I find only 5/8" on Amazon and in the UK I see several for sale and in USA there is Sorby version for $65 and a bunch of 5/8" dia spur drives for ~ $23-25. I'm thinking of buy a 5/8" and turn it down to a 1/2" and save my money. I have a 3/8" center, bought for same purpose of smaller tenons on rungs, but it has a tendency to drill into even small chair rungs when turning.
If you klnow of a 1/2" spur center at a more reasonable price point??? Thanks!