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I'd like to build an exploratory robot that can fit within confined spaces1.

As the robot moves along the inside of a 4" pipe, it's likely to encounter some obstacles that need to be removed. Typically, that will be tree roots up to 1/2" thick. There could also be globs of tissue paper caught by the roots.

The majority of the pipe is made from clay, but a small portion of the pipe is cast iron. The portion that has the obstacles is clay and was laid in the early 1960's.

The pipe to be traversed will likely have fluid (assume water) within it to varying degrees from partially full to completely full. I'm initially planning on tethering the robot with coaxial cable and possibly an additional line like a steel cable. The propulsion mechanism is still being defined, but I expect that the robot will have some ability to fix itself in place while clearing out an obstacle.

The typical way of removing roots and other obstacles is through using a drain cleaning system. The cutting head is usually a pronged piece of metal that can burrow through the obstacle. The problem with this approach is that the pipe is damaged as part of the cleaning process and some amount of roots are left behind.

Another commercial approach that completely clears the line utilizes a spinning wire brush. The downside of this approach is that pipe wall is severely damaged as part of the cleaning.

I would like to devise a cutting mechanism that doesn't significantly damage the pipe but also removes the majority of the obstacles encountered.

What type of cutting, shearing, grinding, etc... mechanism should I consider in order to clear out the path in front of the robot as it explores the line?

1 It's a rather boring application, actually. I want to see how badly the roots are creeping into the sewer line from my house. Ideally, I'd like to clear out the roots too without damaging the clay pipe.

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    $\begingroup$ It's even worse than you think. After you've cut the obstacle you have to move it out of the way somehow which can also be a challenge in confined space. $\endgroup$ – sharptooth Feb 5 '15 at 11:59
  • $\begingroup$ @sharptooth - that's likely my next question in the series if I think there's any other valid answer besides having a stream of water running at the same time. $\endgroup$ – user16 Feb 5 '15 at 13:41
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You should be able to create a non-damaging cutting system if you integrate it with your drive system to control cutting element location. For example, if you use 3+ rubber wheels evenly spaced around the diameter of the device you should be able to reliably fix the device in a static location relative to the pipe walls.

I would recommend 4+ high-speed grinding-type cutters place at say the 0, 90, 180, and 270 degree points. Grinding is slower than cutting but you can remove a larger area of debris more safely than with a cutting device. A grinding device also simplifies debris removal (should wash down the drain). As an alternative, you could have a single (optionally pointed) large grinding tip attached to the front of the device.

In either case, allow at least a quarter-inch clearance between the cutters and the wall to prevent damage at turns. Make sure that the cutters do not extend too far in front of the wheels or you will need to increase your clearance to compensate based on anticipated pipe turns. You can adjust the clearance based on your tolerance for potential damage, but with clay pipes I would be very conservative!

Also, if you want to avoid the additional engineering challenge of drain traps, you should plan to insert the device directly into the pipe below a toilet. Although, since you mention a 4" pipe, perhaps you will be inserting directly into a cleanout on the target pipe.

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I would suggest a Dremel-like cutting wheel positioned at the front which can be rotated around 360˚ such that the wheel stays parallel to the inner surface of the pipe. The small diameter of the cutting wheel will mean you can cut quite close without damaging the pipe. This is assuming you will have a camera and light on the front to see where to orient the cutter.

Another approach would be to have a hole-cutter type blade on the front, mounted just inside another slightly larger cylinder to prevent the blade from hitting the inside surface of the pipe as it moves when biting into roots.

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