On 15.08.13 06:13, John Thornton wrote:
> In the Machinery's Handbook it has charts about various thread limits
> based on amount of engagement and some text that says "In general when
> the engagement length is one and one half times the nominal diameter a
> 50 or 55 per cent thread is satisfactory."
That matches what I've been using, from this table:
----------------------------------------------------
Metric Coarse | BS 4168 Clearance hole:
| | Type of fit:
Size Drill E Core | Close Medium Free
----------------------------------------------------
M2 1.70 61% 1.51 |
M2.5 2.15 63% 1.95 | NOTES:
M3 2.60 65% 2.39 | 3.2 3.4 3.6 2.5mm drill -> 82%.
M4 3.40 70% 3.14 | 4.3 4.5 4.8
M5 4.40 61% 4.02 | 5.3 5.5 5.8 With a nut length of
M6 5.20 65% 4.70 | 6.4 6.6 7.0 1.5 * diameter, 50%
M8 6.90 72% 6.47 | 8.4 9.0 10.0 engagement is stronger
M10 8.70 71% 8.16 | 10.5 11.0 12.0 than the external thread.
M12 10.50 70% 9.85 | 13.0 14.0 15.0
M16 14.25 71% 13.55 | For CI and thin sheet,
-------------------------------------------------- stick near 75%.
E is engagement. Some of those drill sizes are easier to get than others,
so a quick calculation can be made, using:
# d is full thread depth.
Drill_Size = OD - (E*2d) # E is engagement.
# d = kP , where P is thread pitch.
Or transposed for E: E = (OD - Drill_Size)/2d
& k is: ISO Metric 0.613
UNF/UNC 0.613
# British Association BA 0.600
# BSW, BSF, ME (32 & 40 TPI), Whitworth forms 0.640
# BSB, and BSP parallel.
Especially with smaller taps, it's also relevant that the torque
required to drive the tap doubles from 60% to 80% thread engagement,
with half the increase occurring between 75% and 80%. Some say:
To avoid breaking small taps, a guide for Al, brass, and steel is:
< 6 mm: ~ 60% thread engagement
6 - 10 mm: 70%
> 10 mm: 75%
and 75% engagement is as high as I'd go, normally.
The table and equation came from issue 104 of Model Engineering
Workshop, and has worked for me so far.
Erik
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