Basic machining question

[Al_C]

Ever decide you've been doing something wrong for years? Yes, that's the impetus for this thread. Topic: drilling a bigger hole from a smaller one, or just plain being accurate.

The concept is pretty simple. You drill a small hole, say 1/8 inch, and then you drill a bigger one, say 7/16 (or whatever) using the 1/8 hole as your guide. Even using a drill press, I still manage to miss by a bit, far too frequently. Or, I make a dent using the center punch, and the hole ends up being ever so slightly off. So here's your chance to offer whatever tips you may have for ensuring the final resulting hole is dead on. btw - Happy New Year!

[2bking]

Manual drilling is always difficult to get the center accurate. Center punches often don't leave a deep enough impression for the flat point of a standard drill to follow resulting in drill point wonder. A second or third impact on the punch will give better results. There are sheet metal drill bits that help because they have a very sharp point and cut around the edge of the hole first but they can't be used to enlarge a previous hole. You are having problems following an 1/8 hole with a 7/16 bit because the flat on the bottom of the 7/16 bit is bigger than the pilot hole. Try using a larger pilot hole by following the 1/8 bit with a 1/4 bit and I think your results will be much better. If you work is clamped in position while using a drill press, a 7/16 bit is very stiff and will not follow a pilot hole that is off center. The work piece needs to have freedom to self center when the larger bit finds the pilot hole.

[DaveS53]

To improve the accuracy of a hole's location, use a black sharpie to color the area where you want to drill and use a precision machinist's rule (.010 inch increments) along with a sharp carbide tipped scribe, to scribe a cross, marking the hole's center. If you center punch off location, you've got problems. As already noted, use a small drill to start and don't go too much larger at one time. Using 135 degree split point drills to start might also help.

https://www.amazon.com/PEC-Flexible-...achinist+ruler

https://www.amazon.com/General-Tools...+tipped+scribe

https://www.amazon.com/Max-Power-Cob...%3A135+Degrees

[lance corsi]

Your accuracy on aluminum is best found at higher speeds vs those used on drilling steel, etc. Use adequate cutting oil, not Wd-40 or Kroil or PB blaster, etc. As stated above, use a pilot drill large enough to center your larger bit. Hold your workpiece as firmly as practically allowable, and if hand drilling, get a comfortable stance & brace yourself if possible.

[NAZ]

Here's what a former machinist does:

When drilling by hand or in a drill press I mark where I want the hole similar to what Dave describes above. I stopped using dykem years ago and use those huge magic markers now and the scribe I use a a 1/8" TIG welding tungsten that I have sharpened. I start with a very light center punch mark so that if I miss the crosshair I have a chance to fix my mistake. If it's on the money I make a deep impact crater larger in diameter than my pilot drill.

I typically start with a small pilot drill or small center drill -- way smaller than 1/8". The smaller the pilot the less variance around the center punch mark. If I was going to drill a 7/16" hole I'd step up the size about halfway from my pilot to my final size. If using a center drill for a pilot on a 7/16" final size I'd probably not use an intermediate size. Once the final size is drilled I use a countersink to lightly chamfer the edges of the hole. On large holes like when using a hole saw I use a deburring tool to break the edges or sometimes a half-round file. But breaking the edge of a drilled hole is important and so many fail to do this.

If accuracy is critical I use a milling machine and use the DRO to locate where the hole will be. Drilling by hand or even in a drill press with jobber style drills is inherently inaccurate. Just depends on how critical the hole placement is.

[mikeinatlanta]

I also start way smaller than 1/8". When hitting center is critical sometimes I'll custom sharpen a drill bit using the split point fixture to take the chisel edge down to almost nothing. Lets it sink into a center punch dent without walking.

[Gumball]

Ditto on the very small bit for the pilot hole and using a scribe in a sharpie mark to get the "crosshair" in the right place. To that I'll add that I use a spring-loaded center punch that makes a really good divot on aluminum and is actually pretty good on steel, too.

[Al_C]

Thanks, guys! I used "7/16" as a representative diameter, but I think I got useful info. My takeaway: start really small and work your way up to the desired diameter. I've always done the two-step approach; it would appear that I need to think in terms of 4+ steps.

[NAZ]

The takeaway is: to be more accurate you need to minimize error. There is error in every step -- measuring, marking, center punching, drilling. Work to produce the smallest error and you will increase your accuracy.

[mikeinatlanta]

Thanks, guys! I used "7/16" as a representative diameter, but I think I got useful info. My takeaway: start really small and work your way up to the desired diameter. I've always done the two-step approach; it would appear that I need to think in terms of 4+ steps.

Don't think you need to be so concerned with the number of steps. I suggest you look at the size of the chisel area of the bigger bit and make sure it is smaller than the pilot hole. When the chisel is bigger than the pilot it can walk a little easier.

