Service tax on labour charges

We are doing labour work - HEAT TREATMENT. we receive goods which are excisable after full processing. Some of the big customers pay excise duty on them - so we do not charge service tax. We have small customers who send the same goods - but since their turnover is less than 1 crore they do not come under excise and hence dont pay excise duty. Should we charge service tax on such parties. Please clarify and help