Whats the purpose of an arrow spinner? I was looking through the build a longs and saw this    Arrow Spinner.

And how do you use it?

You lay your arrow across the rollers and spin it to make sure your broadhead is mounted straight.

Also you can check for strightness

Not just broadheads but your nocks also. It`s one tool I would`nt want to get along without.

Well I have 2 doz. shafts coming my way. How do you use it to check for straight nocks, and shafts?

Get an arrow straightner, dial indicator type such as a Cavalier, I really like mine.

Michael, you spin the arrow through the washers and if they aren't straight, they will spin at an angle

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If they are straight they will line up with the shaft.
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Bottom line is the straighter the better. If the point of the arrow and the knock are in line with each other and spin true whats in between can be somewhat crooked. Example would be shafts made with reeds etc.
Ron