Generally, should the first drill to create a bore on the lathe be split?

Hey all, I (like many others on manual lathes, I assume) have a set progression of drills I use when opening a bore in solid stock. Today, I had to deviate from it because my normal first drill, a cobalt split point "S" drill (chosen only because I have a ton from an auction) wasn't long enough. So I grabbed something else of similar diameter but longer. It was HSS, but the steel I was drilling was mild, so I didn't expect that to be an issue. It was a much bigger difference than I expected. The drill I chose was 118 degrees instead of 135 like my "normal" starter, and wasn't split.

What was likely the difference between the ease with which I usually open the hole compared to this? Was it the cobalt, the lip angle, or the split point?

The split point makes a big difference in penetration pressure. You'll really notice this if drilling by hand with a portable drill motor.

Generally, the starter drill need only be approximately the diameter of the web of the drill. So size S is pretty large unless your next drill is a 3" diameter Too large of a starter hole can lead to chattering and chipping of the cutting edges of the next drill on entry. You don't need to feel pity for the machine on drilling full diameter after your pilot hole unless you stall it out routinely. All those intermediate passes just kill your profitability on the job.

Once you go split points there is no going back...

Though I am not sure about the idea of a drill progression in most materials. 3/4" and under would be a 1 shot deal, over that might see a 1/4" pilot and then undersized prentice and bored to size and over 1" sees a 1" spade drill and then bored out to size.

How many drills are you using to get to Ø1" for example?

I am assuming that you used a center or spotting drill first, not the split point. This is normal practice to get the hole on center line. I also know that a lot of people use a split point in place of a center drill, but this is not good practice.

To your specific question. The big difference is the split point. Take a look at the ends of the two drills. The split point cuts all the way to the center whereas the jobber drill pushes the point area aside.

Back in ancient history, the 118 degree point was accepted as the best for a variety of materials. Later work as produced other geometries.

DRILL POINT GEOMETRY



by JOSEPH MAZOFF

Tom

Edit: You can convert your 118* jobber drills to split point by carefully grinding the split point geometry, leaving the point angle at 118*. I have done that to a lot of my drills.

Centre drill first is the normal procedure.

Regards Tyrone.

If you are using a split point drill as your first drill, I fail to see the use of spot drill. After all, your boring op will take care of any off center issues.

OK, first you do not say if you are going to finish with a boring bar or if you will just use drills up to the desired size.

If you are going to use a boring bar to reach the final size, then the initial drilled hole only needs to be large enough to admit that boring bar. The boring bar will create a centered bore, even if the initial, drilled hole is an inch or more off center. So, in that case there are no worries about what kind of initial bit you use. Split point will go in easier so you may as well use one.

BUT, if you are not going to finish the bore with a boring bar; if you are just going to make the hole (bore) with drill bits, then there are other concerns. One of the main advantages of boring a part in the lathe is getting that hole well centered. But this centering action assumes a rigid tool (drill bit). You are starting by drilling a small hole and then opening it up with one or more larger diameter drills. If you want the bet centering, then the FIRST drill bit that you use should be as rigid as possible so it does not deflect as it contacts the work and starts the hole. Using a small diameter bit makes this more difficult as smaller diameter bits will be able to easily deflect. If the tip of that bit is not perfectly centered on the work when first contact is made, then it can start the hole a bit off center and the flexibility of the bit will allow it to continue at that off-center location and your initial hole is off center.

Because of this, a shorter bit is best and a spotting bit is the shortest you can get. Even if they are off center (due to the tailstock not centered, drill chuck not centered, bit is a bit bent, or any other reason) they will not flex very much and the hole will be a bit large, but on center. So that gives you the best start. Many will use a center drill to start the hole. This is OK, but that very small diameter at the tip of a center drill can flex. What happens when the larger, main diameter of the center drill starts to bite metal is anyone's guess. Will it self center at that point, like the spotting bit? Perhaps. But that would require pulling the small diameter tip off of the hole it has created and could break it. But perhaps not. It may stay off center and wiggle the whole tailstock while drilling deeper. So, again I say that a spotting drill is best.

Now, should that spotting drill have a split point? I say NO. A split point will start penetrating as soon as it contacts the work. If that initial contact is off center, then it will want to follow that initial point as it orbits around the center axis of the lathe and of the work, pulling the tailstock with it. A chisel point drill will be pushing metal aside at that initial contact and a slightly off center, chisel point will create a dimple that is a bit larger in diameter than the chisel edge. And that dimple will be centered. That dimple should be shallow,perhaps the outer edges of the spotting drill won't even make contact with the work. So this centered dimple will draw the following drill into that centered position and you will get the best advantage of the lathe's self-centering action. If the lathe tailstock succeeds in holding the spotting bit off center, then you will get a slightly larger diameter hole but it will still be centered.

