Viceroy T.D.S 1/1 G.B - Parallel Test Bar
Posted: Mon 09 Apr , 2007 11:51 am
Hi
Without really planning to, I have ended up making a parallel test bar. When I was testing the lathe, I found that the headstock wasn't quite lined up with the tailstock. Tests showed 2 thou" error. I decided I'd have a go to see if I could eliminate this error just to see if I could do it.
To do this, I held a 35mm bar in the 3-jaw chuck, and the other end in the tailstock centre. I adjusted the lateral setting of the tailstock until I was able to turn the bar parallel.
With a dial gauge on the tailstock end of the work, I then released the tailstock centre from the work. If the spindle is perfectly lined up with the tailstock, the dial gauge doesn't move.
The reality is that the dial gauge will always move because the 3-jaw chuck will not hold the work in-line with the spindle. To eliminate this error so I could see the error between the spindle and tailstock alignment, I did the following procedure.
I measured the min and max deflection with the DTI at the free end of the work, then rotated the work until the DTI read half-way between the min and max. This can be checked by rotating the chuck 180 degrees. It should end up with the same reading. In these two positions, the work and the spindle line up when looking down from above the work.
If the tailstock centre is now engaged with the work, any movement of the DTI indicates misalignment between the spindle and the tailstock.
I corrected this by adjusting one of the four bolts that hold the bed to the base. I nominated one (shouldn't matter which) as the only one I would touch. The bolts are obviously designed to be adjusted. There is a large brass adjustment bolt that has a steel holding-down bolt passing through the centre. Turning the brass bolt adjusts the height of each bed foot.
Adjusting the lathe bed alignment moves the tailstock out of alignment. So any adjustment to the bed requires more test cuts to line up the tailstock. The adjustments were itterative and it took me a day to narrow down the errors.
It was at this point that I decided the bar was going to end up too valuable in terms of my time to recycle. So I decided to make a parallel test bar with a morse taper at one end. To do this, I re-mounted the bar in the chuck. I cut a grove to a depth that matches the fat end of a morse taper. I then did a parallel cut and took a series of measurements along it's length. Every point measured 1.2493" (I was aiming for 1.25" but overshot). My test bar is straight and parallel to within +/-0.05 thou" (50 micro inches) over a 9" length. At the start of this little project I didn't know how much I could reduce the errors, but the results are far better than I ever expected to achieve. This little lathe is impressively accurate.
To achieve these results, I found by experiment that the following applied :
Don't touch the lathe when cutting. Leaning on the lathe will definitely cause errors.
Use power feed when cutting. Hand pressure on the wheels or controls causes errors.
Make sure the tool is very sharp so it will cut properly with fine cuts.
Tailstock measurements need to be made with everything clamped ready to turn. The act of clamping changes the position of the tailstock. To complete an adjustment it needed to measure, loosen, adjust, tighten, measure and repeat. It's a tedious process. Eliminating errors in fractions of a thou inch was right at the limits of my DTI and micros.
Mount the DTI on the saddle rather than the cross slide. The saddle is more stable than the cross slide.
Hold the micrometer the same way when measuring. I got different measurements if the micro was horizontal or vertical. I did all measurements with the micro held vertically.
I measured every point three times to ensure I was getting consistent results with the micro. It's easy to get within a thou ". My micro has a vernier scale for 0.1 thou" and I found good technique was essential to get consistent measurements.
Let the work cool after making a cut and before making a measurement. It makes a difference when trying to measure to sub-thou errors.
Only read the measurement after taking the measurement. I found it was easy to take false readings if I was looking at the readings while applying the micro to the job.
I found all of the above made a difference to adjusting the lathe, and making accurate measurements. There's nothing original here but it all had a tangible effect. I've written all of this down because I am sure there will be some readers who doubt that it is possible to reduce errors than 0.05 thou inches on an old lathe designed for educational use. I know I would have been one. There is nothing special about what I have done. The men who made a machine that could still be adjusted to this accuracy 25 years later are the impressive ones.
