Introduction to Spindles
 |
 |
 |
 |
 |
 |
 |
 |
 |
Spindles
There are two broad categories of spindles used on CNC machines: brush types
and brushless types. The brush type spindles, as the name implies, use
commutating brushes to transfer current to the coils that cause the shaft to spin.
The brushes tend to wear over time under normal use, and consequently wear
out faster under heavier applications. Another drawback to brush type spindles is
the noise associated with their commutating brushes. The main advantage,
however, is that they are generally much lower in cost. In fact, in some CNC type
applications, they are considered disposable.
This low cost also tends to make these spindles less precise. TIR, or Total
Indicator Runout, is an industry measure of spindle accuracy. TIR measures how
true the rotation of the shaft is. It is typically measured at a fixed distance from
the tip of the spindle shaft and, as the measurement name indicates, determines
how out-of- round the rotation is. Brush type spindle motors generally have TIR
significantly greater than the brushless type spindles. In many applications, such
as woodworking, this is not of concern.
The other broad category of CNC spindles are brushless, also known as AC
spindles. Brushless AC spindles do not have the maintenance issue of brushes
wearing out or the noise associated with brush type spindles. Because AC
spindles require less maintenance, have a better TIR rating, they are also a more
expensive alternative to brush type spindles. The cost is usually justified when
looking at the overall comparison between the two spindle types.
A summary of the comparison of their features is shown below:
In the category of AC spindle motors, there are 3 types: fixed collet spindles,
manual quick change spindles, and automatic tool change spindles.
The fixed collet spindle requires the collet to be changed with a manual
operation that usually requires a couple of wrenches. This is a fairly simple
process but takes a few minutes each time it is required. The manual quick
change and automatic tool change spindles have an electro/ pneumatic system
that releases the current tool and replaces it with another in a matter of seconds.
The automatic tool change system, as the name implies, is done without any
operator intervention while the manual quick change system usually requires the
operator to activate a signal to release the tool and to activate another button to
reload the next tool into the spindle.
The question of which spindle should be selected depends primarily on the
applications and quantities of parts to be made. The benefit of the automatic
tool change system is that a great deal of time can be saved for parts that are to
be made in large quantities that also require a number of different tools. If the
quantities are not so large, the manual quick change might represent an
economical compromise. In the case where the quantities of the parts are small
or only a small number of tools are required, there is no clear benefit to using
either a quick change or automatic tool change system.
Tool Length Offset
The main difficulty with changing tools on spindles is adjusting the length of the
new tool to match the previous tool so that the software can continue at the
correct height of machining. This task is no longer very time consuming. In the
past, the operator would have to either carefully adjust the tool height relative to
the workpiece with gauge blocks or adjust the height of the Z-axis relative to the
work-piece to re-zero the Z-axis. Although this is not difficult, the task had to be
performed carefully or else the second tool would not be at the correct height
and the part might have a visible defect.
Now, all Techno machines and many other CNC machines come with tool-
length sensors and compensation software. This allows for the tool length to not
only be measured but automatically compensated for in the G- Code software,
with a simple and automatic process.
Spindle Power
The traditional way that the spindle power is “measured” is by the HP rating. This
rating has to be carefully considered since the HP rating is generally proportional
to the spindle rpm. For example, if a 3HP spindle is rated at 3HP at 18,000 rpm,
it would only have 2HP at 12,000 rpm. The speed at which the rating is specified
is therefore extremely important.
The other aspect of spindle power that must be considered is the nature of the
power rating. The power rating on the brush type spindles is generally specified
as a momentary peak rather than a continuous rating. The rating for AC spindle
motors is usually considered a continuous rating but even this might be specified
as a function of duty cycle. Some spindles are rated for 100% duty cycle – able
to maintain the rated power continuously, while others are rated for a 60% or
80% duty cycle. In the latter case, the expectation is that the spindle will be
used to its rated power for a few minutes and then allowed to “rest” for a brief
period before the next part. This duty cycle rating is associated with the required
cooling of the spindle.
Spindle Cooling
Three common methods of spindle cooling include: fan, compressed air or
liquid. There are two types of fan-cooled spindles. First is an electric fan which
will blow air through the spindle body. The second is a fan blade attached to the
spindle arbor. This method of air flow is dependent on spindle rpm. Both
methods of cooling have drawbacks. With fan-cooled systems, the duty cycle of
the spindle is approximately 60% to 70%.
