Tips for centered and symmetrical holes in aluminum

I am pretty capable have new drill bits, drill and drill press, just wondering if the pros can share their knowledge....Any tips for drilling centered holes and making sure everything is symmetrical on an aluminum piece?

Is an adjustable square and pencil the best tool to use when making an intersection on a square piece of aluminum? How can I make sure my holes are perfect...Can you recommend a good center punch and counter sink?

Re: intersection on square piece. Best depends on a lot of things. "Best" as far accuracy on a drill press would be an XY table and a wiggler or edge finder. You'll be contending more with spindle run-out than anything. I mean, there are more accurate methods, but the DP is your limiting factor. The Renishaw probe can stay on the shelf.

Hello! If you are doing layout work with a square, I'd suggest using a sharp scribe. It will leave a finer line than a pencil. Using an adjustable machinist's square with a 45 degree X 90 degree head only allows you to 'eyeball' a line from corner to corner. Using a centering head ''cups' the corner and , I think it is much easier to get the true corner. More so when the edge has been rounded off by deburring. If you are like me ( I'm getting older and the peepers don't work as good) using a center punch with a magnifying lens helps a lot!
With the centering head, you can double check the veracity of the line from corner to corner by indexing the square on the opposite corner of a square piece of stock and either scribe another line or just look closely and see if the first scribed line is right at the edge of the blade.

Forget the pencil. Use layout dye and a sharp scriber. You want the layout lines ad fine as possible. Prick punch the hole locations. If you don't get the punch mark exactly on the intersection of the layout lines you can move it by holding the punch at an angle and striking it, then hold it upright and strike it again. Check how it fits the layout lines and repeat the process until the punch mark is centered on the layout lines.

When you want holes symmetrical about a central hole use a divider set to the hole spacing to mark the hole centers.

Once you have the hole locations laid out go to the drill press. Use a wiggler with needle point to get your part lined up with the spindle. Clamp your part. Put a center drill or spotting drill in the chuck and spot the hole location. Then drill the hole. Repeat for each of the other holes.

The accuracy of the result depends primarily on two things: how accurately you lay out the holes and how accurately you position the punch mark under the spindle.

I suggest you start by having an accurate scale, adjustable square, divider, and scriber. Those are the absolute minimum layout tools you will need. Be sure they are machinist quality tools and not hardware store junk. For layout dye you can substitute a broad magic marker. The only purpose of the dye is to provide contrast so your scribed lines are visible. Get a wiggler and learn to use it. I mentioned a prick punch. It is similar to a center punch but with a sharper taper at the end. It is used for accurately locating the hole positions. The center punch can be used after the prick punch to make the punch mark bigger. You want the diameter of the punch mark bigger than the web thickness of the drill you use to start the hole otherwise the drill may walk. I often omit the center punch step if I am using a small center drill to spot the hole.

Some people use a digital or dial caliper to scribe a layout line parallel to a part edge. Just set the required dimension on the caliper. Slide one jaw of the caliper along the part edge while scribing the line with other jaw. Only angle the caliper enough for one jaw to follow the edge. If you angle it too far the distance you are trying achieve will be too short. Another approach is to use the step measurement function of the caliper and to scribe a line against the fixed jaw. HF (I don't think I am allowed to use the whole name) is selling a digital caliper for about $11 with coupon so there real excuse for not having a caliper. I use the HF calipers for most of my non critical dimensions. My very expensive Mitutoyo stays in its box most of the time.

There is a device called an optical center punch. It consists of a base with a cork face and a hole for the punch. You put the base on the work and roughly align the hole to the scribed lines. Then you insert the plastic magnifier thingy in the hole. Use the index marks on the plastic thingy to get centered over the layout lines. Some have cross hairs, some have a circle for aiming. Hold the base firmly against the part, remove the plastic thingy, insert the center punch and hit it with a hammer. The cork face on the base provides enough friction so it will not move between the aiming and the punching. It is much more accurate than putting in the punch mark with unaided eyes.

How can I make sure my holes are perfect

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First, nothing is perfect. Machinists, cabinet makers, carpenters, all types of craftsmen work to certain tolerances. A rough carpenter may work to _+/- 1/8" or +/- 1/16". A machinist may use a variety of techniques to work to tolerances that are somewhat or a lot better than that: 0.010", 0.003", 0.001", 0.0005", or even better. But there will always be errors, nothing is perfect.

