Metalcutter
Titanium
- Joined
- Sep 14, 2005
- Location
- San Diego
The Drilling Cycle
Two important factors in the drilling cycle contribute to premature drill wear.
Absence of dwell at the bottom of a hole or peck and rapid traverse down to the
exact bottom of a hole for succeeding pecks.
The reaction time of CNC equipment is all but instantaneous. When a drill,
feeds its way down into a hole and reaches the bottom of the hole; or the end of
any peck it changes direction to 'Z' up. It also changes into rapid traverse.
In other words the drill is literally snatched out of the hole.
Twist drill geometry incorporates right hand spiral flutes. The flutes
intersect the cutting edge to form a positive back rake angle. Under feed a
chip flows up the flute and therefore lays over the cutting edge.
This overlay condition resists the retraction of the drill. Through this, resistance forces
develop which work against the cutting edge trying to flake it away from the
drill body.
Drill geometry is designed to support the drill point under
tremendous cutting pressure while spiralling down into the work. Yanking the
working drill from the hole causes these forces to change direction on the
cutting edge at an angle down and away from the drill body creating an effective
high negative back rake.
On retraction the drill shears the chip off using the
fragile cutting edge. The result is either abnormal abrasion, chipping or
breaking off the cutting edges.
Also after leaving the hole a crested bump is
left at the hole bottom because the drill point did not finish making the chip
before retracting. If the returning drill is going in for another peck it could
rapid to the bottom catching this bump and experience excessive cutting forces
again chipping the cutting edges.
A fix for this problem is to program a dwell long enough to hold the 'Z' axis drill
position for one to two spindle revolutions letting the chip thin out to zero.
Retraction at this point would not damage the tool. This length of time at 3000
RPM would be approximately 0.02 seconds for one revolution. To figure this time
increment divide the RPM into 60 seconds. Three hundred RPM would equal 0.2
seconds.
Shallow Holes
Most machining centers have several canned drill cycles which are used to drill
holes. A G81 spotting or shallow hole drill cycle has no programmable dwell,
although popular and okay for through holes it is not a good choice to use in
blind holes.
A G82 drill cycle has dwell that can be programmed. This G82 cycle is a good
replacement for a G81 when used for drilling holes through or three drill
diameters deep. In fact a G81 can be completely eliminated.
Deep Holes
Peck drilling is generally required when drilling holes deeper than three and
one half drill diameters. Most canned pecking cycles allow for starting the
feed rate just above the last peck to prevent the drill from hitting the hole
bottom at rapid traverse rate.
The difficulty is that no dwell is available at any peck bottom except the last. So no effective method exists to prevent the chip breaking action from causing severe tool damage. Some people think this tool damage comes from heavy feed rate. Light feed rates are then programmed to solve a problem which should not exist in the first place. The net result is lower productivity.
A New Subroutine
A temporary work-around is to build a special pecking cycle. The pecking cycle
would incorporate a dwell at the end of each peck for a time length sufficient to
allow one or two drill revolutions.
Another approach would be to change the drill feed to up, or Z plus, for a few thousandths. This would allow the chip to thin out at the end of the peck just before retraction.
This approach would not be a dwell but a feed reversal and would thin the chip and also position the drill away from the hole bottom to prevent work hardening the material.
The permanent fix is to talk the machine tool builders into creating drill pecking cycles in
their controls which could provide programmable dwell or feed reversal between
every peck. This enhancement would allow drilling to proceed at the best feed
rate and production could go up across the board.
An exercise:
Take a one inch travel indicator and mount it to the machine head and point it down, and "touch" something.
Then program a drill cycle very slowly. Make the rapid slow enough to see as well.
Then run it and watch how the drill cycle acts in your machine.
Notice too, the amount of clearance between pecks before cutting takes place again.
That clearance between pecks is important. If the clearance is .030 and you are drilling a .031 hole you'll be waisting time, especially if you are pecking every 1/2 diameter.
The Haas I used once had the ability to change the clearance between pecks. I use about 3-5% of the drill diameter for clearance.
The Special Peck Cycle
When drilling the tough alloys SST, Inconel, and like materials, drills last longer if you can feed reverse before retracting. In these alloys it's probably worth the trouble to figure one out.
