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Counter-sinking lag bolts James 06-30-2006
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Posted by James on June 30, 2006, 2:29 pm


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--



Posted by Bob Morrison on June 30, 2006, 2:39 pm
In a previous post James wrote...
> Summary of Questions:
>
> 1. Am I planning the correct pilot hole sizes ?


The pilot hole should be no more than 60% of the lag shank.

For 3/8" (0.375") this works out to 0.225", or 7/32".

> 2. For drilling through 6 inch timbers, what type of drill bits are better,
> the auger type, or spade ?

Either one works, but you will get a cleaner hole with an auger type bit.
>
> 3. Is the use of the spade bit the correct type of bit for the
> countersinking ?

Again, you can use either one, but the hole will be cleaner with an auger
type bit. You might even consider using a 1-inch hole saw to cut a
groove and then use your spade bit to clean out the hole.

The countersink hole should be just slightly larger in diameter than the
washer under the head of the lag -- you are using washers aren't you?

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

Posted by Brian V on June 30, 2006, 3:22 pm

> In a previous post James wrote...
>> Summary of Questions:
>>
>> 1. Am I planning the correct pilot hole sizes ?
>
>
> The pilot hole should be no more than 60% of the lag shank.
>
> For 3/8" (0.375") this works out to 0.225", or 7/32".
>
>> 2. For drilling through 6 inch timbers, what type of drill bits are
>> better,
>> the auger type, or spade ?
>
> Either one works, but you will get a cleaner hole with an auger type bit.
>>
>> 3. Is the use of the spade bit the correct type of bit for the
>> countersinking ?
>
> Again, you can use either one, but the hole will be cleaner with an auger
> type bit. You might even consider using a 1-inch hole saw to cut a
> groove and then use your spade bit to clean out the hole.
>
> The countersink hole should be just slightly larger in diameter than the
> washer under the head of the lag -- you are using washers aren't you?
>

Make sure you do the counter-sunk holes first. Once you make the pilot holes
it'll be too big for either a spade or auger tip and make a VERY sloppy
hole, of centered as well. If it were me I'd use a forstner (some call em
forsner's) bit for the counter-sunk followed by an auger for the pilot.



Posted by Phil Scott on June 30, 2006, 3:59 pm


>
>> In a previous post James wrote...
>>> Summary of Questions:
>>>
>>> 1. Am I planning the correct pilot hole sizes ?
>>
>>
>> The pilot hole should be no more than 60% of the lag shank.
>>
>> For 3/8" (0.375") this works out to 0.225", or 7/32".
>>
>>> 2. For drilling through 6 inch timbers, what type of drill
>>> bits are better,
>>> the auger type, or spade ?
>>
>> Either one works, but you will get a cleaner hole with an
>> auger type bit.
>>>
>>> 3. Is the use of the spade bit the correct type of bit for
>>> the
>>> countersinking ?
>>
>> Again, you can use either one, but the hole will be cleaner
>> with an auger
>> type bit. You might even consider using a 1-inch hole saw
>> to cut a
>> groove and then use your spade bit to clean out the hole.
>>
>> The countersink hole should be just slightly larger in
>> diameter than the
>> washer under the head of the lag -- you are using washers
>> aren't you?
>>
>
> Make sure you do the counter-sunk holes first. Once you make
> the pilot holes it'll be too big for either a spade or auger
> tip and make a VERY sloppy hole, of centered as well. If it
> were me I'd use a forstner (some call em forsner's) bit for
> the counter-sunk followed by an auger for the pilot.

works great if you like spending a lot money on bits...
especially large diameter long ones.




>
>



Posted by CC on June 30, 2006, 9:33 pm
I agree with Brian. I used a 1 1/4 forstner bit to drill counter sink holes
in 6X timbers that I used for my shed foundation. Make sure you drill the
counter-sink holes first. If not it is really difficult to keep a large
forstner bit centered with a hand drill. Use flat washers under the screw
head.
I used the same size washer as the hole drilled. Made a nice looking job and
tightened down tight without any crushing of the wood fibers with the large
washers
CC


>
>> In a previous post James wrote...
>>> Summary of Questions:
>>>
>>> 1. Am I planning the correct pilot hole sizes ?
>>
>>
>> The pilot hole should be no more than 60% of the lag shank.
>>
>> For 3/8" (0.375") this works out to 0.225", or 7/32".
>>
>>> 2. For drilling through 6 inch timbers, what type of drill bits are
>>> better,
>>> the auger type, or spade ?
>>
>> Either one works, but you will get a cleaner hole with an auger type bit.
>>>
>>> 3. Is the use of the spade bit the correct type of bit for the
>>> countersinking ?
>>
>> Again, you can use either one, but the hole will be cleaner with an auger
>> type bit. You might even consider using a 1-inch hole saw to cut a
>> groove and then use your spade bit to clean out the hole.
>>
>> The countersink hole should be just slightly larger in diameter than the
>> washer under the head of the lag -- you are using washers aren't you?
>>
>
> Make sure you do the counter-sunk holes first. Once you make the pilot
> holes it'll be too big for either a spade or auger tip and make a VERY
> sloppy hole, of centered as well. If it were me I'd use a forstner (some
> call em forsner's) bit for the counter-sunk followed by an auger for the
> pilot.
>


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