Taig offers a top-slide which can be mounted at an angle onto the cross-slide for cutting tapers. It attaches onto the cross-slide by a set of dovetail claws which grab onto a steel dovetail rim mounted on a round island in the underside. These jaws are tightened by means of a tiny hex screw. This flimsy arrangement is a pain to use. When installing the attachment, the mounting screw is concealed inside one of the T-slots of the cross-slide so that one has to fumble around with a small hex key to engage it; the small claws do not have sufficient gripping force so that quite often the whole top-slide moves out of alignment even when only a modest amount of pressure is exerted onto it; and the tool bit is secured with only a single #10 screw and T-nut which provides no room for shimming, and it can also work loose midway through a job.
The obvious solution is to modify the swiveling and tightening mechanism. However, with the top-slide already sitting on top of the cross-slide, there is little room left for any changes.
Luckily I came across a modification by Walter Maisey which has given me an idea. His modification involves making a new mounting plate and a new QCTP tool bit mount. Milling dovetail rails is beyond my current capability so I cannot follow his design. However, I found his method of tightening the top-slide onto the cross-slide by means of a cam to be most interesting. So I went about to implement my own modification around it. I am more than happy with my endeavour and am pleased to share my experience with you.
As I have mentioned in earlier Instructables, I prefer metric to imperial units. However some measurements have to be in imperial units in order to be consistent with the Taig.
This Instructable may appear to be somewhat complicated and long winded, but the actual construction is actually quite straight forward, provided that critical dimensions are followed.
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Step 1: Design
Modification involves three parts, namely a booster plate attached to the bottom of the top-slide mounting plate, a spindle and cam tightening mechanism, and an adjustable tool holder attached to the slide by means of a cantilever mounting block.
1. Booster Plate
The basic idea is to add a new plate under the top-slide. The combined thickness of the original mounting plate and this addition gives enough head room for a stronger and lockable spindle to be installed. This spindle is operated by a cam to tighten and loosen the top-slide on the cross-slide.
I find that there is no need to make this new booster plate to be the same width and length of the existing mounting plate. I decided to simply use a 50 mm (2") disk, 12 mm thick. The disk may look a bit odd when affixed to the rectangular bottom of the top-slide but it is easier to make and install. As long as it serves its intended purpose I can't see any objection. The existing mounting is milled out from its recess. The top 6 mm half of the new mounting disk is turned into a 30 mm diameter plateau which fits into the bored out area. When the new disk is mated with the existing mounting plate, the effective thickness becomes 12 mm (ignoring the thickness of the existing dovetail rails). The centre of the plateau is then bored to 12 mm diameter for insertion of a floating spindle which provides the axis of rotation of the attachment.
2. Spindle and Cam
The 12 mm spindle is drilled across with an 8 mm hole, 1 mm below center height. An 8 mm rod runs through this spindle. This rod is made into a cam shaft by reducing its diameter to 7 mm, offset 0.5 mm from centre, along 12 mm of its length in the middle (see diagram). A hex cap screw head is affixed to one end. The cam is an eccentric section of an 8 mm rod which passes at right angles through the spindle. The rotation of this rod (cam shaft) moves the spindle up or down in its seat thereby changing the clamping pressure of the top-slide onto the cross-slide. The cam shaft is turned with the cap screw head accessible at the perimeter of the disk. Once engaged, the cam needs to be turned by only a few degrees to fully tighten up.
3. Tool Holder and Cantilever
By installing the booster plate, the top surface of the top-slide, where the tool bit is originally mounted, is raised by several millimeters. To compensate for this, I designed a new tool holder. This is mounted onto a shaft attached to a cantilever block which in turn is locked onto one of the T-slots of the top-slide . The tool holder is modified from an original Taig tool post. It is adjustable for height so no shimming is required.
The tool holder may be mounted either in front or on one side of the top-slide to change the orientation of the tool bit, which may sometimes be necessary between jobs.
