Holding Power, Part 2

This is a continuation of Part 1.

Withstanding the Leverage Component

Recall that the leverage effect tends to push the tenon deeper into the chuck at the top and pull it out at the bottom. This pushing and pulling arises from the force exerted upward on the workpiece by the cutting edge of the tool.

The ability of a tenon to withstand this leverage is limited by two factors. One is that the wood can be compressed which may let the tenon slip out of the chuck. The other is that the wood may fracture. That is, part of your tenon may simply break off and let the workpiece go flying.  

For both of these issues, there is an advantage in having a larger area of contact between the jaws and the wood. This implies that the diameter of the tenon should be close to the machined diameter of the jaws so the force exerted by the jaws will be spread out over more wood. This will make the tenon less likely to fail.

Did you notice the difference between the torque and leverage components?  Using a larger tenon will almost certainly increase the torque component but may actually weaken the leverage component. However, the leveraging force that has to be withstood becomes smaller when the tenon diameter is increased, and this will offset some of the weakening that arises because the jaws make contact with less wood.

The bottom line: using a larger tenon increases the torque capability but reduces the ability of the tenon to withstand the leverage component.

Oneway Jaws

The gripping surface of the standard serrated jaws made by Oneway is not circular. Instead, each jaw has a contour that provides very nearly the same same contact area between the jaws and the tenon over all the range of diameters the jaws will grip. Further, the gripping surface minimizes dig-in when the jaws are tightened against a tenon larger than the machined diameter.

It follows that there is no additional gain in torque capability due to dig-in when gripping large tenons.  However, the jaws will be able to supply more torque because the moment arm is greater for the larger tenon.

Also, because the contact area is very nearly the same for large and small tenons, there is no advantage

in using a tenon that is close to the machined diameter of the jaws. Bigger is better for both the torque and leverage components.

The outer surface of the jaws is slightly dovetailed with two rings that tend to lock the jaws inside a recess. My opinion is that these work no better than a standard dovetail.

Oneway also makes pure-dovetail jaws that have no serrations and which are circular. These are called “smooth dovetail jaws” and are the ones that I prefer.

Nova Jaws

The standard Nova jaws are circular and smooth with an inside profile that has both a straight and a dovetail section. The dovetail portion is only about 3/16” wide.

The tenon for the Nova jaws is easier to shape than a pure dovetail. One method is to first cut a straight tenon, perhaps with a parting tool, and then cut a small triangular groove with a spindle gouge to accommodate the dovetail portion. For a soft wood, you can skip the triangular groove and plan for the dovetail section to compress or bite into the wood. The outside profile of the jaws is a pure dovetail.  

A Note on Forming Dovetailed Tenons

With experience, forming a dovetailed tenon becomes second nature but before you gain that experience, some care and attention to detail is in order. Specifically, the angle on the tenon should match pretty closely the angle on the jaws. This angle, typically, is in the range from 7 to 10 degrees.

Fortunately, the angle does not have to be exact. Wood is compressible and will, to some extent, deform to match the jaws when they are tightened. However, there is one situation to avoid.

Do not get the angle too pronounced, or too acute. If you do, contact with the jaws will be made only at the sharp corner of the tenon and the grip will not be really secure. Under the stress of turning, the workpiece can loosen and even though the piece may not come out of the chuck, you will be working at a disadvantage. It is better to have not enough angle rather than too much.

It is a simple matter to see if your angles are too this, too that, or just right. Unless the diameter of the tenon is exactly equal to the machined diameter of the jaws, you will get dig-in at the edge of the jaws, and the mark left behind will indicate the accuracy of the angle. Ideally, the mark should be uniform across the width of the tenon, and if it is, the angle is just right.  

If the jaws are expanded considerably to grip a larger tenon, you can look between the jaws and see the contact between the jaws and the wood. However, this serves only as a rough indication because the wood will be compressed to some extent, and this will tend to make the contact look better than it really is. Of course if the angle is way off, it will be apparent.

In the photos below, the angle is “just right.”  The jaws just grip the tenon very lightly, for the inspection, so that no compression of the wood occurs. Then the jaws were tightened to produce the marks where the jaws dig into the wood. (The sample tenon was jammed against the chuck jaws to take the picture.)

The following photos show a tenon where the angle is too great. Note the gap between the jaws and the tenon near the shoulder. Further, the mark left by the dig-in is more pronounced near the corner of the tenon.

And finally, when the angle is too flat:

Tenon Width and Depth of a Recess

[A possibility for confusion exists here. The diagram at right shows the “width” of a tenon as I use the term in this article. It could also be called the “thickness” or “length.”]

For maximum holding power, the tenon should be as wide as possible so that the jaws have more wood to grip. This makes crushing the tenon or having it break off less likely. This is especially important for wood that is soft or degraded. However, under no circumstance should the tenon be so wide that it bottoms out on the chuck jaws. If it does bottom out, the front of the jaws cannot make contact with the shoulder of the tenon and the holding power will be significantly reduced.  

How narrow can a tenon can be and still allow the jaws to hold the piece securely? This depends largely on the quality of the wood. If the wood is sound and very hard, a tenon as narrow as 1/8” or even less can be used for workpieces that are not too large. However, the type and quality of the chuck jaws must be considered.

