## 1.10 PRACTICE PROBLEMS

1. If a bar of SAE 1040 is to be turned with a high speed steel tool with a feed of 0.015” per revolution, and a depth of 0.050”. Previous experiments have revealed that the following cutting velocities yielded the following tool lives,

90 fpm for 30 min.

80 fpm for 90 min.

75 fpm for 150 min.

a) estimate the cutting speeds to get tool lives of 60 and 120 minutes.

b) calculate the mrr at the two speeds found in part a).

2. Two tools are being compared for their costs. The table below summarizes the details of each tool. Find the economic tool life and cutting speed for each tool, and determine the least expensive tool.

(ans. tool A T = 45.9 min., V = 232.6 fpm, tool B T = 11.73 min., V = 305.6 fpm, both A and B cost \$0.062/min.)

3. What happens to the cutting process as the temperature rises?

ans. As temperatures rise both the tool and work change. Heat causes expansion, therefore the dimensions change, and accuracy decreases. Heat also causes decreased strength of the material. This causes faster wear in the tool, but also makes the work easier to cut.

4. We are going to estimate the effects of feedrate on tool life. Some simple calculations yield the Taylor tool life coefficients of n = 0.4 and a C = 400. Find the change in tool life (in %) when velocity drops by a) 20% and b) 40%. [based on Kalpakijian]

5. Some tools use coatings that reduce the coefficient of friction. How does this affect the cutting process?

ans. Reduced friction in cutting reduces heat in the chip and tool, and this will prolong tool life. The reduced friction also decreases the wear rate and prolongs tool life.

6. Describe the factors that are used to decide when a tool should be reconditioned, recycled or discarded.

ans. Two failures typically occur; wear and fracture. If a tool is worn, and the material and geometry permit, we can recondition a tool - grinding is common. If a tool is fractured or can’t be reconditioned, it can be discarded. In some cases tools contain parts that can be reclaimed, or materials that can be recycled.

7. As cutting temperatures rise materials expand. How does this affect the cutting process?

8. Consider that at a certain velocity we will get the lowest cost per piece. As the cutting velocity rises the cost per piece rises (but we will improve the production rate) what cost components rise or drop?

9. Describe at least two methods that generate heat during machining.

10. How does the heat generated during cutting affect the operation?

11. What are the main failure types found in tools? Where do these typically occur on the tool?

12. What does the parameter ‘n’ mean in Taylor’s tool life equation? How is ‘C’ different?

13. What properties are desired in a material for a cutting tool?

14. What are the main functions of cutting fluids?

15. We have been asked to calculate the cutting speeds that gives the maximum possible production rate and lowest cost for an existing job. The current tool will last for 4 hours if we cut at 300 fpm and 2 hours at 345 fpm. The following things are known about the job.

- the tool costs \$6.50 and has 2 edges that can be reground 5 times before discarding.

- it takes 5 minutes to change the tool, and 10 minutes to regrind it.

- the labor rates for the operators is \$25.00/hr.

- the tool room labor rate is \$35.00/hr for regrinding tools.

(ans. Vecon = 525fpm, Vcost = 404)