In the previous article we looked at how to fix a bowl to a lathe with a block of wood. Although this method eliminates the need for a chuck, it doesn’t present with its own drawbacks. As such, it may be worth considering the jam chuck bowl method as a quicker way to get your bowl attached to the lathe. In the second part of this two part series I am going to take a look at how you can fix your item without the need of a chuck or glue.
Jam chuck bowl method
The initial stages of the jam chuck bowl are the same as the glue block bowl. The one exception is that once I am ready to turn the underside, I need to cut a spigot to create the hold. Having never done this before, I’m not entirely sure of the best size to cut the spigot. I decide to use the spigot I would use on my scroll chuck as a guide. I guess I need a bit more depth for a jam chuck than for my scroll chuck, so cut it 60mm diameter and 10mm deep, initially roughing it out with my bowl gouge, then refining the spigot using my skew in scraping mode, flat on the tool rest. I cut it perfectly square rather than dovetailed, as I would on my scroll chuck.
Laws of the chuck
To achieve the best support when working with a scroll chuck, in either spigot or recess mode, it is important to realise the chuck jaws work in two ways. Firstly, they grip onto the spigot or recess, but they also need a surface to bear against to resist leverage which can easily force a bowl and chuck to part company. This bearing surface is easily formed as a flat for the face of the jaws to seat against. This law must also apply to a jam chuck: if I can counteract the leverage forces with a flat bearing surface, it stands the very best chance of working.
Selecting the jam chuck
I choose a piece of clear pine, European redwood (Pinus sylvestris), for the jam chuck. Firstly, it is cheap. Secondly, being softer than the ash of the bowl, it should have a little give in it to achieve a good tight grip on the spigot. I mark the size on the pine block with dividers and remove the bulk of the recess with a gouge, squaring the edge with a skew in scraping mode. The pine block is around 50mm thick, despite the fact I only need enough to house the 10mm spigot. I would always recommend using overly thick material here because sure enough, I cut my first recess too big. The additional thickness allows me to remove all evidence of the first attempt and try again. My second try is better, but not quite as tight as I would like. My options are to try again or pack it out with tissue paper. Judging by how tiny the difference between it fitting and it not fitting, I decide to try a folded piece of tissue paper. A couple of firm taps with the heel of my hand seats the bowl beautifully in the jam chuck.
Turning on a jam chuck
My first few cuts are very light and tentative. I soon gain confidence when the bowl shows no sign of flying across the workshop. Despite the theory being sound, I fully expect the bowl to come away at least once. However, I am pleasantly surprised by how secure the jam chuck feels. I make sure I pay full attention to all of my cuts to avoid a catch. If something is going to make the bowl leave the jam chuck, it will be a catch. As I progress, my confidence grows, and I risk a few more aggressive cuts. The bowl is as solid as if it were held on a scroll chuck. I turn an ogee shaped bowl down to an even 5mm wall thickness and sand to 320 grit.
Removing bowl from jam chuck
With the bowl turned and sanded, I now need to remove the bowl from the jam chuck. At 5mm, it is quite thin, so I don’t want to use too much force. I decide the best way to do it is to apply a leverage-type force by gently knocking the bowl with the heel of my hand. Sure enough, the levering action gradually twists the bowl from the chuck, but not easily, which is quite reassuring. The tissue paper comes out in a perfectly formed cup and all that remains to be done is to re-turn the bases of both bowls and form a foot. I like a foot on a bowl, but feel free to shape the bowl as you see fit.
Re-turning the bases
The method I always use to finish the underside of bowls is suitable here. It only involves a faceplate with a disc of MDF fixed to it. The bowl is then sandwiched between the MDF and the live centre – essentially between centres. This gives almost full access to the underside of the bowl. The jam chuck bowl is the most straight forward; it involves simply reducing the overall size of the spigot to a foot that is in proportion with the bowl. This is turned with a spindle gouge and just a tiny pip is left under the live centre to remove by hand at the end. The glue block bowl requires a little more work as there is around 25mm of sapele to remove, before refining the curve of the bowl and forming the foot. Regular checks are needed to ensure the curve looks good and a foot can be formed. Once happy with the shape, it can be sanded to the same standard as the rest of the bowl and finished off, away from the lathe.
Summary
From the start, I suspected the glue block bowl would be the easiest and least likely to cause me issues during the turning process. However, the jam chuck bowl proved to be the least hassle of the two. I was pleasantly surprised by the quality of the grip on the jam chuck. My only reservation would be that a beginner who may be more prone to catches, could well struggle with the jam chuck method. This is ironic considering it is likely to be a beginner without a scroll chuck who might want to use this method. The glue block method would certainly be most secure for a beginner, but this method does need forward planning and careful preparation to ensure the blanks are ready to turn on the day and are safe, which may also put people off. A scroll chuck certainly gives the best of both worlds, and I wouldn’t be without mine, but if you want to try something a little different or to challenge yourself, I would highly recommend a jam chuck bowl as an exercise.
Alternative methods of working without a chuck
Some turners prefer not to use a scroll chuck, instead opting for the solid hold of a faceplate and sturdy screws for bowl and hollow form turning. In this case, there is always excess timber to allow the screws to do their job, while still being able to remove all evidence of them. For some, faceplates and chucks are too rigid, not allowing for creative flair. This is particularly relevant with natural edge work where balance of the rim is important. These bowls are often started between centres, until the turner is satisfied with the look of the emerging bowl and a chucking point can be fixed.