Turning Hollow Forms Pith to Pith

I turn a lot of end grain hollow forms, primarily because I am adding a pewter collar to them and so need the wood to be in as stable an orientation as possible.  About 6 months ago I started turning end grain hollow forms from a whole log mounted pith to pith.  I was a bit apprehensive about it at first, worried that the base of the hollow form would be inclined to crack as it had the pith included, but after rough turning  and finishing a number of them I have been encouraged with the results.  I thought I would share the process and some of my thoughts and observations on turning a hollow form pith to pith.

Before I go any further, I do need to acknowledge and thank Ed Malesky from the Turning Arts Group for his advice and the discussions we have had on the topic.  Ed also publishes a blog which you might consider following.

Typically an end grain blank is cut from a log as shown in the diagram below.  As well as a lot of chainsaw and bandsaw work, getting a reasonable size (5 to 6″ diameter) blank for a hollow form means that you need a fairly big log to start with.  Once the blank is cut there is also the danger of it cracking if it is not turned right away due to it being such a thick piece of wood.  For that reason it is also very difficult to purchase an end grain blank from any of the online stores.  The vendors are concerned about larger blanks cracking and most will only offer end grain blanks, or spindle blanks, up to 4″ diameter.

end grain blank in log

So the first advantage of turning hollow forms pith to pith is that you do not need a particularly big log.  Depending on the position of the pith, a log in the 10 to 12″ diameter will easily yield a 5 to 7″ diameter hollow form.   The second advantage is there is not much chainsaw work needed in order to prepare the blank.  One simple crosscut to cut off a section of the log and you have your blank ready to mount on the lathe.  Because of the ease of preparing the blank it is also possible to leave the wood in log form until it is needed, further reducing the likelihood of cracks.

From an aesthetic view, I feel there is a lot more symmetry and balance to a finished vessel turned pith to pith compared to one turned from a blank cut out of a log to exclude the pith.  This symmetry and balance is visible in both the grain pattern and the distribution of the heartwood.  You can see this in the two images below of some rough turned end grain hollow forms.  The one on the left was turned pith to pith and you can see how the annual growth rings flow completely around the form.  By contrast, the vessel on the right was turned from a blank that did not include the pith, and the growth rings tend to run vertically up and down the piece.

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The images below show the difference in the balance of the heartwood.  In the vessel on the left, turned pith to pith, you can see the heartwood distributed evenly all round the hollow form at both the top and the base of the form.  The vessel on the right,  turned from a blank that excluded the pith, only shows the heartwood along one side of the form.

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The following images should provide a photo essay of the process of rough turning a pith to pith hollow form.  The first step is to cut an appropriate length off a leg.  Allow a couple inches extra either side for turning away any cracks on the end of the log and also for holding the log while hollowing.  In a perfect world the pith would be located in the center of the log, however this is pretty rare.  I take the end of the log that has the pith closest to the center, which should yield the largest diameter on that section of the log, and drive a 4 prong spur drive directly into the center of the pith.  This will be the end that is the top of the hollow form.

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I mount the log between centers, placing the live center directly into the pith on the tail stock side.  Depending on how offset the pith the log will not be very well balanced and you need to pay attention to the following:

  • The banjo may not clear, so you might need to start turning at the tail stock side and move towards the head stock side as the log is turned cylindrical and clearance is made for the banjo.
  • Check the clearance on the tool rest by rotating the log by hand before turning the lathe on.
  • The log will be out of balance, so start the lathe at the slowest speed possible and only increase the speed as the blank is turned cylindrical and becomes more balanced.
  • Wear a face shield and take care when turning the bark off.  Depending on the bark you can get some pretty big chunks flying off.  If the bark is loose it can be removed before turning.

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I like to use a spindle roughing gouge to true up the blank, but you could also use a bowl gouge.  I will turn the blank to a cylinder and then true up the end at the tailstock.  I try and get it slightly concave as this is the end I will be mounting my face plate to.

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I then remove the blank from the lathe and attach the face plate with wood screws.  In order to center the face plate I have a piece of scrap wood that has been turned so that it just fits into the opening of the face plate.  It also has a hole drilled in the center to accept a transfer punch.  I place the transfer punch in the dimple in the center of the blank, drop the piece of wood over the transfer punch and then drop the face plate over the piece of wood.  Then I will screw the face plate to the blank.  The face plates I use are made from 1 1/4″ nuts and are not very deep so I need to take care that the dimple on the blank does not restrict the face plate from being threaded all the way back onto my spindle when I mount it on the lathe.

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Once remounted on the lathe it may be necessary to turn away some of the wood at the tailstock side first to get past the cracks that were visible on the end of the log.  Once that is done and I am clear to solid wood I will shape the outside of the form, leaving some bulk at the base to give stability during the hollowing process.   I like to leave the tailstock live center in place for additional security while shaping the outside of the form.

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Then I will drill the form to finished depth with a forstner bit and mark the depth of the hole on the outside of the form.  When drilling I allow for the length of the screws holding the faceplate on plus at least an inch to allow for the parting cut and the base of the hollow form.

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I will then hollow the vessel out.  I try and maintain a fairly even wall thickness and don’t leave the walls too thick.  This is especially important at the base of the hollow form where the pith is.   I have a tendency to walk a fine line here unfortunately.  Because I am adding a pewter collar to the form my natural inclination is to leave a bit of extra thickness on the walls towards the base in order for the piece to feel balanced and not top heavy due to the pewter collar.  I have had a couple of rough turned pith to pith forms crack at the bottom and believe it is mainly due to me leaving to thick a wall at the bottom.

Once done with the hollowing I will shape the bottom of the vessel.Then after double checking the depth of the vessel,  I will make a parting cut about 1/2″ wide leaving a tenon about 3/4″ in diameter.  Finally I will create a dovetail tenon on the very end of the blank.  The diameter of this tenon is restricted by the face plate and is quite a stretch for the jaws of my chuck, but it will be sufficient for remounting the piece at a later date and doing some light turning and sanding.

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In the image below you can see the tenon between the base and the hollow form.  I’ve found that this tenon will invariably bend as the piece dries as it is essentially the pith of the log, however it forms a natural breaking point and any cracks that are likely to develop in the base will generally not transfer to the hollow form.  The dovetail tenon formed by the face plate is also visible in the image.

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The form is then unscrewed from the faceplate, I write the wood species on the underside of the base and then it goes up on the shelf with the rest of my rough turned hollow forms to dry.

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I will cover the final turning of the hollow form in another post, but before I close I want to add a couple observations and things to watch out for.

The image below shows some small cracks at the base of the hollow form.  I’ve found that these cracks can normally be turned away once the hollow form is remounted on the lathe after drying.   To confirm if this is possible I will first take a light and look inside the hollow form to confirm that the crack has not gone all the way through the wall or base.  If I don’t see any evidence of the crack on the inside of the form I will mount it on the lathe and carefully reshape the base to turn away the crack.

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I’ve learnt the hard way not to be greedy.  Turning pith to pith means just that.   The turning must start with the drive center and live center placed directly in the center of the pith.  The two hollow forms in the image below were turned from the same log.  The pith was far from the center of the log and so in order to maximize the diameter of the hollow form, I placed the live center in the center of the log, not in the center of the pith.  Not only is the heartwood distributed unevenly, but the rim of the hollow forms have warped so badly that I’m not sure I’ll be able to salvage these two.  Surprisingly enough, neither have cracked.

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The image below is a great example of the thick base cracking but the crack not extending through to the hollow form.

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The closing shot below shows a group of rough turned hollow forms.  You can clearly see the small tenon separating the heavy base prone to cracking from the hollow form.

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