Concentric Drilling With a Radial Arm Saw

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Introduction: Concentric Drilling With a Radial Arm Saw

About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

This is a piece of round stock into which I have drilled a hole that is perfectly on center.  I could have made the hole as deep as my drill bit is long, but left it shallow.  I did this on my radial arm saw.  What I am demonstrating in this Instructable was inspired by a chapter in the Foxfire Books on boring the barrel for a Kentucky flintlock rifle. 

Step 1: Features of a Radial Arm Saw

My radial arm saw has a 1/2 x 20 threaded shaft on the backside of the motor.  Not only can I attach a saw blade on the front side of the motor, but I can attach a drill chuck or a sanding drum on the backside end.  Further, I can swivel the motor so the shaft runs parallel to the saw arm.  Because the motor moves along the arm on a suspension track, I can pull the motor into something I want to drill.  That would move the motor and bit nearer to the left side of the photo. 

I cut a piece of wood 2 x 3 inches a little over 5 inches long and aligned it under the drill bit so it is parallel to the drill bit.

Step 2: Make an Upright

I cut two pieces of 1/2 inch plywood and attached the first one to the piece of 2 x 3 with drywall screws.  You may notice the bit in the chuck is very small in this photo.  This is the size I would use to make a pilot hole if I were using a brace with an expansion bit.  The small pilot hole marks the center of the drill chuck and also allows the screw threads on the expansion bit to grab the wood and pull the bit through for cutting the hole.

The time I needed to use what I am demonstrating here I had only the expansion bit.  I needed to drill a concentric hole 1/2 inch in diameter up the end of a nicely finished banner pole so it could rest on a fancy brass stand with a vertical 1/2 inch steel rod bolted into its center.   Neither the hole nor the steel rod would be visible when in use. 

Step 3: Clamp and Drill

Use a large "C" clamp to hold the 2 x 3 firmly to the saw table.  Pull the saw motor into the upright so the drill bit makes a pilot hole on center with the drill chuck.

Step 4: Or, Use Another Pilot Bit

I now have a hole saw the same diameter as the piece of round stock in the first photo.  Its pilot drill is 1/4 inch in diameter.  I have placed a 1/4 inch drill into the chuck.  And will enlarge the pilot hole in the previous step to 1/4 inch. 

Step 5: Make a Second Upright

The second upright is like the first, but mounts to the end of the 2 x 3 inch piece nearer to the saw motor.  One feature of a radial arm saw is that the arm can be swung to the side to make room, and then put back into the same precise indexed position.  Swing the saw arm back into position and make a pilot hole in the second upright.  

Step 6: Make the First Hole Larger With the Hole Saw

My saw's shaft turns at about 3,000 rpm.  That is too fast for my hole saw.  I used the pilot hole to center the hole saw mounted in my electric drill. 

Step 7: Enlarge the Other Hole

Move the saw's arm to one side to make room and use the hole saw to enlarge the second hole.  When finished I had two holes the same size as the round stockand on the same center as the drill chuck.

Step 8: Insert the Round Stock and Drill

The round stock has been inserted through the holes in the support.  Push the drill bit and the round stock into one another.  Because my motor turns at 3,000 rpm, I drill in very short bursts to keep the bit from overheating.  Some radial arm saws have a 1/2 x 20 spindle that runs at a greatly reduced speed and is helpful for drilling operations. 

The jig for this operation is specific to the size of the round stock used.  It would be difficult to remove the jig and attach it again later.  Too many things would have moved and the holes in the uprights would no longer be concentric with the drill bit.  But, it is very useful when you have a specific need for a concentric hole in a longer piece of round stock. 

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    45 Comments

    Great tip! Like you said it would be difficult to make something that is reusable, and it's hard to imagine needing frequently enough to justify making something like that, but for an occasional need this is a great tip!

    1 reply

    Thanks. I have used it a couple of times when it solved a real need.

    I don't  think the the one we have has that potion for the drill chuck. Its a 20 year old Craftsman one.

    As for the speeds is it the same or is there some kid of gearing?

    3 replies

    My Sears Craftsman 10 inch radial arm saw was purchased new in 1972.  About that time they also made a 9 inch lighter duty model.  A friend has one and I did not see a 1/2 x 20 thd. spindle on the backside of the motor on his saw, only the mechanism for a manually operated blade brake. 

    As I mentioned, the Montgomery Ward and Sawsmith saws have a second parallel shaft with a mechanism for adjusting the rpm's of that shaft. 

    Phil and All,
    I have a recently acquired 9" Sears radial arm saw. It has the brake mechanism on the motor shaft opposite the blade. I used a block of wood to lock the blade/motor shaft, and removed the brake with the same wrench used for the saw blade. Under that, there is indeed a threaded shaft. I have yet to ascertain the thread size, but I'm betting it's 1/2 inch.

    The thread size is 1/2 inch x 20thd./inch so it will fit a Jacob's Chuck and similar accessories. Congratulations on your acquisition. I hope you enjoy it very much.

    Nice, great way to get extra work out of your radial arm.  I've got a drill press that I love for this but it can be tricky, I'm working adding a laser sight to it, just of set to the chuck, or bottom mounted which would allow me to true things up a bit easier, it seems that that would be easy for your rig, you could put the dot right on the end of the bit.

    I want to thank you.  I hope you find something useful in the things I have posted.  Best wishes.

    From rifle to wood handle...  It's amazing where inspiration can come from.  Good job!
     
    10 replies

    Thank you for looking and for commenting.  According to your profile you may be a little young to remember when the Foxfire Books were making their appearance and creating some stir.  If you find them in a library, they are interesting to read.  They are an attempt to document Appallachian culture and methods before those become extinct.   The  chapter on Kentucky rifles had some really simple and clever ways to do very precise operations.

    I would highly recommend the Foxfire books as required reading for all makers.  You can find them at many libraries and they are for sale on ebay.

     SIGH.. only if I had downloaded all the books that where available for "free" at www.librum.us/index.html , while they where free .  Any it's worth a visit to the site as the books are now available on CD or DVD at reasonable prices, many demos available as well that are fairly usable, but large files. A lot of old tech stuff is available if you have an interest in that.

    It would have been nice to get them for free. I wonder if Google has scanned them...

    At least they are widely available.

    Thanks for your comment.  I just found "Foxfire 5" online.  It is the volume that includes the chapter on gunsmithing.

    Have fun. The books are great reading.

    I would be cautious on some of the information in the Foxfire books. One is a remedy for the flu and it involves drinking gasoline.......

    You have to take the books for what they are: a documentation of how things were done in Appalachia.   There is one volume that has folk tales that they grew up with, clearly not to be taken literally.  Another has interviews of folks who were in their 90s back in the 1960s.  They just tell stories of what growing up in the mountains was like.

    The beauty of the books is that a lot of crafts and skills would have been lost with the passing of these people.  Even though there is no real need to make a chair from woven oak caning, it is a great challenge to do it and teaches you a lot about materials and methods.

    I believe the medicinal cure you write of was drinking kerosene, not gasoline. Although, it is not much better.  Kerosene has also been the fuel of choice for fire-breathers.

    The same caution would most certainly apply to things found on the Internet.  The user always needs to apply some tests for veracity, but most of us know that already. 

     Foxfire 5 is the only one of the series I own, and found the topic of the making of guns interesting. Particularly the means of cutting the rifling  More likely than not the simple methods produced usable guns. The degree of precision I'm unsure of, as the projectile forced down the barrel was really malleable. To be fair even today, the making a gun barrel striag