Deep Hollowing Basics: Review by Carl
“There is no one way to do anything”. (Carl) . “But there is a best way”. (Ron G).
Your experience and budget are always in play. Mostly budget ! You can rack up some large costs for full systems of stabilized deep hollowing set ups, laser or video systems and add some purchased or home made cutting tools and boring bars.
I have included some pictures of brand name devices and have identified them as such. I do not endorse any particular company or product specifically, and all images and references are for example only. There are MANY companies making various of the deep hollowing rigs. Shop around and check on YouTube also for home made devices.
Cutters: To my knowledge, all deep hollowing is done with some form of a scraper. The very topmost part of the work might be available for a gouge, but as your tool needs to “disappear” into a hollowing, you MUST switch to some form of scraper. More coverage of scrapers will be added over time.
Typical 1/8″ x 1/8″ HSS cutter. These are cheap and readily available since they are used in metal lathes, without the curve grind. In this set up the cutter head holder can be easily replaced with more of less angle mounted cutters. The cutter extension from the head is easily adjusted, as is removal of the cutter for sharpening. Typical scraper with burr sharpening is used.
Image shows the boring bar and cutter head with laterally angled cutter as placed inside the turning. An main inside bore out was done earlier with a Forstner bit, driven to a reasonable bottom depth. Remember to accommodate the sharp center tip of your bit, or grind them down if your tailstock quill is sturdy. Once a Forstner Bit is started, it self centers anyway, even without a point. Turning proceeds with shallow cuts made repeatedly starting from the rim. The rim inside is finished first, clearing material towards the base over time, ALWAYS clearing towards more material and strength. A smooth interior can be achieved with even and smooth movement of even these 1/8″ cutters. Larger diameter scrapers can be used for final cuts, including cutters ground with negative rake for the finest finishing passes. Remember to finish the rim most material first, as if you go really thin, you can not go back. There just will not be enough strength to the piece to hold up to the lateral forces applied again at the rim. A steady rest may be added and may help, with a wheel directly in line with the cutter to counter the lateral forces.
There are three main methods of holding a tool for use in deep hollowing:
- Free hand hold of your favorite deep hollowing tool(s) over a normal tool rest in your banjo. Note that this is the hardest technique for anything but “shallow or small” deep hollowing. Catches and blow ups occur when you least expect it, especially at that “one more cut”. The handle of your cutting tool should be a minimum of 4 times the length of the cutter overhang beyond the tool rest support. The “heavier” the shaft of the tool, generally the more stable. 1/8″ HSS cutters take less of a bite and have a lower chance of a catch or dig in than larger cutters.
- Captive Tool devices: besides final rest at the cutting end on the tool rest in your banjo, there is a “captive” mechanism towards the rear of the handle of your tool. The tool rest and the position of the rear captive set is made to be level on a horizontal plane, and set to have your actually cutting edge at the mid-point of the lathe shaft/piece being hollowed. The captive “cage” can be rolling cylinders, to plain steel bars. The entire captive mechanism is fixed to your lathe bed with a bottom plate and through bolt, though a few may mount to your tailstock. Here are links to a couple captive tool set ups. They can also be homemade pretty simply.
Carter Very Large Deep Hollowing Captive Tool Set up. Shown primarily to demonstrate the parts involved in typical captive set ups. Note the addition of a steady rest: essential for most long section deep hollowing where lateral forces can be very large, and may rip your piece out of a chuck. When possible face plate mounting may be more secure if able to be incorporated into your pieces final design.
Jamieson System: uses your banjo and tool rest at the working end, and adds a captive “cage” and torque stabilized cutting tool holder. Laser system or video system can be mounted as illustrated above with laser.
3. Articulating deep hollowing devices: These use a series of bars or blocks to rotate the attached cutter into and around the hollowed form. The boring bar is attached into the top articulator, and the entire assembly needs to be adjustable to the cutter being at midline, just like the captive devices. The articulation should be smooth and free flowing, and enough “reach” needs be present either with the series of articulating bars or blocks, or by moving the entire jig into the piece, once the top most hollowing is accomplished, by sliding the base of the jig inward towards the headstock. As with the captive tool jigs, the base is fixed to your lathe bed with a bottom plate and through bolt, though, less commonly, some may use the tailstock quill to fix the back end of the articulation in place. Here is a typical articulated set up: picture courtesy of Trent Bosch Tools. This set up actually ” captures” the tool at the handle making the entire tool end and handle part of the articulation. Available for different diameter boring bars up to 3/4″.
Home made articulating deep hollowing rig: solid aluminum blocks with inset bearings on 1″ shaft pivots. Fully height adjustable to center cutter on 1 1/4″ shaft into base and beneath bed mounting slide. Post upward holds the laser or in this case, camera. To the left is the boring bar with cutter at the tip.
Now: besides the jig, there is often a desire to visualize how much wall thickness you have left when hollowing. It is really poor form to have your tool abruptly appear out of the hollowing and truncate, sometimes dramatically, your beautiful hollowing.
Most often used are laser pointers. Lasers are aimed from directly above the turning and measured a desired distance away from your cutter. Once the laser beam “falls off” the side of your turning, you are at that desired wall thickness. Laser set ups are inexpensive and easy to mount on almost any deep hollowing set up.
Another method uses a small bulb attached toward the end of your cutting tool. With a bright enough light, you can watch the “color” of the turning from the outside, awaiting glow of the light through the wall of the piece. By achieving a constant degree of glow, a uniform thin thickness can be obtained. Works best on light colored wood and takes some getting used to, but simple and inexpensive.
One “advancement” on the use of the laser is the video camera and a separate view screen. The camera gives a “virtual” image of where your cutter is within the piece, while simultaneously monitoring the outside view of your turning piece.
Deep Hollowing Video Set Up:
Video system with 24″ screen and long, 1″ boring bar. Piece on lathe is Norfolk Island Pine. Ergonomics are perfect to handle the cutter, watching the screen, not bending over, do difficult visual angles. Screen is on a grinder base and is easily moved to best viewing while cutting position (or cutting while viewing?). Cutter head and camera may need to be realigned occasionally to ensure that “setback” remains constant. If different cutting surface is used (different angle of approach), the cutter, set back and camera can be easily realigned by moving the transparency sheet on the monitor screen. Hence, only use of a few pieces of painters masking tape to tack the transparency sheet to the screen.
Draw on a clear plastic “overhead projector” sheet. The black indicates the actual location of the cutter and the red indicates the thickness desired. Cut out towards the outer contour until red edge hits the outside contour of the turning. Easy !
Note that the camera is held over the cutter and moves exactly the same amounts as the cutter itself. The desired turning thickness is set by the offset as indicated in red.
Here is a neat YouTube on various uses of video systems for woodturners, including, but not limited to, deep hollowing.
This is the camera that some of us are using and like for this application. Note that it is analog, with BNC or RCA video out connection and not HDMI or other digital. Your screen needs to take old style yellow “video in” jack. Screen size and mount is your choice. A minimum of a 16″ Flat Screen is useful. More than 24″ is overkill ! The camera should be mounted just like you would a laser pointer, solidly on your moving tool arm, to follow the cutting edge of your cutter from above. We will post a full description and pictures of the set up soon. Feel free to shop around for this camera and the Swann site itself is just a suggestion. It is TINY ! light weight and very easy to mount for this application. It is inexpensive, but may not be readily available. Other camera’s can be used for this application, but this swann minicam has a 54 degree angle of view (narrow) which is ideal for this application. Most security cameras are wide angle.