flat sheet layout of helix?

Our team is using a helical “trough” to guide balls into the gun loading zone. The sides of the trough are easy to layout but the bottom is difficult to do correctly. We have not found a way in Inventor to construct the flat panel layout of the helix - does anyone know the correct method?

An image of the trough with balls is attached.

Note: laying out the helix as concentric circles with a slit works reasonably well but does not fit with our team motto: “Good enough is NOT good enough”

Bob Holmstrom - Mentor - Team 1432
Portland, Oregon

Suggestion: change your team motto.

To quote an earlier post, good engineering often comes down to knowing the difference between “better” and “good enough.”

You have a workable solution to one specific problem, a plethora of other potential problems waiting to be solved, and a finite amount of time and resources. Solve the problem you can with the solution at hand (laying out the helix as concentric circles with a slit), and move on.


I might be wrong but, I believe the sheet metal environment is capable of unfolding only those shapes that can be made with a brake press. Straight-line bends only like and electrical enclosure. Here’s an idea: make your chute from a double-helix of flexible tubing. Easier to build and lighter as well.

Reply to Dave,

While I agree with your response, the motto came about because no one seemed to be able to make a part that resembled the drawing - mistakes and poor workmanship were asked to be accepted as “good enough”.


Reply to Henry,

Inventor can “unroll” a tube that has been truncated at an angle into a flat drawing, so the limit does not appear to be just press brake operations. I was hoping that some knew how to flatten a helix because we have a CNC panel router that can make complicated curves as easily as simple ones.


The sheet metal app of Inventor should have a flatten function for making part drawings, mainly so drilling and the likes can be done on complex shapes before putting into a break.

You should be able to select this funtion and it will correctly flatten your helix. I’m not currently on a computer with Inventor, But once I get home I can check for you exactly how to go about flattening a helix.

Reply to Bob,

OK, I was wrong. I just flattened a helix in IV6 that would be similar to the sides of your chute. Turned out to be just a parallelogram. I could not do the bottom though. I think you can flatten the helix if the face you want to flatten is concentric with the axis. This makes sense to me. If the face was perpindicular to the axis and you had more than one revolution, flattening would result in the object colliding with itself so this might be a built in limitation although I have not tried this in version 10.

Please reply if you find a solution.



It’s easier to make a whole new part for this one…
Or you can try making it in as a sheet metal part, but I don’t know how exact you are going to be able to make it.

But! This is how I would do it:
Measure the length of the outside and inside arcs. Make two concentric arcs in a new part file and then spread them apart so the distance between the two arcs is the width of the channel. Then constrain the arcs so their length is the same as those measured. Inventor has some strange constraints so try a few things before you actually make the part. Then you should be able to extrude it and vwala, you have your part!

I have no clue how well this would work until I actually tried it, but this would be my first choice.

And if you can’t find the measurements or the arc length then could you post the radius and height difference all I can get back to you on how long they are.


I believe you have the solution. I made a helix 1" wide with an axis offset by 1" with the coil distance of 3" (I think - it really doesn’t matter for the sake of discussion). This results in a coil with a 4" diameter. The outside edge measured something like 12.9 somthing. A 4" diameter circle has a circumference of like 12.56. So I think what you suggest would work.

DJ and Henry,

Thanks for your help on this task!

Our helix is 26.5" outer diameter and 7.5" wide (inner diameter 11.5") with a pitch of 11". If I draw the helix in Inventor, and measure the edge lengths I get the following:

outer edge > 83.976 inches (a 26.5 inch circle = 83.252)
inner edge > 37.766 inches (a 11.5 inch circle = 36.1284)

DJ suggests concentric arcs contrained in length and separation. Arc implies circular and so far I have not discovered a way in Inventor to constrain the length of the arc. But working with it bit, it seems that the arcs may need to be sprial arcs not circular arcs.


Reply to Bob,

“so far I have not discovered a way in Inventor to constrain the length of the arc”

I believe that’s true but you could work backward from pi * d = c

“But working with it bit, it seems that the arcs may need to be sprial arcs not circular arcs”

That’s a brain twister for those smarter than I about these matters such as a die designer. On a practical note, the amount of error will probably not matter that much. However, for the sake of discussion and doing thing right, this is a good question. Brings to mind a spring. Does it change diameter as it compresses and lengthens? I suppose yes, if you deform the length enough.

Am I making any sense at all?

As far as i’m aware, it is mathematically impossible to create a true flat helix out of a sheet of metal. Some bending/stretching may have to occur. It could work to use semi- or quater-circle sections and weld them together at the correct angles. Alternately, you can probably just use strips of metal on the underside to support some cloth or another sheer material.

And yes, the diameters of the cutout will be larger on the flat material than the diameter of the true helix. It’s possible to obtain the measurement using the pythagorean theorm. I can post a pic of how to do this if you can’t figure it out.

Reply to Nuttyman54,

“Some bending/stretching may have to occur.”

That’s exactly why I suggested a die designer may have further understanding of how to develop this part. Metal forming operations often involve stretching metal in a controlled manner. The part can be probably be successfully approximated being built by hand from sheet metal but would not end up as a true helix. As far as functioning for the intended purpose - no problem.