Techniques? OK.

First, a pencil will draw a line that is at least 0.010" wide and that is not very precise in machinist circles. It can be too big even for some fine wood work, like cabinet making. Machinists prefer scribed lines. The work is coated with a thin coat of a colored lacquer (die chem) and then a metal or carbide scribe with a fine point is used to draw lines in it. These lines can be around 0.002" or 0.001" wide and they provide a lot more precision. The colored lacquer provides a contrast to the bare metal exposed by the scribe so the lines stand out well.

I have a very good quality Starrett square but the lines on it also have a definite width so setting it to better than +/- 0.005" is difficult. Some here may say they can do better and I do not dispute them, but still the accuracy is limited.

The traditional method for scribing a part for machining is to use a surface plate and a height gauge with a scribe installed on it. The surface plate is extremely flat (+/- ten thousandths of an inch) and the height of the scribe above the plate can be precisely set with the vernier, dial, or digital scale that the height gauge is equipped with. With a method like this the lines can be easily located on the part with +/- 0.001" accuracy and with a good height gauge and some care you can do a lot better than that. In this process the width of the line becomes a major source of error.

For parts that do not need that level of accuracy, I often use a digital caliper to either directly mark the part, using the sharp edge of the jaws, or I transfer the setting from the caliper to one of my good adjustable squares and use that to scribe the lines. Of course, some additional error will creep in when you transfer a measurement like this.

Those are the two ways that I usually mark locations on parts.

The location for a hole is usually marked with two scribed lines that are at right angles to each other, forming a cross. Now a scribed line can be felt with a fine pointed instrument and I keep some good quality prick punches for making the first, SMALL punch mark at the hole's location. The punch is dragged lightly over the scribed lines, moving it perpendicular to both of them until the intersection is located. Then a light blow with a small hammer will make a small punch mark, hopefully on that intersection.

I use a 10X magnifier (Hastings Triplet is best) to examine that first, small prick punch mark to see if it is actually centered on the cross lines. If not, a punch is inserted in it and set at an angle to move the punch in the direction needed. This step takes some practice.

Once that initial, small punch mark is centered, a larger punch can be used to enlarge it to a size appropriate to the drill that you will be using. Generally this is around the web thickness of that drill so the drill sits in the punch mark when starting. Another tip here if your hole has a fairly large diameter is to use a smaller diameter drill to start the hole and then use the larger drill to follow the hole made by the smaller, initial drill. The theory here is that any drill, used with reasonable care and a punch mark for starting, will never drift more than it's radius off that mark and most often it will be centered far better than that. So, for instance, a 1/16" drill will never be more than 1/32" off and most likely it will be within a few thousandths. Then the larger drill will follow that initial hole and it will be centered equally well. But if you started with a 3/4" drill, it could easily drift off by 1/16" or even 1/8" if you are not really careful.

Many will tell you to use a spotting drill and I also like that idea. A spotting drill is a very short drill and it will not flex very much while starting the hole. You only make a dimple with it for the larger drill to follow.

All of the above takes time and if you need to work faster but with looser tolerances, another trick I use for centering holes on a part is to set an adjustable square to the calculated distance as best as I can and then I scribe lines from both opposite edges with it. The center will be half way between these two lines. This works well even when the width of your parts differs by a bit from one part to the next.

Illinoyance said:

Forget the pencil. Use layout dye and a sharp scriber. You want the layout lines ad fine as possible.

There is a device called an optical center punch. It consists of a base with a cork face and a hole for the punch. You put the base on the work and roughly align the hole to the scribed lines. Then you insert the plastic magnifier thingy in the hole. Use the index marks on the plastic thingy to get centered over the layout lines. Some have cross hairs, some have a circle for aiming. Hold the base firmly against the part, remove the plastic thingy, insert the center punch and hit it with a hammer. The cork face on the base provides enough friction so it will not move between the aiming and the punching. It is much more accurate than putting in the punch mark with unaided eyes.

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This is the method I've used for 20 years now... Works very well.. Here is a link to the optical center punch Illinoyance mentiond and I use.. You can find them cheaper. Taken care of they last a long time....

