# Any Tools, tips or tricks to cut a hole exacltly in the center of a square?



## jimtyp (Jan 2, 2008)

I need to cut some holes in the center of square block. Working in G scale makes even being off by a couple of hairs noticeable. Any tools, tips or tricks you can share to help with this?

The best I've done is mark an X from the corners and drill where they meet, but like I said even off a couple of hairs and I notice the hole is not exactly centered.


----------



## monsterhunter (May 25, 2008)

What exactly are you working on?  Can you cut the hole first and then cut the block to the right size?
Dan


----------



## Mike Reilley (Jan 2, 2008)

Drill several blocks as well as you can...using an X mark. Look at your product. Find the one that is centered best. Put it back on the drill press, lower the drill into the hole, and build a jig around the piece that holds the piece. Clamp the jig down. Then make more holes in the center of other square blocks.


----------



## Paul Norton (Jan 8, 2008)

I use your method of drawing an X across the corners, but I use a fine awl instead of a pencil as the lines are far more accurate.












The awl is also used to make a fine whole at the center of the X to guide a small drill bit. Increasing larger bits are used to increase the size of the hole if necessary.


----------



## Burl (Jan 2, 2008)

You didn't say how big the hole needs to be, but I find drilling a small hole and then progressively increasing the bit size will usually give better results.


----------



## SteveC (Jan 2, 2008)

Jim

You didn't mention just what type of material you are drilling (I'll make the assumption like others, it being wood), you also didn't mention what you were drilling with (again, I'll assume a drill press).
[*] Type of wood will affect the accuracy of the hole drilled (softwood & wide grain less accurate, hardwood close grain more accurate).
[*] First measure the diagonals accurately and make sure they are in fact equal, if not you're not square and don't have the true center.
[*] Mark the cross point of the diagonals with a straight edge and a thin knife blade not a pencil, if you want an accurate mark.
[*] Next, accurately mark the drill point with a prick punch.
[*] Make sure that the drill press table is square (perpendictular, at 90°) to the quill, and the center hole in the table is aligned with the center-line of the quill.
[*] Use a brad-point (lip & spur) type drill bit of the size hole you wish.










[*] If the hole is larger than you can get a brad-point drill bit for, use a forstner type bit of the correct size.










[*] After chucking the drill bit, tighten the chuck loosely by hand and check that bit is running true (by turning the quill by hand, NOT under power), then tighten with the chuck key, and check for running true again.
[/list] As suggested by others, make sure the block blanks are all identical in shape and size, then when setting up the the first one to drill create a jig that will accurately hold the blank, allow no movement of the blank while being drilled, and you can remove the finished piece and replace it with the next blank. It's a good idea to make the jig up on another piece of wood that way it is usually easier to clamp the jig to the drill press table.

Remember when setting up the jig on the drill press table to check (and recheck) that the spur on the drill bit accurately aligns to the prick punch mark on the first blank. Also, it's very important to set the "depth stop" on the drill's quill feed so that you drill completely through the blank and slightly into the jig base but not through it. As a safety margin it's always good shop practice to make sure the drill bit is aligned with the center hole of the drill press table just in case the depth stop fails for some reason. If using a bit that's larger than what will fit through the tables center hole, then be extra careful with the feed.

It would also be a good idea to make a few extra blanks, just in case the first one is a bit off. That way you can make any corrections that may be needed.

Hope the above is found to be of use.


----------



## RimfireJim (Mar 25, 2009)

Metal block? You can use your calipers as a scribing tool (OK, all you real machinists can cringe now). Measuring across the block. Divide by two. Set your calipers to the result and lock. Keeping the calipers perpendicular to the edge, scribe a line at the center with one jaw by running the other jaw along the edge. Check by running along opposite edge. Repeat for other direction. Where lines cross is the center. Use a center punch to mark that spot. If you are working with soft metal like brass or aluminum, you can feel the lines and the punch will drop into the cross.
Once the center is punched, use a center drill to start the hole. It won't walk like a drill bit. Follow with a small drill bit, then larger bits to get to finished size.

