[webgeek]

Hi all; this is a stupid question but it's really giving me fits. I've created a fixture with a small pocket 1" square and 1" from the home (0,0) point of the fixture. I've also added three dowel pins, two at the top and one at the left edge that are in alignment with the home point. This means I can slap a piece of metal on the fixture and it's consistently aligned in relation to the fixture. When I start work, I use a laser edge finder to "touch off" the square pocket walls and then I adjust a set amount to home the machine. Finally, I secure material to the machine with 1/4x20 holes tapped into the fixture.

So to use the fixture, I "touch off" and home the machine to the fixture. Then I place the material down and clamp it from the outside. I drill 1/4" holes in the material that should align with the holes in the fixture. Then I bolt the material down to the fixture in addition to the outside clamps. Here lies the problem...

When I bolt down the material, it will subtly shift away from the dowel pins. The problem is that even a few thousands off in initial alignment anywhere is enough for the 1/4" holes to not align properly with the threaded holes in the fixture. This causes the material to shift a tiny bit when clamping and means I can't reproduce positioning consistently.

My question is this: can I just use a larger drill bit to buy me a touch of clearance (3/8 for instance)? Seems odd to me that this is even coming up but danged if I can't ever get it perfect no matter what I do. It seems like the tight bore holes + dowel pins over constrain the material so it shifts. I'd not even ask but I feel like I'm missing something obvious here.

Thanks in advance!

-Mike

[BobWarfield]

Your 1/4-20 bolts are torquing the workpiece away from the positioning dowels. You can move the dowels so that they support the workpiece in the direction they're applying torque. Or, you can add some sort of corner clamp that is opposite your dowels and presses the workpiece tight against them.

A nice toggle clamp bearing against that corner would do. You could then torque down the 1/4-20's without worrying.

Cheers,

BW

[Geof]

Your "misalignment" is possibly due to the 1/4" drill running off.

Do you spot drill first then come in with the 1/4" drill? If you do not spot drill the 1/4" drill can wander as it makes contact with the surface. The wandering may not be much but what happens is the drill starts making the holle off center and the end result is that the hole is not perpendicular.

Having said that you should be using clearance holes anyway. You need some clearance around the bolts so that they are just holding things down, not tending to influence the alignment. And if you have a generous amount of clearance such as 1/64" then the bolts are not likely to affect the alignment.