Another is the noise generated by the arbor fans which tends to be in the upper
70’s to lower 80’s decibel range; however, electric fan-cooled spindles do not
have noise related issues. These spindle offerings usually do not exceed 24,000
rpm and are usually the most economical.
Compressed-air-cooled spindles allow for a 90% duty cycle. These require a
constant stream of clean nonfluctuating source of compressed air. These
spindles are typically used for greater then 24,000 rpm. These spindles usually
incorporate ceramic bearings.
This method is the most efficient and allows for duty cycles of 100%. These
spindles are ideal for very demanding applications such as production of very
hard materials in a 24/7/365 day operation. The constant loads on the spindle
generate a lot of heat, and the only method to remove this excess heat is through
a separate liquid chiller unit.
Choosing a Spindle
There are many spindle options available for Techno CNC Routers. Correct
selection of a spindle is imperative for optimum machine performance. The
electric spindle is the heart of the machine. Many variables must be considered
when selecting the correct spindle such as material to be cut, production
volume, tooling, machine feed rate, and spindle rpm. Generally, each material
and cut has an ideal tool profile and cutting speed. Larger diameter tools require
slower speeds. Smaller diameter tools require higher speeds. Spindle speed and
feed rate for a given cut must be balanced for best quality, tool life and spindle
life. Incorrect spindle speed is a common error in CNC machining. Machine feed
rate and spindle rpm are directly related to one another. The higher the rpm, the
faster the machine must be cutting. The typical question is how fast should I be
cutting? This can be determined by the chip load. Simple formulas can be used
to predetermine feed rate.
Formula:
(chipload) x (# of cutting edges) x (rpm) = feed rate
Many cutter manufacturers supply this information with the specific cutter to be
used for different types of material to be cut. This will get you close and the
optimal feed rate can be fine- tuned at the machine. Typically, feed rates that
are too slow will decrease tool life due to the increased friction. This increased
friction will not only wear out the cutter, but will also heat up or burn the material
being routed.
Other considerations that must be taken is how to enter the part. Ramping into
the part is the preferred method. The ideal ramp should be between 0 and 20
degrees from the table surface. This angle will allow you to enter into the
material at 100% of the feed rate. At any angle greater then 20 degrees, the feed
rate should be reduced accordingly. Entering into the part on a ramp will greatly
increase spindle bearing and tool life.
There are two broad categories of spindles used on CNC machines: brush types
and brushless types. The brush type spindles, as the name implies, use
commutating brushes to transfer current to the coils that cause the shaft to spin.
The brushes tend to wear over time under normal use, and consequently wear
out faster under heavier applications. Another drawback to brush type spindles is
the noise associated with their commutating brushes. The main advantage,
however, is that they are generally much lower in cost. In fact, in some CNC type
applications, they are considered disposable.
This low cost also tends to make these spindles less precise. TIR, or Total
Indicator Runout, is an industry measure of spindle accuracy. TIR measures how
true the rotation of the shaft is. It is typically measured at a fixed distance from
the tip of the spindle shaft and, as the measurement name indicates, determines
how out-of- round the rotation is. Brush type spindle motors generally have TIR
significantly greater than the brushless type spindles. In many applications, such
as woodworking, this is not of concern.
The other broad category of CNC spindles are brushless, also known as AC
spindles. Brushless AC spindles do not have the maintenance issue of brushes
wearing out or the noise associated with brush type spindles. Because AC
spindles require less maintenance, have a better TIR rating, they are also a more
expensive alternative to brush type spindles. The cost is usually justified when
looking at the overall comparison between the two spindle types.
A summary of the comparison of their features is shown below:
In the category of AC spindle motors, there are 3 types: fixed collet spindles,
manual quick change spindles, and automatic tool change spindles.
The fixed collet spindle requires the collet to be changed with a manual
operation that usually requires a couple of wrenches. This is a fairly simple
process but takes a few minutes each time it is required. The manual quick
change and automatic tool change spindles have an electro/ pneumatic system
that releases the current tool and replaces it with another in a matter of seconds.
The automatic tool change system, as the name implies, is done without any
operator intervention while the manual quick change system usually requires the
operator to activate a signal to release the tool and to activate another button to
reload the next tool into the spindle.
The question of which spindle should be selected depends primarily on the
applications and quantities of parts to be made. The benefit of the automatic
tool change system is that a great deal of time can be saved for parts that are to
be made in large quantities that also require a number of different tools. If the
quantities are not so large, the manual quick change might represent an
economical compromise. In the case where the quantities of the parts are small
or only a small number of tools are required, there is no clear benefit to using
either a quick change or automatic tool change system.
