CNC Routing on Thick Hardwood Stock

A CNC operator gets detailed advice on bit selection and machining technique for cutting curves into 1.5-inch thick Walnut for a bar top.July 29, 2012

I recently bought a CNC and have a small job. I am cutting radii in walnut bar tops. The walnut is 1.54" thick. I would appreciate any guidance as to tool strategies and proper bits. At the end of the day I need edges that would be as if I had run them over the jointer.

Forum Responses
(CNC Forum)
From contributor H:
When you say radius, do you mean you are cutting a roundover on the outside? Walnut is very hard so multiple passes is highly recommended, especially if you want a smooth cut.

From the original questioner:
Yes Contributor H. For the moment I am cutting arcs and lines, no profiling. I just need glass-smooth jointable edges.

From contributor H:
Donít have anything of the shelf that will cut 1.5 thick. It would have to be custom.

From contributor M:
Amana has an insert roughing tool for shaping material that works very well and is not ridiculously expensive. RC 2350 or RC 2354. This tool will not leave a nice edge and has the potential of blowing out some serious chunks if you take too much at once, but will shape your work quickly for finishing passes.

Vortex has a 4460 slow upcut finisher, a 1/2 inch diameter with 1-3/4 flute length that would suit your needs in both left and right hand if your spindle goes both directions. You will likely need to climb cut the last pass or two with a sharp bit, maybe a pass at 1/16 then 1/32. Steel would be better than carbide if you can find it but the 4460 will do fine.

From contributor H:
We do indeed cut a lot of material like that but aren't looking for dead smooth surfaces since we will be working the edges in the profiling processes later on.
For what it's worth we do this in multiple passes at about 1/2" depth of cut per pass. We use a 1/2" HSS 2 flute upcut spiral bit to get all the way through while climb cutting. Then we do a final conventional cut in one or two passes (material thickness being the defining factor).

We hold the material with vac pods so there is a little material deflection away from the bit on the climb cuts. This leaves the part slightly over sized for final trimming with the conventional cut pass. If the wood were more firmly held in place I would probably do a .03 offset on the climb cuts to get the same oversize result.

We use the HSS bits because of the vac pod hold down system. Since there is a limit to their holding power we run at high RPM and slow IPM feed rates. This overheats the bits and HSS holds up better than carbide under those circumstances. We do occasionally break a bit due to the vibration but it's usually when the bit is reaching the end of its usable life anyway. We typically can cut around 40 hardwood mouldings before the bit gets to that point.

If I needed that jointed edge finish described in the original post I think I would do a .03" offset on our standard climb cutting process and then switch to a carbide finishing bit, perhaps using a 3/4" bit for the larger cutting arc.

From the original questioner:
I plan to start on some practice blanks.

From contributor J:
You could always use your CNC to cut a pattern and use a router with a bearing bit and bring the tool to the work. Use a jigsaw to cut almost to the line of course. If you use a big router with a nice chunky bit you should get a jointed edge.

From contributor R:
I used to run a CNC at a stair company. We cut a lot of stair treads at 1.5" and thicker. Often, they required a returned edge so smooth quality was critical. For these cases, I would make multiple cuts stepping down 1/2" with each pass, but the cuts were made 1/32" outside the final lines. A final pass was made to the finish dimension. The light cut means you can cut the entire edge at once.

Even if you can't find a bit with 1 1/2" cutting length (and you can) you could use a 3/4" diameter cutter with a 1/2" shank and make two passes. The wider cutting edge means that the cutter can work below the surface. Often, you can cut below the surface and the shank will not quite rub, but having a larger cutting diameter insures a clean edge.

From the original questioner:
That's exactly what I have in mind. I did locate an Onsrud 1.625 finisher bit that I will use to cut the last 32nd.

From contributor G:
I do a lot of thick solid wood (1 3/4" plus) on my router and have been using one of the Amana insert-type bits - it has about a 2.5" LOC. The cut is fairly smooth and it tears out far less than a standard 1/2" or 3/4" upcut spiral. Sanding is required for a truly smooth finish, but very often it leaves no visible lines. If you are boring with it the first insert gets fried pretty fast, but swapping it is fast and easy. Just be careful not to cam out the little #15 torx head, I did that the other day.

From the original questioner:
I purchased the Onsrud 60-254 finisher and completed a full depth final, removing the last 32nd. The joint is as good as a Martin!