The following materials are used:
Aluminium disk 50 mm in diameter, 12 mm thick or larger
Aluminium block 25 mm x 25 mm x 50 mm or larger
8 mm round steel rod 30 mm or longer
8 mm round steel rod 40 mm or longer
#10-32 1/4" long cap screw
M3 screw 6 mm long
12 mm round steel rod 18 mm or longer
3 x M4 countersunk machine screws
2 x #10-32 1" long cap screw
3 x #10-32 5/8" long cap screws
Step 2: Construction: Booster Plate
The booster plate is made from a round block of aluminium. First, the block is drilled and tapped with a 3/8"-24 hole in the middle. It can then be screwed onto the chuck arbor and then machined to a disk of the required size, 50 mm in diameter, 12 mm thick.
An 8 mm hole is drilled from the circumference midway across the disk, past the centre to a depth of 40 mm. This hole will accommodate the cam shaft. In order to drill the hole accurately, the disk is mounted edgewise on the milling attachment. I drilled a 6 mm pilot first and then widened it to 8 mm with an endmill.
The disk is then placed onto the three-jaw chuck, and the centre hole is bored to 12 mm in diameter. This hole holds the locking spindle of the attachment.
On the top surface of the newly prepared disk, the outer 10 mm from the circumference is machined down to a thickness of 6 mm, leaving a 30 mm plateau in the middle. This top-hat shaped plateau will later mate into the axial recess of the original mounting plate, and the resulting combined thickness of the two plates will be 12 mm (ignoring the thickness of the dovetail rails which are not touched), giving enough room for the cam operated spindle to be installed.
When the plateau is mated into the original mounting recess, its top edge will partly get in the way of the top-slide lead screw. A 1.5 mm 45° chamfer is required to avoid this.
Step 3: Construction: Spindle and Cam
The spindle allows the attachment to be rotated to any desired angular position, at the same time securing it onto the cross-slide by cam action. It consists of an 11 mm tall 12 mm diameter cylinder connected to a 6 mm tall 4.5 mm diameter pin at the bottom. This is turned as one piece from a short length of 12 mm steel rod. The bottom pin is threaded #10-32 at the tip for 3 mm. The spindle is inserted into one of the T-slots of the cross-slide to secure it. It must be allowed to rotate freely so that the angle of the top-slide can be changed at will. A square nut is not appropriate so a common #10-32 nut, with its hex corners rounded down to about 8.5 mm in diameter is screwed onto the bottom pin and secured permanently with loctite.
An 8 mm hole is then drilled across the spindle, 6 mm from the top. I used the milling attachment for this operation, first making a pilot hole with a centre drill, drilling to 6 mm and finally milled to 8 mm.
I proceeded to make the cam shaft from a piece of 8 mm steel rod. The rod is drilled and tapped #10-32 at the end to accept a #10-32 cap screw about 6 mm long. The cap screw is later screwed in and secured with a small 3 mm set screw across its thread. Make sure that the set screw is recessed into the rod. If not, then any protruding portion must be trimmed flush with the surface of the rod. This cap screw head will allow a hex key to engage and rotate the rod.
The rod is mounted onto the four-jaw chuck, off centre by 0.5 mm. At 19 mm from the cap screw head, it is turned down to 7 mm, for a section 12 mm long. The rod is then cut to 40 mm in total length, forming a cam shaft for the spindle. This eccentric section allows the spindle to be lowered or raised by up to 1 mm, depending on the angular position of the rod. This action then either loosens or tightens the top-slide onto the cross-slide.
The dimensions for the spindle and cam shaft are critical and must be strictly followed.
Step 4: Construction: Tool Holder and Cantilever Mounting Block
The tool holder is modified from an existing Taig tool post which I have recovered from an unsuccessful ball turning jig made earlier. Or it can be made quite easily from a block of aluminium. 21 mm is cut from the bottom of the Taig tool post, and 5 mm is cut from the back (opposite side of the tool slot), reducing it to 25 mm wide, 20 mm deep and 24 mm tall. The mounting hole is widened to 8 mm to accept a shaft of the same diameter connected to the cantilever block. A #10-32 tapped hole is drilled on the left side (top view) of the mounting hole, 4 mm from the edge. This is for the tool-bit adjustment screw. Another #10-32 tapped hole is drilled on the left side, 12 mm from the top and 12.5 mm from the front. This is for a set screw which locks the tool holder in position once height adjustment is made.
