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Bob Gerenser

DIY hollowing camera set-up

Jul 7, 2016

Bob Gerenser

gerenser 2016-07

Bob Gerenser demonstrated a high tech solution to aid in the hollowing process.

This article written by Mike Lanahan appeared in our 2016 July issue of Chips & Chatter.

Bob Gerenser demonstrated a high tech solution to aid in the hollowing process. Experienced turners like Mahoney or Jackoffsky, who have 1,000’s of hours hollowing experience, have developed a sense of where the hollowing cutter is within the turning. Trent Bosch developed and sells a system he calls the Visualizer, which uses a video camera in place of a laser on a captive hollowing system, and lets the turner, regardless of his hollowing experience, “visualize” where the cutter is within the piece being turned, much as very experi enced turners have developed this ability. Trent Bosch’s Visualizer sells for $650, and is a great idea. Bob put together his system for about $25-$30 but was reluctant to publicize it, and cut into Trent’s intellectual property marketing, until an article (“Frugal Camera Hollowing Rig” by Tom Schneider) in the online e-zine More Woodturning, August 2015, let the cat out of the bag. 

Why a hollowing aid valuable? Well, if it is not obvious, while hollowing you most often can’t even see the cutter. Bending over and trying to peek inside is uncomfortable, leads to poor ergonomics, and is futile, regardless of the natural temptation to do this. It also gives little or no clue to wall thickness. 

Doesn’t a laser do the same thing? The use of a laser is a great help to knowing where the cutter is within the piece being hollowed. The limitation to a laser is that it is a point of light, that is adjusted to only one point on the cutter head, or the desired wall thickness away from a point on the cutter. As the hollowing moves from the neck, to the side and to the bottom, the position of the laser needs to be readjusted to maintain the desired reference to the contact point on the cutter. 

How does the video system work? Replace the laser with a small video camera (Bob uses an endoscope camera, see equipment reference list, below) which is in a fixed position mounted above the cutter in a captured hollowing system (Bob’s system is The Monster). Feed the video signal to a monitor or laptop, and adjust the position so the cutter is visible roughly in the center of the screen, and secure the adjustments. Position the camera such that the image on screen is one to one scale with the real world. For instance, if you are using a 5/8” diameter cutter, the image on the screen should also be 5/8”diameter. As the captured system is moved around, the cutter remains stationary on the screen as everything in the background moves around. Use a dry erase marker to outline the cutter on the screen of the monitor/computer. Also mark the desired wall thickness around the outside of the cutter, if desired. Now when the cutter goes into a piece being hollowed, the surface and edges of the piece being hollowed are visible on the screen, and the outline of the cutter and wall thickness are visible as well, allowing the cutter to be “visualized” within the hollowing piece. Keep hollowing until the wall thickness reaches the edge of the turning, and presto, you are there. And Bob’s your uncle. 


  1. Check the position of the cutter on the screen relative to the marked outline often as the cutter is removed, to clean out the shavings inside. If the adjustment gets loose, or gets knocked out of alignment, you are worse than flying blind. 

  2. Make sure you don’t use a permanent marker! Seems obvious. Of course you can tape transpar ent sheets to the screen, and mark with anything you like. 

  3. Have the screen close to the turning, and pointed in the direction of the headstock is the most natural and ergonomic placement. If it is far off-axis, just because there was a table to hold the monitor, it is quite disorienting. 

Thanks to Bob for demonstrating his cost saving hollowing aid. 

Equipment List 

Video Camera:

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