Drilling hole precisely

[John Almberg]

I am trying to make a sheave for a wooden block. Something like this, except the sheave is made out of oak, not plastic:

http://relimg.reliancemarine.com/46.jpg

I am cutting the 2" blank out of an oak plank using a hole cutter. The hole cutter makes a pilot hole in dead center. I need to expand this hole to 3/8", but the bigger hole also needs to be dead center for the sheave to turn smoothly on the bronze pin.

I am using a drill press to drill both the blank and the 3/8" hole. After doing several experiments, I can get *close*, but I'm having trouble getting the 3/8" hole perfectly centered.

All I'm doing now is eyeballing the tip of the drill bit over the pilot hole. You'd think this would work, but the result is always a smidgen off centered.

Is there a trick that would help?

Thanks in advance: John

[pat warner]

Probably a reamer is called for, but what about some heroics to start the hole with a 3/8" drill? Indexing, locating and holding the work is a trick indeed, See one solution (http://patwarner.com/holding_the_work.html). The work has to be immobilized for any accuracy to be consistant.

[Shawn Pixley]

For real precision, I'd turn it on a lathe. For practical work, have you considered modifying a hole saw to cut the outer diameter and the pilot at the same time? I think you could modify a hole saw to get a 3/8" bit in there. If niether of those options has appeal, I'd make a jig to align it on my drill press. Or you could cut it out of thicker stock by partially cutting with the hole cutter. Then switch bits without removing the stock and drill the pilot. Then you could rip through the bandsaw to the desired thickness. Just quick thoughts before coffee

[Bill ThompsonNM]

Try gluing the oak blank to a carrier board with hot melt glue or wood glue and a paper separator . Fasten the board to your dp table and use the hole cutter (not through the carrier!) then without unclamping the carrier drill the 3/8 inch hole. Now separate your blank from the carrier with a sharp whack with a chisel.
(alternatively-- use a lathe to create your piece)

[Steve Ryan]

Use a 2 1/4hole saw, or one that will give you a slightly larger blank. Enlarge the hole to 3/8, put a 3/8 bolt in your drill press and sand to final size.

[glenn bradley]

Bill's got it but, here's a variation. Put a bit in your DP that matches the center bit on the hole cutter. Lower the bit into the center hole of your object to align it. Clamp your object down to the table securely. Change to a 3/8" Forstner bit and drill the hole again.

[John McClanahan]

How about a step drill, like these. http://www.harborfreight.com/3-piece-titanium-nitride-coated-high-speed-steel-step-drills-91616.html

John

[Jim Summers]

I wonder if a 3/8 forstner bit and clamping the workpiece will help? My forstner bits are 1/2 and I don't think it is as long as a 3/8 twist bit. I have a cheap set of brad points also but they wobble all over the place.

HTH

[Jamie Buxton]

How 'bout replacing the pilot bit with a 3/8" diameter? Here's a 3/8" bit with a 1/4" shank: http://www.amazon.com/Vermont-American-10424-Shank-Speed/dp/B0002YUTNC

[Brian W Evans]

If the diameter doesn't have to be precise, you could get close to center like you're doing now and then use a jig like this on a disc sander:

http://images.meredith.com/wood/images/2006/10/m_discsandingjig.gif

to make sure the hole is centered. You'd need to replace the wood screw with a 3/8" dowel or pin, of course. Similar jigs could be made for the band saw, spindle sander, table saw, etc. if you don't have a disc sander.

Here's a link to the Wood magazine article (http://www.woodmagazine.com/woodworking-plans/sanding/one-way-to-make-a-perfect-circle/).

Here's another one (http://www.woodmagazine.com/woodworking-plans/sanding/disc-sander-circle-jig/).

[Dan Cameron]

Use a 3/8 bit with a 1/4 sank in the holeaw. They are readily available.

[Mark A Johnson]

To start with, how are you holding down the material? Once you start machining the part, the key to keeping center is not moving the part. There are tools that will improve your ability to center. If your removing the piece and loosing center, your original marks are gone. Cutting two blocks simultaneously and marking them exactly.

From there you can use a wiggler to find exact center point on your drill press. Google "Starrett Wiggler" and you can find direction on usage. The link shows you the tool. If you can't make it work let me know and I'll cut it for you on my CNC.

Mark

http://littlemachineshop.com/products/product_view.php?ProductID=2279

[Terry Beadle]

+1 with Dan Cameron

There exists a "center finder" that you buy from Enco or other machinist suppliers. You put it in your drill press and it will give you the exact center.

Another simple jig is to turn a short piece of hard wood to a fine tip. It needs to have a shaft OD to match the hole from the hole cutter. To use, put the short jig fine point up in the hole. Move the work piece with the jig point until it matches the tip center of the 3/8th bit. Clamp as needed.

[Chris Fournier]

This job screams out as a lathe project. Otherwise you could use the hole saw as you have been doing and then increase the diameter of the centre hole with the apropriate countre bore bit. The link shows the bit I am talking about. Offshore imports can be had for cheap.

http://www.google.ca/imgres?imgurl=http://www.scorehi.com/counterbore.jpg&imgrefurl=http://www.scorehi.com/Gunsmithing_Tools.htm&h=584&w=762&sz=21&tbnid=gyzSxlVbwtzHbM:&tbnh=109&tbnw=142&prev=/images%3Fq%3Dcounterbore%2Bbit&zoom=1&q=counterbore+bit&usg=__mfwffgxfdzi5c75wo428FK57q2o=&sa=X&ei=Y1qPTYejDpC-tgfA2OC-DQ&ved=0CDwQ9QEwBg

[John Almberg]

Wow, I was hoping for one good trick and got a dozen.

I don't have a lathe, but I am using my drill press (Delta) to cut the groove in the sheave, using one of those Microplane rasps. I guess if I cut the hole as close as I could *before* cutting the groove, 'turning' the groove would eliminate the wobble.

If that doesn't work, I have a whole list of ideas to work with.

Thanks!

[John Almberg]

How 'bout replacing the pilot bit with a 3/8" diameter? Here's a 3/8" bit with a 1/4" shank: http://www.amazon.com/Vermont-American-10424-Shank-Speed/dp/B0002YUTNC

Ah... kill two birds with one stone. I like that!

[mreza Salav]

To enlarge a hole by drill here is a simple trick I have shared many times:

Before cutting the 2" using the hole saw, first use a 3/8" bit (or whatever size you need) to drill half-way the hole in the center (not all the way).
Typically, there is a small dip in the middle already at which you can put the tip of the pilot 1/4" bit. After cutting the 2" disk use that half-way cut 3/8" hole to place your 3/8" bit and complete the hole.