Drilling feed & Speed

[BanduraMaker]

I've got to drill about 48 3/16" holes today or tomorrow to hold tuning pegs in an instrument I'm building. I'd like to do this on my machine if possible but there's a few issues.

I've got a spindle on my machine that has an absolute lowest speed of about 5000 rpm - I also have no idea how much (or little) torque I have at that speed. I'm thinking I should probably try around 8000rpm just to be safe.

So, with such a high rpm, what's a recommendation on a feed rate? I'll be using a brad point bit for the drilling that's always been good for this stuff.

I'll be running a test first and was thinking of starting at 50ipm with a .250 depth before clearing chips. Total depth is about 1.25"

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[CarveOne]

If your software has it, use the peck drilling toolpath. Vcarve Pro has it and it allows the peck depth to be set to whatever you want it to be. More pecks but shallower pecks won't bog the rpm so much and it clears out the chips faster.

[BanduraMaker]

An update:

I tried peck drilling on a test piece and that resulted in slightly "loose" holes. To elaborate, these holes will be holding tuning pins and after peck drilling, the pegs were loose. I ended up using 5000 rpm with a straight drill at 75ipm. Doing a straight drill ended up with much tighter holes.

First two holes I did at 4000rpm and the spindle stalled when it broke through the other side of the pin block on the first two holes. I did a feed hold and upped my spindle speed to 5000rpm and completed the job. What I didn't realize was that when I stalled my spindle, I also lost about .2" of Z due to lost steps.

Luckily, I was over drilling enough that the holes are deep enough. I think next time, I'll just start with 5000rpm and try and keep the depth such that I don't break through.

FYI, the material was about .3" of Bolivian Rosewood followed by about .75" of hard maple. Bit was a high dollar brad point of some sort with probably about 350 or so holes drilled so far.

Interestingly, I did a test with a cheapo twist drill in my test block (in case it broke) and it was super loose - like throwing a hot dog down a hall way.

[SquibLoad]

You might try doing it with a 1/8" endmill as a pocket instead of a drill bit. I was doing this very thing tonight (string peg holes) on a bridge for another luthier friend. He needs accurate 3/16in holes for a tight fit with the pegs. Worked like a champ. Used a longer reach 1/8in 4FL endmill, take about .2-.25 DOC passes, 5K RPM, about 10IPM for these small holes in hardwood. Worked great. I've never had very good luck with regular or brad-point drills, escpecially jobber-length drills, as they follow the grain too much and wander, making an egg-shaped hole. I've had better luck with end mills. If I must use a drill, I use a screw-machine length bit, or even better, a center drill if it is a very shallow hole. If you need it REALLY accurate, try milling very slightly undersize with the endmill, and then using a reamer to finish to accurate diameter. Using that with a piloted tap guide should keep it straight.

[CarveOne]

An update:

I tried peck drilling on a test piece and that resulted in slightly "loose" holes. To elaborate, these holes will be holding tuning pins and after peck drilling, the pegs were loose. I ended up using 5000 rpm with a straight drill at 75ipm. Doing a straight drill ended up with much tighter holes.

First two holes I did at 4000rpm and the spindle stalled when it broke through the other side of the pin block on the first two holes. I did a feed hold and upped my spindle speed to 5000rpm and completed the job. What I didn't realize was that when I stalled my spindle, I also lost about .2" of Z due to lost steps.

Luckily, I was over drilling enough that the holes are deep enough. I think next time, I'll just start with 5000rpm and try and keep the depth such that I don't break through.

FYI, the material was about .3" of Bolivian Rosewood followed by about .75" of hard maple. Bit was a high dollar brad point of some sort with probably about 350 or so holes drilled so far.

Interestingly, I did a test with a cheapo twist drill in my test block (in case it broke) and it was super loose - like throwing a hot dog down a hall way.

Drill bits are something that varies a lot in the finished size. You could measure them until you find one of the perfect size, but then it will drill a larger hole due to run-out in the collet or from a slight curve in the drill bit itself. Brad point drills tend to be even worse, based on the ones I have. I wouldn't put one of mine in a router for fear that it would turn into a fan blade before it even touched wood.

Try a 135 degree split point drill, and if you can, use one that is slightly under size. The split point may not follow the grain so much because it is center cutting. Standard drill tips just rubs its way through the wood and can walk in any direction that is the least resistance.

