holes are tapered

I have an ox machine. No matter what machining operation I do for making holes in 10mm PVC , i get a tapered hole. The top part of the hole is about 0.15mm larger than the bottom part.
I have used a few different feeds but no change and have checked that the bit itself is not tapered.

Any ideas?

 

There is no melting. It is producing nice chips. I'll try reduce any flex in z axis.

 

It was a deflection issue. I was cutting the PVC with a 1.5mm depth of cut. As a test, I changed the depth of cut to 0.1mm. There was basically no tapering! The hole was still undersized because I previously oversized the tool diameter in CAM. Once I adjust this back down to the right diameter, I should be ok I reckon.