Making a Poor-Man's Taper Attachment

Tooling for my Taig lathe

Part of the Making Stuff collection
by Douglas W. Jones
THE UNIVERSITY OF IOWA Department of Computer Science

Copyright © 2018. This work may be transmitted or stored in electronic form on any computer attached to the Internet or World Wide Web so long as this notice is included in the copy. Individuals may make single copies for their own use. All other rights are reserved.

Version 1

Mounting Bars
the bars for the taper attachment
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
arrangement of taper attachment before starting to turn
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:

A finished taper
after the final pass of taper cutting
To turn the taper, proceed as follows:

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:

All of this tooling has seen significant use I am confident that I can use this method to accurately turn just about any taper shallower than about 30°. My experience with turning the 30° taper on the spindle center convinced me that that was close to the limit for tapers I could turn with this setup.

I described most of the material above in an May 31, 2017 posting on Nick Carter's Taig Lathe and Milling Machine Blog

Version 2

Mounting Bars, With Pins
the bars for the taper attachment the bars for the taper attachment
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.

How do you cut up a chunk of tungsten-carbide? With a cheap diamond cutoff disk. I chucked the mandrel that came with the disk in a lathe collet, draped the lathe bed with paper towel to catch the debris (a mix of tungsten carbide and diamond dust), clamped the tungsten carbide chunk in the toolholder, and then used the cross feed to slowly feed it into the cutoff disk. Simple. The bad thing about this approach is that the mandrel that came with the pile of cutoff disks couldn't hold the disk very well centered, so it only cut during a fraction of each revolution.

In any case, after cutting, it's easy to use the face of the cutoff disk to polish the cut end, the same way you would sharpen a carbide lathe tool. When done, throw away the diamond and carbide contaminated paper towel, oil the lathe bed and cross slide, then wipe it clean with a new piece of paper towel before oiling it again.

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 top of the left pin the top of the right pin
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
using calipers to set the 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:

5×sin(15°)=1.553 (rounded)

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
a turned taper still in the lathe
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.)