问题 问答题 简答题

线切割进给速度对工件表面质量的影响?

答案

参考答案:

(1)进给速度过高。这时工件蚀除的线速度低于进给速度,会频繁出现短路,造成加工不稳定,平均加工速度降低,加工表面发焦,呈褐色,工件的上下端面均有过烧现象。

(2)进给速度过低。这时工件蚀除的线速度大于进给速度,经常出现开路现象,导致加工不能连续进行,加工表面亦发焦,呈淡褐色,工件的上下端面也有过烧现象。

(3)进给速度稍低。这时工件蚀除的线速度略高于进给速度,加工表面较粗、较白,两端面有黑白相间的条纹。

(4)进给速度适宜。这时工件蚀除的线速度与进给速度相匹配,加工表面细而亮,丝纹均匀。因此,在这种情况下,能得到表面粗糙度好、精度高的加工效果。

阅读理解

“Enough” with the multivitamins already. That’s the message from experts behind three new studies that tackled an often debated question: Do daily multivitamins多种维生素make you healthier?

“We believe that the case is closed - supplementing(补充) the diet of well-nourished adults with (most) mineral or vitamin supplements has no clear benefit and might even be harmful,” concluded the authors of the editorial summarizing the new research papers. They urge consumers to not ‘waste’ their money on multivitamins. “The ‘stop wasting your money’ means that perhaps you’re spending money on things that won’t protect you long term,” editorial co-author, Dr. Edgar Miller said, “What will protect you is if you spend the money on fruits, vegetables, nuts, beans, low fat dairy, and things like that. Exercising would probably be a better use of the money.”

The strong message was based on a review of the findings from three studies that tracked multivitamins link to cancer protection, heart health, and brain and cognitive(认知的) measures. The first study looked at vitamin supplementation’s role in preventing chronic(慢性的) disease. The next study looked at whether long-term use of multivitamins would have any effect on slowing cognitive decline. The third study looked specifically at multivitamins and minerals role in preventing heart attack. “The three studies found no difference in rates of chronic disease, heart attack and the need for hospitalization between vitamin-takers and placebo(安慰剂)-takers.” Dr. Edgar Miller stated.

One expert agreed some nutrient-deficient people may still benefit from multivitamins. “There might be an argument to continue taking a multi(vitamin) to replace or supplement your not healthy diet,” Dr. Edgar Miller added. He also notes that vitamins can benefit people with celiac disease and those who are pregnant.

小题1:This text is likely to be selected from a book of           .

A.medicine

B.education

C.food

D.business小题2:Which of the following is NOT Dr. Edgar Miller’s opinion according to the passage?

A.Vitamins should not be used for chronic disease prevention. Enough is enough.

B.Having a balanced diet and exercising would probably be a better way to keep healthy.

C.The three studies do not provide support for use of multivitamin supplements.

D.Taking vitamins to replace or supplement your healthy diet is necessary.小题3:What can be inferred from the text?

A.Vitamin supplements have proved harmful to the health of adults.

B.Vitamin supplements are beneficial in certain conditions.

C.Nowadays taking vitamin supplements is common to most people.

D.Daily multivitamins will make you healthier.小题4:The author’s purpose of writing the text is most likely to___________.        .

A.persuade

B.describe

C.inform

D.instruct

单项选择题