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Why center drills in a lathe and not a drillpress

Bill D

Diamond
Joined
Apr 1, 2004
Location
Modesto, CA USA
Most advice I see says to start a hole in the lathe with a center drill then switch to the twist drill. For the drill press they normally say center punch then spot drill or directly to the twist drill. Do they assume a Dp owner will not even know what a center drill is?
Only using smaller drills and working up in size if the hole is large.
I did use a centerdrill to locate a start hole in my DP this morning. I only used the short nub to make a dimple not the full center portion
Bill D
 

mhajicek

Titanium
Joined
May 11, 2017
Location
Minneapolis, MN, USA
This has been discussed here before. A center drill will work for starting a hole, but its intended purpose is to make a center hole for use with a live center or dead center on a lathe. A spot drill tends to work better for starting a hole. Ensure that the tip angle of the spot drill is slightly larger that the tip angle of the drill.

Many carbide drills are intended to be used without a spot.
 

CalG

Diamond
Joined
Dec 30, 2008
Location
Vt USA
Look at the pointy end of any "center drill". There is nothing magic about it. Nothing more than a stubby twist drill.

Using one in a drill press is sort of dumb, as the quill and spindle are loose enough to "not hold center".

There you are, following a prick punch (or nothing at all) , just with a stubby drill.
 

L Vanice

Diamond
Joined
Feb 8, 2006
Location
Fort Wayne, IN
Look at the pointy end of any "center drill". There is nothing magic about it. Nothing more than a stubby twist drill.

Using one in a drill press is sort of dumb, as the quill and spindle are loose enough to "not hold center".

There you are, following a prick punch (or nothing at all) , just with a stubby drill.

When I use my vertical mill to drill a hole, I often first use a centering microscope to line up on the scribe marks from my height gage, then start the hole with a combination drill and countersink (AKA center drill). If I take the time to lay out scribe marks, I can't see then poking them with a prick and center punch and losing location accuracy. I have screw machine length split point drill bits that will go into the material where the mill points them, but I feel better about keeping location by first using the combo drill, which is stiffer than smaller drill bits.

Another special use of the combo drill is creating the locating hole for the 60 degree point of the special set screw that fastens Hardinge crank handles to their lathe feed screws. People break the cranks off and I have repaired quite a few Hardinge screws. Again, this is a mill job that requires better accuracy than a drill press can provide.
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Larry
 

bellaireroad

Aluminum
Joined
Oct 4, 2015
Look at the pointy end of any "center drill". There is nothing magic about it. Nothing more than a stubby twist drill.

Using one in a drill press is sort of dumb, as the quill and spindle are loose enough to "not hold center".

There you are, following a prick punch (or nothing at all) , just with a stubby drill.

Dumb if you have a crappy DP, not if your DP has .001 runout [emoji57]


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Bill D

Diamond
Joined
Apr 1, 2004
Location
Modesto, CA USA
I got my dp down to 0.002-0.003 runout and said. good enough. A week later I jammed a big drill in steel and it was bent out by more then 0.007.
Bill D
 

mnl

Cast Iron
Joined
Sep 7, 2007
Location
Maryland near DC
I’m with Larry. I tend to use a center drill to hit the intersection of the scribe lines. Why a center drill? Because the tip is smaller and it’s easier to see the mark. Of course this isn’t for precision work, if it were I would clamp it down to the mill and do it by numbers.
 

jerholz

Cast Iron
Joined
Sep 24, 2015
Location
Dallas, Tx
Ok, I'm just an amateur, but I thought that the way a center drill works in a lathe is that there are cutting edges on the flutes. That way even if the tailstock is not perfectly centered, the hole and the countersink are guaranteed to be concentric with the spindle axis. If the tailstock is off center you just get a bigger hole.




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CalG

Diamond
Joined
Dec 30, 2008
Location
Vt USA
Ok, I'm just an amateur, but I thought that the way a center drill works in a lathe is that there are cutting edges on the flutes. That way even if the tailstock is not perfectly centered, the hole and the countersink are guaranteed to be concentric with the spindle axis. If the tailstock is off center you just get a bigger hole.




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You might want to carefully observe what happens when the drill point contacts the moving work.
Compare that with what occurs when a rotating split drill contacts a stationary workpiece.

And for a real eye opener, see what happens when you first drill, using the lathe, into a piece of stock that had been cleaned up but a tiny tit has been left due to inaccurate tool height.

Words can not do what 1/2 hour of careful observation can.

eta

Spot drills are not cheap, but they break off less often than do lathe center drills.

A single .315 carbide spot drill should last even a careless machinist a lifetime.

Anyone here ever " strike a center" with a graver? Ahh, but that is lathe work, and off topic.

We was talk'in press drills.
 

CalG

Diamond
Joined
Dec 30, 2008
Location
Vt USA
Ever tried to center punch on the lathe ?
Need I elaborate :codger:


Prior to mounting the work, yes! Many times. The essence of four jaw work piece hole making, boring or threading..

I even have a wag tail indicator that "hovers" around the tail center point when everything is "Just Right"
 

sfriedberg

Diamond
Joined
Oct 14, 2010
Location
Oregon, USA
Just went through this with a student at the high school. Needed an F tap drill in the end of a lathe part. Even after facing off the part nicely, the F drill (which is not a tiny drill!) still decided to twiddle around like someone shaking their finger, due to deflection and a not-perfect chisel tip. A #4 center drill put a spot right on center, with none of the deflection nonsense, which the F drill followed with no trouble. Spot drill would have done equally well as the center drill.

This particular misbehavior doesn't take place on a drill press, where the tool is turning rather than the part. The two situations are not "the same except for observer reference frame." Seems like a subtle point, but it makes a big difference in practice.

Also, as a general rule, lathe parts are expected to have better concentricity and feature positioning than drill press parts. Which encourages getting holes started precisely on the lathe.
 

Mark Rand

Diamond
Joined
Jul 9, 2007
Location
UK Rugby Warwickshire
Centre drills are for drilling centre holes.

They don't have the correct point geometry to be followed accurately by another drill.

Use a spotting drill.
 
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jim rozen

Diamond
Joined
Feb 26, 2004
Location
peekskill, NY
Holes drilled in a drill press are mostly the result of scribed layout lines. The intersections are prick punched (under a microscope if available) and the centerpunched on top of those. The resulting mark is drifted to be close to the line marks, again a microscope is good for this.

Then to the drill press.

A center drill is rigid enough so it can substantially ignore the punch and start wherever it wants. Worse, the large body diameter obscures the punch mark.

Use a small diameter twist drill - one whos lips are mostly captured in the conical punch mark. It will 'pick up' the mark and start the hole true to the mark, irrespective of the drill press spindle vagaries. Once started and made a few diameters deep, a larger pilot drill picks that hole up and drills straight and true, but IN THE CORRECT LOCATION.

Once a lathe centerdrill starts a hole in the wrong spot, you can't drift it back.

Also, real precision lathe centering work is not done with centerdrills in the tailstock - it's done with single point centering tools in the toolpost.
 








 
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