Lattice Tooling
I started making lattices using the method described in David Springett's book, Woodturning Wizardry. This involves mounting the work off-centre on a faceplate and turning the lattice grooves using a specially ground chisel and a flat tool rest that can be mounted extremely close to the work. I shan't go into the details, but I do recommend the book, not just for this but for the many other wonders within.
I decided that if I was going to progress then I would need an eccentric chuck to mount the work, and so I bought the Robert Sorby eccentric chuck (no minor investment), and an X-Y table to mount lattice cutting tools made from high speed steel. With this arrangement you can achieve offsets of up to 35mm and a maximum of 12 equal rotations, each of 18 degrees. With this kit one can do useful work, but the method is limiting, slow and fiddly. I use it now as a toolholder for very small diameter work.
I spent many weeks developing methods and tooling with which I was able to make some progress, but the truth is that apart from the valuable learning I gained the tools and methods are all forgettable and I shan't burden you with any of it.
I needed an arrangement that would give extreme rigidity, large eccentric offsets of up to 130mm and an accurate rotary table.
And this is what I have been using to make the latest lattice bowls, and it works better than I had ever imagined.
To mount the work I have a rotary table bolted on a 45 degree cast iron block which clamps to an X-Y table with the axis of rotation at the same height as the lathe spindle. The rotary table has a No 2 Morse taper so I can mount chucks via an adapter that has the same thread as the lathe spindle. I can now mount the work in any way that I would if I was plain turning with the work rotating.
The tool holder was the bit that took a while to develop, and could not be made until I had the Myford ML7 engineering lathe with a Z-axis attachment for milling the dovetail slide. It is really just a very rigid, precisely settable fly cutter that can cut radii up to 130mm, and can be set repeatably to about a thousandth of an inch.
The small HSS tool is mounted in a tool block, which can be replaced with one to take different sized tools. The tool block is mounted on a dovetail slide which is locked by two cap head screws. A micrometer screw with an M8, 1mm pitch thread is used to traverse the tool along the dovetail slide.
The counterbalance weight (there is an additional weight, unseen, on the other side of the faceplate) permits rotational speeds of about 400 rpm at all positions of the tool.
Update 21 July 2013
I agonized for some time over how to make a lattice where the cutter rotates at 90 degrees to the axis rotation of the work. Using the method described above will allow inclinations of 15 or 20 degrees without too much difficulty, but to get to 90 degrees requires more bed clearance than I have. The Myford Mystro has 140mm clearance and I needed another 100 mm.
In the end the solution was rather obvious. By rotating the head through 90 degrees and using the auxiliary support I was able get to 245mm clearance. Because of the fixture weight I had to add an additional support, but the arrangement in the pictures below does the job and only takes a couple of minutes to set up. The picture also shows the work held in Masterchuck, which does have a rather large over-hang; a Patriot chuck gives a little more clearance if required.
The circular grooves to be cut on the inside of the bowl are made using a purpose made rigid boring bar (picture below) with a 3mm square section HSS tool ground to about 40 degrees.
Update 15 January 2014
The set-up described above is fine if you want to make lattices that are essentially flat, or very close to it. To make something that follows the contours of a closed form bowl more closely then you have to be able to set the work at an oblique angle (typically 15-30 degrees to the vertical) as shown in the photograph below. I managed to pick up a small tilting rotary table for very little money and it does the job very well indeed.
The difficulty was to find a table small enough not to lift the work centre too high and yet rigid enough to do the job. I have now used this arrangement on many pieces and it seems to work quite well.
Update 23 September 2014
I am not going to write that the cheap 4” tilting rotary table I bought for a song and described above has not given good service, because that is far from true. However, even after having replaced the table clamp, which was hopeless, with the two tommy bar clamps shown in the photograph, and the tilt clamp bolt which sheared off, it still suffered from poor worm and gear matching giving unacceptable cyclic errors which became apparent in more critical work. The threaded nose had to be (unhelpfully) secured with a 5/16 UNC thread rather than a Morse Taper. So, having been extensively used, it will now be taking an honourable retirement.
I splashed out on the 4” Vertex tilting rotary table above (20 times the price and 100 times better) which has a high quality feel and appearance and extra mass, which is much better able to resist the intermittent cutter forces sometimes present with off centre turning. To mount a chuck or face plate on the table I used a threaded Morse taper (MT2) with an M5 tapped hole to take a small locking collar (shown below). This secures the Morse taper in the table and enables it to be easily rotated and locked in the required starting position.
Update 9 October 2014
I bought a set of three division plates to turn my two rotary tables into cheap dividing heads, to reduce the chance of making setting errors. These came as a kit including handle and fasteners and were made by Soba. I have bought a lot of their stuff in the past and it has been mostly serviceable.
It is a fact that you rarely get better quality than you pay for, and this was no exception because there was a lot wrong with the kit. Fortunately, what really mattered, spacing and concentricity of hole pitch circles with centre hole, was good, so I spent a couple of days putting the rest right rather than sending it back.
Cutting lattice grooves is highly repetitive work and there is no doubt that the probability of an error using a rotary table is much reduced with division plates. Both of my tables now have their own set of plates.
Update 14 November 2015
I recently bought a large compound cross slide, mostly so that I could cut at larger oblique angles and as in this case to make a lattice on a concave surface requiring a large oblique angle and overhang. It has the added advantage of being far more robust and massive than the table I was using, and so much safer.
Update 23 January 2021
I am still using this cutter mount today. I have enabled the cutter head to be set further out by drilling a hole and adding extra counter balance weights, and I have another two toolholders for larger and much larger cutters. The large cuts are not all made in one go, the cutting forces are just too great, they take two passes, the first with a smaller tool. I wrote back in 2013 that this set-up worked better than I had imagined. Seven years later and that statement is still true. The 21.7.13 update above was one of the many blind alleys I have spent time and effort in over the years. The next real advancement, and again this was a huge leap forward to a process I still use today involved the use of a high quality Vertex tilting rotary table shown above. The vertical table is redundant.