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cdc02254 (Aerospace) (OP)
17 Mar 06 6:20
Can anyone point me in the right direction of a Minimum thread depth chart?.  I am tapping a m2x0.4 hole into a piece of HE15 but am limited on depth what is the minimum length of thread neaded to have contact in the tapped hole?

Thanks

CDC02254
HemiBuell (Mechanical)
17 Mar 06 8:25
I do not know the answer but this question has been answered many times on this board.  Try a search, or the machinery's handbook.
rb1957 (Aerospace)
17 Mar 06 9:23
no apologies for stating the obvious, but the minimum thread depth is "as small as you can design".

the next question is how strong do you need it to be ?

the rule of thumb is three threads develop the full strength of the bolt.

if i was designing something with minimal thread engagment i'd either consider the effects of vibration VERY carefully, even if you don't think you'll have vibration, or consider the effects if this thread backs out, or add positive retention device (lock washer) ... but i doubt you've got room for this.

good luck
dik (Structural)
17 Mar 06 9:49
RB... have to be a little cautious about the 3 thread rule... generally OK if bolts are matched to nuts, but often bolts are inserted into a lower strength material and maybe half a dozen threads are appropriate...

Dik
CoryPad (Materials)
17 Mar 06 10:19
thread725-35222

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

wgchere (Mechanical)
17 Mar 06 11:19
I was always taught two rules of thumb (not from the same people, though) for minimum engagement
1. 5 full threads
2. one diameter of engagement--if the screw is 5mm then it needs 5mm of engagement
rb1957 (Aerospace)
17 Mar 06 11:31
you're right dik, the strength of a bolt and nut depends on both elements.  we'll often put steel bolts ('cause they're readily available) into heli-coils which ('cause these are placed in Al plates) are effectively Al threads.
unclesyd (Materials)
17 Mar 06 15:52
If you really want to get involved with thread engagement here is a good place to start.

http://www.hexagon.de/dose/dose1e.htm
diamondjim (Mechanical)
18 Mar 06 6:22
One advantage of the helicoil is that
the thread in the aluminum is larger
than the thread of the bolt so you
have to examine two different sizes
for strength calculations.  Heli-coils
generally come in lenght of 1 times
the diameter, 1.5 times the diameter
and 2.0 time the diameter without
being a special order or design.
FACS (Mechanical)
18 Mar 06 7:12
I use the three thread rule only when pressed. Then I try to use as many fasteners as practical.

Sometimes its better to use many small fasteners instead of one large fastener.

If you can't use three threads, then you usually have a problem. Remember you can always use fine threads.

Charlie
www.facsco.com

Cockroach (Mechanical)
20 Mar 06 0:58
But you have asked for the minimum allowable thread depth, i.e. length of thread, to be tapped.

Wgchere is quite right, typically one diameter is full length of engagement.  This is usually used in the thread programs when length of engagement is unspecified.

Otherwise you need to know the loads applied to the threading and determine length of thread based on shear.  Since the internal thread profile has greater shear area than the external counterpart, and I mean always, you can base your calculation on the external threading alone.

The Machinist Handbook covers this fairly well for Acme and Stub Acme threads, not so great for Unified National and Metric edition.  However, there are plenty of programs readily available to which you can apply the numbers specific to your case for a quick answer.

I'd concur with others and say one thread diameter.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

harrisj (Automotive)
21 Mar 06 7:09
An M2 hole is pretty small. Controlling depth of full thread to 2mm is asking for trouble (de-burr the hole and you've lost 25% of your thread). In a soft aluminium alloy you won't be able to inspect or measure it effectively. Maybe if it's screwing on a maker's nameplate it would be OK but I wouldn't rely on a 2mm deep M2 hole in Al for anything worthwhile.
wgchere (Mechanical)
21 Mar 06 11:33

Quote:

2. one diameter of engagement
But a 2mm screw isn't really much of a workhorse anyway, right? I mean, your tensile load is going to be pretty light.
alexit (Mechanical)
21 Mar 06 12:14
M2x0.4 means 1X Diameter engagement is only 5 threads, as harrisj stated a standard corner break 0.2-0.4 x 45° removes a full thread from that.

Our corporate standard is 2X Diameter for anything smaller than M5...
Cockroach (Mechanical)
21 Mar 06 15:23
Good points, HarisJ and AlexiT.

Yeah, don't think a guy would want to stand the whole house over the M2 X 0.4.  Thread shear value is good, but tensile load below the minor diameter of the pin (i.e. screw) would certainly lead to a failure.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

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