As already mentioned, final hand reaming to size is probably the end solution to getting a really consistent close fit.

[BanduraMaker]

Hmmm... I think you guys might be making this into a bigger deal than it is or perhaps you're using the wrong bits.

The bit I used is this one: Fisch Vortex D Brad-Point Bits - Fine Woodworking Tool Review and although it didn't get a great review in the linked article, my experience with it is great. The hole size has been very accurate with this bit and it cuts through hardwood like buttah.

I bought it about 13 years ago and only use it to drill tuning pin holes and so far, about 350 to 400 holes later, it's still working well....which is good as I just looked online and it's about $7.50!

Before I went CNC I always did this on a regular old drill press at about 1000rpm and the only times I had problems was when the bit didn't pull straight out of the work but instead lifted a little bit and I didn't catch it soon enough or I retracted too many times.

Even then, the pins have held enough to stay in tune. Out of those 400 tuning pins (about 55/instrument) I might have had to fix one or two of them that didn't hold adequately (You fix them by bending a little tube out of hardwood veneer to line the hole). Don't forget that if it's too tight, it gets difficult to tune the instrument correctly.

I was so stressed over doing this that I shot a video while doing it so that I'd have a record of me destroying an almost finished instrument. I've edited out the first two holes and me fixing the spindle speed thing because it was mostly nothing going on. The run time on this job was under 3 min. One thing you'll notice with this bit is that the chips are just flying out of the hole which is what I think made a single pass possible.

"http://www.youtube.com/watch?v=6Vq1k4rvf3E"]Drilling - YouTube

Using an 1/8" EM to interpolate does sound pretty interesting and I'll definitely try a test of that but I think it's overkill as in the random sample of holes on this instrument, they all held the peg plenty tight and agin, this job only took 3 minutes. 48 holes (this is an instrument for my kid, about 3/4 of normal size so I couldn't fit all the strings in) of interpolation is going to be running for a while!

So again, I think the key here was to use a high quality bit, do a single deep pass at a reasonable feed and to not break out the other side if using a spindle that's weak at low RPM.

[ger21]

I think the 5K rpm might dull the brad point kinda quick. I've been drilling with short length bits from Mcmaster-Carr at 5K with very good results. However, you might get a little tearout on the face. What you might want to try is maybe drill .05 or so with the brad point, and finish with a regular drill bit.

Here's the McMaster # of the 1/4" bits I use. The flute length may be too short though.
28765A24

[SquibLoad]

Using an 1/8" EM to interpolate does sound pretty interesting and I'll definitely try a test of that but I think it's overkill as in the random sample of holes on this instrument, they all held the peg plenty tight and agin, this job only took 3 minutes. 48 holes (this is an instrument for my kid, about 3/4 of normal size so I couldn't fit all the strings in) of interpolation is going to be running for a while!

About 12-15 seconds per hole including rapids. Much less if you use a higher feed rate. I was in no hurry, so I just tried 10ipm to start with and left it there as it worked fine and was plenty fast enough. Whole routine was under 2 min.

[BanduraMaker]

Aren't you supposed to be on vacation Ger? Good point on the dulling aspect. I am feeding pretty fast though. I forgot to touch the bit to see how hot it was afterward.

As to the end mill thing, these holes have to be about an inch deep so I'd need to use a deep reach EM which would worry me regarding rigidity. Perhaps I'll try it one of these days. 10ipm seems pretty slow but perhaps that's what you need to hold a tight tolerance?

I wonder how hard it would be to make a removable drill attachment using a hand drill or something like that. Something with a repeatable mount.

[BanduraMaker]

Perhaps something like this: Through-Hole 'V' point Boring Bits - Toolstoday.com - Industrial Quality Boring Bits

or this:http://www.woodcraft.com/Product/208...-BP47657R.aspx
would be better for high speed boring? Are those bits made for high rpm?

[ger21]

I'd use one of these. They would be more precise than the V bit, which might wander.
Is 5mm close enough? They do have a flat on the shank for set screws, so I'm not sure if that's a problem in an ER collet??

We've had a set of these on our machine for over a year drilling laminate and particle board, and they're still drilling clean holes. I think machines like ours typically spin the bits around 4000 rpm.