The dimensions for toolpost tool holders are published in catalogs, so the
lathe user can compare the dimensions to the lathe dimensions.

If one chooses a QCTP series/size that's too large for a particular lathe,
then the setup may not accept suitably sized cutting tools for that lathe.

A small QCTP will have an advantage of allowing the user to use larger
cutting tool sizes, in some cases.

Taking the TOC top of compound (plinth) distance to the lathe spindle
centerline height will show which particular QCTP series/size will be best
suited to that lathe.

The important dimension is the height of the tool holder's shelf/cutout for
the cutting tool to rest on. This dimension determines how large the cutting
tools can be, by adding the desired cutting tool height to that dimension,
before the top of the cutting tool can no longer be adjusted down to the
spindle's centerline.

All the common dovetailed QCTPs allow the holders to slide to the bottom,
stopping on the TOC.

If one chooses a B or 200 series QCTP for a small lathe, the holders may
prevent 3/8" cutting tools from meeting the spindle centerline (tool tip too
high).. since the holder may come to rest on the TOC. Then the user would
need to use smaller cutting tools (possibly 5/16 or 1/4").
Choosing a toolpost series that's too small could prevent the cutting tool
tip from reaching the spindle centerline (without adding a riser block).

The primary consideration, IMO, is scale.

Choosing a B/200 series for a 9" lathe doesn't guarantee more rigidity. One
wouldn't expect to run 1/2" or 3/4" cutting tools on a 9" lathe, and they
likely wouldn't fit due to the limited spindle centerline height over the
TOC.
Considering other factors such as motor size and the lathes's overall
rigidity, should determine which size cutting tools are appropriate for a
specific size of lathe.

Choosing a B/200 series for a 12" lathe is generally more reasonable.
Using an A/100 series on a 12" lathe may be completely satisfactory and
perform just as well, depending upon the usual chip loads and type of
workpieces.

Reviewing the dimensions of tool holders will be the determining which
holders are going to be suitable for future purchases, but also for the
initial QCTP choice.

Some other factors might be which sizes, and how much existing tooling one
already has in the shop, and perhaps a less common need to use some unusual
sized tooling such as a large boring bar that was only $5 at a garage sale.

--
WB
..........


"Lloyd E. Sponenburgh" lloydspinsidemindspring.com wrote in message
. 3.70...
I'm about to take the plunge, but the Aloris catalog only lists specific
lathes, not a way to calculate which toolholder one needs for a particular
compound height.

Does anyone know the formula, or how much a (say) BXA allows one to adjust
the holders vertically once on the post?

LLoyd