Regards,
Stan Dornfeld
Two important factors in the drilling cycle contribute to premature drill wear.
Absence of dwell at the bottom of a hole or peck and rapid traverse down to the
exact bottom of a hole for succeeding pecks.
The reaction time of CNC equipment is all but instantaneous. When a drill,
feeds its way down into a hole and reaches the bottom of the hole; or the end of
any peck it changes direction to 'Z' up. It also changes into rapid traverse.
In other words the drill is literally snatched out of the hole.
Twist drill geometry incorporates right hand spiral flutes. The flutes
intersect the cutting edge to form a positive back rake angle. Under feed a
chip flows up the flute and therefore lays over the cutting edge.
This overlay condition resists the retraction of the drill. Through this, resistance forces
develop which work against the cutting edge trying to flake it away from the
drill body.
Drill geometry is designed to support the drill point under
tremendous cutting pressure while spiralling down into the work. Yanking the
working drill from the hole causes these forces to change direction on the
cutting edge at an angle down and away from the drill body creating an effective
high negative back rake.
On retraction the drill shears the chip off using the
fragile cutting edge. The result is either abnormal abrasion, chipping or
breaking off the cutting edges.
Also after leaving the hole a crested bump is
left at the hole bottom because the drill point did not finish making the chip
before retracting. If the returning drill is going in for another peck it could
rapid to the bottom catching this bump and experience excessive cutting forces
again chipping the cutting edges.
A fix for this problem is to program a dwell long enough to hold the 'Z' axis drill
position for one to two spindle revolutions letting the chip thin out to zero.
Retraction at this point would not damage the tool. This length of time at 3000
RPM would be approximately 0.02 seconds for one revolution. To figure this time
increment divide the RPM into 60 seconds. Three hundred RPM would equal 0.2
seconds.
Shallow Holes
Most machining centers have several canned drill cycles which are used to drill
holes. A G81 spotting or shallow hole drill cycle has no programmable dwell,
although popular and okay for through holes it is not a good choice to use in
blind holes.
A G82 drill cycle has dwell that can be programmed. This G82 cycle is a good
replacement for a G81 when used for drilling holes through or three drill
diameters deep. In fact a G81 can be completely eliminated.
Deep Holes
Peck drilling is generally required when drilling holes deeper than three and
one half drill diameters. Most canned pecking cycles allow for starting the
feed rate just above the last peck to prevent the drill from hitting the hole
bottom at rapid traverse rate.
The difficulty is that no dwell is available at any peck bottom except the last. So no effective method exists to prevent the chip breaking action from causing severe tool damage. Some people think this tool damage comes from heavy feed rate. Light feed rates are then programmed to solve a problem which should not exist in the first place. The net result is lower productivity.
A New Subroutine
A temporary work-around is to build a special pecking cycle. The pecking cycle
would incorporate a dwell at the end of each peck for a time length sufficient to
allow one or two drill revolutions.
Another approach would be to change the drill feed to up, or Z plus, for a few thousandths. This would allow the chip to thin out at the end of the peck just before retraction.
This approach would not be a dwell but a feed reversal and would thin the chip and also position the drill away from the hole bottom to prevent work hardening the material.
The permanent fix is to talk the machine tool builders into creating drill pecking cycles in
their controls which could provide programmable dwell or feed reversal between
every peck. This enhancement would allow drilling to proceed at the best feed
rate and production could go up across the board.
An exercise:
Take a one inch travel indicator and mount it to the machine head and point it down, and "touch" something.
Then program a drill cycle very slowly. Make the rapid slow enough to see as well.
Then run it and watch how the drill cycle acts in your machine.
Notice too, the amount of clearance between pecks before cutting takes place again.
That clearance between pecks is important. If the clearance is .030 and you are drilling a .031 hole you'll be waisting time, especially if you are pecking every 1/2 diameter.
The Haas I used once had the ability to change the clearance between pecks. I use about 3-5% of the drill diameter for clearance.
The Special Peck Cycle
When drilling the tough alloys SST, Inconel, and like materials, drills last longer if you can feed reverse before retracting. In these alloys it's probably worth the trouble to figure one out.
Regards,
Stan Dornfeld
