1/2" plate to an inside radius of 15-1/16", your formula for a full 360 degrees would be

2*Pi*(15.0625+.25) = ~92.21". From there it shouldn't be too hard to figure how much to take out to yield a .1" gap on each half. You could do it by either calculating the degrees of arc per cut inch of plate, or cut inches of plate per degree arc. But this is all a THEORETICAL exercise in determining the proper cut length.

From a practical standpoint, you have to look at how accurate the cutting process is, how accurate and consistent the rolling process is (you can get different size cylinders when rolling with the grain, against the grain, or across the grain in the same size plate). You also need to know whether your cutting or rolling operators are adding a "fudge factor" on their own. Nobody wants to be the guy that cut a piece of material too short and has to answer questions about why material is being scrapped (or at least shelved and new material needs to be cut).

At the end of the day, you're fabricating, not machining. That's why we have heavy gloves, facemasks, and grinders.

-TJ Orlowski