View Full Version : Picking up punch marks?

05-10-2009, 02:21 AM
I'm in the process of mounting a couple pieces of "boat jewelry" on my boat. Since these gadgets get mounted with 5 each 1/4-20 machine screws, and there's no such thing as a plumb, square, or level surface on a boat, and the gadget (16ST winches) are mounted on a wood block that kicks them back 5 degrees from being square to the mounting surface, I made a drilling jig for getting the holes in the right place and at the right angle. (That sound you hear is my H.S. English teacher burning out the trunnion bearings in her coffin!)

The project at hand is to make the backing plates which fit under the deck and are drilled and tapped to take the 1/4-20 machine screws. The holes got to be at the right place and at the right (5 degree) angle in the right direction.

Anyhow, what's the best way to align a punch mark under the spindle of a mill? I have a wiggler set and an edge-finder set. Both have pointy things which look like they should be useful for this, but I don't think I'm doing this the right way.:confused:

I've been using the wiggler with the needle point gadget as follows. First, I get the needle lined up by bringing the spinning tool against a square edge until it stops wiggling. Then I try to line it up with the punch mark with a 10X magnifier. This sort of works, but getting at the point I'm lining up to with a glass is sometimes problematical. There's always a clamp or something in the way. It seems like there should be a better way!:rolleyes:



05-10-2009, 02:48 AM
I usually use the pointy wiggler. If I see it move as I lower it into the punch mark, or if it swings in an arc after I raise it out of the punch mark, then it's not centered. I don't have to see if it hit's perfectly, I only have to see that it moves.

A 1/4-20 hole will be .2570 (tight clearance) to .2660 ( loose clearance), so you can be off .0075 and still be in the ball park. That much deviation is visible with the naked eye.
( http://www.diyfab.com/reference/tap-and-clearance-hole-sizes/ )

Shim one edge of the plate to get the required 5 degree angle.

Good luck.


05-10-2009, 07:42 AM

Just use your parallel/cylindrical edge-finder as normal.

See the following sketch which should be self-explanatory.

Pick up the end and edge of the job, allow for half the edge-finder diameter plus 0.996" per inch of distance "D" up/along the slope. There is no need to make any "slope allowance across the slope.

Put the 1/4" (0.250") "steps" in and start the holes off with a centre-drill on the "flat" of the "step" as normal and you will very little - if any - "wander" of the centre-drill. Follow up then with your required drilling sequence.

I'd suggest doing one step and hole at a time.

You should be able to do the whole job with the job clamped to the mill table (with packer/parallel strips as required).

Counter-bore if or as required.


05-10-2009, 09:29 AM
Anyhow, what's the best way to align a punch mark under the spindle of a mill? I have a wiggler set and an edge-finder set. Both have pointy things which look like they should be useful for this, but I don't think I'm doing this the right way.:confused:

I use one of these nifty devices.



05-10-2009, 10:01 AM
Close enough projects get the craftsman lasertrac cross hairs on the bridgeport, More delicate things get the camera with cross hairs.. Center drilled items get bit ran backwards and centered.

They make a nice scope to mount on a drill press, it is like $75.. I started making one from a pistol scope, a telescope 90 adapter.. then lost interest in the project.

ON the lathe? or a quickie hole? a step drill does a non-wandering hole. IE: unibit.. short and does not deflect and one cutting edge does not wander.

Rich Carlstedt
05-10-2009, 12:59 PM
Here is a 5- 10 second allignment process.
Changing tools to get allignment is a pain in the butt.
Take a look at your adjustable square. Note is has a little ball with a scriber anchored into the base and is used to scribe layouts !
Get a chunk of clay, ( not play dough!) and kneed it till soft.
wrap it around the ball and encapsolate it.

When you have your tool in the mill, whether it be a drill, or endmilll, what ever, and you are ready to center it on the punch mark, wrap the clay around the drill, with the scrib pointed at the work piece.
try to center it a bit and and turn the spindle on briefly. recenter the pointer.
DO NOT worry about where the ball is, only the bottom endpoint !
Now turn on the mill ( ~500 rpm ) and using your finger tip, gently nudge the pointed end until it runs without runout. It is really very easy to do !
Now with your magnifying glass, bring down the scrib and allign it to the punch mark. Have the power off.
turn on and recheck for runout, and if OK, stop and remove the ball of clay.

Keep it handy on your work table.
It makes any setup a dream and special tools.


05-10-2009, 01:49 PM
My experiences with this are that threaded backing plates are a PITA. It's much easier to drill the mounting holes through the deck, transfer punch a 1/8" piece of stainless and drill clearance holes on the drill press. W/ the lack of plumb on a boat, getting 5 1/4-20 bolts of any length to all be parallel enough to thread into the backing plate is a neat trick. Drilling jigs help, but the holes will still need to be oversize.

Perhaps this is a fiberglass hull w/ a thin deck; to me, doing this in wood w/ 2" long bolts would just be an exercise in frustration. If you really want to thread the backing plate, drill the holes in the hull and then mark and thread them.

- Bart