Comparison of Mounting Methods for Turning Slender *LINK*
Dennis in Southern Oregon
If one attempts to turn a spindle down to a smaller and smaller diameter, eventually one reaches the point where chatter marks begin to develop and further turning leads to increased chatter and possibly whipping of the spindle. The minimum diameter that can be achieved before this happens depends on several factors including the method used to mount the spindle. There have been posts from time to time comparing various mounting methods, but I have not seen any that give quantitative results. As an engineer (long retired), this intrigued me to the point that I decided to see if I could shed some light on the subject. I carried out a mathematical analysis of the problem based on standard engineering equations for beam deflection and followed it up with controlled experiments to try to confirm the calculations. An article, giving full details of the project can be found at the link shown below. The main results are summarized here.
I compared five mounting methods, which are listed below in order of increasing performance. The numbers in parenthesis are the locations of the critical points along the spindle.
1. One end in chuck, the other free. (100%)
2. Between centers—the most common method, and the standard of comparison here. (50%)
3. One end in chuck, the other supported by a center. (59%)
4. Both ends in chucks. (50%)
5. Both ends in chucks and the spindle under tension. (50%)
In each case, it was assumed that no other support was provided. (No steady-rest nor steadying hand wrapped around the spindle.) All results are based on a simple cylindrical spindle and will not apply exactly to arbitrary spindle profiles, although the ranking order will remain as shown. Mathematical analysis of Method 5 is impractical so only experimental results are presented and they are limited to one particular example case. For each mounting method, there is a critical point along the spindle where deflection is greatest for a given tool force. The locations of these points are noted in the list above as a percent of the distance from the drive end to the outer end of the spindle.
The premise underlying the analysis was that for a given type of wood, spindle length, type of cutting tool and possibly other factors, chatter begins when the deflection of the spindle caused by tool pressure reaches a certain amount, independent of the mounting method used. On this assumption, mounting methods that yield higher stiffness will perform better than ones that yield lower stiffness. Furthermore, with this assumption, one can use well-known engineering beam deflection equations to calculate the relative minimum diameters that can be achieved for a given spindle length by the various mounting methods and, conversely, the relative lengths of the spindles that can be turned to a common diameter. The experiments were carried out to test this premise and, in fact, the test results seem agree quite well with the calculated ones as shown in the following figures. Note that these are relative results, with Method 2 being the standard of comparison.
In order not to bias the results, and to allow a meaningful comparison of calculated and experimental results, I was careful to keep conditions as similar as possible for all of the test runs. No hand support was allowed and only a regularly ground and honed skew was used. The speed was kept at the maximum speed of my lathe (about 3200 rpm). In all cases a skilled turner could improve on these results by relaxing these restrictions.
Methods, 4 and 5 require a freely rotating chuck or equivalent at the outer end of the spindle. Previous posts have mentioned the use of a regular chuck attached to a live center for this purpose, but the large moment of inertia of the unit would require great care when starting and stopping the lathe. My solution was to make a simple “chuck” from a piece of 1-inch diameter round aluminum bar stock from a local hardware store. Shown below, they were made on the lathe by drilling and tapping one end to thread onto my Oneway live center and boring out the other end to the desired tenon diameter. The bore depth should be about 1.5 times the tenon diameter or more and the tenons used should be turned to be a force fit into the chuck so no wobble is possible. These aluminum chucks weigh only 1.2 ounces and probably add little to the moment of inertia of the live center itself. The method used to apply tension for the experiments with Method 5 is described in the Article. By the way, there is a typo on the second figure above; the tension for Method 5 was 55 pounds.
A further point worth considering is that if one is turning spindles that require tenons on both ends, Methods 4 and 5 have another advantage over Methods 2 and 3, because with Methods 2 and 3 the total unsupported length of the spindle will be increased by the lengths of the tenons that are not supported by chucks. For example, if you are turning a 20-inch long spindle that requires 1-inch tenons and use Method 2, you would actually have a total length of 22-inches to deal with. By my calculations, this would lead to a 7% increase in the minimum diameter that could be achieved. Therefore, the comparisons of the performance of the various methods given in the original post understate the benefits of the higher-numbered methods in situations where tenons are required on the finished turning.
