Center drilling in steps or 1 pass?

Jmanb13

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I just wanted to know if there is a best practice for drilling on a lathe.

I know to start with a center drill to get the hole started. After that should I just chuck up the bit size I want and go to town or should I start with a smaller bit and then size up in steps?

Also, my assumption is that drill bits are more of a "close" but not perfect size hole? If I wanted a perfect hole I would use a reaming tool correct?
 
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A lot depends on the material , the tolerances , the power of the lathe. Ive always used the step on holes larger then 1/2" , even then its up to the material . If it cuts easy , let chips break often live chips cut grab and hurt. Ive cut big holes in materials with big machines capable to turn 2" drills and bigger , but it was to be bored bigger so if it wasnt straight it didnt mater. In my shop I always step up beyond the 1/2 " hole. If reaming I always step drill the hole. Your drill bits will work easier and last longer too. Less resharpening even.
 
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For me... I step drill. Starting with a center drill. I had been taught by an old machinist, that each of the drills should be slightly larger than the width of the chisel point of the next large drill you will be using in the step. So, for example: if my final bit is a 1/2" drill. I look at the width of the chisel point and pick the next smaller bit so that it's diameter is slightly large than the width of the chisel point on the 1/2" drill. For a 1/2" hole, I'd personally do it in at least two steps depending on material (Center Drill, Intermediate Drill, 1/2" Drill).

If the accuracy of the ID needs to be close, I drill it close and then ream to final size.

If Concentricity is important, I drill, then bore, and possibly ream (if the ID is reasonable for me to ream)

Hope this helps.
 
Not too much to add to the advice already given. I may depend on your machine power. I don't own many reamers so usually drill close then bore to final dimension.

If you haven't seen the thread on the discussion of centre drills vs spotting drills for ...well starting. For 30 years I have been using a centre drill to spot for starting drill bits. And often noticed that one flute would catch and drill a bit off. Based on the information in this thread I ordered some spotting drills, and have immediately noticed an improvement. Now a lot of my work is drilling holes well under Ø 0.25".

http://www.hobby-machinist.com/thre...-with-center-drills-vs-spotting-drills.57023/

David
 
I, too, am impressed with a spotting drill. Its cut is broad enough to let the center of the drill bit start the cut rather than the flutes which occasionally will catch and start the hole off center.
 
Found out about spotting drills about 15 years ago. Since then the only use I have for center drills is for setting up to turn between centers. Couple licks with a diamond hone and a spotting drill is good as new. I have a few different sizes but the cobalt 1/4" does most the work.
 
Jmanb13, I didn't mean to redirect your thread by mentioning a spotting drill. If you have any other questions to get back on track, please ask.

David
 
I just wanted to know if there is a best practice for drilling on a lathe.

I know to start with a center drill to get the hole started. After that should I just chuck up the bit size I want and go to town or should I start with a smaller bit and then size up in steps?

Also, my assumption is that drill bits are more of a "close" but not perfect size hole? If I wanted a perfect hole I would use a reaming tool correct?

The guys have given you excellent advice - start with a spotting drill if you can and if you need to drill a large hole then using a pilot drill that is sized to span the web of the main drill helps. It helps because the center of a drill does not cut; it locates the drill but it requires more pressure to move it through the cut and the pilot obviates this need.

The problem with using pilot drills is that they tend to be small drills that must also be pushed through the cut and the pressure tends to bend them and make those small drills wander. This will lead the main drill that follows to follow the same wandering path. For this reason, if you need an accurately placed hole it is often better to spot and follow with the main drill. You will also find the main drill will also produce a more accurately sized hole because it is larger, stiffer and is also not bouncing around inside the hole.

All drills deflect, even large ones. To minimize deflection, use enough pressure for them to continuously cut but no more than that. A properly sharpened drill will output equal sized curled chips from both flutes and you should see these chips continuously moving. If they stop moving, raise the drill and clear the chips. When you go back in, use more pressure initially to get the drill past the work hardened surface and then watch for the chip flow. With large drills, slow the speed way down and increase feed pressure to keep chip production going. This maximizes the cutting action and minimizes heat production. Done well, a 1/2" drill will only feel slightly warm immediately after the cut.

If you need an accurately sized hole or a hole with a good finish then reaming will do that. However, it may not be a straight hole. Reamers follow the hole so if the drill produced a wandering hole then the reamer will produce an accurately sized and finely finished wandering hole. If you need an accurate, straight and properly sized hole with a fine finish then you should drill the hole, bore it straight and then ream it. The problem is that quite often we do not have the right sized reamer, in which case the only option to get an accurate hole is to bore it.

Keep in mind that drills are like all cutting tools - if they chatter, slow down the speed and increase the feed. Since both flutes must cut, both flutes must be sharpened identically. Unless you are good at drill grinding by hand, buy a Drill Doctor or some other machine/jig to aid you.

Making a hole is easy. Making a straight hole is a bit harder. Making a straight accurate hole with a fine finish is much harder. And then you have to make sure its located in the right place! Fun, isn't it?
 
Not too much to add to the advice already given. I may depend on your machine power. I don't own many reamers so usually drill close then bore to final dimension.

If you haven't seen the thread on the discussion of centre drills vs spotting drills for ...well starting. For 30 years I have been using a centre drill to spot for starting drill bits. And often noticed that one flute would catch and drill a bit off. Based on the information in this thread I ordered some spotting drills, and have immediately noticed an improvement. Now a lot of my work is drilling holes well under Ø 0.25".

http://www.hobby-machinist.com/thre...-with-center-drills-vs-spotting-drills.57023/

David

Jmanb13, I didn't mean to redirect your thread by mentioning a spotting drill. If you have any other questions to get back on track, please ask.

David

Thanks for the tip! And no worries about redirecting the thread. Finding out there is a totally different way to do something is just as good as learning how to do it the same way. It is funny because I ran across that thread through the "similar threads" at the bottom right after I posted this one :) Guess the forum is working well.

I've also noticed while i'm drilling that it seems 1 flute sends out a nice long curl while the other one is hardly doing anything. Just a few small chips here and there.

I'll definitely look into the spotting drills.
 
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