Using your available tools and equipment, accurate holes are not possible. Of course I and many others have seen it done on occasion. To do this takes extraordinary skills and frankly, a great deal of luck, not to mention, perfect eyesight. I have all of the tools and devises mentioned in this thread and have used them all. You can get close sometimes, but never spot-on. Today, I use a BP with a DRO, center or spot drill every time with every hole and use collets to hold the drills never a drill chuck. In that way, I can hold +- .002. Perhaps others can do better, but I cannot.

aaronhl said:

I am pretty capable have new drill bits, drill and drill press, just wondering if the pros can share their knowledge....Any tips for drilling centered holes and making sure everything is symmetrical on an aluminum piece?

Is an adjustable square and pencil the best tool to use when making an intersection on a square piece of aluminum? How can I make sure my holes are perfect...Can you recommend a good center punch and counter sink?

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Have a look at this video. It shows how accurate you can do this.



Manual Sheet Metal Layout Lesson! - YouTube

Great information, reevaluting my tolerances is a great idea. I can see now how the pencil is not as accurate and can measure metal different;y that wood (using lower tolerances with wood)
I think a few simple tools with the dye like scriber, prick punch and compass will help. The video posted above is very helpful.

Can anyone recommend brand names for a machinists ruler, scriber, prick punch, etc

Recently did some holes/slots for a guy that said his Sharpie was more accurate than my BP, CNC, or Moore Jig Bore... No joke... He had a bad case of Meth Mouth, but I got paid...

Another very useful tool for marking-out is the jenny or odd-leg caliper; it performs the same function as some have described with digital calipers, marking a line parallel to an edge.

aaronhl said:

Great information, reevaluting my tolerances is a great idea. I can see now how the pencil is not as accurate and can measure metal different;y that wood (using lower tolerances with wood)
I think a few simple tools with the dye like scriber, prick punch and compass will help. The video posted above is very helpful.

Can anyone recommend brand names for a machinists ruler, scriber, prick punch, etc

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Starrett, Mitutoyo, old Lufkin, old Brown & Sharpe, PEC.
For the punches, brand is not too critical. I still have the prick punch I made over 60 yr. ago.

Fowler and SPI are ok.

Some Chinese import stuff is pretty good, unfortunately you can't tell how good it is until you buy it and test it. I have HF 6" calipers ($11)that I use for non critical work while my $200 Mitutoyo sits on the shelf.

Perfect guys this is a great start

In machining, there is nothing that is "perfect". It's all about "tolerances". In other words, plus or minus decimal equivalents, fractional dimensions, or metric dimensions. Example - + or - .001", + or minus 1/64", + or - 5mm, etc. Finding center of a work piece is best accomplished by measuring from all linear sides as you do your layout using metalworking die (Dykem) and a good quality scriber. Make sure that the lines you scribe meet identically from all sides. To punch the work piece most accurately, use an optical center punch (ON MARK Optical Center Punch - Model: OP-270) available on Amazon.com.
With proper care, you should be able to make your holes concentric within about .005".
The following links are for the optical center punch and metal working die.

ON MARK Optical Center Punch - Model: OP-27: Material Handling Equipment: Amazon.com: Industrial & Scientific
Amazon.com: dykem: Industrial & Scientific

Drilling is a life-long study.

What to do, depends a lot on what you are trying to accomplish.

One thing I can recommend is: use the countersink first. Don't drill then countersink after. Make the countersink first.

To make really accurate holes using a drill press, the best results come from using a jig with a drill guide bushing in it. Learning how to make such jigs and align them is an art.

For making a non-precision hole, use a good quality scribe, punch and ball peen hammer.

Clamping is important. People who don't clamp end up as YouTube fail videos. Clamping is an art.

My main drill press has a light built right into the arm so the workpiece is brightly illuminated.

Tram the table so that it is orthogonal to the spindle.

If you learn how to sharpen a drill bit, you will get much better results. Unfortunately, it is really hard to sharpen drill bits. It's an art. Deckel makes a grinder that can be used to sharpen drill bits using an accessory part. Unfortunately, Deckels cost $5000 and the drill sharpening accessory is $1000.