Depth of cut with sprial bit

[Dave McKeithan]

Hi folks, I haven't posted too much here, but I sure have learned a lot from this site. Hoping someone can help with a question.

I'm making a 50" round solid walnut table top that is 11/2 thick. It's too heavy to cut the diameter on my band saw, so I plan to rough cut it with a jigsaw and finish with a circle cutting jig and my router. I have a 1/2 inch carbide upcutting spiral router bit that will cut up to 2" in length. These bits are expensive, so I wanted to know how much depth of cut I can take on the full 1 1/2 thickness of the table. I'm only planning to leave about 1/16 inch of stock to remove, but of course with a jig saw it will be irregular and probably not square with the faces of the table. Your suggestions would be appreciated.

Dave.

[Mike Hollingsworth]

1/16 with a sizable router sounds ok to me, but look forward to issues at the 2 places where you cut against the grain at 45º. All hell breaks loose.

[Ted Derryberry]

You certainly don't want to take more than the radius of the bit, and much less would be better. Your goal of 1/16" is good. I suggest setting your circle jig bigger than your final dimension and working your way in with several passes. Alternatively you could work your way down by taking passes at less than full depth, say 1/2" at a time. You'd probably still want to make a very shallow final pass at full depth to leave a cleaner edge with less sanding needed. Where you really need to be careful is when you're running with the grain as it's likely to splinter at that point on deeper cuts.

[glenn bradley]

I try to remove no more than 1/4" square per pass with straight bits; 1/4" deep for 1/4" bit, 1/8" deep for 3/8" bit, 1/16" deep for 1/2" bit, etc. I use the circle jig right off and cut 1/4" deep circle on my blank. I then use the jigsaw to cut all the way through at the outer edge of this 1/4" deep "slot". I then use a flush bit with the bearing riding on the original router cut's wall.

[Jamie Buxton]

When you're making a deep cut like that, with the router hanging off the edge of the work, an issue is that the router rocks a bit. As it rocks, the tip of the bit can chowder the edge. I'd hook the router to an offset plate or structure to help prevent that rocking.

[John TenEyck]

I find spiral bits work great for mortising, but not much else. I would use a 1/2 or 3/4" shear cutting straight bit instead. Leave 1/8" or so with the jigsaw and then route with a climb cut, taking 3/8" deep per pass until all the way through, then route back the other way around.

John

[Steven Harrison]

I used a 1/4" upcut spiral on a round serving board taking off 1/16-1/8" every pass and it worked great. I would probably take off more on the light side with your bit size. I didn't bother to rough out the shape first, just went to cutting with the router. I think the extra material helped support the router and keep it from tipping.

[mreza Salav]

I would cut (as you describe) using a jig-saw but then use my router and make progressively small plung cuts going around the circle to cut to the line. I'd not do the full thickness even if just biting 1/16".

[Brad Shipton]

The problem with spiral bits and hand held routers is you cannot go fast enough and most of the time you have fairly ineffective dust collection. On an industrial grade CNC, one would run the bit somewhere between 16,000 - 18,000RPM, cut somewhere around 600-800ipm+ in two or three passes. If the wood were softer you can go much faster. Slower is actually not better because you have too much friction from all the little chips around the bit. All those little chips around the bit combined with a very low chip load leads to heat build up, dull bits, and burnt bits. If you want, you can check your chip load to see. CR Onsrud has all sorts of documents for that. Here is an intermediate CNC cutting out a thick piece---> https://www.youtube.com/watch?v=_RfemZVl0fU A heavier machine will cut quite a bit faster because you can with a 1/2" diameter. An upcut bit will be best as the the chips are more likely to end up evacuated as you cut. I only add this for information.

The general plan sounds reasonable.

[Lee Schierer]

1/16 with a sizable router sounds ok to me, but look forward to issues at the 2 places where you cut against the grain at 45º. All hell breaks loose.

