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All About Hallowing Rigs. Part 2

A piece of turned wood being hollowed using a rig

In the previous article, we took a detailed look at the various hollowing rigs available, including the advantages and disadvantages of each design. Their use can be very advantageous for accuracy, especially when turning larger objects. In this second article, we will take a look at different cutters, how to look after them, and how to measure your cutter.

Cutters

In the beginning, the tools of choice were a machinist’s tool bit, mounted either forward, or canted to the side, and a scraper bit that could be mounted. Many fine hollow forms can still be created with these tools alone. Today, there are so many other choices that you’d be foolish not to give them a try. The many carbide cutters available can be mounted to the hollowing bar, as can the shrouded versions. Because these cut much more than the machinist’s tool bit, you get curls and curls coming off, rather than small chips of wood. Suffice to say, you’ll have a world of cutting tools to bring to bear with the proper adapters for your hollowing rig.

Whether you use a carbide cutter, shrouded or not, or a machinist’s tool bit, adjust the cutter height to be on dead centre, or slightly above centre for your inside cutting. I find that keeping the hollowing rig level with my cutter, adjusted slightly above centre, allows for the best operation. There are other opinions out there, so feel free to weigh those as you hone your own style.

a cutter near a piece of wood
I set my cutter working edge height to be ever so slightly above the centreline of the work
various cutters for cutting within a bowl
There are an assortment of cutters available. These are my workhorses

Cutting and cleaning

Depending on your design of turning and type of cutter, you’ll have to clean the debris at varying intervals. Hollowing through a smaller hole will prevent the cuttings from exiting the turning easily. As they build up inside, they will prevent a good cutting operation from happening. Stop regularly and clean out the debris for best cutting. An air hose inserted into the opening will usually blow out the chips. Be careful of your eyes and other people around you since the debris will be flying out and can easily be a hazard even if you’re using PPE. Your style of hollowing will probably vary depending on whether you are a centre steady user or not. Starting at the centre and working out towards your desired wall thickness in stages will work nicely. Working an inch deeper at a time will leave sufficient stock to support the cut while you work. After you’ve achieved your desired wall thickness, work on the next inch down fairing the next wall thickness cut to the last. Using your same technique as hollowing the inside of a bowl will work nicely for you.

a piece of wood with shavings being cut on a lathe
The debris needs to be cleaned periodically to allow for effective cutting. An air hose works well

To centre steady or not

Among the hollow form turners, there is a differing opinion on whether to use a centre steady. Those that do, typically create their outside shape to its conclusion prior to hollowing. Of course, they may need to leave some meat at the bottom for work holding strength, but their planned use of a centre steady allows them to create and refine their form before they begin their hollowing. Those in the other camp who don’t use a centre steady do their hollowing as they create their shape. By working step by step in the depth, they always have sufficient support to do their hollowing as they progress. Much like thinning the walls of a bowl in stages, this method prevents chatter in unsupported areas by working in stages, arriving at the final form at the same time hollowing is completed.

Either way will work. Both camps have plenty of their own good reasons for their choice. Personally, since I own several sized centre steadies, I usually employ them since they are already sitting beside the lathes. There are several styles of centre steadies available. There are some that fully encompass the turning and there are some whose framework doesn’t completely surround the work. The rollers can be positioned properly, but the mechanics allow for adding and removing the steady without dismounting things. This has an advantage, but does take a bit of extra fiddling around to get things adjusted properly.

a centre steady for woodturning
Centre steadies are available in several designs and in sizes appropriate for your work and lathe
a centre steady with a large piece of wood
One of the newer centre steady designs whose frame doesn’t surround the work

Measuring thickness

One method is the use of a laser beam to indicate the wall thickness. Mounted above your cutting bar, you adjust the position of the laser with respect to the cutting edge, so you’ll know when that thickness has been obtained. The usual method is to set the laser position away from the cutter edge by the dimension of the wall thickness desired. While cutting, the laser beam shows on the edge of the turning. When the beam falls off the edge, you’ve achieved that dimension of wall thickness.

There are some tricks to setting the dimension based on the area of cutting and the type of cutter being used, but those are easily learned at the lathe. An even lower-tech option is the tried and true callipers method. Of course, you’ll need to turn off the lathe to measure, but you can use the larger callipers for measuring down into your form to indicate wall thickness. These callipers come in many sizes and shapes, and you can even use a bent coat hanger to check thickness.

a selection of thickness gauges
Wall thickness can be measured with any of the standard measuring tools that will reach
a piece of hollowed wood being checked for thickness with coat hanger rod
My favourite is the simple coat hanger or welding rod closed to a gap and then compared as slid inside

Your shopping checklist

Are you buying for now, the future, or both? The mechanics are your choice, but the bar size depends on how big and how deep you intend to go. The original style, the scissors style or the two bar weldments will all do the same thing for you when sized properly. You choose your style and the size to cover your needs.

  • Will you need or want a centre steady? They are usually sized based on the swing of the lathe, and don’t easily adapt as the hollowing rigs might.
  • Is your current banjo and toolrest heavy duty enough for your needs, or will you be in the market for a specific toolrest for hollowing? These are incredibly sturdy and will serve you well. They also have specific designs to help prevent falling off the edge.

Your cutting choices are your own, but the most economical to begin with will be the machinist’s tool bits and scrapers –  they are very easy to grind and very modestly priced. Later on, you can consider the carbide cutters. Pricier and a bit trickier, so get your basics before you launch into these. If you are new to the market, don’t be afraid to consider used equipment. Properly cared for, there is little to go wrong or wear out on hollowing equipment, so bargain pricing may be had when buying second-hand. What about home built?

I have used and own some home-built versions of hollowing systems. If they are built properly, they can be as functional as the commercially available units. Do take care that any home-built units are of proper materials and have had quality welding done on them. Also be certain that the hardware is of the proper strengths. Commercial units will have grade 3 or grade 5 hardware to be certain it is up to the loading it will see. Be certain that home-built units have the appropriate hardware used.

Summary

Not every woodturner will have a need for a hollowing system, but I must admit they are a lot of fun when you do. If you make hollow forms of any kind, you’ll find the advantages of a hollowing system priceless. Once set up, the ease of ‘steering’ the cut with only fingertip effort. None of the systems I am familiar with is inexpensive, but they are a once in a lifetime buy if you select properly. Before you take the plunge, seek out a fellow turner who has a hollowing system they are willing to let you try out. With their assistance, spend a bit of time creating something from a fresh cut end grain mounted blank, and I’m betting you’ll be in the market for a hollowing system soon.

a series of deep turned vases.

Further reading

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