4-14_drill_point.jpg

[Straversi]

I like to mark the location with a center punch and then use a countersink bit in a hand drill to deepen the mark. The countersink bit has a very sharp point and it doesn't wander or grab. It's easier to get the drill bits to stay in these deeper guide marks.
-Steve

[carbon fiber]

***READ THIS:*** Here's my best tip, make the crosshair mark way larger than the hole. That way you can see from start to finish that you're in center. I use a machinist's rule and a fine line sharpie. For both the pilot hole and the final size hole, stop a couple times (while still on the chisel point, before you get to the full width of the bit) and look to see if you're still centered in the crosshair. (should look like four even slices of pie, divided by the crosshair) While on the chisel point, you can "move the hole around" a little to "re-center" if you get off by angling the drill slightly in the direction needed. Once you get to the outer edge of the chisel point, the hole is where it is. You can use a center punch to start but it's still going to be up to your drilling to keep the hole centered. I don't rely on the center punch to keep the hole straight, it's really only to keep the bit from walking as you start the hole. I don't rely on the pilot hole to keep the full size bit centered either. Visually check center as you drill with the chisel point until you get to full width of the bit.

[DaveS53]

Once a small hole is drilled, a larger bit will always drill in the same place, as long as the work piece is free to move on the drill table. Just don't step up too much at one time.

An accurate center punch mark is essential, unless you're drilling on a milling machine, with the work piece fastened down. If you're doing that, then the whole process changes. Use an edge finder to locate two edges, then move the mill table to the desired location. Use a 90 degree spot drill to drill slightly larger that the desire hole and the hole can then be drilled in one step, leaving a small chamfer on the edge. That's how you drill a hole to be tapped as the next step. You want a 90 degree chamfer on the leading edge of a tapped hole, large enough reach the root of the thread. Take a close look at any nut and that's what you find, with the chamfer on both sides. If a hole needs to be precisely sized, it's drilled under size and then reamed or bored to the final size.

https://www.cnccookbook.com/when-to-use-a-spot-drill/

[RoadRacer]

I’ll second lance and suggest using cutting fluid every time. I’ve seen arguments over it but I’m a big fan, makes cutting so much easier and the bits last longer. Right now I have a small can of Tap Magic that use whenever drilling or tapping.

[lance corsi]

Actually, prolly half the holes in the world are in the wrong place & the other half are worn out! It's not a bracket, it's just a holder for your hole!��

[edwardb]

I'll throw this idea out there as long as you professional machinists don't laugh at this former IT guy. I do everything mentioned here. Draw a cross hair (usually with a fine point Sharpie), use an automatic center punch with several hits if needed, start with a small bit and move up gradually, and typically use a drill press with the piece clamped down. In steel this almost always gets the hole right where I want it. But with aluminum, just not unusual at all for the bit to wander a bit and not hit the cross hair exactly. Especially with smaller size drill bits. What I do is check after each time to see if I'm on the mark. If not, I'll take a small needle file and "adjust" the hole over back to the cross hairs. Then go to the next size bit and usually it will stay centered. I find I'm able to almost always get the holes right where I want them.

My other hint, and I don't think this has been mentioned, is I use a step drill bit whenever possible after the initial hole is drilled and in the right place. I find they stay centered up in the hole more reliably than a regular drill bit. Step bits are typically for larger holes and have a limited selection of sizes compared to a whole box of twist drills. But I use them whenever I can.

[carbon fiber]

Most people don't have a milling machine or a full set of reamers, spot face bits, etc. Most don't even have a higher end drill press, and cheaper ones aren't very accurate. With lots of holes you drill you can't get them in a press or mill anyway. The crosshair method works with just drill bits and drills. I'm doing what edwardb is talking about (re-centering the hole as needed) with the chisel point of the drill bit instead of a needle file.

[mikeinatlanta]

Most people don't have a milling machine or a full set of reamers, spot face bits, etc. Most don't even have a higher end drill press, and cheaper ones aren't very accurate. With lots of holes you drill you can't get them in a press or mill anyway. The crosshair method works with just drill bits and drills. I'm doing what edwardb is talking about (re-centering the hole as needed) with the chisel point of the drill bit instead of a needle file.

You are correct. Fact is when I want to be really accurate I use guide piece with a drill bushing. Assume most don't have drill bushings. Use pilot hole freehand for medium accurate.

[cgundermann]

Large center point with fine sharpie (to stay center as you enlarge), automatic center punch several times, start out very small and as you increase and step up - drift the bit more towards center if you need to bring it true. Love WD40 as a cutting oil and also like step bits and have found the true V (non-step variable) bits also help to lean your hole back to center as you approach your final drilling diameter.

Chris