Yajurveda 35/5 says "तप॑से॒ शू॒द्रं" means श्रम, i.e. one who generates food etc. through hard work n शूद्र is the one who does hard work etc. like crafts. The word तप is used in the वेदमन्त्र for ईश्वर who creates all the things of the world with infinite power.
Once the Vedic Samskara (यज्ञोपवीत संस्कार) of the Shudra's son begin with proper Vedic rites, he gets the title of Brahmans, Kshatriyas and Vaishyas by virtue and deeds.
Every person is born a Shudra. It is only on the performance of the यज्ञोपवीत संस्कार later that he becomes a द्विज. Every one attains his or her varna only after completing education.
Vedas do not discriminate between different classes.They consider everyone as equal. Yajurveda says that O ईश्वर make me so gentle that all Brahmans, Kshatriyas, Vaishyas and Shudras have affection for me.
प्रियं मा कृणु देवेषु प्रियं राजसु मा कृणु।
प्रियं सर्वस्य पश्यत उत शूद्र उतार्ये॥
O Ishwara! Please make me beloved among enlightenment men, male me beloved among the princes and make me dear to everyone who sees, be he Shudra.
There is no one superior or inferior in the Vedas. All are equal just like brothers.All should help each other to attain the pleasures of this as well as the other world.
प्रियं मां दर्भ कृणु ब्रह्मराजन्याभ्याम्शूद्राय चार्याय च।
यस्मैं च कामयामहे सर्वस्मै च विपशयते॥
|| कृण्वन्तो विश्वमार्यम् ||