Drilling Question - Offcenter Hole

Trying to drill the hole highlighted in blue. Any suggestions for how I go about doing this accurately. Our team has access to a drill press and a mill that would both be capable of doing this kind of work. Any advice helps. Thanks.

Edit: If it changes anything. All other holes have already been drilled. I’m modifying KOP for more elaboration.

If you can use a dial test indicator to find the center of the hole, you should be able to then switch to an endmill to make the hole. An endmill will drill in the center every time, just make sure to use oil and peck.

I would use a 1" endmill first, followed by a 1.125" endmill to get the final size. A boring head will work too.

Depending on how precise you need it, you can get sort of close by using a drill bit and manually plunging and moving to find the center of the hole.


I will have to research how exactly each of those things you described work, but I appreciate the response.

Dial test indicator: dial indicator. Used to find things like edges of holes. You could also (depending on part geometry) use an edgefinder to locate the hole from the edges of the part. The drill bit suggestion will also work. Once you’re done, remember to lock the mill table in position.

Then you use an endmill (NOT repeat NOT a drill bit) to make the hole. Specifically, a center-cutting endmill, somewhat less than 1.125" (I’d say to run 0.5" or 0.75" first, but it’ll depend on what’s available). The reason is that drill bits have angled points, and will immediately try to go sideways into the hole that you already have. There’s a non-zero risk of snapping a drill bit, and a near 100% chance that the hole is ruined. Endmills have flat cutting surfaces and won’t hop into that hole.

Then you use a bigger endmill to finish the hole. Or a boring bar (essentially, a lathe cutter mounted in a fixture that allows you to slide it for really big holes on the mill).

That said, if this is to scale… That smaller hole near the side could cause you some issues, as it won’t be fully consumed by the large hole. You might want to shift left by a few thousandths of an inch if you can, to make sure you remove it completely.

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The method I would use would depend on the accuracy required…both in position, and hole size and finish. If you are trying to fit a 1-1/8" OD bearing to a press fit, then it would be kind of tricky. If that much precision is not required, a hole saw without the pilot might work, if used in a mill, and you have a good way to clamp the work piece to the table.

If you have an end mill of the size required, that would be preferred. The boring bar will work to enlarge the hole, as mentioned, and could give you the precision you need for a press fit.

I doubt you could perform this operation with a drill press.

Thank you so much for clarifying those terms. Is an endmill something I could get at a normal hardware store such as Home Depot?

Home Depot/Lowes/Ace? Well… Not in person. Home Depot will sell them online (calling them “specialty drill bits”).

If you have a Harbor Freight, they sell a set of endmills, up to 3/4" diameter.

Otherwise, ask around for who knows where to buy machining supplies–you may need to go to McMaster, MSC, or Grainger (who ALL have endmills of various sizes and uses, and boring bars too). Amazon does sell them if I’m not mistaken.

BEFORE you get the endmill, though–check your shop to see if there are any around. I’d be surprised if there weren’t a few in the shop, given that you have a mill–see the link I put last paragraph for pictures. Also check on collets for the mill, to make sure that you have the right size(s)–you’ll need the collets to hold the endmill properly. (I don’t trust a drill chuck in a mill with an endmill. Too easy to mess something up.)

Thanks again. I’ll look around our mill storage now that I know what I’m looking for.

If you have someone who knows how to use the mill in your shop, you can ask them about what the terms mean and what tooling to use. Learning to use a manual mill alone will take some work, but should be done if nobody is around to help and you have some hours to spend on Youtube.


It would be really good if you could find somebody local to help. They could look have the tooling you have with the mill. Probably best if you google end mills to read about them. End mills have one to four flutes on them. For a plunge cut (think drilling) you will want a center cutting end mill. You will also need a collet to hold the end mill in the mill. 1-1/8 collet is getting on the large size for typical highschool mills.

Regardless you want to clamp the work really well to the mill. You want to use the right cutting speed. 150 FPM for Aluminum. That translate to about 500 rpm for a 1-1/8 tool.

Do not use a regular 1-1/8 twist drill on sheet metal.

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Another option in lieu of an endmill would be an annular cutter, however the prices of these two would probably come out about the same.

The only real thing I have to add here is: assuming this is a plate of some sort. option 1: support the back side with a squared block of aluminum or (wood?) the plate will will want to flex when punching that big of hole. You are going to need to go slow with minimal pressure to get an accurate result. Option 2: If support on the back side is not feasible. Punching a hole with a 5/8" or 3/4" end mill then taking it to final size with a boring head, but this is a lot of steps (and perhaps equipment).

edit: if you can swing this part on a lathe at low rpms you could cut that hole with a boring bar, however this would require a 4-jaw chuck to position correctly, or at the very least a super-glue arbor.

Per the original post, this is on the KOP chassis. Takes the lathe right out of the equation for this particular part.

I agree with the advice you’ve gotten. However, there could be another option that might be easier/cheaper if you don’t have access to some of the machining mentioned here: a knockout punch. You can get these anywhere that supplies electricians.

You didn’t say what material/how thick, nor specify the tolerances you need. These things could change this answer. But, it’s worth thinking about in some scenarios.

So a low budget hardware store solution. It you have a the right collet, I would use the mill and clamp the bit as close to the spade head as possible. Use a wood cutting spade bit. Open open the center of your work piece so the off center hole does not push the tool off. Back up the work piece with a block of wood so the pilot (tip) of the spade bit has something to drill in. Clam the work piece to the table really well. Make sure you set the table locks. Run the mill (drill press) as slow as it will go. Advance the bit into the work piece slowly. It will have a tendency to grab on the existing hole. Absolutely use safety glasses and keep your hands away from the work. Be careful. Be very careful.

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Practically, yes… but perhaps not entirely :slight_smile:

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oh I missed that, for some reason I thought this was for a tough box mini plate or similar, not a drive rail.

All good ideas. One thing I would add. Using your milling machine clamp or bolt a piece of wood on TOP of the part you want drilled. Use an edge finder to locate the hole. Use a step drill of the desired diameter or a hole saw. If the wood is on top it will act as a guide as you drill through.

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Came here to post basically this.

Consider removing the center drill for the hole saw. With proper fixturing it is not needed.

Annular cutter are great option for drilling accurate holes for flanged bearings. They have a 3/4" shank so they are easy to use with standard collets in a milling machine. Otherwise there are plenty of options for shanks to fit most common larger drill presses that take tapered shank bits. They however cannot be used with hand drills and probably not a good option for lighter duty drill presses.

As long as the workpiece is clamped securely they have no issue cutting holes over existing holes or cutting partial holes.

These Hougen Rotabroach sets will scare the pants right off of you if you use a hand drill but they work in them (you really want to use a drill press or a mill, but a hand drill with a forward handle is useable in a pinch if you know what you’re doing). They also work well in a low power drill press. The nice part about the low power drill press is that if the cutter binds the drill press will stop rather than grabbing the work and throwing it on you. It is nice to be able to replace a cutter head for 20-30 bucks too. The arbor fits perfectly in a 1/2" chuck.