They are polished smooth so the coin will slide in easier. These dies are easier to use because they have a grove machined at the top to hold the coin
UNIVERSAL Coin Ring Folding / Reduction Dies Explained; Coin Ring Tools
Coin Ring-Making Tools
3 Double Sided 17° Fold Over Reduction Dies 1.0 to 1.5 Coin Ring Tools
One Universal Folding/Reduction Die .9″ x 1.0″ @ 17 degrees – Coin Ring Tools & Custom Made Coin Rings – Jason's Works
This die can be used to start the fold process. It has a grove machined at the top to hold the coin flat that makes it a lot easier to get started
1.0 & 1.1 DOUBLE FOLD OVER REDUCTION 17° DIE COIN RING TOOLS STABILIZING LIP
17 Degree Universal Reduction/Folding Die Set 5 Pieces, Stainless Steel: : Industrial & Scientific
17 Degree Folding Die Set 5 Pieces Reduction/Folding Die Set + Folding Cone, Stainless Steel Coin Making Tool Made in The USA : Tools & Home Improvement
17 DEGREE Folding Die (0.70″ x 0.80″) for making coin rings Stainless Steel – Legacy Brand Coin Ring Tools
One Universal Folding/Reduction Die .9″ x 1.0″ @ 17 degrees – Coin Ring Tools & Custom Made Coin Rings – Jason's Works
A rundown on the different reduction die sizes
1.0 & 1.1 DOUBLE FOLD OVER REDUCTION 17° DIE COIN RING TOOLS STABILIZING LIP
One Universal Reduction Die .7″ x .8″ @ 17 degrees – Coin Ring Tools & Custom Made Coin Rings – Jason's Works