Yes you need to be careful where the grain changes direction. Here is a graphic that shows how you should the trimming of your circle with your router. (ignore the round over bit as it pertains to any bit profile)
Routing curves.JPG

[Anthony Whitesell]

The problem with spiral bits and hand held routers is you cannot go fast enough and most of the time you have fairly ineffective dust collection. On an industrial grade CNC, one would run the bit somewhere between 16,000 - 18,000RPM, cut somewhere around 600-800ipm+ in two or three passes. If the wood were softer you can go much faster. Slower is actually not better because you have too much friction from all the little chips around the bit. All those little chips around the bit combined with a very low chip load leads to heat build up, dull bits, and burnt bits. If you want, you can check your chip load to see. CR Onsrud has all sorts of documents for that. Here is an intermediate CNC cutting out a thick piece---> https://www.youtube.com/watch?v=_RfemZVl0fU A heavier machine will cut quite a bit faster because you can with a 1/2" diameter. An upcut bit will be best as the the chips are more likely to end up evacuated as you cut. I only add this for information.

The general plan sounds reasonable.

I agree on the dust collection, but what do you mean "you cannot go fast enough"? You can not make the router traverse fast enough or the bit does not spin fast enough. You note a CNC would spin 16k-18k. The Bosch 1617 spins up to 25k. The one PC and Dewalt I looked are almost the same. So I assume you mean you cannot traverse fast enough.

I just finished the the first of four panels, each have 13 slots 28" long 1/4" wide and 3/8" deep. It cut 1/8" deep great, as well as 1/4" deep. It cut the whole 3/8" in one pass the one time I tried it, and it wasn't too happy about doing so. I went back to 1/8" passes. Slightly more time involved, but not worth wrecking the bit. I do wish they made carbide bits with a shorter cutting length and longer shank. I rarely ever use more than 3/4" of the cutting edge. I would gladly have on hand 3/4"CL and 1 1/2"CL bits if the former was less expensive and saved me some $.

[Brad Shipton]

Anthony, the RPM of the bit can be the same, but I have never seen someone push a handheld router at the speeds one does with a CNC. Chipload is a function of rotation speed, cut width, feed speed, depth of cut, and the number of flutes on the bit. My guess is the chip load one achieves with a hand held router can be less than 1/100th of what is recommended. I pasted two examples from a calculator I use below where I assumed a handheld feed speed of 100ipm, two different RPM settings, 1/4" cut depth, and full width cut. The common spiral sold by the stores we go to seems to be a two flute bit upcut, so I used that. I think that might be a fast feed speed using a handheld router. You can see the chip load is higher when the RPM is reduced, but is still quite a bit lower than suggested in the table. The Vortex table (CR Onsrud has similar) are just guidelines, but it is a good start point. My CNC is a bit undersized, so I fight with this quite a bit.
Chipload-01.jpg
Chipload-02.jpg
Chipload-03.PNG

[Steve Peterson]

I see no issues with a 1/2" diameter router bit cutting cleanly for the 1-1/2" depth of the table. My biggest worry would be the jigsaw cutting cleanly without the blade wandering too far inward. It might be safer to aim for 1/8" clearance which the router bit should easily be able to handle, although maybe not all in one pass.

Another option might be to ditch the jigsaw altogether and use a circle cutting jig with the router. Cut 1/4" depth per pass all the way around and slightly outside the line. Follow up with a full depth finish pass on the line.

[Brian Holcombe]

Upcut spiral seems like a poor choice on this setup. I would imagine a straight flute is much better. Also, as noted above watch for the change in grain direction. That's something to take quite seriously!

[Dave McKeithan]

I see no issues with a 1/2" diameter router bit cutting cleanly for the 1-1/2" depth of the table. My biggest worry would be the jigsaw cutting cleanly without the blade wandering too far inward. It might be safer to aim for 1/8" clearance which the router bit should easily be able to handle, although maybe not all in one pass.

Another option might be to ditch the jigsaw altogether and use a circle cutting jig with the router. Cut 1/4" depth per pass all the way around and slightly outside the line. Follow up with a full depth finish pass on the line.

I already made the cut with the jigsaw, checking frequently to see that the blade didn't cut inside my line. In my experience a jigsaw never cuts square to the face of the cut, especially with thicker material. In this case the taper was in the direction away from my cut line.

Thanks for all the responses, lots of good advice here. I will especially pay attention to the direction of cut on the end grain.

Dave.