Technical Question on Roll Bending

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STRR
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Technical Question on Roll Bending

Post by STRR » Fri Jun 15, 2012 8:42 am

I have a question on the technical aspects of a pyramid type, 3 roll bender. I know someone here, has the knowledge I am lacking.

I have seen many different 3 roll bending machines. My question concerns the distance between the 2 bottom rollers (dies). It would appear the rollers are spaced for the effect they will have on the radius being bent. Being farther apart, would provide for larger radii, while being closer together would have the opposite effect and make the radii shorter. I am sure there are many contributing factors such as material type, dimension, and profile. All of these factors provide infinite complexity to the problems of bending.

I am positive there are formulas for calculating the effect & results in different spacing of the lower roller dies. Does anyone have SIMPLE formulas that do not need an engineering degree to understand and use? A simple logic based explanation would be great if simple formulas are not available.

Thank You,
Terry Miller

f350ca
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Re: Technical Question on Roll Bending

Post by f350ca » Fri Jun 15, 2012 8:55 am

Terry
On slip rolls for flat stock, its the position of the 3rd roller that determines your radius. The distance between your fixed rollers affect the downward pressure the third roller has to impart to bring the metal to the yield point while bending it. The further apart your fixed rollers are the less pressure to bend. Increasing the diameter of the rollers gives more tractive force.
Greg

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Bruce_Mowbray
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Re: Technical Question on Roll Bending

Post by Bruce_Mowbray » Fri Jun 15, 2012 10:00 am

Terry,
It is the diameter of your "top" roller that determines the smallest diameter you can roll. The wider spacing of the bottom rollers will give you more capacity in the initial "pinch" but once the material starts to bend, with subsequent passes, the forming rollers gets closer to the pinch roller and die roller. Except for a chart that shows how much force is required to bend metal of a certain length around a selected diameter in one pass, the information you are seeking depends on your roller diameter and the type of material you are bending and the capacity of you machine.

On the three roll machine, each roller has its purpose. There is an idler (generally the top) roller, which is the actual "die". This roller is usually fixed except in "slip" machines where it can be lifted on one end in such a way to remove full round rings. The pinch or drive roller (usually in front), which is the roller that pinches the material against the idler roller and forces the material through the machine. This roller is adjustable to accommodate different plate thicknesses. And, the forming roller (usually in back), which forces the material around the die roller. This roller is also adjustable from making no contact with the material being bent, up to full contact with the die roller (less the material thickness). When the pinch and forming rollers are at their maximum travel, they essentially just have enough clearance to turn without touching.

The "spacing" that you mention is determined by the design of the machine and machine size/capacity, limited only by the size of the rollers. In the manuals I have seen, including the one for the 10" Bertsh plate roll that I use, there is mention of the operator making a material characteristic "reference" chart to follow as the operator rolls different metals. This will reduce the about of trial and error in future operations. Even 2 plates of the same material and thickness will roll slightly different even with the same settings.
Bruce Mowbray
Springville & Southern RR
TMB Manufacturing & Locomotive Works

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Charlie W.
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Re: Technical Question on Roll Bending

Post by Charlie W. » Fri Jun 15, 2012 10:37 am

Your question is a little like when someone ask " how big is big enough"? It is difficult to answer without a lot more information. Bending can be accomplished many different ways and the answer is dependent on many factors.

About 40 years ago I built several, old fashion, High-Wheel bikes. I had to bend the rims and frame.

Image
Image

I made a 3 wheeled roller to do the bending. I was 18 years old and only had a 10" Logan lathe and Delta drill press back then. It was made from junk I had laying around so it is not an optimal design but it worked fairly well. It had one adjustable roller. The stock was sent through the rollers and one roller was adjusted a little tighter and another pass made. It took about 5 passes to get the rim formed into a circle. Don't forget that the stock needs to be a little longer on each end because when the stock comes off of the roller it leaves a straight section at each end of the piece. Even though you might be able to have 3 fixed rollers, It is easier to bend the stock using multiple passes since there is spring-back in the stock and it is difficult to predict the curve you will get. My roller had an adjustable center roller but I have also seen one of the end rollers adjustable. The end rollers were comvex and the center roller was concave to fit the rim shape.

