[chuckcobra]

I am new to CNC and have a new to me PCNC 770 Series 3 and am having trouble drilling a 1/2" thru hole about 2" deep in 6061 aluminum.

My approach is to spot drill the hole, then drill a full depth pilot hole with a 5/16" drill. The problem occurs with the next step of actual drilling of the 1/2" hole with a new HSS drill bit. The 1/2" drill bit either stalls the spindle motor or the Z axis won't push the drill through the material (and seems to keep trying and maybe losing steps until I hit the stop button).

The last attempt was at 990 rpm and feed of 3.75 ipm using a deep drilling cycle with full retract and .25" peck cycles. Both the spindle pulley and the PathPilot are set on the low belt speed range.

Any suggestions on speeds and feed that I should try?

I would also really like to get this operation done while in the high belt setting.

[Superman]

Spot drill to about 0.200" dia
No pilot drill
1/2" drill to depth, peck with full retract @ S1200 F0.05/rev(=F2.4/min), peck Q0.2, flood coolant on

The higher RPM allows the chip to spiral up the flutes, shallow peck for shorter chip, coolant to stop chip heating up & sticking to the tool.
I'm a metric guy, HSS drilling is around 60 m/min(alum), 25-35 for soft steel, 10-15 for hard steel(4140 etc)
Do a search for "cutting speed nomograms chart"

[peteeng]

Hi Chuck - Your problem is the pilot hole. A chisel point drill is designed not to pilot. If the pilot is too big as in your case it does not "shear the metal" it digs in and jams. Peter

[RussMachine]

Your RPM is too slow.
Tormach 770 motors are only 1 HP, and the ideal torque is at about 1600 RPM.
I would definately bump your RPM up to at least 1400, and feed at 2.8 IPM, peck about .070 deep.
Tormachs are notorious for stalling, due to the weak spindle motors.
You have to baby the feeds and speeds.

[chuckcobra]

Guys,
Thanks to you all for the responses and I will do some more testing tomorrow using the ideas suggested.

I do have a couple of follow up questions before I jump into another round of testing:

1. Is the idea of drilling the 1/2" x 2" hole in the PCNC770 in the high belt position completely unreasonable?
2. If the motor's ideal torque is about 1600 RPM, how does that convert to spindle speed? Is it 1600 x pulley ratio?
3. It sounds like the "peck cycle" might be a parameter to play with. Is there a rule of thumb (or short explanation) for adjust the pecking depth and / or dwell time based on other speeds / feeds?
4. If I wanted to play with a pilot hole, is there a rule of thumb for the size of the pilot based on the final drill size?

[peteeng]

Hi Chuck - 1) a peck cycle is to reduce the chip size and to clear the bit vs having string. The drill parameters are the same peck or no peck, comes down to how the chips are being made in the material, their size and the peck length required to clear them and 2) If you must pilot measure the core of the bit at its tip and make it smaller then the core. You can get pilot point drill bits vs chisel points. Or spilt points. Or measure the pilot size on a center drill (although a centre drill is not intended to be drilled after centering). Make sure your drill is sharp...Peter

118 vs 135 Drill Bit Point Angles Explained. (with Pics) (finepowertools.com)

Spot Drill vs Center Drill. (Differences and Uses) (finepowertools.com)

[mactec54]

Hi Chuck - 1) a peck cycle is to reduce the chip size and to clear the bit vs having string. The drill parameters are the same peck or no peck, comes down to how the chips are being made in the material, their size and the peck length required to clear them and 2) If you must pilot measure the core of the bit at its tip and make it smaller then the core. You can get pilot point drill bits vs chisel points. Or spilt points. Or measure the pilot size on a center drill (although a centre drill is not intended to be drilled after centering). Make sure your drill is sharp...Peter

118 vs 135 Drill Bit Point Angles Explained. (with Pics) (finepowertools.com)

Spot Drill vs Center Drill. (Differences and Uses) (finepowertools.com)

The angle of the drill cutting edges makes no difference when you are using CNC Machines

Peck Drilling is super important when you don't have Hp to drive the spindle he should use .030"-peck max may need less than this, and he will drill that hole without any problems

If he still has stalling problems, then he should use a number of different size drills to get the job done (still using a peck cycle) like .375" then .437" then .5" using a pilot to rib/web thickness would not help in this case when you don't have enough Hp to do the job

[RussMachine]

Use Low gear, at 1600 RPM for the most Torque.

[chuckcobra]

Success with the 1/2" x 2" deep drilling operation on the PCNC 770.

I'm still dialing in the final parameters that I will be using, but the basic concept (which was recommended by several) was to increase the RPM and decrease the peck depth.

Last run today was started with a spot drilling .030"deep. This was followed directly with the 1/2" HSS drill at 1600 RPM (in high belt), 2.8 IPM feed and .025" peck depth with full retract.

Did not need to use a pilot drill. And the machine appeared to have no trouble keeping speed in the high belt position.

Thanks to all for the suggestions.