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The reaming process is to be prepared so that all working tools and materials are readily available.
For the determination of the cutting values it is important to know that reamers are made of tool steel or high-speed steel.
Therefore, reaming is done with low cutting speed (v) and high feed rate. As a guideline, v can be taken as m/min.
The necessary rotational speed (n) is calculated by conversion of
The feed is manually effected and to be selected so as to ensure the surface finish required (no scoring). Prior to reaming the surface must have been finished (admissible height of roughness Rz 20) since otherwise the machining allowance would not be sufficient to remove the tool marks completely.
A fitting hole of 32 mm dia. is to be produced by reaming. What rotational speed is to be selected for reaming?
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================= r.p.m. |
The direction of rotation of the work spindle is of special importance for reaming. When reaming, the workpiece must always rotate in opposite direction to the cutting edges (clockwise rotation), also when the tool is retracted, since otherwise the cutting edges would chip.
The reaming technique always requires prior machining of the hole to be reamed.
Within the technological sequence, reaming is included as follows:
1. Centring
2. Drilling
3. Boring/Counterboring with spiral-fluted counterbore
4. Reaming
5. Final inspection.
Why is the direction of rotation of the work spindle important
for
reaming?
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What are the operations connected with reaming within the
technological sequence of operations for the production of a fitting
hole?
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With the boring/counterboring operation the hole is to be provided with a machining allowance for the subsequent reaming operation. The surface must have been finished (no major tool marks).
The recommended values for machining allowances can be taken from the respective table.
Example:
A fitting hole of dia. 16 F8 to be produced by reaming shall have a machining allowance of 0.2 - 0.3 mm. The boring/counter-boring tolerance, consequently, is 15.8 (upper deviation) and 15.7 (lower deviation).
The final size of a reamed hole does not only depend on the correct diameter of the reamer but also on the type of the material to be reamed, the clamping facilities of the reamer, the position of the tailstock and the type of the coolant and lubricant. The same reamer, for example, may produce different diameters into tough steel and brittle grey cast iron.
Therefore, a test hole must always be produced.
In the event of dimensional variations, the tool is to be changed.
What is the machining allowance for a dia. 24 H7
hole?
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What factors are influencing the accuracy to size of a
hole?
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For the selection of the coolant and lubricant it is essential that
- reaming of grey cast iron is dry reaming,
- addition of spirit is recommended for reaming of aluminium,
- steel should be reamed using cutting oil,
- the use of a water-in-oil emulsion (diluted soluble oil) permits closer reaming of the hole.
The following main tools and accessories are to be made available for machining (centring, drilling, boring, reaming):
- Chuck with internally turned, soft chuck jaws for clamping of the workpiece.- Centre drill with drill chuck for centring the workpiece.
- Twist drill for rough-machining of the hole.
- Internal side-cutting tool (boring tool) or spiral-fluted counterbore (three-lipped twist drill) to produce the hole with machining allowance for reaming.
- Reamer for finishing the hole (see Fig. 2).
- Taper sleeves for location in the tailstock.
- Coolant/lubricant.
- Vernier caliper for dimensional checks in the 0,1 mm range.
- Depth gauge (for stepped holes or blind holes).
- Dial gauge for radial and axial run-out checks.
- Limit plug gauge for initial and final inspections.
Figure 11 Limit plug gauge
1 go end, 2 not-go end, 3 nominal size marking
Figure 12 Taper plug gauge
1 testing area, 2 fitting marking, 3 nominal size marking
The size of the reamer and of the limit plug gauge must be determined on the basis of the working drawing and must correspond with each other. Reamers with a smaller quality index (e.g. dia. 30 H7 required, dia. 30 H6 available) may be used.
Machine reamers are clamped in the tailstock. Prior to reaming it is to be made sure that the tailstock is in central position, i.e. that the axis of rotation of the workpiece and the centre line of the tool are in line. If necessary, the tailstock is to be laterally re-aligned as per Fig. 13 since otherwise the nominal diameter cannot be maintained because the tool will cut at one side of the hole only.
Figure 13 Tailstock adjustment
1 workpiece held in chuck, 2 reamer, 3 tailstock sleeve, 4 centre offset, 5 adjustability
Minor centre offset between the workpiece and tool can be compensated for by the use of a floating head as per Fig. 14.
Figure 14 Floating holder (floating
head)
1 floating sleeve, 2 holder, 3 locking bolt
Labour safety recommendations:
- For transportation cover the cutting portion of the reamer to prevent injuries.- When checking the hole by means of the limit plug gauge make sure that the cutting portion is sufficiently off the work area or is covered.
- Don’t do any measuring or testing unless the work spindle is at standstill 1
- Centre with high rotational speed rates!
Firm clamping of workpiece and tool are essential.
- Wear safety goggles to protect against flying chips (boring).
- When working with spirit (to be used for aluminium) take precautions against fire - easily inflammable!
What can be done if a centre offset between the workpiece and
tool is found when
reaming?
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