Image

Image

Charlie W.
Last edited by Charlie W. on Mon Jun 18, 2012 10:07 am, edited 2 times in total.

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tornitore45
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Re: Technical Question on Roll Bending

Post by tornitore45 » Fri Jun 15, 2012 12:57 pm

Formulas?

If I ahd a roller mill am had to bend one hoop...
I would progressively bend until I got the radius desired.
If needed 100 I will do the second with the first setting an see, pretty soon they will all come out the same.
Mauro Gaetano
in Austin TX

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Bruce_Mowbray
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Re: Technical Question on Roll Bending

Post by Bruce_Mowbray » Fri Jun 15, 2012 1:03 pm

Be careful about doing the second one at the final setting of the first. A sheet of steel will bend more if it is done in one shot than if you were to do it in multiple bends. Each bend work hardens the material a little bit. So, after multiple cycles of bending and releasing, you acutually make the metal springy (non tech term) and the metal springs back. Do it in one bend cycle and it is not as springy and it ends up bending more. Leart that one the hard way. :wink:
Bruce Mowbray
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TMB Manufacturing & Locomotive Works

STRR
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Re: Technical Question on Roll Bending

Post by STRR » Fri Jun 15, 2012 1:10 pm

Thank You for all of your replies.

f350ca, I understand your comments on ease of bending and size of the lower idler dies. Concerning minimum radius - the farther apart the lower dies are, the minimum radius attainable, will be larger than if the lower dies were closer together. An extreme example would be: you'll never be able to get a 1' circle, if the lower dies are 2' apart. While the larger distance would certainly make bending easier, per pass, it would also take more travel from the upper die to get the bend due to the flexing vs. bending of the metal across the span of the lower dies. Farther apart will also make longer straight/flat ends before the bend.

Bruce, You are correct with your comments. But, we are talking about two different roll benders. You are referring to a Pinch roll and I am referring to a Pyramid roll.

Charlie, How did you decide on the distance between your two fixed roll dies? How did you decided on the diameter of those same dies? Your work is extremely nice and I would have never guessed the builder to be only 18. WOW.

Charlie's bender is "upside down" from most pyramid roll benders. It really doesn't matter since the orientation does not affect the bending process. I have seen several pyramid roll benders with adjustable bottom roll dies. I am assuming the different settings would be for different materials, different profiles, or more likely different radii bends. Making multiple passes is definitely the best way to get the most accurate radius. I'm just trying to find some way to compute the best dimensions for my needs rather than by trial and error.

Thank You All Again,
Terry

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Charlie W.
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Re: Technical Question on Roll Bending

Post by Charlie W. » Fri Jun 15, 2012 2:47 pm

STRR wrote: Charlie, How did you decide on the distance between your two fixed roll dies? How did you decided on the diameter of those same dies? Your work is extremely nice and I would have never guessed the builder to be only 18. WOW.
I didn't take a very technical approach to building the bender. I found 2 scrap plates and just placed the three rollers where they seemed to fit well and looked about right. The rollers were turned from some square stock I found somewhere. I made 7 large rims and 7 small rims with it. It was pretty crude but did the job.
STRR wrote: Charlie's bender is "upside down" from most pyramid roll benders. It really doesn't matter since the orientation does not affect the bending process. I have seen several pyramid roll benders with adjustable bottom roll dies. I am assuming the different settings would be for different materials, different profiles, or more likely different radii bends. Making multiple passes is definitely the best way to get the most accurate radius. I'm just trying to find some way to compute the best dimensions for my needs rather than by trial and error.

Thank You All Again,
Terry
I am not sure what you mean by upside down. The roller can be mounted in any position you want. I bent the 1st frame with a carriage wheel steel-tire bender. The other frames I made were done with something sort of like a conduit bender. I mounted the bender to a fixed object, place an extension pipe over the frame tube and bent it a bit and worked my way down the frame. I made a wood pattern/gage to check the shape. It was a a trial and error process.