I have tried using a succession of drill bits to open the initial hole but I find that you only need two diameters. The first bit's diameter should be around the web thickness of the final bit to make the job easier at the large diameter.

So for best centering in the lathe I would:

1. Start with a spotting bit with a chisel point. This can be a large diameter bit for maximum rigidity. But keep the size of the chisel point on that bit in mind. I would want that point to be smaller than the next drill bit's diameter.

2. A standard bit that has a diameter about the size of the web on the final bit. This should follow the start made by the spotting bit. If you start with a spotting bit, then the type of bit on this drill does not matter: split point or chisel point will both work.

3. A final diameter bit to finish the hole. Again, the type of tip does not matter because it will be in the previously drilled hole and never touch the work but see my final thought below.

The reason why I do not use a number of small increment drill sizes is that when cutting a small amount with each size, they can grab in the sides of the hole. This can make a rough finish in the hole and even pull it off center or make a cam shaped, not round hole. You are better off using all or most of the cutting edges of the final drill as that will give you the best centering action, keeping it on the center line of the initial, small hole.

A final thought: the second drill bit does not need to reach the full depth of the final hole. It only needs to give the final bit a good start. So it should provide a guide hole that takes the final bit to the point where it's lands are in contact to the side of the drilled hole for a distance that is large enough to provide guidance. This distance would be around the diameter of the final hole or somewhat more. So final bit diameter plus final bit tip length would be the minimum depth for this drill. If that final bit is going to go past past the depth of the web sized hole, then a split point on it, the final bit, will be an advantage.

Yes, sorry, I guess I meant the first *through* drill. I always use a spotting drill first. If it will be bored to finish, I guess I see the point that it might not be necessary, but I always thought that by not spotting, and having the drills wobble a bit because of an off-center pilot, I was increasing the odds of snapping a bit, so I got in the habit of always doing it.

I have a drill doctor (which I know many of you don't like, but for smaller diameter bits, I've saved shipping a lot of replacement drills over here!), so I can always regrind jobbers into split points, which I now think I might do for some common "entry" drills.

My normal process is to spot, drill through with something like an "S" bit, and then jump to a 7/8" (only because I have a good cobalt one and most of the rest of my MT bits are HSS), then the final size, then usually boring. My smallest good boring bar needs close to 7/8", so that works well (though there have been MANY times where I wish I could fit a bar into a smaller bore, I really need to invest in a good smaller one).

When I was greener, and only used to drilling in drill presses, I did many more steps. Once I got the advice on here to stop doing that, I've never gone back. The holes are better, the chips more impressive, the time less, and it's a lot more fun to watch.

So in the specific case that made me ask this question, I was taking solid 6" stock (8" long, maybe) and prepping it for a mandrel to turn between centers (tube bending dies). In this case, I chucked it with some rough adjusting based on an indicator, center drilled, ran the tailstock center in to stabilize for rough facing and turning cuts, then started drilling. The first drill (as I said, I grabbed for something other than my normal since my normal screw machine starter bit was too short) was 3/8" or so. It drilled pretty much on center (wasn't wobbling in the cut). Then when I went to the 7/8", it bit off center and wobbled a lot. My plan was (after hogging with the 7/8" cobalt drill) to drill with a 63/64" in prep for the 1" reamer, then put the part on the mandrel.

In practice, however, the 7/8" bit so far off center that the 63/64" didn't remove a lot, and the reamer didn't cut anything. The bore is spirally and ugly. In this case, it will be finished out later to 1.280 or something, so I don't care, but I don't like that the process didn't work like I expected.

It seems possible, based on the explanations, that where I went wrong was that the 3/8" long bit I used was too big to let the 7/8" bit bite enough to drill accurately? Think I would've been better off in that case if I'd followed the center drill with something like 1/4"? Even less? I get that we don't want lots more than the web, but isn't there going to be a diameter vs depth problem at some point as we reduce the drill size? An 8" long 1/4" hole is 32 diameters!

Probably exacerbating the situation is the fact that the 7/8" bit was hand sharpened. Although I've gotten pretty good at reproducing the geometry of each lip, I don't yet have a good way to make sure that the lips are equal. So maybe one lip was longer, and it bit first, pushing the drill off center until the other lip finally engaged the work? What's everyone's best trick for making sure the two lips are the same depth when hand sharpening?

Thanks for all of the help. These are rookie questions, I realize, but most of what I've learned has been on PM, with no one local to show me where I'm going wrong, so I have to question stuff I've been doing for a few years to make sure I'm not copying bad habits!

EPAIII said:

A final thought: the second drill bit does not need to reach the full depth of the final hole. It only needs to give the final bit a good start. So it should provide a guide hole that takes the final bit to the point where it's lands are in contact to the side of the drilled hole for a distance that is large enough to provide guidance. This distance would be around the diameter of the final hole or somewhat more. So final bit diameter plus final bit tip length would be the minimum depth for this drill. If that final bit is going to go past past the depth of the web sized hole, then a split point on it, the final bit, will be an advantage.

