We have a 276 and are running wide soft maple flooring with 6" three blade straight knives. Every so often it will tear or chip out leaving a product so rough we can't sell it. We have tried to slow it down with some success but it still is a problem. We have done everything we can think of to stop this problem, including changing the pressure shoe, sharpening the knives, checking for play in the outboard bearings, and running as tight and slow as possible. We also have some mill marks that are a pain to deal with. Help.
(Solid Wood Machining Forum)
From Dave Rankin, technical advisor:
Tearout comes from several possible things. If this problem is somewhat sporadic I would discuss the problem with the people that dry your lumber. Many drying companies will rush the time in the kiln and the wood suffers because of it. This lack of proper conditioning will result in more tearout than normal. Second it is possible that the grain in the wood is more likely to tearout due to nature. Some wood has tighter grain while other wood will be looser grain.
Tearout can also be increased with reduced holddown control. The work piece needs to be held secure with all of the holddowns. Sharp tools are required. Some of the information that you did not provide:
Hook angle of the head - recommended 5-12 degree
Back clearance angle - recommended 20-22 degree
Side clearance angle - recommended not to exceed 5 degree.
Jointed or non-jointed finish - if jointed, the land on the knives must be less than 1/64" to provide a good finish.
Feed rate - this will vary according to the number of knives that are jointed.
RPM of the spindles - this will help in determining the feed rate.
After you have exhausted all of your options you may then consider special tooling. The use of shear angle heads has proven to reduce tearout in many maple applications.