Planer vs. spiral insert heads -- for planers

Which provides the best finish--a planer type or a spiral insert knife head? January 16, 2001

What difference in finish can be expected from a planer type head (such as Terminus) versus a spiral insert knife head (such as DML Spiral Pro)?

Forum Responses
A Terminus head uses a series of single knives that go the entire width of the head. The DML Spiral Pro uses small inserts that are staggered in a spiral around the head.

Both heads are excellent for roughing wood. The spiral design is quieter. The carbide used in a spiral head is normally C2 grade, while the Terminus or Great Lakes carbide is C4 grade. The C2 does not last as long as the C4. This means that the C2 inserts must be flipped more often. For a normal 9" width head, the spiral head has about 64 inserts while the straight head has no more than 4 knives. The time to flip inserts differs from 5 minutes to 1 hour.

For most woods, the C4 carbides run longer that the C2. The finish quality for roughing is not much different. The finish quality for a finished surface varies depending on the insert used. The straight design produces a very clean full width cut. The spiral design can use two different inserts. The first insert is straight across and can cause minor lines in the wood surface. The second insert is slightly rounded at the edges. This allows the overlap between the inserts to blend well. If the inserts are installed correctly, there is generally no blend line.

Dave Rankin, forum technical advisor

Carbide or diamond knives start out much duller than HSS. As dulling of HSS occurs, a fresh joint brings back the finish quality. While HSS is capable of crowding 20-26 knife marks into an inch in hardwood because of its keen edge, carbide or diamond is capable of 12 at most. So high speed steel straight knives win every time for finish quality.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
I think it should be noted that if your wood has a lot of knots or dried glue, then the C-2 grade will be more forgiving than the C-4 as it has a more shock resistance with 6% cobalt versus the 3% cobalt found in a typical C-4. However, not all manufacturers’ C-2 and C-4s are the same. I am basing this on the C-2 and C-4 grades that I manufacture.