Do you need a cdl to drive a welders truck?

If you're carrying quantities of oxyacetylene and oxygen which require the display of placards, and/or the vehicle has a Gross Vehicle Weight Rating of more than 26,000 lbs., then yes you do.

If it's under 26,000 lbs. GVWR and you must show hazmat placards for oxygen and acetylene, you need at least a Class C CDL with a hazmat endorsement. If you're not carrying those commodities in a quantity which requires placarding, you don't need a CDL.

If the vehicle is a single vehicle or combination of over 26,000 lbs. GVWR/GCWR, and there is no trailer or a trailer in tow is under 10,000 lbs. GVWR, then you'll need a Class B CDL (with hazmat endorsement if applicable).

If the vehicle is a combination with a higher than 26,000 lbs. GVWR, and the trailer in tow has a GVWR of more than 10,000 lbs., a Class A CDL is required (with hazmat endorsement if applicable).