I have heard quite a few good things about tangential or diamond tools. They are reported to be simple to use, cut aggressively and leave good surface finish. The tool bit is also simple to grind and be reground. I decide to try it out but first of all I need to make an appropriate tool holder. I have reviewed on the web many ideas on how to make one.
The tool holder is designed to mount the tool in an upright position, tipping the bit forward and left typically by 12°. The angle seems to be not particularly critical and some designs have angles up to 16°. The top of the tool bit is ground diagonally across sloping down at 30°. Looking down from the top, this ground area has a diamond shape, hence the tool is also called the diamond tool. In order to achieve this rather complicated configuration, the holder has to be milled, drilled and ground with precision to the required angles. To achieve rigidity, the most common holder is made of steel and is inserted in the tool-post just like any other tool bit. Making a tool holder this way poses significant problems for me. I don't have any suitable equipment to mill small pieces of steel with compound angles. Also holders have to be made in pairs, one right handed and one left handed. This means that any difficulty is simply doubled.
However, there is a simpler way of achieving this. Why not mill a slot in a rectangular block of aluminium with the appropriate angles at one corner, and use the block as the tool post and bit holder. To make it for the Taig lathe, only one small block of aluminium plus a few screws, a long screw with handle nut to lock the holder onto the cross slide, and two button head cap screws to hold a short 6 x 6 mm hss tool bit, are required.
This project mainly requires the use of the milling attachment with the milling vise of the Taig lathe.
This project is completed in less than a day, and I am very happy with the result.
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Step 1: Mill the Sides
I take a small block of aluminium and trim it down to a square pillar measuring 31 x 31 x 37.5 mm. I do this on the lathe, mounting the raw material on the four-jaw chuck and trued all six surfaces. The actual dimensions are not really critical, larger or smaller by a few mm will also do. The only requirements are that the horizontal cross section must be square, and that the footprint after milling the sides is sufficiently large to mount securely onto the cross slide.
Before the block is milled to shape, I drill a 5 mm mounting hole from the top through to the bottom. This has to be done first because after the sides of the block are milled, it may be difficult to grip the block for the hole to be properly and squarely drilled. I mount the work piece on the four jaw chuck and offset the rotational centre to 12 x 12 mm from one corner. I then drill the hole through.
I install the milling attachment onto the cross slide. This is locked with two long screws accessible at the back. Both screws are recessed into the heavy aluminium body, and one is particularly difficult to engage owing to its depth. A short length of 12 mm cylinder, drilled through with a 5 mm hole, inserted into the recess helps the hex key locate and engage the screw head. The attachment is normally attached with the T-slots parallel to the cross slide, i.e. at right angles to the lathe bed. A sliding plate secured with cap screws at the back helps keep this in alignment. If the plate is slid upward to clear the cross slide, the attachment body may be rotated up to about 15° either side. This is very convenient for this project. I set it to 12° and the attachment is locked securely. I then attach the milling vise across the attachment T-slots, using an engineer's square to true it up.
Two faces of the work piece have to be milled. They are those furthest away from the mounting hole drilled earlier.
I then secure the work piece in the milling vise, long side across. Putting a 12 mm end mill into the three jaw chuck I proceed to mill the entire face down to the edge of one side.
The work piece is then rotated for the other required side to be milled in the same way.
The work piece now takes the shape of an inverted trapezoid, with two faces sloping inward at 12°, 31 mm square at the top and 24 mm square at the bottom. The through hole drilled earlier is now at the centre of the bottom face.
Step 2: Mill the Tool Slot
The next stage is to mill the slot where the tool bit is to be mounted. This slot is cut into the sloping corner of the work piece from top to bottom. When the tool bit is mounted with its nose up, its orientation will be dictated by the geometry of the block. Its tip will slope forward by 12° and left by also 12°, just the proper angles required. I use a 6 mm square tool bit. It will be secured with two button head screws so it will be milled just under 6 mm deep each side. A bigger or smaller bit may be used; you just have to cut the slot to size accordingly. I prefer a bigger yet manageable bit with a larger frictional area against the tool holder body.
The two bottom cap screws mounting the milling vise to the milling attachment are removed. The two top screws are loosened and the vise is swung upward on the right hand side by 12°. I mount the work piece so that the sloping corner edge runs horizontal and at right angles to the lathe bed. The work piece is now positioned correctly for the tool bit slot to be milled.
Using a 6 mm end mill, I carefully mill a square notch into the corner just under 6 mm deep into both sloping faces.
Step 3: Install the Tool Bit
Part of the tool slot towards the top has to be milled away to provide clearing room for chip removal. I return the milling vise to horizontal, re-attaching the two bottom cap screws and nuts. The main attachment is also returned to its normal position perpendicular to the lathe bed, I then secure the work piece in the vise, rotating it diagonally so that the tool slot faces outward. With a 12 mm end mill in the chuck, I mill flat about 15 mm of the protruding corner.
The tool bit is secured onto the now almost complete holder by two M5 button head screws. These screws have large head flanges and they are used to clamp the tool bit onto the tool slot. Two 5/32" holes are drilled 15 mm from the bottom, 17.5 mm from the straight side and tapped M5. The tool bit is slid into position between these two screws. To help improve friction between the tool bit and the aluminium block, a small piece of 600 grit sand paper, 12 mm wide 15 mm long, is folded and placed underneath the tool bit before tightening the screws.
The top 6 mm of the sloping sides are then milled flat with the back of the tool post. This provides a little more clearance for the tip of the tool bit when in use.
The main body of the tangential tool holder is now complete.
Step 4: Completion and Test
The tool holder is locked onto the cross slide with a long bolt. This bolt is made with a flanged steel pin turned to 3/16". The top 8 mm is threaded to #10-32. The flange is ground on two sides to about 8.5 mm in width.
A handle nut tapped #10-32, together with a 8 x 3 mm washer, screws into this bolt to secure the tool holder onto the cross slide.
I will not go into any details on how the bolt and handle nut are made.
After this Tangential Tool Holder with tool is finished, I tested it by turning a few scrap rods of steel. I am pleasantly surprised by how smoothly the tool cuts, generating long continuous strands of coiled chips. The surface finish is also better than I expected. The above photo shows an example where no filing or touching up has been applied.
This tool can be used to turn and face without changing the position of the holder. Also, it functions both as a right-hand or left-hand tool. To make it left-hand, simply loosen the locking nut and rotate the holder 90° clockwise. The tool then faces the other direction to turn and cut into shoulders without further change of position.
I am more than happy with this project.
[P.S. September 1, 2017]
The two M5 Phillips button head screws locking the tool bit in position has since been replaced with recessed hex button head screws. These enable a higher torque to be applied which makes the tool piece much more securely seated and not easily displaced by constant knocking on the tool tip by rough work.