So now I have a parallel bar as shown in the images. I am cutting the morse taper at the headstock end so I end up with a perfectly centred hole at the tailstock end. This will make it a little awkward to cut the morse taper. I am holding the work in the three jaw chuck because I don't yet have a driver dog (but I have a driver plate).
I am planning a couple of little project with morse tapers. I intended to use the taper turning attachment. This comes with a roller follower which runs along a straight edge. The straight edge is set to the angle of the taper. The roller follower would be great for copying curved shapes. For straight tapers, any eccentricity of the roller, dirt or marks on the straight edge would appear on the work. I decided to make a skate to get smoother results.
The result is shown in the image. I used brass to avoid wearing the steel straight edge. The brass skate is hinged on a roller bearing from a racing kart engine that I blew up (another story). The brass has square ends to push away any dirt or swarf on the straight edge. It should smooth out any minute bumps on the straight edge.
When I did some measurements, I found that I can use the top slide to cut a full length of a No.3 morse taper. I then decided not to use the taper attachment. The top slide should give a straighter and smoother line.
The hard part is setting the taper angle. For 4" travel, I need 0.1" offset. The photos show a DTI mounted on the top slide. I am using my parallel test bar to measure and set the right angle on the top slide. The parallet test bar provides the reference I need to get the morse taper right.
Setting the correct angle is going to take a bit of effort. Tightening the top slide clamping screws rotates it a little, so it's back to the measure, loosen, adjust, tighten, measure cycle. I have found that it helps to tighten and loosen the screws in the same order.
Once the morse taper is cut, I will flip the work and finish off the end of the bar. It won't matter if the end is not quite concentric.
So at the end of this exercise, I have a very accurate lathe and a test piece to prove it. The parallel bar is already proving useful to help set up the morse taper. Knowing how accurate the bar is will make it a useful reference and accessory to help make other projects. I have learnt a lot while making this simple bar, mostly about how good the lathe is.
Without really planning to, I have ended up making a parallel test bar. When I was testing the lathe, I found that the headstock wasn't quite lined up with the tailstock. Tests showed 2 thou" error. I decided I'd have a go to see if I could eliminate this error just to see if I could do it.
To do this, I held a 35mm bar in the 3-jaw chuck, and the other end in the tailstock centre. I adjusted the lateral setting of the tailstock until I was able to turn the bar parallel.
With a dial gauge on the tailstock end of the work, I then released the tailstock centre from the work. If the spindle is perfectly lined up with the tailstock, the dial gauge doesn't move.
The reality is that the dial gauge will always move because the 3-jaw chuck will not hold the work in-line with the spindle. To eliminate this error so I could see the error between the spindle and tailstock alignment, I did the following procedure.
I measured the min and max deflection with the DTI at the free end of the work, then rotated the work until the DTI read half-way between the min and max. This can be checked by rotating the chuck 180 degrees. It should end up with the same reading. In these two positions, the work and the spindle line up when looking down from above the work.
If the tailstock centre is now engaged with the work, any movement of the DTI indicates misalignment between the spindle and the tailstock.
I corrected this by adjusting one of the four bolts that hold the bed to the base. I nominated one (shouldn't matter which) as the only one I would touch. The bolts are obviously designed to be adjusted. There is a large brass adjustment bolt that has a steel holding-down bolt passing through the centre. Turning the brass bolt adjusts the height of each bed foot.
Adjusting the lathe bed alignment moves the tailstock out of alignment. So any adjustment to the bed requires more test cuts to line up the tailstock. The adjustments were itterative and it took me a day to narrow down the errors.
