The trickiest part of turning a bat is that down near the handle, a bat is relatively thin compared to its length. As you start cuttingtowards the handle, the bat will vibrate and cause the tool to bounce and make spiral chatter marks. The first thing to do is to make sure your tools are as sharp as possible. The second thing to do is to find a way to support the stock in the middle. The best and most convenient way for the experienced turner is to put your hand right behind the cut and support it. This may be a little daunting for the novice and you can use a store bought or shop built steady but I find support from behind to be the most versatile and effective way to prevent chatter marks.