Down-Cut Spiral Bit Sizes for CNC: 1/16 to 3/8 Inch
By Premium CNC & Router Bits for Precision Cutting | Published: 2026-11-02
Category: How-to Guides
Learn how to choose down-cut spiral bit sizes for CNC work. Match diameter, length of cut, and reach to your material and top-edge finish for clean results.
Quick answer
Choose the largest down-cut spiral bit diameter your smallest feature and spindle can handle, then match length of cut and reach to your material thickness. For clean top edges in plywood and melamine, a 1/8″ or 6mm bit covers most work; step down to 1/16″ only for fine detail.
Why diameter and length of cut matter more than overall length
Down-cut spirals push chips downward, so the top veneer or laminate stays supported during the cut. That is why they produce clean top edges in plywood, melamine, and MDF. But the bit only cleans the edge along its length of cut. If your material is thicker than the length of cut, the lower portion of the edge may still tear out.
Diameter affects both finish and speed. Smaller bits leave a finer edge in tight curves and small features, but they deflect more and require slower feed rates. Larger bits remove material faster and stay rigid, but they cannot cut tight inside corners or fine details. Match the diameter to the smallest radius in your design, not to the largest straight run.
When long-reach bits help and when they hurt
Long-reach down-cut bits extend the cutting edge below the collet, letting you reach deeper without the shank rubbing. They are useful for deep pockets, mortises, or thick stock where a standard-length bit would bottom out. However, extra reach reduces rigidity. Use the shortest reach that clears your workpiece, and reduce feed rate or depth of cut when you extend reach.
If your work is mostly 3/4″ plywood or 18mm MDF, a standard-length bit with enough length of cut is usually stiffer and safer. Reserve long-reach bits for the jobs that actually need them.
Step-by-step bit selection for clean top edges
- Measure your smallest inside radius: Check your CAD or drawing for the tightest inside corner. The bit diameter must be no larger than twice that radius. For a 1/16″ radius, use a 1/8″ bit or smaller. If the radius is smaller than your smallest bit, redesign the corner or plan a secondary operation.
- Match length of cut to material thickness: Choose a bit whose length of cut equals or exceeds the material thickness. For 3/4″ plywood, a 1″ length of cut works; for 1/4″ stock, a 3/16″ or 5/8″ length of cut is enough. A bit with a shorter length of cut than the material will leave tearout on the lower part of the edge.
- Check reach for deep pockets or thick stock: If the shank would contact the workpiece before the flutes finish the cut, switch to a long-reach bit. Use the shortest reach that clears the material. Excess reach increases deflection; reduce feed rate by 20–30% when using a long-reach bit.
- Confirm shank size and collet compatibility: Verify your router or spindle collet matches the bit shank. The 1/4″ shank is common on many CNC routers; 1/8″ and 6mm shanks require the corresponding collet. Never use a reducer sleeve without checking runout; a poor fit ruins edge quality and can damage the bit.
- Test the top edge on scrap: Run a short test cut in the same material and direction. Inspect the top edge for fuzz or chip-out. If the edge is not clean, try a smaller diameter, a slower feed, or a down-cut bit if you were using up-cut. Down-cut spirals excel on top edges, but they can pack chips in deep slots; clear chips with air or adjust depth per pass.
Product facts to guide your choice
- 1/16″ Long-Reach Down-Cut Spiral Bit (AstraHP Coated): 1/16″ tip diameter with 3/16″ length of cut and 1/2″ length of reach. Ideal for fine detail and thin stock where a larger bit would chip or not fit, but the short length of cut limits it to shallow edges.
1/16″ Long-Reach Down-Cut Spiral Bit (AstraHP Coated) - 1/8″ Long-Reach Down-Cut Spiral Bit (AstraHP Coated): 1/8″ tip diameter with 1″ length of cut and 1/4″ shank, also available in 5/8″ length of cut. A versatile size for clean top edges in plywood and melamine up to about 1″ thick; the shorter 5/8″ option adds rigidity for thinner stock.
1/8″ Long-Reach Down-Cut Spiral Bit (AstraHP Coated) - 6mm Down-Cut Spiral Bit – 1/4″ Shank (AstraHP Coated): 6mm cut diameter with 1″ length of cut and 1/4″ shank. Slightly larger than 1/8″ for faster material removal while still fitting many CNC collets, and well suited to grooves and dados in metric designs.