The tool holder is mounted onto the top-slide by means of a cantilever block. This block is 25 mm thick, 27 mm tall and 50 mm long; the last two dimensions are not critical. Referring to the diagram shown, two rectangular areas are milled out of this block from one end. The set screws which tighten down the tool-bit in its holder are exposed and accessible from the top by means of two M8 holes. The bottom area accommodates the tool holder. An M8 threaded hole is drilled on top of the milled out area, 12.5 mm from the front (where the milled sections are), 12.5 mm from one side. This accepts a shaft 8 mm in diameter which fits into the mounting hole of the tool holder. Finally a 3/16" hole is drilled, 12.5 mm from the front, 4 mm from the left side (top view, milled area is front). This accepts a #10-32 cap screw 5/8" long which engages the threaded hole in the tool holder. Turning this screw moves the tool holder up or down on the shaft.
Two 3/16" mounting holes are drilled through the cantilever block, 32 and 41 mm from the front, 12.5 mm from one side. The top 7 mm is widened to 8 mm to allow the mounting cap screw heads to be recessed and concealed. Two 1" #10-32 screws engage square nuts in the top slide to lock down the cantilever. The mounting hole closer to the front is designed to allow the block to be mounted at right angles on the side of the top-slide if necessary so that the orientation of the tool-bit can be changed.
Step 5: Change to Original Mounting Plate
Having made all the necessary parts, the next step involved destructive modification of the original top-slide mounting plate and there is no way to go back once it was started. So I took extra precaution to make sure that the cam tightening arrangement worked as designed, and the tool holder and cantilever block combination secured solidly and properly into one of the top-slide slots.
I needed to remove the existing dovetail fitting from under the original mounting plate. Ideally I could mount the plate off-centre onto the four-jaw chuck and machine the inside out. Unfortunately the chuck does not have enough turning capacity and I do not want to invest in a pair of risers just for this project. So I decided to mill it out by hand. It was a tedious process and with clumsy fingers on the turn wheels I managed to clean out the cavity to a depth of 6 mm. This was not without leaving a few jagged edges here and there. Since all these would be hidden from view once the new plate is installed, it really doesn't matter.
Having made sure that the booster plate mated neatly with the existing mounting plate and the top-slide lead screw is not obstructed, I clamped the two plates together, drilled three 1/8" holes holes 120° apart, 5 mm from the edge of the new plate (see attached diagram ). I then widened these three holes to 4 mm and countersunk them on the booster plate, and tapped the three holes in the existing plate to M4. The two plates were then attached with M4 countersunk screws, secured with loctite in the threads.
I then milled the half of the cam shaft opening on the edge of the existing mounting plate to complete a thorough 8 mm hole for the cam shaft to go through to the spindle.
Step 6: Final Assembly
I mounted the tool holder onto the shaft of the cantilever block, put in the adjustment cap screw from the top and the locking set screw from the side. I put a suitable tool bit in the tool slot.
The assembly is then mounted onto the top-slide with the two mounting cap screws engaging square nuts in the T-slot. The spindle is then inserted into the centre of the booster plate and the cam shaft is inserted. The complete unit is slid onto a T-slot of the cross-slide. The top-slide rotates to any angle easily. With a hex key in the head of the cam shaft, the top-slide is locked tightly in position by turning only a few degrees.
Step 7: Conclusion
I have run a few trial turns with this modified top-slide and the results are very satisfactory. The top-slide is solidly secured to the cross-slide; the tool holder is stiff and can be easily adjusted for height. Changing mounting position and slide angle is a breeze. The only downside is that the tool bit can only be mounted in two orientations and cannot be swiveled. However, there are few cases in which I have to do this. Even so, I can always grind a new tool bit for the occasion. I am very happy with this modification.