How to accurately drill holes using a simple divider tool

By Carl Heideman
Apr 14, 2024 | Tech Tips, drilling | Posted in Shop Work | From the Dec. 2016 issue | Never miss an article

Photography by Carl Heideman

A custom bracket with mis-drilled holes is pretty much worthless. Want to save yourself a lot of time and aggravation? We have found that a common set of dividers–a simple tool that usually costs less than $15–can be a real time-saver, accurately locating holes while minimizing the need to take measurements. Here’s how we used a divider to quickly locate three holes in a custom bracket.



Our favorite tools for transferring holes from one bracket to another are aptly named transfer punches. However, we didn’t find them very useful this time: As we went to transfer holes from a cast bracket to our fabricated piece, the original casting simply would not lie flat and any rocking hurt our accuracy.



An inexpensive set of dividers does the trick here. We use them all the time to measure the distance between holes. Rather than try to measure from center to center, we measure from edge to edge on the same side of the holes. It’s easier to measure to the edge, and the distance is the same as long as the holes are the same diameter–in this case 10mm.



We then lay the divider on a ruler to get the space between the hole centers: 1¾ inches.



Some quick math told us that if we wanted to center our holes in a piece of 3-inch-wide steel, we’d need them to be ⅝ inch from the edge. We set our divider to ⅝ inch and scribed lines along the edges. Double-checking our measurement told us our lines were 1¾ inches apart. We also scribed a line 90 degrees from the parallels. The intersection of the lines would determine where our first two holes would go.



Now we needed to space the last hole, so we again used the divider to measure edge to edge.



This time there really wasn’t a need to determine the dimension. We just stuck that third hole along one of the lines that determined one of our first two holes. Now we had our three holes marked. We just had to drill at the intersecting lines.




Center punching really helps the accuracy of drilled holes. We use both a spring-loaded automatic center punch (top) as well as a traditional punch and hammer (bottom). We actually center-punch each hole twice–first by hand with the automatic center punch, then once again, making it more pronounced, with a hammer and traditional punch.



Our quick work has yielded three clearly marked and punched locations for our drilling. While the part is accurate, it should have a little style. Radiused edges make for good style–and reduce the chance of a stress riser. The first step is to set our dividers to ⅝ inch and mark radiuses for the two outboard corners.




We then drilled our holes and sanded the radiuses as scribed.

STEP 10:


While there was more to do in the next steps of our bracket project, this baseplate matched the casting that it was replacing. It bolted in place on the first try with no need to oval any holes. The whole process took us about 20 minutes with very simple tools. You can do this.


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jkstill New Reader
11/30/18 3:48 p.m.

Nice article, thank you.

Another similar method that I have found that works:

Cut the heads off appropriately sized bolts, and grind the resulting end to a point.

Insert the bolts, then line up the workpiece and give it a good tap with a deadblow hammer.

The bolts will leave a nice dimple for drilling points.

When alignment is more critical, drill the first hole, and get a bolt through it, making it easier to line up the piece correctly, then tap it with the hammer.



Torqued New Reader
11/30/18 4:19 p.m.

When transferring dimensions from the dividers to a rule such as a carpenter's square, you can increase your accuracy a little by starting with the dividers at the one inch mark instead of at the edge of the square, then just subtract the inch for your spacing calculations.  I find it easier to place the divider precisely in this way, especially if the rule is well used and maybe has the edge dinged up a little. 

BimmerMaven New Reader
12/1/18 10:05 a.m.

Harbor Freight 6 in dig calipers, $10-20, have sharp points and can be used to scribe as a divider would; saves measuring and errors in doing so.  as shown, the dividers are 1 13/16 apart.

Brotus7 HalfDork
12/1/18 10:13 a.m.
BimmerMaven said:

Harbor Freight 6 in dig calipers, $10-20, have sharp points and can be used to scribe as a divider would; saves measuring and errors in doing so.  as shown, the dividers are 1 13/16 apart.

Agreed.  Have a pair of cheap calipers that double as scribes.  You can color the metal in the approximate area of interest with a sharpie marker before scribing to help it show up (or marking fluid/dykem).

spandak Reader
12/2/18 11:34 a.m.

Good tip

Now how do you keep the drill bit from walking or otherwise being not center? Center punching helps but on some of the larger holes I find the drill bits don’t move symmetrically about the center through the material, even after drilling a pilot hole. I hope that makes sense...

tyronejk New Reader
12/2/18 9:16 p.m.

In reply to spandak :

A pilot hole that's at least at wide as the web thickness of the final drill bit should keep the drill from wandering.  If the drill still goes in off-center, I'd guess that something is wrong with the drill (i.e. ground asymetrically).

Danny Shields
Danny Shields GRM+ Memberand HalfDork
12/3/18 5:23 a.m.

Good short article with terrific clear photos! I could use more detail on Step 9, "sanding" the corners round. Does that just mean taking the part to the bench grinder and grinding the corners down to the scribed radius line?

Brotus7 HalfDork
12/3/18 6:13 a.m.

I like to use center drill bits when making pilot holes. They're pretty short and stout, so they don't flex much and tend to follow the center punch pretty well.

Carl Heideman
Carl Heideman
12/3/18 6:43 a.m.

Thanks for the kind words.

Brotus:  I use the center drill bits all the time.  They don't flex at all. They just break if you go too hard on them. That's how I made the part, but we left the drilling out of the story to keep it short. Machinists often disapprove of the use, but they work so well  

Danny:  I usually sand on a combo 6" disk/belt sander   You'll see it in a lot of the stories I write.  An electric 4" hand held grinder works pretty well too if you clamp the part down.



BA5 GRM+ Memberand Reader
1/21/19 7:09 a.m.

Another trick: keep in mind where/when your car was made.  If the car was made in a place or time where they were likely using metric, then it can be useful to convert your measurements (especially if they're a somewhat oddball fraction) to metric. 

For example, that measurement you just took on your Honda probably isn't 1-11/64", it's 30mm.  For bolt patterns and things like that you can assume that you're mostly going to be using pretty round whole numbers.  

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