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Jan Ysselstein

Rolling Mill Question

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I am trying to get an approximate fix on the upward pressure on the bottom roller on the popular Hugh Mc Donald Rolling Mill. As I look at various videos I see what appears to be quite a variety of effort made to push down on the pedal. Owen has also made reference to "flex" in the frame working as an advantage for him.

Does anyone have a guess based on the math of he lever system or any other means of measuring ?

I am gathering some parts for an experiment in rolling hot steel..having an idea of the roll pressure may save me quite a bit of time (and $). The roll tightening mechanism I am thinking of has no give, but only a set gap...am I going to break something or do I ned to build in some flex?

Thank you for any discussion on this topic.

 

Jan

 

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I'd work backwards from what your drive system can produce to figure out how much load that can possibly create (without stalling).

 

hp=(4*pi*aPN)/33,000

 

Where:

hp = horsepower

pi = 3.14...

a = .5* (Roller_radius*reduction amount)^.5 in feet

P = the load on the rollers (the part you're looking for) in pounds

N = rpms

33,000 is from 33,000 ft*lb/min = 1 hp

 

So: P=hp*33,000/(4*pi*a*N)

 

This is for both rollers being driven, as they are in industry. I don't know exactly how it changes for a single roller being driven and the other idle, but I think that it would change the 4 in the equation to something slightly different. I don't think it is going to be a critical difference.

 

Source: Mechanical Metallurgy, 3rd ed., George E. Dieter, pages 613-615.

 

If you have a 2 hp system and 4 inch rollers running at 20 rpm to reduce the thickness by 1/8th inch then that would make the stalling load at 2*33000/(4*pi*(.5*(4/12*.125/12)^.5)*20) = 8,913 pounds. Again, that is to limit out your torque. Actually achieving your reduction may require less work, but this way you won't break your equipment. Hopefully I got all my algebra in there correct.

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Jerrod,

 

Thank you. A reduction of 1/32" per pass would be enough for me as I am only using it as a flattening machine. I can easily forge to about 1/4".....if I can roll the rest of the way, the yield per bar will go way up. So 4 passes at 1/32" each will be enough for me , using 3" rolls. The thickness after a pass ( pass #1) will be 87% of the original thickness...4 to 5 passes will get me to 1/8" even if each pass only reduces the thickness by 13%.

Interesting the "P" goes up as the rpms go down...I would have thought that giving the rolls more time to complete the deformation would have lowered P.

 

Jan

Edited by Jan Ysselstein

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I thought that was interesting too, but it has to do with the hp being static, and that is a measurement of ft*lb/min. The whole thing isn't overly intuitive.

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The other way to get to the pounds needed is to use the equation:

P=(2/3)*(sigma0*width)(roller_radius(change_in_thickness)^.5)Qp

 

Where:

sigma0 is the uniaxial flow stress for the material (good luck finding that in a table)

Qp is found in a chart (page 613 in the above referenced book) and ranges from about 0.8 to 4.0 based on the reduction percent and the ratio of the roller diameter to the final thickness.

 

Generally this is the method used to determine the load, which then tells you how to use the previously noted equations to establish your hp requirements.

 

Given your 3" rollers and 1/8" final thickness and an intended reduction of 13% your Q value would be about 1.0. Your flow stress is likely to be 20-35 ksi. That means your max load via that calculation would be around (2/3)*(35000*width)*(1.5*(.25-.125)^.5)*1.0 = X pounds depending on width. For 1" width that would be 12,375 pounds to do it all in one pass. if you go in 1/32" bites it is only 6,187 pounds.

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Thanks Jerrod,

 

Based on reducing to .87 of the original thickness I will be taking 29 degree (correction 15 deg. per roll) angle bite into the metal as explained in this PDF on rolling.

http://eng.sut.ac.th/metal/images/stories/pdf/03_Rolling%20of%20metals.pdf

 

That also surprised me as a larger bite than my guess was. I may have to settle for a 10% reduction per pass. The bearings will take it but we will see about the rest.

Jan

Edited by Jan Ysselstein

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Jan,

 

I built a roller a few years ago using the Mc Donald plans as a guide and adapted the design to suit the steel I had available. I don't have a copy of the plans any more but I seem to remember that using his design for the foot lever, ie the way its set up and the length of the lever, I and sure he mentions that there is a 60 / 1 advantage. Another thing he mentions regarding the lever is to set it up so that when its pushed all the way to the floor the forces are directly pushing up through the bar to the bottom roller, otherwise you are using all your strength to hold the bar down. I use mine by taking small bites at a time, probably in the region of 1mm.

 

As Owen mentions flex in the frame which is not a bad thing or you start to break welds or bolts, depending on design. I built mine for drawing out sword blades and with a bit of practice you can taper these as well.

One thing that did surprise me was that I always shot blast my steel clean before grinding and the scale from putting a bar through the roller is much harder to remove than from the power hammer and thicker.

 

Here is a link to some photos and spec's, http://www.britishblades.com/forums/showthread.php?174695-Rolling-mill

 

Mick.

Edited by Mick Maxen

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I no longer have a rolling mill as My one ended up with a friend, I should make another they were very handy bits of kit.

The mill I had generated around 6 tonne (6000kg) of upwards force from my weight acting on a 60 to one lever, and though the mill did theoretically lock out at full force, in reality some of the sideways force would bypass the lock out and could lift me off of the ground. the sideways generated forces from rolling are beyond my maths.

