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I would like to suggest that we have a fret slot race.
Winners keep the losers machine.
Nelson

Bob, what are you using for CAD/CAM?

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"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

Bob - surely something as simple as fretslots you could do with a few lines of G code written in less than 5 minutes....

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"Act your age, not your shoe size" - Prince

Of course- but I won't be able to run that path reliably without snapping the cutter at that feed rate. A fret slot isn't just a slot, it's a slot being cut with a high aspect ratio cutter being driven at below its optimal surface speed with almost nil chip evacuation in a material that densely repacks chips behind the cutter...so some finesse to account for those can pay off

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

I stand corrected on that feed rate. That's just...remarkable.

I'd like to know what you mean by "finesse"? Is this something we could apply?

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

I've been cutting my fret slots pretty conservatively at 20 IPM and .02 per pass. It takes about 10 minutes at that rate to cut a banjo fingerboard--probably twice the time it takes me to cut one on a table saw. But, off the CNC it's dead accurate, simple to change scale length on a whim, and I cut the pocket for the fifth string "pip" using the same bit in the same toolpath. Since I do my fingerboards with closed end fret slots, I don't have to cut binding channels or deal with open fret slot ends later. All in all, it's quicker for me on the CNC and a neater job at the same time. A gang saw would be quicker, but not nearly as flexible. Most of my fretboards are flat, but when I do a radiused board, an engraving tool path will follow the radius and give constant depth fret slots.

I've found that with my shallow passes and a dust foot on the machine, sawdust packing in the slots hasn't been a problem.

Gang saws aren't as accurate as a good CNC. Now here's a question for you - how accurate do fret slots REALLY need to be...I mean, there is a big rounded surface on top of the fret. Want to bet on what kind of tolerance there is on where the string really breaks over the fret...?

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http://www.PeakeGuitars.com

I made up a spreadsheet once showing the +/- 1cent tolerance for each fret. It's pretty wide on the lower frets of a bass. It's been too long since I looked at it to remember the upper fret specs for a guitar. It was interesting to see though.

The feed rate's a lot like being able to hold your breath for four minutes- semi-impressive, but nobody'll pay for it

So far as the technique goes, it's all about avoiding big changes in force with those cutters. They don't really dull and chip like larger cutters, their only failure mode is snapping from what I've seen.

When you're trying to prevent gouging, you're really trying to lower the jerk** in your toolpath. Smooth acceleration makes smooth motion. With the small cutters, you need to try and lower the 'jerk' in the force on the cutter, so you make your toolpath so the stresses on the cutter are either constant or changing smoothly. So it's more like trying to get a constant or smoothly changing material removal rate, while factoring in your depth of cut to account for the 'leverage' on the cutter.

I can't go much further without releasing some trade secrets, but that's enough to replicate the result if you're willing to do some calculations and some figuring. Remember, also, that a lot of this stuff matters a whole lot less at 10,000RPM than it does at 90,000RPM though it's oddly useful at low RPM as there's significant bending stress from poor surface speed of the edges rather than high feed rate.


**jerk is the derivative of acceleration, how fast your acceleration is changing...'snap' is the derivative of jerk, and my favourite engineering term

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Bob, could you sum all the above by just saying "take it easy and use common sense with tiny cutters? I'm not sure there are any "trade secrets" needed.

OK, I trimmed all that pointless detail out:

Just cut real fast and don't break the tool. It's common sense!

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Won't a compressed air jet keep the chips out of the slot?

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"Act your age, not your shoe size" - Prince

Chris, I think compressed air would be worth a try.
The ebony shavings would probably have less tendency to "weld" in the slot if the cutting action was kept cool and the shavings evacuated quickly.
Nelson

Directing air via a loc-line system gets it in the general area but to be really effective I think you'd need something more precise. On that note, anyone willing to share photos of said system?

Hi Sheldon,
Here is how Gene Baker is doing it with the HAAS.
In this shot the nozzle is not very close. He can move it in and out as necessary.
I'm sorry I can't tell you feed rates and such.

The density they pack with is pretty impressive. At full density (one-pass full depth) it takes a few seconds to blow them out with 100PSI coming straight from an air gun into the channel, and it can require some actual scraping or picking at the dust to get it to release even then.

With a bunch of shallow passes, you could blow them out with your breath. Like anything else in machining, speed tend to exacerbate the problem.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Interesting edge clamps. I've never seen those before.

Hi Kevin,

What power is your laser? Is it a CO2 job? I've tried cutting fret slots with mine (I've owned a 45W Epilog 32EXT for years) but have not had good success in Ebony. Some success in Cocobolo. Do you use vector cuts (and play with the power/speed/pulse rate to get the depth you want) or raster engrave the slots?

I am a brand new CNC owner, and just ordered up a bunch of 0.023" bits to sacrifice

BTW, thanks all for sharing all this information - you are a great bunch of guys and I sincerely thank you all!

Cheers,
Dave F.

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Cambrian Guitars

"There goes Mister Tic-Tac out the back with some bric-brac from the knick-knack rack"

Bob - ever tried a "Cold Gun" like the one Travers Tool sells? These get so cold frost forms on the plastic Loc-Line, and they run on compressed air.

http://www.travers.com/skulist.asp?Requ ... ice=$30.79

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"Act your age, not your shoe size" - Prince

Yeah, I've got one. 'Vortex tube' is the more generic term for them. I haven't found it to be all that useful, though. Maybe if a shop had enough air to spare that they could get 60+ PSI of cold air firing out of it?

Everything I've considered it for was better served by having more room temperature air available, or by using lubrication. It'd be cheaper to cool the liquid if you needed more cooling even after using mist lube.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Has someone tried tilting the router 90 degress and use a .023" circular saw?

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Or an angle grinder

Essentially every guitar company. I think Taylor was first, though they were using a right angle head last time I checked. They're probably using a motor mounted at 90 degrees now, since right angle heads tend to wear out every few months at factory use levels.

If it weren't for the blind slots thing, I'd have probably set up a small motor with a saw. Fret slots would be a relatively low gain use compared to other things if you could index the head...

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Are blind slots a Thorn patent or are they public domain?

To resurrect and old thread -

I'm getting ready to slot a fretboard. My setup is:

18000rpm spindle
.024 2 flute carbide EM (square) with a roughly .1" max DOC.
My slot depth will be .056"
My plan is to do two passes, vertical plunge and a depth .028" per pass.

According to a feed&Speed calculator on line, with a chip load of .001"/tooth, I need a feed rate of 36ipm which just seems fast to me (for 18000rpm). Dropping the load to .0005/tooth is (obviously) 18ipm. Do these feed rates sound reasonable?

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