Workshop Wednesdays – Routing on the Lathe – Part 4

Monday 9 July 2018

Whereas it is relatively straightforward to move a router along the outside of a spindle, vessel or bowl by using a shield around the cutter as a guide or using a template to run the base of the router setup against, internal curvature is rather more problematic, as we will demonstrate here. We are talking about shallow bowl or platter shapes, which seldom have a regular concave internal shaping. So, we need to consider two possible solutions. One, for a non-regular arc and two, for a regular arc profile. A vessel with a 'return' lip would pose a problem and would probably be impossible for the cutter to reach inside.


The first thing that became evident was the ready-made aluminium routing device with its flat-fronted base and lack of a central row of bolt holes for rotational purposes, ruled it out as means of doing this work. However, you could fit a secondary base underneath and drill it to secure it and with a central row of holes. Instead, the homemade setup held more promise. The bolted on, twin contact points could be removed and the slightly flat nose on the base could be rounded, so it would run more precisely against a template. It is important for the cutter to be presented to the workpiece in a similar arc to the bowl profile

Template method


The first method we tried was to mimic a non-regular curve using a template, which was drawn by guestimate on a thin piece of ply and reshaped on a disc sander until it fitted side-to-side. It seemed to be a good fit…


… but when we cut out the actual template, it didn't seem to be exactly the same from side-to-side. Instead, a half template proved much more reliable and it could, in theory, be flipped over to draw out a full 180 degree mirror image template


However, an early attempt at using the template was adrift from the shape we had drawn out. We needed to find a better way of following the curve. So a piece of spray-mounted paper was used with the stationary cutter tip rubbing inside the bowl and a felt-tip pen used to draw marks on the paper


Here you can see the felt-tip pen marking against the rounded 'nose' on the base of the routing device. The device is moved around and more marks made and then joined up to create a continuous curve


The curve was then cut out carefully on the bandsaw and the edge cleaned up with the help of a fine rasp. The trick was then to clamp it carefully in the correct position to the MDF base, which was fixed on the lathe bed. You may have spotted the fact that the curve we cut out doesn't allow for the cutter projecting into the bowl. This can be adjusted on the drawing before cutting out so the curvature matches more closely


Once the template is cut, you need to position it. We found this to be fiddly and it requires a bit of trial and error and dry fit runs to get it in the right position. Once done, it needs to be securely fixed in place. Once this is done, remember to keep the pressure down so the baseplate stays on the platform and into the template, so the jig does not deviate from the cut template

Rotation method


This is, in theory, easier to get right if your bowl has a constant or nearly regular internal curve. The homemade jig had a handy set of holes, which we could use to hold it down to the baseboard with its underneath T-nuts


A bit of experimentation showed us the most likely position to bolt into. Swinging the device from side-to-side before making and switching on, seemed to work but it was slightly 'heart-in-the-mouth' until the first pass was made for real


Spurred on by the success of the first cut with a 60 degree bevel bit, we adjusted the position on the register plate behind the chuck and made a second pass…


…and a third – this was proving to be easier than we thought. Any slight depth variation didn't seem to be a problem


The result so far, stopping and switching off at the end of each pass not overrunning the centre point. It was obvious we couldn't do intermediate passes without ruining the middle where they would all meet up


So, we did a series of intermediate stopped passes instead. The cuts were quite neat – there was just a bit of fluffing at the rim of the bowl, which needed to be rubbed off


A very promising result, but we didn't want to stop there. There was still scope for more experimentation


The next trick was to adjust the rotation point on the base of the device so the centre would cut deepest and fade out quickly as it went outwards


Here are the combined techniques to show what can be possible. Obviously, you need to try for yourself and pick the effect that works best for the turned piece you want to decorate with extra detailing