Making a Poor-Man's Taper Attachment

Version 1

Mounting Bars

A classic taper attachment for a lathe involves a rail mounted behind the lathe bed. To turn a taper, the cross-slide is disengaged from its lead screw and connected to a shoe that rides the rail on the taper attachment. This rail is angled to the lathe bed at the desired taper angle, the tool is adjusted to meet the workpiece at one end of the taper, and then the carriage is driven along the length of the taper.

To make my taper attachment, I first built a pair of bars that could clamp to the lathe bed. I used 1/2" square aluminum bars, with clamp shoes made from 3/4" extruded aluminum channel that was a snug fit on the bars. The shoe that rides on the rear of the lathe bed is fixed permanently to the bar by a press-fit pin and a screw. That clamp is held parallel to the bar by a 1/8" spacer.

The front clamp is free to pivot on its mounting pin, but it has a captive nut (a swage nut) driven into it that mates with a cap screw through a hole in the bar. To mount the clamp bar on the lathe bed, the rear shoe is engaged, then the front shoe it pivoted into position before the mounting screw is inserted and tightened.

After the bars were completed, I mounted both of them on the lathe and scribed a reference line on the bar tops parallel to the lathe bed.

Setting up to turn a taper

I decided to dispense with the complexity of disengaging the cross-slide lead screw. The Taig cross-slide can be disengaged from the screw, but it isn't easy. Instead, I opted to make taper-turning rely on two-handed lathe operation, using one hand to drive the carriage down the ways while using the other hand to back off the cross slide. This means that the taper attachment simply blocks any motion of the carriage that would tend to cut too far into the workpiece.

To cut a taper, I clamp the two mounting bars to the lathe bed as far apart as I can and then use C-clamps to clamp the ruler from my combination square to the bars. To set the angle, I measure the distance between the bars and then multiply this by the tangent of the desired angle. One end of the ruler is set on my scribed reference line. The other end is offset from the scribed line by the tangent times the distance between the bars.

I made a follower for the carriage out of two pieces of hot-rolled steel bar. The bottom piece fits in the T-slot, while the top piece clamps down over it. At the far end, a 1/4" steel pin through both bars rides against the ruler. To set up for turning a taper, I do the following:

• Set the point of the tool against the starting point of the taper. In the photo, I'm setting up to turn the 15° taper on the rear of a piece of tooling to fit the headstock. It's already drilled and tapped for the drawbar I'll use to mount it.

• Set the follower in place with the follower pin snugly up against the ruler and tighten its clamp screws.
  

A finished taper

To turn the taper, proceed as follows:

• Make a series of face cuts, driving the cross slide until the stop bumps the ruler, then backing off and advancing the carriage for the next cut. The result will be a series of small stair steps along the length of the taper, with the step size determined by how far you advanced the carriage for each face cut.

• Return the tool to the starting point of the taper and slowly advance the carriage while loosening the cross slide. Let the ruler push the cross slide back towards you.

The photo shows the 15°taper I was cutting, immediately after the final cutting pass. I'm cutting fairly hard steel, so I'm using a brazed carbide cutter. All of the swarf from the final pass is balanced on the cutter in this photo — it's a mixture of cutting oil and very fine gritty bits of steel because I made the final pass very slowly in order to get a nice finish to mate with the lathe spindle taper.

This setup has worked very well. So far, the primary use I've had for it is to turn 15° tapers on tooling that fits the lathe's spindle taper, but I've used it three times:

• To make a spindle depth stop.
• To make a spindle center.
• To make a fly cutter.

Version 2

Mounting Bars, With Pins

After using the taper attachment several times, I decided to improve it. I made a new bar longer than the first bar in order to improve my ability to make steep tapers, but more important, I added a pair of pins as reference points for setting up tapers. For pins, I cut up the shank of a broken tungsten-carbide micro drill. The drill shank was 1/8" diameter and around 1" long, long enough to make two pins.

In the photo, you can see that I drilled holes for the pins centered in lines scribed parallel to the lathe bed. I clamped a scriber to the lathe carriage to get these lines exactly parallel to the bed, then center punched them carefully. I used the microscope (you can see the lens poking down from above in the overview photo) to correct the center punching so it was close to exactly where it belongs, then used a micro drill to make short pilot holes. Despite all my careful planning, the holes ended up about 0.01" off from each other relative to the lathe center line.

Left and Right Pin Alignment

The photos here were taken through my microscope using a hand-held iPad camera. The microscope has a reticule measuring scale with 0.001" graduations, and I turned the scale so it was perpendicular to the axis of the lathe. Note that the microscope images are inverted: The zero on the scale is on the outboard side (away from the lathe centerline) and the images show the curve of the outboard side of the top of each pin.

The microscope was mounted on the lathe carriage for these photos, and aside from moving the carriage to look at the two pins, the microscope was not touched. The measurements were quite repeatable. Moving the carriage back and forth between the pins did not change the relative position of the scale. This means that the microscope scale is in exactly the same position relative to the lathe centerline in both photos. I also checked that unclamping a bar from the lathe and re-clamping it did not change the reading so long as it was clamped tightly. As you can see, the pins are now within 0.001" of each other relative to the lathe centerline.

How did I get the two pins within 0.001" of each other after drilling holes that were off by about 0.01"? By using a ball peen hammer and a long but very slim punch to swage the aluminum against the sides of each pin, pushing it toward the correct alignment. You can see a smooth background outboard of the right pin, while the background outboard of the left pin is full of out-of-focus sparkles. These are the dimples made by the punch. The dimples made to push the right pin into position were on the other side of the pin and are visible as sparkles around the base of the pin in the oblique views at the top of this section.

It took a fair number of hammer blows to push the pins into alignment, but not one hammer blow landed on the pins themselves. The entire process involves distorting the aluminum around the pins in order to center them.

Using it

Setting up a Taper

To setup to turn a taper, first set the tailstock and cross slide roughly where it will go and figure out where to attach the bars where they will permit enough carriage travel. In this case, the right bar ends up with the tightest constraints, just fitting between the tailstock and the carriage. I put the left bar behind the chuck in order to make the new alignment pins exactly 5 inches apart, center to center, then tightened the clamps.

With this done, I set the taper using my dial caliper. I had to back off the tailstock to do this. In this case, I wanted a 15° taper for yet another device to fit my lathe spindle, and with 5 inches between my alignment pins, here's the necessary math:

I set set my caliper to 1.553 inches and locked the slide. With the left end of the ruler from my square lightly clamped to the left bar, I pushed the right end out using the inside jaws of the caliper until the other inside jaw fit tightly against the alignment pin, using the jaw of the caliper to keep the caliper square with the ruler. After checking that the left end of the ruler was still snug against its pin, I tightened that clamp and then clamped the right end.

The Result

At that point, I oiled the dead center, snugged the tailstock back in place, and installed the taper attachment's follower on the cross-slide. Actually turning the taper was no different from version one, although using a freshly sharpened brazed-carbide bit produced much nicer looking swarf than my earlier efforts, as you can see in the photo. (As a general rule, if a cheap brazed-carbide bit has a sharp edge fresh out of the box, it's an accident.)