Workpiece hardness correction
CORRECTION FOR SORT OF ALLOYS
Al alloys
Material kvM
Al alloys wrought non-hardened HB 60 2,6
Al alloys wrought hardened HB 100 1,0
Al alloys wrought non-hardened HB 75 0,9
Al alloys wrought hardened HB 90 0,6
Cu alloys
Material kvM
Brass for automatic machines HB 110 1,8
Brass HB 90 1,0
Bronze electrolytic Cu 0,7
DEFINITION OF BASIC CONCECUTTING GRADES PRAMETCHOICE OF TURNING TOOLCHOICE OF MILLING TOOLCHOICE OF DRILLINGWEAR OF CUTTING INSERTSGRADE GROUPS EQUIVALENT TABLES
Insert grade Feed
f
[mm.rev-1]
Cutting speed
v
c [m.min-1]Creep-resistant alloys on basis of Ni, Co, Fe, Ti
Internal Peripheral
Correction on sort of alloy
S
16 ÷ 20
8030 8030
0,05 ÷ 0,08
Sort of alloy kvHB
21 ÷ 25 0,06 ÷ 0,09
Insert grade Feed
f
Treated steels 46 ÷ 50 HRC
DEFINITION OF BASIC CONCEPTSCUTTING GRADES PRAMETCHOICE OF TURNING TOOLCHOICE OF MILLING TOOLCHOICE OF DRILLINGWEAR OF CUTTING INSERTSGRADE GROUPS EQUIVALENT TABLES
78
ESC
ESC
6 Drilling
6.3 Drilling of holes with larger or smaller diameter than nominal drill diameter
By means of misalignment of drills with cutting inserts we acquire the possibility to machine holes with a diameter (larger or smaller) differing from a nominal drill diameter. It is generally accepted that the misalignment (displacement) of drill axis with nominal diameter Dc towards the hole axis (axis of revolution) by the value + x or – x, results in a hole with diameter Do = Dc ± 2x, thus larger or smaller one than the nominal drill diameter is. Both cases are shown in the following Figure.
In the Figure a, the drill was misaligned by a value of +x (in direction of peripheral insert from the workpiece axis of revolution; on the contrary in the Figure b, the drill was misaligned by a value of –x in direction towards the workpiece axis of revolution.
In the case according to the Fig. a, it comes to the formation of a larger hole with the diameter of Do = Dc + 2x In the case according to the Fig. b, it comes to the formation of a smaller hole with the diameter of Do = Dc - 2x.
The magnitude of misalignment -x is before all restricted by the difference between the nominal drill diameter Dc
and the diameter of its body D1 – thus by the clearance between a drilled hole and a drill body.
Fig. a Fig. b
At drills with cutting inserts Pramet, the holes of diameters in the range of 16 ÷ 59 mm can be drilled in this way.
The possibility to drill holes with a diameter of Do that differs from the nominal drill diameter Dc extends the application fi eld of drills with cutting inserts. Pre-adjustment of the position of drill body enables the tolerance reduction of drilled holes.
From the technological point of view, two differing cases must be distinguished:
a) Stationary drill – it is used on turning lathe – the workpiece performs the main rotary motion, the tool usually does the feed b) Rotating drill – it is mostly used in machining centres where the drill performs the main rotary motion, the feed
is mostly performed by drill or also by workpiece
6.3.1 Stacionary drill
At drill clamping it is necessary to maintain the insert edge position (tool tip) which shall be parallel with the axis of the transverse feed (infeed) of a machine. The misalignment has to be carried out in direction of tool tip, and drill axis and hole axis have to be at the same line. A larger hole diameter is achieved by the misalignment in direction of a peripheral cutting insert. The maximum allowable misalignment in the perpendicular direction to the misalignment direction is 0,03 mm.
DEFINITION OF BASIC CONCCUTTING GRADES PRAMETCHOICE OF TURNING TOOLCHOICE OF MILLING TOOLCHOICE OF DRILLINGWEAR OF CUTTING INSERTSGRADE GROUPS EQUIVALENT TABLES
79
ESC
insert – depending on the “effi cient length” tip xL of a cutting insert. At cutting inserts with shape WCMT which are used on drills Pramet, the value xL is indicated in the following Figure. The maximum value +x can be determined for a certain drill diameter Dc from the condition that both drill edges must remove the whole hole material with an increased diameter of Do = Dc + 2x.
Problems with the determination of maximum drill misalignment +xmax are displayed in the following Figure.
A/ Without misalignment ≈ drill axis in the hole axis Dc ... drill diameter [mm]
Do ... hole diameter [mm]
xLo ... effi cient length of peripheral edge [mm]
xLv ... effi cient length of internal edge [mm]
x´Lv ... overhang of internal edge over the drill axis [mm]
It is usually accepted xLo = xLv then it is accepted
In case without misalignment, both edges operate with a certain overlapping xp
D
cx
Lo+ (x
Lv -x´
Lv) - x
p= 2 D
c2x
L- x´
Lv- x
p= 2
DEFINITION OF BASIC CONCEPTSCUTTING GRADES PRAMETCHOICE OF TURNING TOOLCHOICE OF MILLING TOOLCHOICE OF DRILLINGWEAR OF CUTTING INSERTSGRADE GROUPS EQUIVALENT TABLES
80
ESC
ESC
6 Drilling
B/ Misalignment by value of +x
By drill misalignment +x, the axis of revolution shifts in accordance with the Figures 4-9 along the internal edge, its effective length shortens consequently. Furthermore, the effective length of each edge shortens by a value of radius of the nose curvature rε.
Then it means according to the following Figure:
for xLo = xLv = xL is accepted:
2(x
Lo+ r
ε)+2(x
Lv- r
ε- x´
Lv-x
max) = D
c+ 2x
maxx´
Lx
max= (xL - r
ε) - - 2 D
c4
It is necessary to reduce the maximum misalignment with regard to a possible drill springing-back (pressing off), for instance at drilling in an inclined concave or convex surface. At the same time the infl uence of the hole depth ≈ relation L/D of a drill must be taken into account.
At drills with small diameters in the range of Dc = 16 ÷ 25 mm, where cutting inserts VBD WCMX 0302 and WCMT 0402 are applied, it is necessary to reduce the values xmax with regard to the fact that the axis of revolution moves with an increasing misalignment in direction to the centre of the internal cutting insert and the edge length xLv increases and it comes to a friction of machined material. Consequently, a very unfavourable stress of cutting insert develops and a risk of its destruction increases.
The maximum misalignment value –x (Do < Dc) is determined by the requirement that the tip of the internal cutting insert is not moved to the right direction from the axis of revolution, that no material remains in the middle of the hole bottom which could cause stamping and practically could aggravate or disable a drilling (x´Lv < 0).
Maximum values +xmax a -xmax for different drill diameters with cutting inserts Pramet are given in the following Table.
Simultaneously, the values of hole diameter D0min and D0max are also mentioned which can be achieved by the respective misalignment. It concerns the values for stationary drills. The data “A” are valid for smaller tool overhang and drills for hole depth 2,5D, and the data “B” for tools with a larger overhang and for a hole depth 3,5D and larger.
DEFINITION OF BASIC CONCCUTTING GRADES PRAMETCHOICE OF TURNING TOOLCHOICE OF MILLING TOOLCHOICE OF DRILLINGWEAR OF CUTTING INSERTSGRADE GROUPS EQUIVALENT TABLES
Range of diameters
D0 [mm]
A +x B +x -x A D0max B D0max D0min
16