formula for thread on a lathe

[cob]

does any know what the formula is how to find the rpm and feeds when doing a thread on a lathe

[sinha_nsit]

Choose the rpm to suit the desired cutting velocity.
Cutting velocity (approx) = ND/300 m/min where D is diameter in mm,
or equal to ND/4 ft/min where D is in inch.

Need not worry about the feed. Just specify pitch (F-word in a threading cycle); the control would automatically calculate the required feedrate.

[cob]

did not quiet understand what you ment but say I wanted to do a tread of 1/2 -13 material is aluminuim and I am working with inches .
how would I calculate that rpm

[angelw]

Originally Posted by cob did not quiet understand what you ment but say I wanted to do a tread of 1/2 -13 material is aluminuim and I am working with inches . how would I calculate that rpm

Determining the RPM to use in a screw cutting operation often involves more than just calculating a RPM based on the surface speed for the workpiece and cutting tool material.

The formula to calculate the RPM using an imperial (inch) system is as follows:

RPM = CS X 12 / pi /D Where:
CS = Material Surface Speed for given cutting tool material
D = Workpiece Diameter
pi = mathematical constant (3.143 approximately)

1. Surface speed (CS) is normally specified in feet/min or meters/min.
2. Rotating cutting tools and workpiece diameters are normally specified in inches or mm.

Because items in 1 and 2 are specified in different units, one must be converted to the units used in the other. Accordingly, that's the reason for multiplying CS by 12 in the formula above; to convert feet/min to the same units used to specify the workpiece diameter D.

This formula can be simplified by dividing 12 by pi to obtain a constant to use in the formula.
12/pi = 3.8197

The formula for imperial calculations becomes:
RPM = CS X 3.8197/D
and can be further simplified by rounding 3.8197 to 4.0 to obtain a close approximation.
RPM = CS X 4/D

As mentioned earlier, there are other factors to consider. The slide velocity of the carriage carrying the cutting tool is a product of the RPM and the feed rate in feed/spindle revolutions; in the case of a screw thread, the lead of the thread. The slide velocity must not exceed the maximum slide velocity specified for the machine, in most instances this will be the Rapid Traverse speed of the machine. Once the RPM has been calculated using the above formula, multiply the result by the lead of the thread; for a single start thread this will be the same as the pitch, and compare the result with the maximum allowable slide velocity of the machine to ensure that its not being exceeded.

An error in the thread lead occurs when the slide accelerates from zero to the correct slide velocity at the beginning of the thread, and during deceleration to Zero at the end of the thread. The length of the Acceleration/Deceleration zones increase as the RPM and thread lead increases. The error resulting from Acceleration/Deceleration is control specific, but must be considered when programming the start position of the threading tool and how large an undercut into which to run the tool at the end of the thread. Sometimes there will have to be a compromise between the RPM for the best cutting speed and factors such as maximum slide velocity and slide Acceleration/Deceleration.

Regards,

Bill

[cob]

WOW
thanks for taking the time to explain all this.
i though it was a simple formula. but i guess i have to read this over and over till i get this
thanks

[angelw]

Originally Posted by cob WOW thanks for taking the time to explain all this. i though it was a simple formula. but i guess i have to read this over and over till i get this thanks

It really isn't that complex, but to avoid problems you need to consider all aspects involved. The points I raised regarding maximum slide velocity and Acceleration/Deceleration normally only become applicable in practice with very small diameters when the spindle RPM are quite high.

You can calculate the maximum RPM for any particular lead of thread`using the following formula for an imperial set machine.

RPM = MV X 12 / TL Where:
MV = Maximum slide velocity of the machine in Feet/Min
TL = Thread Lead
Lets say that the machine has a Z Rapid Travers speed of 32 feet per minute (10 meters per minute approx.) and that the lead of the thread`is as in your example, 13 TPI (0.0769"), then the formula would be:

RPM = 32 X 12 / 0.0769
RPM = 4993

Using your example of 1/2 13 thread in aluminum, and a surface speed of, say, 1400 f/min, then using the formula:

RPM = CS X 12 / pi /D Where:
CS = Material Surface Speed in f/min
D = Workpiece Diameter
pi = mathematical constant (3.143 approximately)

You get the following:

RPM = 1400 X 12 / pi / .5
RPM = 10,695

As can be seen from the above, the resulting RPM is way in excess of the max RPM that can be used with a feed rate of 0.0769" per rev for a machine with a max slide velocity of 32f/min.

Regards,

Bill