Cutting really straight holes?

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ttabbal

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I know drill bits will wander a bit, but I'd really like these to be pretty straight. The application is a plinth for my lathe. The holes are being used to hold it down with T-nuts in the lathe cross slide. Depth is about 2.3 inches, clearance hole for 3/8" bolts. I will be cutting on a Bridgeport mill, so rigidity is reasonably good. Material is 7075-T6 aluminum.

I have a decent set of drill bits, but I've seen them wander a little in deeper holes.

I have a boring head and bars, Criterion and Borite cobalt. 3/8" seems pretty small for a boring head though. And I would need to pre-drill enough to fit the bars. Depth is a bit of an issue as well. My smallest diameter bar has a min diameter of .25" and max length of under 2". I could perhaps come back from the other side, but having to indicate the hole again on the other side feels error prone. More due to the operator. :)

I do have some carbide end mills that seem like I could plunge cut with. 3/8" dia, just long enough to work. I've been leaning toward these and cleaning the chips out with compressed air periodically. The flutes are only about 1" long.
 
If you've got some O1 steel, you could made some D-bits. They don't tend to wander as much as a drill bit, but they do take some time to make.
Perhaps acquire an appropriately sized reamer for the job?
 
For this application, I wouldn't be too fussy about it. They are just through holes and you actually want a little bit of room to situate the plinth accurately on the cross slide.

7075 machines like mild steel so its nice to work with. I would spot it and drill with your on-size drill without using a pilot; that will give you your best accuracy. Calculate the proper speed and then use just enough feed pressure so that the drill cuts continuously. The hole will be pretty accurate as a result.
 
For this application, I wouldn't be too fussy about it. They are just through holes and you actually want a little bit of room to situate the plinth accurately on the cross slide.

7075 machines like mild steel so its nice to work with. I would spot it and drill with your on-size drill without using a pilot; that will give you your best accuracy. Calculate the proper speed and then use just enough feed pressure so that the drill cuts continuously. The hole will be pretty accurate as a result.
 
If your endmill is long enough, drill slightly undersize then use the endmill to finish it.

Like Mikey said though, the clearance holes should be oversize anyway. If you really want to position the part accurately, drill and ream two dowel holes in it and the T-Nut. Your T-nut would have to be made with no slop in the T-slot as well.
 
ttabbal said:
So... Stop overthinking it and drill the !#@$# holes already? :)

I can do that.
Click to expand...

It's always wise to really think things through, especially the first time you do something or when the project is a long time in the making. You did fine; discovering you screwed up after you screw up is not that much fun.

I wanted to tell you that when you need a hole to be fairly straight (drills only make fairly straight holes), try not to use a pilot drill unless the drill is large, like over 5/8". Pilot drills are properly sized when their diameters just span the web of the main drill. Hence, they are sort of bendy and flimsy so when drilling pressure is high enough they will wander. Accordingly, the main drill that follows will also wander. Using the main drill alone requires more pressure but if you pay attention you will see and feel it cutting. I tend to prefer a little less speed and a little more feed pressure to keep the drill cutting continuously. It produces less heat, less wandering and more accurate holes.

Remember too that reamers will follow the pre-reamer hole. If the pre-reamer hole bends, the reamer will bend to follow it. If you need a hole that is accurate and straight, bore it. Reamers are best used when a good finish is required. Ideally, they also produce accurate holes but only if the proper pre-reamer drill is used and the reamer is sized correctly for the material and the hardness of that material. I have a lot of reamers but often find that I don't have the exact size I want so I learned to bore accurately.

There is a lot more to making holes than you might think. You asked the right question, Trav.
 
I've never done much more than basic drilling. A little boring on the lathe, but I know there is a lot I don't know about this stuff. :) I have had some accurately center-punched holes not line up quite right on the other side before, so that's why I asked. I do think those used an incorrectly sized pilot based on the info I've found since, and were on a somewhat flexy, trashy, drill press. I don't use that POS anymore.

So on the mill, if I do want a very accurate and straight hole, the boring head is the way to go. I'll have to break it out and do some practicing. I want to get better with it. Sounds like it's overkill for this project, but I know I'll need that eventually. For this it sounds like the center drill followed by the main drill is probably the way to go. I have a better machine and drill bits than I did then, so it should be better. I think I'll drop some extra 7075 in the vise for some tests to determine what speed to use.
 
Do you have any spotting drills? If not, you might want to consider buying some. They work better for accurate hole starting than center drills, although center drills will work. Choose the spotter based on the drills you intend to use. If using 118 degree drills, use a 120 degree spotter. If using 135 degree split points, use a 140 degree spotter. I prefer cobalt spotting drills - they seem to last forever.

Cutting speed for aluminum is about 350-400 sfm for drilling. 7075 is harder than 6061 so I would go with 300 sfm. A free running clearance hole for a 3/8" bolt is 0.3970", which is an X-size letter drill, or 0.3860" (W-drill) for a close fitting through hole. If you use that, then rpm is over 2800 rpm; sorta' fast. At that speed a sharp drill, fed manually, will walk a hole right through your work piece. If you aren't comfortable at that speed, slow it down and feed by feel and you'll do just fine. Use some lubricant - WD-40 or any cutting oil and feel the tip cut.

Boring is one of the most complicated operations there is. There is a whole lot more to it than you might think. Just about anyone can use a boring head and make a hole but making it come out to a specific size, now that is not as easy as it might seem. Not going into that here but yeah, practice drilling and boring before doing a real project.
 
I don't have spotting drills. I really should get a couple though. Same with good letter drills, I have a good fractional set, but the letter set is HF and I am not happy with them.

Thanks for mentioning the speeds, I looked it up and was surprised at the speed, but got distracted with work. :) Sounds like the recommended speed is actually that high. I'll start my testing higher than I expected then.

That was what I came up with after holding a boring head and experimenting with it, it's more complicated than it looks when I start thinking that I want a hole exactly Z size at location X/Y. I think I need some practice time before I can really trust myself with it.
 
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