Manufacturing (machining time for turning)

In summary, the problem at hand involves determining the minimum time required to machine a large forging, taking into consideration the use of a 1" micrometer, the dynamic shear strength of the material, and the desired deflection due to cutting forces. The equation VT^n = C is being used to approximate tool life, with values of n = 0.151 and C = 120.3 already determined. The Brinell Hardness of 300-400 corresponds to a Specific Energy of 3.6-4.4 Nm/mm3, with an average value of 4 Nm/mm3 being used. Other calculations include determining the depth of cut, Pc, and Fc, Ff, and Fr forces.
  • #1
nardoo
1
0
Hi everyone,

Following is a problem that I have made some progress on but am now stuck. I can see that I probably need to use the orthogonal model to approximate turning but I cannot see how do it, there seems to be too many unknowns. I am confused by how n and C relate to the model or how to use the information given in part (c) of the question. Finally, the deflection... I don't know what that corresponds to, is it the chip size?

Thanks for any help on this.

Problem:
You are using a 1” micrometer.
Katrin would like to know the minimum time required to machine a large forging. The 8 ft long forging is to be turned down from an original diameter of 10” to a final diameter of 6”. The forgin has a BHN of 300-400. The turning is to be performed on a heavy duty lathe, with a 50 hp motor and continuously variable speed drive on the spindle. The work will be held down between centres , and the overall efficiency of the lathe is 75%.

The log is made from medium carbon 4345 alloy steel. The steel manufacturer, some basic experimentation, and established knowledge of the product and its manufacture have provided the following info:

(a) a tool life equation developed for the most suitable type of tool material at a feed of .02ipr and a rake angle of alpha = 10 degrees. The equation VT^n = C generall fits the data, with V= cutting speed and T = time in minutes to tool failure.


(b) Two test cuts were run, one at V = 60sfpm
Where T = 100 min and another at V = 85 sfpm where T = 10 min


(c) The dynamic shear strength of the material is on the order of 125,000 psi. Jay decides to make two test cuts at the standard feed of .02 ipr. He assumes that the chip thickness ratio varies almost linearly between the speeds of 20 and 80 fpm, the values being .4 at the speed of 20 fpm and .6 at 80 fpm. The chip thickness values were determined by micrometer measurements to determine the value of Rc ( Rc is chip ratio )

(d) The log will be used as a roller in a newspaper press and must be precisely machined. If the log deflects during the cutting more than .005” the roll will end up barrel shaped.


How should I proceed to estimate the minimum time required to machine this forging, assuming that one finishing pass will be needed when the log has been reduced to 6” in diameter? The deflection due to cutting forces must be kept below .005” at the mid log location.

Assume Fc * .5 = Ff and Ft * .5 = Fr and that Fr causes the deflection


What I have already worked out:

(1) in the taylor tool life equation, n = 0.151, C = 120.3

(2) Brinell Hardness of 300 – 400 corresponds to Specific Energy (U) 3.6 – 4.4 Nm/mm3 – I used an average value of 4 Nm/mm3 in subsequent calcualtions

(3) depth of cut = 2 in = 50.8 mm (i'm not in the states)

(4) Pg =

Pc = (50)(0.75) = 37.5

(5) Pc = URmr
Rmr = 37.5/4 = 9.375 mm3/s
 
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  • #2
(6) Fc = 2.3 * Pg^0.5 = 2.3 * (37.5)^0.5 = 13.97 N Ff = 0.5 * Fc = 6.985 NFr = 0.5 * Ft = 0.05 (I don't know how to calculate Ft)(7) Deflection due to cutting force must be kept below .005” at the mid log location. I don't know what this corresponds to.
 
  • #3


(6) Vc = 37.5/9.375 = 4 mm/s

(7) Vf = .02 * 4 = 0.08 mm/s

(8) Fr = 0.08 * 4 = 0.32 mm


I can see that you have made some good progress on this problem and have attempted to use various equations and information to estimate the machining time for turning. However, I can also see that you are stuck and unsure of how to proceed further. Let me provide some guidance and suggestions to help you move forward with your calculations.

Firstly, you are correct in thinking that the orthogonal model can be used to approximate turning. This model is based on the assumption that the chip thickness ratio (Rc) is constant throughout the machining process. In your calculations, you have already determined the values of Rc at different cutting speeds, which is a good start.

Next, you need to consider the dynamic shear strength of the material (125,000 psi) and its relationship to the cutting forces. The cutting forces (Fc, Ff, Ft, and Fr) can be estimated using the specific energy (U) and the depth of cut (50.8 mm). You have already calculated the specific energy based on the Brinell Hardness of the material and have determined the depth of cut.

Once you have estimated the cutting forces, you can use the equation given in part (c) of the problem to determine the chip thickness ratio (Rc) at different cutting speeds. From there, you can use the values of Rc to estimate the cutting speeds at which the chip thickness ratio will be 0.4 and 0.6, as mentioned in the problem.

Now, let's consider the deflection of the log during the cutting process. This deflection should be kept below 0.005 inches to ensure that the log is precisely machined. You have already made an assumption that Fr causes the deflection and have calculated its value. However, you need to consider the deflection due to all the cutting forces (Fc, Ff, Ft, and Fr) and not just Fr alone. You can use the assumption given in the problem that Fc * 0.5 = Ff and Ft * 0.5 = Fr to estimate the total deflection.

Finally, you can use all the information and calculations you have made so far to estimate the minimum
 

1. What is machining time for turning?

Machining time for turning refers to the amount of time it takes for a machine to complete the turning process on a workpiece. This process involves rotating the workpiece while a cutting tool removes material to create the desired shape.

2. How is machining time for turning calculated?

Machining time for turning is typically calculated by dividing the total length of the workpiece by the cutting speed of the machine. This calculation can also take into account other variables such as the depth of cut and feed rate.

3. What factors can affect machining time for turning?

Several factors can affect machining time for turning, including the type of material being machined, the complexity of the workpiece, the cutting tool used, and the speed and feed rate of the machine. Other factors such as machine condition and operator skill can also play a role.

4. How can machining time for turning be reduced?

There are several ways to reduce machining time for turning, including using high-speed cutting tools, optimizing the cutting parameters, and using advanced machining techniques such as multitasking and high-speed machining. Improving the efficiency and accuracy of the machine and operator skills can also help reduce machining time.

5. What are the benefits of reducing machining time for turning?

Reducing machining time for turning can lead to increased productivity and cost savings for manufacturers. It also allows for faster production of parts, which can be beneficial in meeting tight deadlines and fulfilling large orders. Additionally, reducing machining time can improve the overall quality and consistency of the machined parts.

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