It was at this point that I decided the bar was going to end up too valuable in terms of my time to recycle. So I decided to make a parallel test bar with a morse taper at one end. To do this, I re-mounted the bar in the chuck. I cut a grove to a depth that matches the fat end of a morse taper. I then did a parallel cut and took a series of measurements along it's length. Every point measured 1.2493" (I was aiming for 1.25" but overshot). My test bar is straight and parallel to within +/-0.05 thou" (50 micro inches) over a 9" length. At the start of this little project I didn't know how much I could reduce the errors, but the results are far better than I ever expected to achieve. This little lathe is impressively accurate.
To achieve these results, I found by experiment that the following applied :
Don't touch the lathe when cutting. Leaning on the lathe will definitely cause errors.
Use power feed when cutting. Hand pressure on the wheels or controls causes errors.
Make sure the tool is very sharp so it will cut properly with fine cuts.
Tailstock measurements need to be made with everything clamped ready to turn. The act of clamping changes the position of the tailstock. To complete an adjustment it needed to measure, loosen, adjust, tighten, measure and repeat. It's a tedious process. Eliminating errors in fractions of a thou inch was right at the limits of my DTI and micros.
Mount the DTI on the saddle rather than the cross slide. The saddle is more stable than the cross slide.
Hold the micrometer the same way when measuring. I got different measurements if the micro was horizontal or vertical. I did all measurements with the micro held vertically.
I measured every point three times to ensure I was getting consistent results with the micro. It's easy to get within a thou ". My micro has a vernier scale for 0.1 thou" and I found good technique was essential to get consistent measurements.
Let the work cool after making a cut and before making a measurement. It makes a difference when trying to measure to sub-thou errors.
Only read the measurement after taking the measurement. I found it was easy to take false readings if I was looking at the readings while applying the micro to the job.
I found all of the above made a difference to adjusting the lathe, and making accurate measurements. There's nothing original here but it all had a tangible effect. I've written all of this down because I am sure there will be some readers who doubt that it is possible to reduce errors than 0.05 thou inches on an old lathe designed for educational use. I know I would have been one. There is nothing special about what I have done. The men who made a machine that could still be adjusted to this accuracy 25 years later are the impressive ones.
So now I have a parallel bar as shown in the images. I am cutting the morse taper at the headstock end so I end up with a perfectly centred hole at the tailstock end. This will make it a little awkward to cut the morse taper. I am holding the work in the three jaw chuck because I don't yet have a driver dog (but I have a driver plate).
I am planning a couple of little project with morse tapers. I intended to use the taper turning attachment. This comes with a roller follower which runs along a straight edge. The straight edge is set to the angle of the taper. The roller follower would be great for copying curved shapes. For straight tapers, any eccentricity of the roller, dirt or marks on the straight edge would appear on the work. I decided to make a skate to get smoother results.
The result is shown in the image. I used brass to avoid wearing the steel straight edge. The brass skate is hinged on a roller bearing from a racing kart engine that I blew up (another story). The brass has square ends to push away any dirt or swarf on the straight edge. It should smooth out any minute bumps on the straight edge.
When I did some measurements, I found that I can use the top slide to cut a full length of a No.3 morse taper. I then decided not to use the taper attachment. The top slide should give a straighter and smoother line.
The hard part is setting the taper angle. For 4" travel, I need 0.1" offset. The photos show a DTI mounted on the top slide. I am using my parallel test bar to measure and set the right angle on the top slide. The parallet test bar provides the reference I need to get the morse taper right.
Setting the correct angle is going to take a bit of effort. Tightening the top slide clamping screws rotates it a little, so it's back to the measure, loosen, adjust, tighten, measure cycle. I have found that it helps to tighten and loosen the screws in the same order.
Once the morse taper is cut, I will flip the work and finish off the end of the bar. It won't matter if the end is not quite concentric.
So at the end of this exercise, I have a very accurate lathe and a test piece to prove it. The parallel bar is already proving useful to help set up the morse taper. Knowing how accurate the bar is will make it a useful reference and accessory to help make other projects. I have learnt a lot while making this simple bar, mostly about how good the lathe is.