Is there such a thing as a 1/2" shank 6mm spiral cutter?

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23 Jun 2013
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I've been routing out 6mm (well, 6.7mm, but hey ho) dadoes to slide in cabinet backs and drawer bottoms. Having just broken my bit, I did a bit of work to see what might be the problem. My collett seems ok, and there aren't any worrying marks on the broken bit (it's a trend 6.3mm double-flute bit with a 1/2" shank), so I'm going on the assumption that I was putting too much stress on the bit. From reading around, I can see why a single flute or better yet - a spiral down-cut bit would be much faster, cleaner and more efficient. However, I'm at a total loss to find a reasonably priced bit that will do ~6mm dadoes in a 1/2" shank. I could simply swap out to a 1/4" shank as my router (the Hitachio M12V) has both, but I'd much prefer to go with the larger shank for more stability etc. Am I missing something here? I'd figured this would be a pretty typical piece of kit given how often 6mm plywood serves as cabinet pieces!
I’ve never seen one. Most (all?) the spiral bits I’ve used have been solid carbide. I suspect there are a few reasons why noone makes a 6mm bit on a 12.7mm shank - 1. Machining away that much carbide would be hard work, and costly, and 2. The stress concentration around the step in diameter could weaken the bit.

Just fit the 1/4 collet and use a 1/4 shank bit.
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The way my buddy makes these is place a half inch slug of carbide in the Sneeburger 5 axis CNC grinder. It will take a while. When I ran Homag CNC routers, we used 1/4 inch shank upcuts.

If your breaking cutters, it’s either a jerky movement of the router or you feeds and speeds are off. If the cutter is full, that also increases the cutting force.

Carbide cutters are expensive. Just Is If your cutting back grooves for cabinets, your likely going to have a very narrow back edge. This is tricky with a hand router due to support. If you come at from the other side your going to need a sizable fence.

Outside of a CNC router, a standard cabinet side presents as a tall unstable cut on a shaper. A table saw or router table is the best solution.

if you don’t have a router table set one up with some MDF trash wood.

The biggest issue with cut down carbide cutters is the neck down from the shank to the cutter. It’s an area of concentrated stress. It makes them susceptible to breakage.

I bought my Wadkin PK in a surplus RAF auction online. Eventuality got it to the states and home. In the boxes was a blade I have never seen before. Solid 6mm plus with a flat top grind made entirely of high speed steel.

This thing cuts a perfect back groove like no ones business! Like I said I have never seen one before or since. No name on it. Could ne a Wadkin item or it a special RAF item. Not sure.

I am restoring the PK now and I got it running on the Oliver 270 saw for now. Since trying it, I have used it exclusively for back grooves at home.

I would recommend a dado but aren’t those outlawed in England?

I would look into the orange router bit line. CMT to be exact. I have had good luck with them.