There are many ways to create hollow woodturnings, ranging from the very traditional gouges to some of the more modern implementations of carbide cutters.
With that said, what tools are used is dictated mostly by the types and depths of hollowing being done. A bowl is a hollow turning as much as a lidded box – it has a different aspect ratio, but it is certainly a hollowing. For the most part, hollowing of those types of items is pretty straightforward and is accomplished using the standard woodturning tools.
What about far deeper hollowed forms such as vases, urns and the like? In this two-part series on hollowing, I will take a detailed look into some of the techniques and tools available for clear out cavities for a woodturning project. For those who already own a hollowing system, you might pick up a few tips on setup, but we’ll mainly focus on the basics for those who haven’t yet committed to a system.
My goal is to lay out enough of the basics to be helpful in the selection and setup of a hollowing system for newcomers. The cutters and scrapers used in the hollowing rigs are a topic of their own. Nearly every offering in the cutting and scraping arena will fit on or can be adapted to nearly all of the hollowing systems.
Why use a hollowing rig?
These take the tools out of the hands of the turner and provide the mechanical support to control the tool forces. This support allows the user to simply guide the cutting process by manipulating the bar. Their use varies, but all share the goals of making deeper hollowing easier and safer. This is especially true when you wish to hollow through a relatively narrow opening.
The mechanisation of the various offerings differ somewhat, but they all allow for greater reach over the toolrest while restraining the rotational torque that presents itself while cutting.
Safety
One of the benefits of the hollowing systems available is that they take advantage of good support and leverage control. When hollowing with hand-held turning tools, the user can overreach, creating potentially unsafe conditions because of the leverage issues and forces involved. When this leverage is combined with a fast-spinning material, the potential for injury is considerable.
When using any of the hollowing systems, you still need to be aware of the overhang limitations and leverage effects. Stay within the bounds recommended by the manufacturer, and apply those recommendations to the safer side if you think appropriate.
By far and away the most critical issues are bar support on the front end and the locking mechanism for the bar in the rear support of that design.
Be certain that you are supporting the front of the bar. Don’t ever let it slip off the front rest on either side. If your design includes using a bar capture system on the rear, use the safety pin, locking mechanism, or whatever you are provided with by the manufacturer to ensure that the back end of the bar always stays within their trap. A mishap at either end can create unsafe conditions in the blink of an eye. Of course, all of the standard good turning safety practices apply.
You should be using the appropriate PPE whenever you are turning.
Question your needs
As you think about selecting any of the available hollowing systems, you probably should ask yourself several basic questions. The answers to these will usually point you to the hollowing system(s) that can meet your needs best.
- How big will your typical hollowed turning be? Working on a mini-lathe versus an extended bed large swing lathe can help point you correctly to the right sized unit
- How deep or, more appropriately, how far over the rest will you need to work? Hollowing 150 or 200mm deep is far less demanding than hollowing half a metre or so down into a vessel.
- How thin will the walls of your work need to be? Nearly any rig will let you work to whatever wall thickness your skills will allow, but bigger work with thin walls may benefit from a laser, and possibly a centre steady. Both of these are handy to have, but are additional costs that may not be essential if you do thicker walled work that isn’t particularly deep.
- Are you going to be working through a small opening? While most hollowing rigs aren’t often used for a bowl type turning, they are usually used to hollow vessels. If the opening at the top of the vessel will be small with respect to the size of the turning, your choices for a hollowing system might be impacted.
- Perhaps the most difficult question is, will your system need to grow? You may work on a mini now, but there may be a larger lathe in your future. Many systems are adaptable to a certain degree, so thinking through this a bit may prevent the purchase of another unit later on.
Sizes of a hollowing bar systems
There are many implementations of ‘non-hand-held’ hollowing systems. These enable the capture and control of the back end of the bar via a trap or other mechanical system, with the front-end riding on a toolrest. The basic features that the shopper should be aware of are the size of the system and the type.
When considering size, the key is how deep you intend to hollow, and how big your lathe is. Nearly all the hollowing systems are sized based on the lathe they will be used on. The bar sizes I’m familiar with range from 5/8in to 1¼in diameters. The 5/8in bars are for the depth of about 9in or less, while the 1¼in bars are rated for 33in or less.
Obviously, a table-mounted mini-lathe will be far smaller than something intended for a lathe with a 24in or larger swing. When you are selecting your future hollowing system, pay attention to those that will upsize to a larger lathe. Depending on the manufacturer, you can get toolrests that will fit into a 5/8in mounting and shim to a 1in. Depending on the style, your system may work on a 10 or 12in swing lathe and be able to be adapted to a 24in swing.
The bar size may be a factor if you use your smaller rig on a larger lathe. Remember, the diameter of the bar and the mass will be the main determinant of how far you can reach over the rest, as well as how much dampening your rig will provide.
Types of hollowing systems
Capture hollowing rig
The capture hollowing rig first began as a bar with an outrigger shape to prevent rotation. It still works extremely well. There is a trap at the back that the outrigger travels in, now with a safety catch pin to prevent it from disengaging from the guide trap.
Welded bar
An alternative to this design is the side-by-side welded bar design. It uses the same concept of a trap at the back to support the back end, with the side-by-side bar preventing rotation due to the guide trap. This system also uses a safety locking pin to prevent slipping out of the guide trap.
Scissor type
The third common system is a scissors type of system. It avoids the need for a guide trap at the back end. It is held at the back with a support post fastened to the bed of the lathe. The front of the hollowing rig uses the same support as the other systems, whilst the toolrest provides the height adjustment for the cutting edge.
Which is better?
I have systems of all three types and they all work nicely. The scissor types I own are a bit less heavy duty by design and are quite nice to use. For the longest reach, I use my largest dual welded bar design.
Toolrests
You can use your standard toolrests to support the hollowing rigs, but take care to work directly over the toolpost. The rest is the strongest there.
Be careful if you have cast rests. They are prone to break if you load them too much away from the toolpost. I favour using the banjo and toolrest specifically made for the hollowing process. Compact and strong, they feature movable pins to control the edge point. Not only does the pin prevent falling off, but it also allows the bar to be levered against it.
That should give you plenty of information about hallowing rigs to thank on for now. In the next article, we’ll be looking at cutters, how to maintain them, and different ways to measure bowl thicknesses.