monophoto
Member
A concern that has been discussed a lot on this forum is the issue of tailstock wobble on midi-lathes, and the impact on drilling accuracy. I recently had to deal with this issue while making some small boxes, and thought that it might be helpful to post an illustrated discussion of what this problem is all about.
The tailstock casting on most lathes includes a rectangular protrusion on the bottom that aligns the tailstock to the bedways. As shown in the sketch, that protrusion (depicted as the dashed blue rectangle in the tailstock) ideally should be exactly as wide as the spacing between the bedways. However, many lathe owners find that their tailstock can wobble a bit - it rotates very slightly about a vertical axis that roughly passes through the center of the tailstock. This is because that protrusion is very slightly more narrow than the spacing between the bedways. The sketch of the 'less ideal lathe' shows that if the tailstock can rotate slightly, the axis of the tailstock may not align with the axis of rotation of the headstock. This is purely a consequence of less precise machining, and based on the comments on this forum, is not uncommon on midi-lathes produced in the Far East.
The amount of error that this wobble causes is typically quite small, and usually isn't a problem for most turning tasks. Where it is more likely to be a problem is when drilling - that small inaccuracy causes the drill bit to try to enter the turning slightly off-center. The photo shows the result of a test in which a brad-point bit that was not aligned exactly to the center of the turning carved a path 5mm in diameter around that center. This can cause the hole to be larger than the diameter of the bit. And because the bit will try to center itself in the turning, the bit to flex while drilling - and the resulting metal fatigue can cause small bits to break. While this is usually not a major problem for those who make pens from kits, the folks who make kitless pens need to achieve a higher degree of accuracy when drilling, and subsequently threading, concentric holes. That's why most kitless penmakers use machinists' lathes rather than wood lathes. However, there are a few members of the IAP community who have the skills and patience to do kitless pens with wood lathes in spite of this handicap.
The degree of angular misalignment between the tailstock and the axis of the headstock is a function of the degree to which the protrusion on the bottom of the headstock is smaller than the spacing of the bedways. However, the amount of centering error that can occur at the face of a turning depends on the ram extension, and on the size of accessories between the end of the ram and the turning - which in the case of drilling includes the jacobs chuck and the drill bit. As the ram is extended, and as the bit becomes longer, the centering error increases. I did some measurements on my 12" Turncrafter lathe and found the following:
So this begs the question of how to deal with this problem. Some possibilities include:
The tailstock casting on most lathes includes a rectangular protrusion on the bottom that aligns the tailstock to the bedways. As shown in the sketch, that protrusion (depicted as the dashed blue rectangle in the tailstock) ideally should be exactly as wide as the spacing between the bedways. However, many lathe owners find that their tailstock can wobble a bit - it rotates very slightly about a vertical axis that roughly passes through the center of the tailstock. This is because that protrusion is very slightly more narrow than the spacing between the bedways. The sketch of the 'less ideal lathe' shows that if the tailstock can rotate slightly, the axis of the tailstock may not align with the axis of rotation of the headstock. This is purely a consequence of less precise machining, and based on the comments on this forum, is not uncommon on midi-lathes produced in the Far East.
The amount of error that this wobble causes is typically quite small, and usually isn't a problem for most turning tasks. Where it is more likely to be a problem is when drilling - that small inaccuracy causes the drill bit to try to enter the turning slightly off-center. The photo shows the result of a test in which a brad-point bit that was not aligned exactly to the center of the turning carved a path 5mm in diameter around that center. This can cause the hole to be larger than the diameter of the bit. And because the bit will try to center itself in the turning, the bit to flex while drilling - and the resulting metal fatigue can cause small bits to break. While this is usually not a major problem for those who make pens from kits, the folks who make kitless pens need to achieve a higher degree of accuracy when drilling, and subsequently threading, concentric holes. That's why most kitless penmakers use machinists' lathes rather than wood lathes. However, there are a few members of the IAP community who have the skills and patience to do kitless pens with wood lathes in spite of this handicap.
The degree of angular misalignment between the tailstock and the axis of the headstock is a function of the degree to which the protrusion on the bottom of the headstock is smaller than the spacing of the bedways. However, the amount of centering error that can occur at the face of a turning depends on the ram extension, and on the size of accessories between the end of the ram and the turning - which in the case of drilling includes the jacobs chuck and the drill bit. As the ram is extended, and as the bit becomes longer, the centering error increases. I did some measurements on my 12" Turncrafter lathe and found the following:
- The protrusion on the bottom of the bedways is about 0.15mm more narrow that the spacing between the bedways.
- That amount of error is enough to cause as much as 0.48 degrees of misalignment between the tailstock and the spindle axis. Note that aligning the drill bit to the turning can zero out this error, and this maximum occurs if no particular care is taken to center the bit on the turning.
- When drilling using a standard 7mm brad-point pen bit in a jacobs chuck and with the tailstock ram extended to its maximum, the centering error can be as much as 2.5mm as shown in the photograph. Again, this is a worst-case number, and more accurate centering is possible.
- I also found that the spacing between the bedways on my lathe varies across the 18'" length of the bedways - the spacing is wider at the headstock end. This means that the amount of backlash that is possible also depends on where on the bedways the tailstock has been placed.
So this begs the question of how to deal with this problem. Some possibilities include:
- Purchase a lathe produced from a more precise machining process. I suspect that a Robust or OneWay lathe would perform much better than a less-expensive midi-lathe produced in a factory in Asia.
- It may be possible to shim out the protrusion on a specific lathe, but that's beyond my skill set. As my measurements indicate, it would not require a lot of shimming to prevent the tailstock from rotating, but achieving that amount of correction permanently would take the skills of an experienced machinist.
- The simplest approach is to mount the turning on the headstock spindle using whatever approach is preferred, and using the toe of a skew chisel, cut a small dimple in the center. Then, align the tip of the drill bit to that dimple before locking the tailstock down to the bedways. Aligning the bit to the exact center of the turning aligns the tailstock exactly to the rotational axis of the headstock, and essentially eliminates the error.
- The spur on a brad-point or Forstner bit can easily be aligned to a small dimple, but aligning a standard twist drill is more fiddley. My experience is that a twist-drill bit is more easily aligned to a starter hole created by a center bit aligned to that dimple. Also, if the desired diameter of the hole is large, it is easier to align successive bits in steps rather than to try to go directly to a large diameter bit from that initial starter hole.