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With making a cylindrical bore true to size as well as with the preparation of plain pin connections, the following steps are required:
5.1. Clamping
The workpieces, as far as possible, should be clamped in such way, that they can be drilled, counterbored/countersunk and reamed in succession without the clamping being loosened in between.
5.2. Scribing/prick-punching
This operation has to be carried out as described under “Drilling and counterboring/countersinking”; it can also be done before clamping.
5.3. Drilling
Since the reamer must remove material from the inside of the bore, the bore roust be made smaller than the nominal diameter of the bore true to size indicates. This difference is called “undersize”
Empirical values for undersizes of bore holes in steel are:
|
N in mm |
U in mm |
|
up to 5 |
0.1 - 0.2 |
|
5 - 20 |
0.2 - 0.3 |
|
21 - 32 |
0.3 |
|
33 - 50 |
0.5 |
N = nominal diameter
U = undersize
With tough materials and light metal, the undersizes are larger than with steel.
The diameter (D) of the drill is calculated by the following formula:
|
D = N - U |
D = diameter of the drill
The calculation of the rotational speed (n) of the drill is explained in the lesson “Drilling and counterboring/counter-sinking”
General formula:
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V = cutting speed (approx. 22 m/min)
p = 3.14
5.4. Countersinking
The bore must be spot-faced by a 90° countersink on either side. In doing so, the diameter of the countersinking (Ds) is to be calculated with the help of the following formula:
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Ds = N + 0.2 m |
Ds = diameter of the countersinking
The rotational speed for countersinking bores up to a diameter of 10 mm can be approx. 350 r.p.m. with larger bore holes it must be lower.
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n » 350 r.p.m |
5.5. Reaming
With through bores, always hand reamers can be used; for bores with pockets, the reamer must be spiralfluted (blind holes must be reamed by machine reamers only.) Tap wrenches are used as auxiliary means
Figure 11 - Reaming operation -
clockwise rotation
Lubricating and cooling agents are to be seen from the following table:
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Steel: |
cutting oil |
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Aluminium alloys: |
spirit |
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Chromium-nickel alloys: |
colza oil, petroleum |
5.6. Cleaning the bore
After reaming, the chips and remaining oil have to be removed from the bore with the help of compressed air or brush.
5.7. Checking/pinning
Standardized bore holes which are made true to size are checked with the respective plug limit gauge, nonstandardized bore holes true to size may be checked with the help of an internal micrometer.
Pin connections are checked as to functioning after the pins are set in.
Figure 12 - Testing by plug limit
gauge
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