For gripping narrow tenons, I prefer smooth dovetail jaws that have a crisp, sharp corner at the front of the jaws. In my opinion, serrated jaws are less than ideal for this application.  

Similar considerations apply to the required depth of a recess. Soft or degraded wood will require a deeper recess, and it is important for the chuck jaws not to be worn or rounded where they make contact with the interior wall of the recess. Under favorable circumstances, a recess no deeper than 1/8” will do just fine.  

Seating the Tenon in the Jaws

When mounting a workpiece on the chuck, it is important for the shoulder of the tenon to seat firmly against the front of the jaws. Further, the shoulder should be well-defined and cleanly cut, and it should be square to the rotational axis of the lathe spindle.  

For all but the smallest pieces, I always use the tailstock to put pressure on the workpiece as I tighten the chuck jaws. Using the tailstock, for me, is easier than holding the piece with one hand and trying to tighten the jaws with the other. And there is another reason that you may not have considered.

Tightening the jaws does not pull the shoulder of the tenon against the front of the jaws. This is true even for dovetail jaws even though, at first glance, it might appear otherwise. The angle of the dovetail is too shallow and the friction too great for the tenon to slide in the jaws as they are tightened.  

In light of this, the shoulder will never press against the jaws with a force greater than what you apply when you are mounting the workpiece on the chuck. If you press with only a couple of pounds, that will be it. If you want firm seating, you must use the tailstock.

How tight should the jaws be?  

The jaws should be tightened “pretty tight,” but be reasonable. Don’t crush the tenon, which is more likely to happen with spalted or degraded wood. Also, make allowances for a tenon that is rather narrow; it will be easier to crush.  

Over-tightening is not good, but you can err in the other direction. I once went through a spell where bowls would come out of the chuck for no apparent reason. I checked everything – even sharpened the serrations of the chuck jaws – but later discovered the problem. I was not tightening the jaws sufficiently due to a loss of strength in my hands. The loss of strength was caused by breathing the fumes from the candles on my birthday cake.  

Here’s another tidbit:  when turning green wood, check the tightness of the chuck jaws every so often.  Water will squeeze out of the tenon and the jaws will gradually loosen.  

Pushing the Limits

Question:  What can happen if the tenon diameter is too small for the workpiece?  Well, that’s not too hard to answer – the workpiece may come out of the chuck. Part of the tenon may simply break off (leverage effect) or the jaws may spin on the tenon (torque effect).  

I’ve come up with a rule of thumb to estimate how close to the limits I’m working. It involves the number 5. If the diameter of the workpiece is greater than 5 times the tenon diameter, I know I’m near the limit. This applies to bowls, platters, and so forth, and the limiting factor is the ability of the chuck to supply the required torque to the workpiece.

This even works for spindles. If the diameter of a spindle exceeds about 5 times the diameter of the drive center (spur drive or stebcenter), you will be pushing the limit and the drive center is likely to spin on its seat.

Going in the other direction, suppose I have a vase 10” tall mounted on a chuck using a 2” tenon.  If I make a cut near the rim of the vase, the point farthest from the chuck, I will also be working near the limit.  The distance from the chuck to the rim of the vase is about 5 times the tenon diameter. In this case, the limiting factor is the leverage effect.  

I’ve watched many turners exceed the number 5 and get away with it, especially when turning a finial or similar piece. It is not uncommon to see someone mount a spindle blank 6” long in a scroll chuck using a 1” tenon and then proceed to turn the entire length of it without tailstock support. Even if they do it successfully by taking very light cuts, you can often hear the wood vibrating, and sometimes they will even mention the problem with vibration. They are definitely pushing the limits, and they are no doubt well aware of it.

This rule, the number 5, is not cast in stone. After all, I just made it up. And success, or lack of it, will depend heavily upon other factors such as the soundness of the wood, the sharpness of the tool, and the skill of the turner.  

Exceeding the Limits

Suppose you have a big lathe, a bowl blank 15” in diameter, but a chuck whose jaws only open to 2.5”.  Can you be reasonably certain that you can true up the blank and turn the bowl using the relatively small chuck. My answer is, “Maybe, but probably not.”  

The diameter of the bowl blank is 6 times the diameter of the chuck, which is really pushing the limits. It would be better to use a larger chuck if one is available, or use a faceplate or glue block if you don’t have a larger chuck.  

However, if you use tailstock support for most of the turning, including the hollowing, you can proceed with much greater confidence. Tailstock support extends the capabilities of a chuck dramatically because it essentially removes the leverage component from the equation.

To wrap it up ...

So, is there anything useful to be gleaned from all this discussion? Well, perhaps. If you realize which component, torque or leverage, is going to be more important for a particular workpiece, you can size the tenon to favor that component.  

For example, when turning a disk-like piece such as a large-diameter blank for a shallow bowl, the torque component is going to be most important. In this case, use a tenon that requires the jaws of the chuck to be almost fully open.

On the other hand, if the workpiece is small to medium in diameter but extends way out from the chuck, the leverage component will be dominant. Use a tenon the same size as the machined diameter of the chuck jaws. That is, one that requires the jaws to be almost fully closed.

Recognize when you are working near the limits and adjust your technique to compensate. Take light cuts with sharp tools. Also, keep the lathe RPM on the low side so that if the piece does come out of the chuck, the excitement will be minimal.  

And finally, use tailstock support whenever possible.  

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