[mitis]

Hello folks.

I've recently had the situation to need to counterbore many holes in a 2000 mm (78'') part for screw heads. The through hole is ф6,5(1/4 '') and then we need to counterbore to 13 mm(0,51 ''). The depth was originally 5 mm but I later decided to give one more as screw heads weren't getting enough in so it became 6 mm(0,23 '').

The problem is that I cannot seem to find the proper cutting mode. I started much higher having some 3000 RPM and 220 mm/min (8,66 '' / min) because I truly don't have experience in this process. The sound and vibrations which resulted from the first trial were very noisy and I pulled things down.

By trials, I reached the following:
RPM = 1000
Feed = 30 mm/min (1,18 '' / min) which seems very slow to me. I mean, it is the only mode that the machine doesn't seem to have problems with. And for my great surprise, the RPM seems to be very important as well.

My co-worker was very little slower than the machine by doing the counterbores manually on a drilling machine with fixed Z coordinate.

My CNC is a self-made router with rigidity above the average. We designed it for extensive use with aluminium and so far it satisfies our ideas. The spindle power is 1.2 kW.

I would be interested in getting some raw data which has worked for you. I wasn't aware the counterboring process requires so much spindle power

[awerby]

It sounds like your spindle just doesn't have the low-speed torque to drill half-inch holes effectively in aluminum. That's not really surprising. If it's the typical Chinese-made 1.2kw spindle I'm thinking of, it was designed to take fairly light cuts at high speeds. You can lower the frequency to make it go slower, but that also reduces its effective torque, so it's bogging down. You might have better luck with a helical routine, using a smaller endmill to move around the holes removing material from the edges of the holes at much higher RPMs.

[mitis]

@awerby, thanks for replying. You are not the first to mention endmills to be used in contrast to the special counterbore tools, I haven't tried that.

As for the spindle, it's not from the low-cost Chinese, which are made for soft materials and can't work well under 8000 RPM (they normally reach 24 000 rpm). I took a look at these before taking the decision and they were surely excluded from my list. Instead, I put a 4th servo motor for the spindle. It is 1.2 kW, 4 Nm. Most spindle motors' nominal speed is 3000 and they can up to 4000. I am not saying the machine is not doing what is needed, the sound is just pretty noisy and the result conical surface is not impressive as well. The motor can overload up to 300% for 10 seconds, so I believe that's not the case. At least, I made same pre-calculations and the theoretical power for this counterboring process never exceed 500W but of course theory is always outnumbered by practice.

[mactec54]

mitis

What Cam software do you have, as you can do a spiral Bore in most cam software, which is the easy way to do a counter-Bore, here is an example, this will do a .510 bore with a .312 cutter, you can try this out, part X0Y0 is center of the hole, adjust speeds feeds to suit your machine

[AARONT]

Use a 1/4" carbide end mill and interpolate your counterbores with a 10% doc (.025" depth). Run it 18000 to 23000 rpm with a feed of .002" IPT ex. 4 flute end mill 18000*.002*4=144ipm. If your cnc will interpolate that quickly, you will be flying through your cbores. Use a mister or at minimum strong air blast for chip removal and keeping tool cool.