Tool Length Offset
The main difficulty with changing tools on spindles is adjusting the length of the
new tool to match the previous tool so that the software can continue at the
correct height of machining. This task is no longer very time consuming. In the
past, the operator would have to either carefully adjust the tool height relative to
the workpiece with gauge blocks or adjust the height of the Z-axis relative to the
work-piece to re-zero the Z-axis. Although this is not difficult, the task had to be
performed carefully or else the second tool would not be at the correct height
and the part might have a visible defect.
Now, all Techno machines and many other CNC machines come with tool-
length sensors and compensation software. This allows for the tool length to not
only be measured but automatically compensated for in the G- Code software,
with a simple and automatic process.
Spindle Power
The traditional way that the spindle power is “measured” is by the HP rating. This
rating has to be carefully considered since the HP rating is generally proportional
to the spindle rpm. For example, if a 3HP spindle is rated at 3HP at 18,000 rpm,
it would only have 2HP at 12,000 rpm. The speed at which the rating is specified
is therefore extremely important.
The other aspect of spindle power that must be considered is the nature of the
power rating. The power rating on the brush type spindles is generally specified
as a momentary peak rather than a continuous rating. The rating for AC spindle
motors is usually considered a continuous rating but even this might be specified
as a function of duty cycle. Some spindles are rated for 100% duty cycle – able
to maintain the rated power continuously, while others are rated for a 60% or
80% duty cycle. In the latter case, the expectation is that the spindle will be
used to its rated power for a few minutes and then allowed to “rest” for a brief
period before the next part. This duty cycle rating is associated with the required
cooling of the spindle.
Spindle Cooling
Three common methods of spindle cooling include: fan, compressed air or
liquid. There are two types of fan-cooled spindles. First is an electric fan which
will blow air through the spindle body. The second is a fan blade attached to the
spindle arbor. This method of air flow is dependent on spindle rpm. Both
methods of cooling have drawbacks. With fan-cooled systems, the duty cycle of
the spindle is approximately 60% to 70%.
Another is the noise generated by the arbor fans which tends to be in the upper
70’s to lower 80’s decibel range; however, electric fan-cooled spindles do not
have noise related issues. These spindle offerings usually do not exceed 24,000
rpm and are usually the most economical.
Compressed-air-cooled spindles allow for a 90% duty cycle. These require a
constant stream of clean nonfluctuating source of compressed air. These
spindles are typically used for greater then 24,000 rpm. These spindles usually
incorporate ceramic bearings.
This method is the most efficient and allows for duty cycles of 100%. These
spindles are ideal for very demanding applications such as production of very
hard materials in a 24/7/365 day operation. The constant loads on the spindle
generate a lot of heat, and the only method to remove this excess heat is through
a separate liquid chiller unit.
Choosing a Spindle
There are many spindle options available for Techno CNC Routers. Correct
selection of a spindle is imperative for optimum machine performance. The
electric spindle is the heart of the machine. Many variables must be considered
when selecting the correct spindle such as material to be cut, production
volume, tooling, machine feed rate, and spindle rpm. Generally, each material
and cut has an ideal tool profile and cutting speed. Larger diameter tools require
slower speeds. Smaller diameter tools require higher speeds. Spindle speed and
feed rate for a given cut must be balanced for best quality, tool life and spindle
life. Incorrect spindle speed is a common error in CNC machining. Machine feed
rate and spindle rpm are directly related to one another. The higher the rpm, the
faster the machine must be cutting. The typical question is how fast should I be
cutting? This can be determined by the chip load. Simple formulas can be used
to predetermine feed rate.
Formula:
(chipload) x (# of cutting edges) x (rpm) = feed rate
Many cutter manufacturers supply this information with the specific cutter to be
used for different types of material to be cut. This will get you close and the
optimal feed rate can be fine- tuned at the machine. Typically, feed rates that
are too slow will decrease tool life due to the increased friction. This increased
friction will not only wear out the cutter, but will also heat up or burn the material
being routed.
Other considerations that must be taken is how to enter the part. Ramping into
the part is the preferred method. The ideal ramp should be between 0 and 20
degrees from the table surface. This angle will allow you to enter into the
material at 100% of the feed rate. At any angle greater then 20 degrees, the feed
rate should be reduced accordingly. Entering into the part on a ramp will greatly
increase spindle bearing and tool life.
| Industry Links |
