A novel quick change tool post.
There has been a number of designs of quick change toolpost (QCTP) that are based on a pillar, securely mounted on the top slide, onto which the tool holder slides and is then clamped to the post by a pinch bolt. The height of the tool post can be simply set using a set screw.
This design is appealing since it can be made just using a lathe. Other designs of QCTP usually require a milling machine to cut dovetails. The design is also very rigid since the toolpost sits directly on top of the top slide whereas most of the other QCTP systems the toolholder is hanging off the side of the top slide. Conventional pillar type QCTPs can only accommodate one tool in each holder because of the pinch bolt and because there would be no way of independently setting the tool height for each tool.
Conventional designs use the pinch bolt to clamp the tool holder to the pillar. A little lateral thinking suggested an alternative approach of making the pillar expand to grip the tool holder. By doing this each tool holder could accommodate two tools thereby halving the number of toolholders required. Furthermore by arranging a small step on the top of the top slide surface then each tool can be adjusted independently. When the toolholder is turned so that it is facing the axis of the lathe then its adjuster screw is resting on the step. When the tool holder is turned around then the other tool in the holder is facing the axis and its adjuster is on the step. The only limitation here is that the difference in height of the tool tips must be less than the thickness of the step. In practice for 1/4" HSS tools the tips of the tools is usually within 2 mm of each other.
This design is appealing since it can be made just using a lathe. Other designs of QCTP usually require a milling machine to cut dovetails. The design is also very rigid since the toolpost sits directly on top of the top slide whereas most of the other QCTP systems the toolholder is hanging off the side of the top slide. Conventional pillar type QCTPs can only accommodate one tool in each holder because of the pinch bolt and because there would be no way of independently setting the tool height for each tool.
Conventional designs use the pinch bolt to clamp the tool holder to the pillar. A little lateral thinking suggested an alternative approach of making the pillar expand to grip the tool holder. By doing this each tool holder could accommodate two tools thereby halving the number of toolholders required. Furthermore by arranging a small step on the top of the top slide surface then each tool can be adjusted independently. When the toolholder is turned so that it is facing the axis of the lathe then its adjuster screw is resting on the step. When the tool holder is turned around then the other tool in the holder is facing the axis and its adjuster is on the step. The only limitation here is that the difference in height of the tool tips must be less than the thickness of the step. In practice for 1/4" HSS tools the tips of the tools is usually within 2 mm of each other.
The toolpost pillar
The toolpost pillar is 25 mm diameter and about 32 mm high. It has a 45 degree conical face at the top end and it has 6 evenly spaced slits running lengthways along the body to within 4 mm of the bottom. This stands on the step plate at the bottom. On top of the pillar is a conical collar and the assembly is bolted to the top slide with a 10 mm bolt.
The components of the pillar. From left to right: the 10 mm bolt, the conical washer, the pillar, the step plate. Note the pin in the step plate. This fits into a small hole drilled in the top slide to prevent the step plate turning.
The toolholders
Each toolholder is made from a block of mild steel 50 x 50 x 25 mm. It is bored out to 25 mm through the centre to fit over the pillar. There are two adjuster screws to set the height. The toolholder slots are milled in the lathe by clamping the tool to the pillar at the correct height and using an 8mm slot drill mounted in the lathe chuck. The milling process can be speeded up by drilling a 7 mm hole to remove most of the metal first. The tools are retained by 5 mm socket head screws.
Further development
After making this toolpost I subsequently reduced the height of the top slide. This in turn allowed a modified version of the tool post to be constructed, see here. This improved toolpost has a greater range of adjustment for tool height variations.