With the smaller rollers of the mc donald mill the rollers will tend to slip on the steel (2" rollers), my mill had slightly bigger drive rolls (3")and it would jam and break something back up the drive chain, I ended up having a sacrificial weld that would break when the mill jammed.

The parting force generated by pulling a wedge (for example a cold spot) into the rollers could be quite massive, industrial powered mills are big bits of kit.

speed is also quite important, needs to be fast enough to not cool the steel but slow enough for you to feel happy and safe with it. I cant remember the rmp exactly either 22rpm or 26 rpm at 3" so 3 to 4 inches a second, a bit faster would be fine.

good luck they are great tools but have their risks.

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Thank you for the replies..this brief conversation has started to clear up some of my erroneous concepts..I hope it goes on. I am going to attach a photo of someones rolling mill ( right now I don't know who, but I will find out and give him the proper credit)...this mill shows the method of roll gap adjustment I am planning to use......any suggestion why not are welcome. I did change the angle of the bite to 15 degrees because there are two rolls doing the work . My rolls will only be 3" wide for the test, most bars end up about 2" wide.

Mick that scale phenomenon is a bit scary...maybe a little flux can stay...very clean build by the way.

DSCN0866_0008roll copy.jpg

Edited by Jan Ysselstein

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Jerrod,

 

"This is for both rollers being driven, as they are in industry. I don't know exactly how it changes for a single roller being driven and the other idle, but I think that it would change the 4 in the equation to something slightly different. I don't think it is going to be a critical difference."

 

The initial test will have no rollers being driven..the material will be pulled through the rolls by holding on to the cooler end and rolling the hotter end. The heat loss experienced by the material just resized should make it ( may make it) strong enough not to yield. Very much like a drawing bench. I may have to oil or moisten the rollers on the go, to get enough cooling. I realize as I get thinner and thinner this method has less chance for success...let's see what happens.

IF this does not work I will go to driving the process by powering the rollers with chain or belting.

Jan

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I have an old chart that may be of use.

Tensile strength of hot steel.jpg

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Thanks Ric,

 

I have experimented with plastic modeling clay in my jewelry rolling mill (junk) and it worked at about 1/2"..the friction of the sleeve bearings is large enough to become a factor with very thin material.

Jan

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In my limited experience with rolling mills I think you may find that you're not only going to need driven rollers, but a bit more torque than you could accomplish manually pushing a bar through. I've never hot rolled though.

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Jerrod,

"pulled through", looking at the above chart the material on my side of the rolls would about 2x as strong as that which needs compresing...

The other thought is ( looking at all this math ) what if it does not work but almost does...what would be the best action...drop the idea or something short of a chain drive.

 

The angles are now at 15 deg and the real presssure will be at about 7 degrees. My just told me the rolls have arrived.

 

Jan

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I meant pull, yes. I just know that the cold rolling I have done in the past was driven and I could not have stopped it manually once the rollers got a hold of it. And the machine seemed to be working hard. I've been working on a rolling mill design for home building (fixed/adjustable gap like the image you posted above, not the foot actuated type) and will be putting a hand crank option as well as the motor. I think the hand crank spinning even one roller will be easier to pass the metal than pulling it through. I could be wrong though. I actually hope that you find that I am wrong, that would be quite encouraging for my project.

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The mill I use most is a 4" dam x 6" wide at 33 RPM driven by a 7.5hp motor.

Torque estimated at 1300 foot pounds. I can stall it with some materials taking a big bite.

I have other larger mills, but this works well.

 

Ric

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Jerrod,

 

For the test, the pulling mechanism is a 2" hydraulic cylinder.( 24" long ) .once a speed gets established and the pulling force ( based on the psi of the fluid ) I may go to a screw type actuator . The rollers look good, the bearings are not what I expected and may eventually need to be replaced by needle bearings. The rollers are basically a piece of DOM, machined smooth and machined for the bearings to fit.

 

If all goes well, I should be able to get a 45" ( the calculation says 56 but there are always losses) bar from a 1200 gram melt 1.5" wide and 1/8" thick....do I hope this thing works, yea. I will post a pic of some of the parts I have collected. So the sequence will be, heat the metal and make sure there is a "cold zone at the close end....bring down the top roller into that transition zone( cold to hot) )...then begin pulling immediately ( and hope nothing breaks).

 

Jan

Edited by Jan Ysselstein

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I was fortunate enough to talk to one of the guys that built the rolling mill I was using (again, cold rolling) and he cautioned to never use bearings, always bushings. He said he has seen bearings get dimples from metal being put in quickly and that is basically like hammering the rollers apart. The dimples in the roller races then really hinder efficiency. Sounded like the voice of experience when he was explaining it. Very nice of him to tell me too.

 

Pulling with a hydraulic cylinder...Good call. B) I'm sure that wouldn't have occurred to me before tying a bunch of other bad options first.

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Jerrod,

 

That is good to know, thank you. This rolling process has to start at a low spot ( thin spot ) on the bar ( these are usually at the bar ends ), I don't think I can indent the hot bar to create a low spot with a manual screw...but I would like to have that feature. I may have to create a starting "groove" on the press, then go to the roller. Maybe the top gap control will have to be a short stroke hydraulic cylinder ( bushings) and depend on stops for depth control ( no distal taper here ).

 

Jan

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