Since you mention "off by a few hairs", I figure you're talking metalwork and not wood. "A few hairs" is on the order of .005 inch (and I have thick hair); trying for better than that in wood is hopeless - too many variables.


----------



## Les (Feb 11, 2008)

Posted By jimtyp on 05/11/2009 10:19 AM
I need to cut some holes in the center of square block. Working in G scale makes even being off by a couple of hairs noticeable. Any tools, tips or tricks you can share to help with this?

The best I've done is mark an X from the corners and drill where they meet, but like I said even off a couple of hairs and I notice the hole is not exactly centered. 










Jim:

If you're doing precision work (under 0.005"), or tighter, start by laying out two perpendicular lines on the metal plate such that one bisects the other and all lines from the center are of equal length. (Elementary geometry text will give you all you need to know.) Then by the same procedures, create a square with a scribe and straight edge, or a machinist's layout square _known to be accurate. _A surprising number aren't. Now check your work in all possible dimensions: the sides should all be equal, the diagonals you've created equal but check 'em anyway. If all is good, you've got a square with a true center for a hole.

Since you seem to be having recurrent problems, it's time to look at your tools:

1) are your dividers truly ground to concentric points? Ditto your center punch. If you're using a standard jobber's drill, is it ground to center so it cuts w/o drifting? Are you using a center drill when staring the hole? Is it nice and sharp? Is your center punch one intended for fine layout, or is it a common metalworking center punch? Is it sharp, or can you see light reflecting off the point.(Dull).

To use a center punch takes a little practice: what you do is approach the intersection of the lines from the bisected angle direction with your punch held about 45 deg. Touch it down, let it rotate up, and lightly tap it. Closely inspect the tiny mark you made. Is it right? If not, you can use the punch, held at an angle, to 'drift' that hole one direction or another. That's why 'delicate' is the watchword here. Once that prick mark is where you want it, hold the punch perpendicular to the work and tap it lightly with a light ball peen (no claw hammers allowed.) Inspect the somewhat deeper prick mark. Happy? Reset your punch and give it a firmer tap. You're intention is to leave a clean, conical hole, not drive the punch through the material.

2) Someone has already advised you to check your drill press. If you're not using one, but say a hand drill, forget the whole question. Here's a good way, if you have a dial indicator: Chuck up the arm of the indicator, by sequentially snugging each jaw of the chuck in turn. Set the indicator for zero, if it's a +\- or some halfway number if it isn't. Then rotate the chuck slowly by hand in a half circle. Is the needle still reading about the same, +\- .002"? Drill presses are not normally considered high-tolerance like a mill, though some can be. If it is, then set your indicator 90 deg from where you started (by rotating--don't take it loose from the chuck.) Note/reset the dial to suit. Repeat as above for another 180 deg. You should have about the same number. Think of it as checking a line right to left, and front to back on your drill press table. While you're at it, lower the table, extend the quill to the max, (don't lock it) adjust your dial indicator and move the quill r/l, frt/bk. Lots of wobble? There's likely your problem: you can't drill precision holes with a sloppy quill.

3) Next, chuck up a center drill, a short stubby thing with two ends. Use the smallest one and start it on the center-punched hole and run it as deep as you like. Note, if you're intended hole is smaller than that drill's diameter, just make a clean impression and quit. Go to your twist drill to finish. If you're trying to drill a really large hole, start with a pilot hole that suits.

Step 3 illustrates why cheap twist drills won't do. You have to have a drill bit that cuts on center, and most jobber's drills will. It's up to you to notice that the chips from each flute are equal. If they aren't, sharpen that bit or get another one.

4) Lastly, deburr with a hand deburring tool. All you want to do is knock the rough edges off, not make a miniature countersink in the hole.

Hope this helps.

Les


----------

