Newbie looking at 6040 (China CNC) for polypropylene 15mm sheet

[Zeeflyboy]

Normal double sided carpet tape works but you can actually hear the difference compared to a proper clamped mounting in the amount of vibration it allows. You can also see chips "dancing" on the surface which shows the part is vibrating... I wouldn't recommend it for where tight tolerances or good finish is required - much better to get a good fixture plate sorted out.

I use the TPA-2 here (in stock now, though with a smaller stylus I think which may actually be better for our purposes) http://www.kurokesu.com/shop/TPA2

As mentioned though, in this particular scenario it won't do much you couldn't do with a tool steel rod and a micrometer. If you don't already own one you also need to feature in the cost of a decent dial indicator to allow you to get the ball centred.

For your particular scenario here I may even be tempted to machine a small vacuum fixture plate which will hold the parts in alignment and hold down the part whilst allowing you to machine the exterior in a single operation. I think you will never be happy with finish unless you allow it to machine the full exterior depth in a single op. Even a tiny misalignment will show in the finish.