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Counter-sinking lag bolts James 06-30-2006
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Posted by James on July 1, 2006, 12:48 pm
Thanks for info on the Forstner bit. Looks like a real specialized bit,
and it would do a neat, clean cut for the counterbore.

--James--



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Posted by Bobk207 on June 30, 2006, 3:41 pm

James wrote:
> I am building an outside shed, using large timbers and lag screws of two
> sizes, 3/8 inch and some 1/2 inch. In some places, I want to
> counter-sink the lag screws. I want to see if I am on the right track.
>
> For the 3/8 inch lag screws, I plan to use a 1/4 auger drill bit for a
> pilot hole. Would that be the right size ? To countersink the head of
> the screw, I plan to use a 1 inch spade drill bit.
>
>
> For the 1/2 inch screws, I plan to use a 3/8 auger drill bit for the pilot
> hole. Again, is this the ideal size ? To countersink the screw head, I
> would use a 1 1/2 inch spade drill bit.
>
> I will be using washers, so the countersunk holes must accomodate the size
> of the washers.
>
>
>
> Summary of Questions:
>
> 1. Am I planning the correct pilot hole sizes ?
>
> 2. For drilling through 6 inch timbers, what type of drill bits are better,
> the auger type, or spade ?
>
> 3. Is the use of the spade bit the correct type of bit for the
> countersinking ?
>
>
>
> I know these are rather "green" questions, but I learn from this group.
>
> Thanks !!
>
> --James--

The holes you are suggesting are just a little too big. The pilot hole
size should be a function of the wood species as well. That said,
since you're most likely building with a softwood I would cheat
sligthly smaller.

I usually measure the root diameter of lag & go just slightly smaller
(in softwoods) . This way, I get a lttle bit of compression of the
wood & it makes for a nice tght assembly. Hardwoods I go the other
way; esp oak!

I want to have as much thread as possiblw w/o danger of splitting the
wood or breaking the lag.


A slight 'nit' about the OP's teminology........

a countersink is a tapered feature desinged to accept the tapered head
of a flat head bolt or screw.......... usually 82 deg or 100 deg or
other

the feature the OP is creating; a flat bottomed hole is actually
refered to as a counterbore

cheers
Bob


Posted by Bob Morrison on June 30, 2006, 5:35 pm
In a previous post Bobk207 wrote...
> The holes you are suggesting are just a little too big. The pilot hole
> size should be a function of the wood species as well. That said,
> since you're most likely building with a softwood I would cheat
> sligthly smaller.
>

Bob K is correct. Here's a quote from the National Design Specification
for Wood Construction regarding the holes for Lag Screws:

"(a) The clearance hole for the shank shall have the same diameter as the
shank and the depth of penetration as the length of unthreaded shank."

(b) The lead hole for the threaded portion shall have a diameter equal to
65% to 85% of the shank diameter in wood with G> 0.60, 60% to 75% in wood
with 0.50<=G<=0.60, and 40% to 70% in wood with G<=0.50 (see Table
11.3.2A) and a length to at least the length of the threaded portion. The
larger percentile in each range shall apply to lag screws of greater
diameters."

So it seems you will need 3 drills: one for the pilot hole, one for the
shank hole, and one for the counterbore. You don't say what kind of wood
you are using, but Douglas-Fir had a "G" of 0.50 and Hem-Fir has a "G" of
0.43. The typical pressure treated timbers are Hem-Fir, but look for a
grade mark to be sure.

--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com

Posted by Jordan on June 30, 2006, 10:03 pm
Bob Morrison wrote:
>
> So it seems you will need 3 drills: one for the pilot hole, one for the
> shank hole, and one for the counterbore.
>

But, not in that order!
I used to have a job making timber children's playground equipment, and
counterboring was standard procedure. The holes were further plugged
with plastic caps.
Drilling the hole before the counterbore I found is not good, pilot or
no pilot. Unless of course you have some fancy guided counterboring
drill (unecessary).
Counterbore first, then drill for the bolt.


Posted by Stormin Mormon on June 30, 2006, 11:13 pm
Some grease injected into the bolt holes sure makes bolts go in
easier.

--

Christopher A. Young
You can't shout down a troll.
You have to starve them.
.

In a previous post Bobk207 wrote...
> The holes you are suggesting are just a little too big. The pilot
hole
> size should be a function of the wood species as well. That said,
> since you're most likely building with a softwood I would cheat
> sligthly smaller.
>

Bob K is correct. Here's a quote from the National Design
Specification
for Wood Construction regarding the holes for Lag Screws:

"(a) The clearance hole for the shank shall have the same diameter as
the
shank and the depth of penetration as the length of unthreaded shank."

(b) The lead hole for the threaded portion shall have a diameter equal
to
65% to 85% of the shank diameter in wood with G> 0.60, 60% to 75% in
wood
with 0.50<=G<=0.60, and 40% to 70% in wood with G<=0.50 (see Table
11.3.2A) and a length to at least the length of the threaded portion.
The
larger percentile in each range shall apply to lag screws of greater
diameters."

So it seems you will need 3 drills: one for the pilot hole, one for
the
shank hole, and one for the counterbore. You don't say what kind of
wood
you are using, but Douglas-Fir had a "G" of 0.50 and Hem-Fir has a "G"
of
0.43. The typical pressure treated timbers are Hem-Fir, but look for
a
grade mark to be sure.

--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com



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