Since it was a bike rim, I drilled all the holes for the spokes before bending it. If I had been more experienced at the time I would have added about 4 exter inches on each end where it was straight so I could cut that section off. Instead it was made to length. I pushed down on the rim to form an oval and welded the two ends together. After grinding the welds, I opened up the bender, placed the welded rim in it and bolted it back together. Then it was a simple matter to crank the rim through and it shaped the 2 straight sections to for a nice round rim.

By the way, I use to ride it. This was taken a few years after I made it. My 2 sons were over a couple weeks ago and rode it. The first time it had been out in several years.

Image
Charlie W.

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Re: Technical Question on Roll Bending

Post by STRR » Fri Jun 15, 2012 3:29 pm

Charlie,

You can say "crude" all you want. Your finished product is spectacular. I am a bicycle fan, more or less, and that high wheel could have come straight out the the 1890's.

My comment about upside down was meant for others trying to understand what I am looking for. The most usual configuration is the adjustment and adjustable roll, being on top with the two fixed rolls on the bottom. As I said, it doesn't matter what orientation it is in. The important thing is the relationship between the three rolls. Your "trial and error" sure produced as you wished. You discovered the inherent flaw of pyramid roll benders: Flat/straight ends. Your solution was a perfect one. Unfortunately, I am going to have to work out that problem. Thus, the questions as to the distance between the lower rolls. Needing to get the desired radius with the smallest/shortest straight ends.

Thank You for your contribution. It is a pleasure to read your comments and see your photos.

Good Luck,
Terry

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Re: Technical Question on Roll Bending

Post by f350ca » Fri Jun 15, 2012 7:07 pm

Terry
What are you wanting to roll?
This is one I built a number of years ago. Think its has 12 inch long by 4 inch dia rollers. Thats 5/16 plate I'm rolling in the photo. Can take some measurements and photo's if this is the sort of machine your after. It leaves about 3 inches at the end of the curve, if Im rolling rings I just leave the material long and cut the straight sections off.

https://lh6.googleusercontent.com/-Mhoo ... GP1255.jpg
Greg

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Re: Technical Question on Roll Bending

Post by atomarc » Fri Jun 15, 2012 8:37 pm

Here is the one I built!

Stuart
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roll1.jpg

STRR
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Re: Technical Question on Roll Bending

Post by STRR » Fri Jun 15, 2012 10:07 pm

Thank you for the photos. Two very nicely built benders. Both of these show how closely you can place the lower roll dies and obtain a very tight radius bend, with very little straight left on the ends.

To answer your question; I am bending rail. Lots of it. 8 lb, 10 lb, and 12 lb. I believe I will have to roll the curved pieces into a uniform radius and then reverse the bend to straighten them out. When I begin to lay the rail, I will have to bend in spirals and constant radii curves. I am planning on three sets of die rolls or maybe one set with the capability of adding form "washers" to obtain the proper profile for the larger size rail.

Now you see why I have asked about the distance between the lower dies. Large radii and different sizes of material. I may be able to get away with one distance setting between the lower dies but it may be better to have up to three settings. One for each size rail, as each size will bend differently. Not to mention, all the rail was not made by the same manufacturer nor have they endured the same aging due to weather and use. This could be a nightmare. I see a lot of "trial and error" with mostly error being the result. I'm trying to hedge my bets by building the bender as best I can. Thus minimizing the possibilities the problems are in the machine instead of the operator. I believe I can cope with the ends by using a small hydraulic bender I have already constructed and works with fairly good results. It's just way too small to work as much rail as I have to get done.

With the lack of plans and the few documented bender builds I have been able to find, I will do my best to photograph and document my build for future builders. I know there will be more people like me with the same questions.

Thank you all for your information, photos, support, and suggestions. I appreciate them all and I hope you will not be offended when and if I don't use everything suggested. Please keep them coming. I need all the help I can get.

Cheers,
Terry

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