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I drilled several holes in my desk to put some grommets using a 7/8" hole saw bit. Unfortunately, I realized afterwards that the hole was supposed to be 1" to fit my grommets.

Then, despite knowing better, I used my 1" hole saw bit in an attempt to make the holes the correct size. Although I was able to make the hole larger, the new hole did not come out clean at all.

The obvious solution to this would be to clamp a piece of plywood behind the hole to use as a pilot. However, the position of the holes make this solution pretty unfeasible - they are in the back of the shelf of a large hutch - i would need to get a very long piece of wood with some huge clamps in order to make that solution work (unless I am missing something):

enter image description here

What is the best way of fixing this? I could get the next larger size of grommet (1.5") and make the hole larger, but which bit would be the best for this task? I have never used a Forstner bit - but it looks like it might be promising. Thoughts?

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I have to check but I am sure we have covered this. This one is close woodworking.stackexchange.com/questions/410/… but there an option not mentioned there. – Matt Mar 7 at 14:53
    
Do you have a slightly longer drill bit which you can use as pilot drill (the one coming with a hole saw usually is embarrassingly short)? If it's longer than the work piece's thickness, this is a breeze, just need to put any old piece of wood under the hole. – Damon Mar 7 at 15:18
    
@Damon - I don't think this is a feasible solution - check my update above with additional details about the positioning of the hole. – William Mar 7 at 15:28
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If the holes are in back, and the edges will be covered by the grommet anyway, this may not be worth trying to fix. – keshlam Mar 7 at 16:28
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I would think that an 8' (or so) 2x4 could be clamped horizontally behind where the holes need to be, and used as a backer for the pilot bit. If you've built this desk, I'd guess you've got at least 2 bar clamps deep enough to span the hutch riser + 2x. (I'm envisioning that the holes all go at the grey drawer level - the 2x can run behind those drawers.) – FreeMan Mar 7 at 18:16
up vote 18 down vote accepted

Lee Valley (I'm sure you can get it elsewhere) sells a product called an Oops Arbor that allows you to mount two hole saws to a single arbor. The smaller one is held further out from the larger one allowing it to support itself as it drills.

from http://www.leevalley.com/en/images/item/woodworking/drilling/30n0390-inset.jpgFrom http://www.leevalley.com/en/images/item/woodworking/drilling/30n0390s.jpg

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That is exactly what I need! Thanks so much! – William Mar 7 at 19:10
    
Never saw those! Awesome – bowlturner Mar 8 at 2:31
    
I've seen hole saws before where the height of the blades increased as the diameter decreased, but I never realised why — until now. Thank you! – SQB Mar 8 at 8:36
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If only you could use that to make the holes smaller. – Matt Mar 8 at 11:18
    
This is what you use to make holes smaller, @Matt – FreeMan Mar 8 at 13:12

If you are unable to move the piece or provide additional access (as you describe), then another approach might be to cut a wood plug the same diameter as the current hole and glue it in place in the hole. Once it is set up you can begin drilling again using the larger bit.

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And as a bonus - if the holesaw in question uses a centering bit, the plug will have a centered hole for cutting the larger hole in the original surface. – arootbeer Mar 8 at 11:51

If you and can disassemble the piece and have a router with a rabbet bit, you can enlarge the hole by going around with depth set less than the full thickness of the material and the bearing against the edge of the hole.

Then flip the piece over, attach a bottom-bearing flush-trim bit, and set the depth so it bears against the edge of the rabbet you just cut.

You can calculate the width of the cut by subtracting the diameter of the bearing from the diameter of the rabbeting bit and dividing by two. E.g. if your rabbet bit is 3/4" diameter has a guide bearing of 1/2", it would give you the extra 1/8" you need, as would any combination of bit/bearing where the difference in diameters = 1/4".

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With your 1.5" holesaw, cut a nice hole in a chunk of scrap plywood. Clamp this to the location of the old hole, and drill away.

(Unless you plan to wrestle your desk to a drill press, the forstner will do you no good at all.)

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I don't think this is a feasible solution - check my update above with additional details about the positioning of the hole. – William Mar 7 at 15:28

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