View Full Version : Finding exact center

December 31, 2010, 11:45 PM
I have a DPMS 308 upper with the factory tubular fore end.
Apparently it's one that doesn't leave the factory with a swivel stud.
How does one find the exact bottom-center of the fore end so I can drill a hole for a swivel stud?

December 31, 2010, 11:55 PM
More than one way to skin this cat.What do you have access to?
I'd use a Bridgeport.You may not have one.
Tell what you have.Example,a real nice table saw?Lets see,make a little hardwood block,very square,with a dimension 1/2 the dia of your forend.
Its now a gage block.You need a flat,small piece of steel,like a lathe tool bit or piece of key stock,the right flat wood chisel or a plane iron would work.you need flat,sharp piece of steel that is sharpened from one side only.
It is your scribe.
Move the rip fence away from the blade so your table is your surface plate.
Lay the upper on the table,and clamp the flat top to the rip fence,using the rip fence for an angle plate to index the upper.If you want,a little white shoe polish would be layout die for contrast.Lay the flat side of your scribe blade,sharpening bevel up,on top of your gage block.Make a little scribe line where you want the hole.There will be some error,its OK,turn the whole thing over and do it again.You will have two parallel lines,each slightly off center.
Center is between them.Use a level across the receiver when you put it in a drill press.I would use a pointer in the drill press and a loupe to center on the marks.(If you have long hair and the drill press is running you could get scalped.)
Secure the vise with two clamping points,making sure it stays centered,Check and adjust,A little mallet tapping is OK.
A center drill is a good thing .its a short drill that makes a good start .It wont walk.Lacking one,short chuck a circuit board short,stubby drill.an undersize one.Make a pilot hole.Then finish drill.
I recommend you begin by running the quill stop on your drill press upto just let your drill touch the workpiece,then creep it down maybe 1/64th at a cut.It will control the drill breakthough,and minimize any sudden grab or fracturing of the graphite.I think Clark Custom guns.of 1911 fame,makes these forends.Call them for drilling advice.
This layout idea I gave you,it a way of thinking about what you have to work with.Its not "THE" way. can use a piece of bent wire in a spindle like a test indicator to center a workpiece within .002 or .003.

I'd go humble and pay a smith before I would use a hand drill,though.

January 1, 2011, 02:09 AM
I have a table saw and a drill press.
I was thinking of this (type) of tool I found online:


Need to check if it will apply to the diameter of my fore end.

January 1, 2011, 05:21 AM
I have not used one of those.Looks like a reasonably sound principle and should work well enough to locate a sling swivel .I'd turn it 180 degrees to see if it reads the same.
As I said,more than one way.
That wire in the spindle thing is like using a test indicator to center.Bend it and sweep it to one vise jaw,Get the wire to just miss by a whisker,then rotate it to the other jaw.You can see when the wire sweeps to just barely miss both jaws.

January 1, 2011, 10:33 AM
A tool like that well get you within a few thou.
cheap one, shipped.