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сверлильный
Drilling Tool Selection
Fig.1 Twist Drill
Fig.2 Center Drill
Cutting Parameters
Select parameters following this sequence: depth of cut, feed rate, cutting speed. Holes below 35 mm can be completed in one drilling pass. For holes over 35 mm, two-stage drilling is recommended. The diameter of the first drill takes 0.3~0.7 times the final hole size.
Feed rate can be estimated at 0.01~0.02 times drill diameter. Typical values stand at 0.15~0.50 mm/r for cast iron and 0.10~0.35 mm/r for steel. Cutting speed should stay moderate: 25~30 m/min for low-carbon steel and 20~25 m/min for cast iron. Lower speeds bring stable drilling on milling machines.
Common Defects and Solutions
| Machining Defect | Root Cause | Preventive Measure |
|---|---|---|
| Inaccurate hole position | 1. Incorrect marking and punch marks
2. Long chisel edge leading to poor centering 3. Errors in coordinate movement |
1. Improve alignment accuracy during positioning
2. Grind the drill chisel edge 3. Calibrate milling machine coordinates |
| Hole deflection | 1. Asymmetric cutting edges
2. Excessive feed bending the drill 3. Non-perpendicular workpiece end face 4. Misalignment on cylindrical surface drilling |
1. Grind drills correctly
2. Control feed rate 3. Pre-drill guide pits on uneven end faces 4. Pre-locate with center drills |
| Polygonal hole shape | 1. Excessive drill relief angle
2. Unequal length of two cutting edges |
1. Reduce drill relief angle
2. Keep cutting edges equal and symmetrical |
| Rough hole wall | 1. Improper cutting fluid and cutting parameters
2. Excessive depth of cut 3. Worn dull drill bits 4. Clogged chip flutes |
1. Optimize cutting fluid and cutting parameters
2. Grind or replace worn drills 3. Select properly sized drills and retract drills for chip removal on schedule |
Reaming
Tool Selection
Fig.4 Reamer
Cutting Guidelines
- Cutting speed and feed rate
For standard high-speed steel reamers: cutting speed Vc ≤10 m/min and feed rate f ≤0.8 mm/r for cast iron. For steel workpieces, Vc ≤8 m/min and feed rate f ≤0.4 mm/r.
- Reaming allowance
Allowance must remain moderate. Insufficient allowance fails to remove residual material from previous operations and causes heavy reamer abrasion. Excessive allowance destabilizes cutting, raises cutting heat and expands measured hole diameter. Surface roughness also worsens.
Critical reminder: Never reverse the reamer during retraction. Reverse rotation lets chips scratch hole surfaces and break cutting edges. This mistake frequently appears among new operators.
Common Reaming Defects and Solutions
| Machining Defect | Root Cause | Preventive Measure |
|---|---|---|
| Over-sized hole | 1. Missing diameter check for reamer
2. Misalignment and reamer runout 3. Excessive cutting speed 4. Overlarge feed rate |
1. Check reamer dimensions before machining
2. Clamp reamer tightly to control runout 3. Select suitable cutting parameters |
| Under-sized hole | 1. Worn dull reamer
2. Elastic recovery of steel after large-allowance reaming 3. Kerosene lubricant used on cast iron |
1. Replace worn reamers timely
2. Adjust allowance and lubricant based on workpiece material |
| Crooked hole axis | 1. Worn guide section on reamer
2. Bent hole from prior operation; low reamer stiffness cannot correct deflection |
1. Inspect and maintain reamers regularly
2. Straighten hole axis with rigid end mills before reaming |
| Poor surface roughness | 1. Reamer reversal during retraction
2. Improper machining allowance 3. High cutting speed generating built-up edge 4. Unsuitable cutting fluid |
1. Retract reamer while maintaining forward rotation
2. Optimize parameters and cutting fluid to avoid built-up edge |
Boring
Tool Selection
Fig.5 Solid & Single-point Boring Tools
Fig.6 Double-edge Boring Cutters
Cutting Parameters
Common Defects
Common Boring Defects and Solutions
| Machining Defect | Root Cause | Preventive Measure |
|---|---|---|
| Out-of-tolerance hole size | 1. Incorrect adjustment of boring cutter radius
2. Measurement error 3. Tool deflection from long overhang 4. Tool tip wear |
1. Adjust boring cutter radius
2. Perform precise measurement 3. Improve boring bar rigidity 4. Regrind cutters and apply cutting fluid |
| Large circularity error | 1. Workpiece deformation during clamping
2. Poor spindle rotation accuracy 3. Elastic deflection of boring tools 4. Unlocked worktable 5. Loose workpiece clamping |
1. Optimize clamping for thin-walled parts
2. Calibrate spindle accuracy 3. Improve tool rigidity 4. Lock worktable 5. Optimize clamping point layout |
| Excessive perpendicularity error of hole axis | 1. Improper selection of positioning reference
2. Dirty reference surface 3. Misaligned spindle zero position |
1. Select proper positioning reference
2. Clean contact reference surfaces 3. Calibrate spindle zero position |
| Oval hole shape | Perpendicularity error between spindle axis and feed direction | Recalibrate spindle zero position |
| Tapered hole | 1. Tool wear during cutting
2. Vibration caused by loose fastening screws |
1. Regrind cutting tools
2. Tighten boring tool fastening screws |
Вывод
Four core requirements govern hole machining: dimensional tolerance, geometric accuracy, surface roughness and positional precision. All parameter adjustments target these four standards.
Drilling performs rough opening, reaming finishes small and medium holes, and boring handles high-precision hole patterns. Combined, these three processes cover most hole-machining tasks on milling machines. Hole machining relies on solid fundamentals. Proper tool selection, matched cutting parameters and familiarity with typical defects yield consistent quality after repeated practice.
