Today I was working on a hollow form when the lathe started to growl – first when I was cutting the end grain then getting progressively worse. I checked the chuck to make sure it was tight on the lathe and the piece itself by trying to move the whole thing. I checked the tightness of the jaws. Everything seemed fine. I did the same with the handwheel on the outboard end. Everything still seemed fine. It certainly seemed like it was originating in the arbour somewhere; I even checked the motor and arbour pulleys. After some head scratching I thought I should double check the chuck mounting to the arbour. My lathe has an arbour that has a locking groove that accepts locking set screws in the chuck. When I loosened them I realized the chuck was slightly loose. Obviously I had dropped the ball last time I installed the chuck. The set screws had eliminated any play but the chuck wasn’t firmly seated against the arbour. I retightened things and the noise went away.
This made me think I should discuss a few things that should be considered when mounting a chuck on a lathe and when mounting a workpiece in a chuck.
Obviously, the chuck should be properly seated against the arbour face. Some turners like to slam the chuck home, some even turning the lathe on to run the chuck in, which certainly tightens the chuck but over time damages the threads and certainly makes it un-necessarily difficult to remove the chuck, which is also hard on the machine. When I was an apprentice machinist I had my knuckles rapped (literally) for doing this. It was pointed out that as long as the mating faces are clean (so make sure you wipe the faces) and the chuck is firmly seated, the job is done. I’ll admit it may seem like a fine line between the two but there is quite a difference. Of course, if you have locking set screws, tighten them as well: they are a backup in case things come loose, as in my case. Ideally, a firm push with a wrench (or the chuck wrench if no flats on the collar) against the spindle lock is far more effective than the “slamming home” technique. In some cases this will actually make it tighter. This is especially important if you have a reversing feature or are turning a very large diameter piece where the momentum can loosen the chuck when stopping the lathe.
This also brings to mind an accessory that I see available for supposedly making it easier to remove the chuck from the lathe. Some catalogues list a plastic ring that slips over the arbour and cushions the chuck-to-arbour joint so that the chuck doesn’t lock on when jambed home. First, this is treating the symptom, not the problem; the problem being that the chuck has been over-tightened. Second, a metal face to metal face connection provides a solid joint whereas a plastic ring in between makes things a little mushy by comparison. Solve the problem by not over tightening the chuck in the first place and save yourself a couple of bucks at the same time.
When mounting work in a chuck it is a good idea to tighten the chuck gradually using all the chuck key holes progressively. This balances the lateral play in the scroll, distributing the force equally between the jaws (another old machinist’s trick). It is not necessary to overtighten the piece but it must be solid. Do this by using firm, equal pressure on the chuck wrench each time until you reach refusal. If you are turning wet wood, stop the lathe and recheck the jaws frequently at first and less often once you are sure things are stable.
Preparing the work for mounting in the chuck is important. I am not a fan of using the dovetail method of chucking unless I absolutely have to. Wood is strong in compression but weak in tension. An expanding chuck in a dovetail socket puts the wood in tension and therefore increases the chance of the the wood breaking and the piece departing from its moorings. Not a good thing. Some turners argue that wood is lost by using a tenon and that they either like the dovetail socket as a feature or they can easily turn it into part of the foot. With proper planning, there is no wood lost when using a tenon either – I do it all the time. In contrast, the maximum holding power of the chuck can be put to bear when using a tenon and as long as you plan the diameter of your tenon to be within the line of your form, the foot of the vessel can be completed on reverse chucking the same as a dovetail. When making a tenon, make sure it is suitably large enough (and the jaw diameter large enough) for the workpiece. A 20 inch platter needs more than a 2 inch diameter tenon. When preparing a tenon it is important to cut the tenon rather the scrape the tenon (or dovetail). A scraped tenon has two areas of tear-out 180 degrees apart that can be weak and mushy so the jaws cannot provide equal pressure. Depending on the orientation in the chuck and the degree of tear out, the worst scenario being that only two jaws are doing all the work. A cut tenon is far more equal in hardness around its circumference (plus it is actually round) so is much more stable with equal pressure on all 4 jaws. The real holding strength of a chuck is where the axial face of the jaws meet the face of the work (90 degrees from the tenon). Therefore, whether a dovetail or tenon is used, the adjacent face of the work must be clean, flat and 90 degrees to the axis of the work. If you err, err on the concave side so that the jaw face bears cleanly on the face. Also, make sure the inside corner is clean and sharp for the same reason. Check your jaw profile. My Oneway requires a straight tenon 90 degrees to the work piece. I have some Technatool chucks for a couple of mini lathes that require a dovetail on the inside end of the tenon (which has proven to be a lovely stress raiser causing the tenon to sheer off at that point). Whatever your jaw profile is, make sure you cut the tenon to match rather than counting on the jaws to crush their way into the wood. It may seem as though the jaws are really biting into the wood but this will not allow the jaws to seat firmly against the wood on the tenon or the face.
As always, I encourage your comments and questions, so please refer to the tag line at the bottom of the article to post a comment.
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