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Has anybody else tried routing fret slots with cutting fluid? I just tried an experiment using 91% isopropyl alcohol as a cutting fluid. I did a series of slots at 5ipm, 15ipm, 20ipm and 40ipm with the following setup:
- 0.023" 3-flute bit (from Precise Bits)
- 2 passes, 0.035 depth of cut (0.070 total depth)
- 30,000 RPM spindle speed
- Material: some sort of ebony
- A healthy squirt of isopropyl
I hadn't intended to go at 40ipm (programming error), but it seemed to work fine. After blowing out the slots, there were some chips to fish out, but it was more like scraping out a paste than the really tightly packed debris when I cut dry. The alcohol evaporates almost instantly with a little compressed air. (I believe 90% or higher concentration will evaporate completely without rusting anything.)

What are the chances that the fumes will ignite?

I've wondered about this. I've been using alcohol as a coolant for aluminum for a while without any problems. I figure that with all of the drinking and smoking that goes on, occasionally you'd hear about someone blowing themselves up while doing shots of Everclear.

I suspect that alcohol vapors need to be pretty well enclosed before they'll ignite and that you'd need to use an awful lot of alcohol to get an ignitable vapor concentration in a normal shop setting.

Just did a quick test - I filled a shallow aluminum pan (~6" dia) with 91% isopropyl alcohol, let it sit for a little bit for fumes to accumulate, and then waved a lit match about 1/2" above the surface. Nothing. I finally just stuck the match in the alcohol - it actually took a little cajoling to get a steady flame going. Hard to imagine getting any sort of ignition (of fumes or liquid) in a normal machining scenario.

Not trying to be inflammatory but what's the point of the cutting fluid with wood? At 30krpm, you can cut full depth slots at 40ipm with no fluid anyway (just make sure you limit jerk on the bit).

I use an air nozzle pointed at the tip of my bit and when I have it aimed correctly, the fret slots come out nice and clean and ready to go.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Whenever I try slot routing, even with compressed air blowing in the slot, I still seem to get highly compacted chips in the slot. I've been afraid to go too fast. Also the chips have been a pain to scrape out. I must be missing something in the process. Maybe going faster is the key to chip ejection?

That could be it.

I currently cut my fret slots with a 2flute bit at 18krpm (fastest my spindle will go) at 20ipm at a DOC of .030 with an air nozzle pointed right at the tip of the bit. You can see a pic of what it looks like on the opening screen of my website: http://www.birkonium.com In that picture the fret slots aren't stopped but it works the same either way. Now that I know the trick of how to mount my air nozzle I rarely do this anymore but in the beginning I was having some difficulties with dust remaining in the slot due to poor aim. Pressure is set pretty high, I think around 40 to psi or so.

That said, I've also cut fret slots at a full .080 depth at 25ipm using a path inspired by Bob Garrish with the same results. The only reason I haven't changed this in "production" is that it's a pretty big change to my model that I haven't gotten around to yet. Perhaps today I'll do it.

With a 30k spindle, you should be able to cut at 41ipm. Since you're using 3 fluted bits, you should be able to go at 54ipm.

I suppose it's possible that part of the problem you're having is due to the 3 fluted bits but I'm pretty sure Bob uses those with the full depth slotting without any difficulties.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

+1 to what Andy has said. A lot of it comes down to the bit you are using. Down cut bits won't eject chips, up cut will help to get the chips up and out of the way but bring on the possibility of tear out and edge fuzz, straight bits are in the middle. Then of course you have your flute choices, the more flutes you have the less room you have or chip evacuation. I've always found smaller chips compact more easily than larger ones. I know you prolly know all this but just thought I'd point it out anyhow.

Also like Andy mentioned take the time to get your air hose positioned correctly. Think about how you will be cutting and where the chips will naturally want to go and aim your air to assist them on their journey out of the slot, rather than having to fight them. Use the momentum they have coming off the bit already. Best of luck to ya

Sounds like my problem is getting the air aimed correctly. On my little hobby machine, I'm just hand-holding a standard air nozzle. I guess I need to be a bit more methodical about this. Until I get may air setup fixed up, though, the alcohol seems to work like a champ.

There's a much simpler solution: run the last pass twice. It recuts the chips and turns the hard-packed gunk into fine dust that blows out easily (ie: with your lungs).

I never did full depth passes, I did multiple passes at really high feed rates. Well, multiple depths anyway- it's one single curve so they're not so much 'passes' as there is no retract.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services