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I'm not doubting you, but are you saying you can push a 1-1/4" bit through with no existing bore, other than a bit of a hole to get it started on center? I'll have to try that one! I guess I always assumed a standard chisel point wouldn't be center-cutting enough to do that?

No cutting tools are really center cutting, the SFM is near 0 and your are effectively driving a chisel through the material which is why the split points make such a noticeable difference with the narrower web.

Since you mentioned that the hole was 6" deep and your drill wasn't long enough, personally I would have just drilled it from both sides. It is must easier to push a stout jobber drill compared to something like an aircraft length drill and drilling 4" with the S and 7/8" and flipping it over and repeating that might actually save time rather than having to deal with chip evac on such a deep hole.

Crazy duplicate deleted

if you have a nice big radial drill, or heavy drill press, it will go right through with no pilot. Like drilling boiler drums for tubes... they don't baby it up from a little drill, jam it down and drill, there's 50 more to do.

If you do not have a heavy drill press, then piloting makes sense to cut the down pressure required by teh "scraping cut" taken by the web thickness "point". Step-drilling gets taken to extremes by the home shop crowd, stepping up even by 1/8" at a time, is hard on the "corners" of the drill. I've chipped corners, but I have never yet split a drill up the web by too much down pressure. Maybe I just never have used a big enough drill press.

the other problem with step drilling is that the drill wants to "screw itself in": and not cut well with the tiny steps hobby guys tend to use, Lots of reasons not to do that.

If you pilot, just relieve most of the web thickness, and then drill through full size if you have the power.

If you pilot the deep hole you refer to, then drilling from both sides should follow the pilot drill and meet up well Dunno if it will be straight or perpendicular. that's what boring is for.

On a related note, I just went looking for a small inserted boring bar and found what I think is a price mistake (smoking deal?) on Amazon. I got one and there's one left, if someone wants it:
Ultra-Dex HM6J STUPL2 Heavy Metal Boring Bar to Hold a Positive TPMT 21.51 at -3 Degree Lead, Left Hand, Coolant Thru, .375" x 4.5", .41" Minimum Bore: Amazon.com: Industrial & Scientific

3/8" Ultra Dex heavy metal .410 min bore for $36!

So, let me ask, what's your general go-to diameter for the first through hole? Assuming I'm following it with a finish drill from somewhere between .5" and 1-3/8"? Or are you actually measuring the web to use one starting drill if the finish will be 3/4" and a different starter if it'll be followed by 1-1/4"?

JasonPAtkins said:

So, let me ask, what's your general go-to diameter for the first through hole?

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What's the size of the hole ? That's the size drill I'd use. Screw this foolin' around crap.

I think part of your problems with drill wander is how they are sharpened. If you research drill problems, you will find that incorrect grinding will produce exactly the type of problems you are having.

Tom

JasonPAtkins said:

So, let me ask, what's your general go-to diameter for the first through hole? Assuming I'm following it with a finish drill from somewhere between .5" and 1-3/8"? Or are you actually measuring the web to use one starting drill if the finish will be 3/4" and a different starter if it'll be followed by 1-1/4"?

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The drill I pick for the first through-hole is the size of the the hole I need to drill. If I need to bore it (for finish, location, or size reasons), then it's the biggest size that I can fit my boring bar into.

Assuming your lathe isn't tiny, drilling a pilot hole only helps to chip the next drill, unless the material is extremely hard or something.

I split everything even centerdrills.

The chisel point does not cut. It pushes the metal aside, towards the actual cutting edges. The power needed for this is probably a big part of how much power you need for a particular drill. My SB9 has a 1/2 or 1/3 HP motor and it can push a 1", chisel point drill. And I did say that a split point could be an advantage if this is the case.

I find that I can get a boring bar in that 1" hole easily.

JasonPAtkins said:

I'm not doubting you, but are you saying you can push a 1-1/4" bit through with no existing bore, other than a bit of a hole to get it started on center? I'll have to try that one! I guess I always assumed a standard chisel point wouldn't be center-cutting enough to do that?

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If you do want to drill a pilot hole, do use a spot drill to make a starting dimple, then switch to the full diameter drill you intend to use, and drill until it has entered to full diameter. This will make a proper cone for the big drill to start in after you have drilled the pilot hole. This is especially important for spade drills as they will not like starting on any kind of a pilot hole if that entry angle is not made while the center of the drill is pushing metal.

I use a pilot hole fairly often on a deeper hole that will be enlarged with a standard twist drill. Although it does waste some time, I can then feed the bigger drill at quite a heavy feed rate and make up for the time spent drilling the pilot. Otherwise, the big drill will just push the stock through the chuck (I'm using one piece smooth hard jaws all the time on the small and medium lathes).