Drilling! - small holes on the mill and centre drills

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Jonny:
Chris a centre drill is mainly used on mills and lathes to aid the centring of a twist drill after. If tailstocks out the drill will probably start off centrailish and cut large.
Normally used as you say to cut a cone but in your case 0.8mm you will struggle with a BS1 centre drill just touch the round leaving an indent. Swap for twist drill and get a feel for whether its cutting or not, no good just forcing it, it will blunt the drills quicker plus generate heat.

Nearly forgot, mount the round in mill vice on a suitable parallel packer and centre the X and Y to suit, even lock the beds, easy.
If its wandering off after indentation it will suggest a mill problem/s somewhere whether that be quill play, lack of torque, other mill flex some where, too fast as in motor speed or drills bending and not cutting, etc.

What else sounds like your twist drills are not sharp and scrap unless you can sharpen at correct angle and equal else it will cut large.

raynerd:
Well thanks for the replies. I didn`t think about using a jig, that could be useful and is perhaps the way to go although I did except that I could get the job done with just an indent and dilling directly in that.

Kvom - milling a flat is what I end up doing but what annoys me is that I then either need to mill a flat, drill and then turn it down to diameter with the hole already there. I have now had a few situations where it has been at diameter and me needing to put in the hole and hence why I decided to post but yes, a flat is what I have been doing.

OK, well some good tips and I`ll let you know how I get on, certainly a lot here to think about.

Lew_Merrick_PE:
There are really two problems here. (1) Locating your position dead center on your round part; and (2) drilling a small hole without wandering.

(1) There are several ways to get to dead center over your shaft. If you have a DRO or TravaDial (yes, I am still in the dark ages), an edge finder used from either side of the bar will get you within .0002 (0.005 mm) without too much trouble. In the days before TravaDials (long before DRO's), we used to have a widget that was a piece of v-block (about .100 inches or 2.5 mm thick) mounted in a round bar. The v-block had spot for a test indicator to "touch" on and the shank had a collar that would hold the indicator. When both sides of the v-block read the same on the indicator, you were dead center on the shaft. I found my "center indicator" a while back -- and packed it up in one of the 3000 or so boxes still in storage, so I don't have a picture to post. The drawings for making it were in one of my apprentice milling books, but I have probably "passed that along" quite a while ago (they used to be really common).

(2) The first problem with small diameter drill bits is being sure that you have them properly sharpened and centered. Most drill bit grinding units stop at 3/32 inch (2.3 mm). There are several "hand honing jigs" floating around. The one described in Guy Lautard's The Machinist's Bedside Reader (#1) works. There are several variations on this design that were in various apprentice handbooks in days of yore. The one I like best uses a bushing with a guide pin so you can have several for different diameter and twist rates to locate the tip. The "point" being that you hone one side of the drill and then spin it 180° to hone the other side. Typically, by the time you have 1.5 diameter of the drill (past the lips) in the hole, it should be self-guiding -- so long as you do not collapse the column of the drill bit with too much force.

As noted above, centerdrills are shorter and stouter than equivalently sized drill bits. Anytime you need accuracy of placement with a drill bit, a centerdrill should be your first step of operation.

Finally, beware of cheap drill bits. They are almost never straight and even less likely to have been ground correctly. You are far better off going to some major manufacturing facility where they "surplus" dull drill bits and dig through the bins there than to buy the cheap sets. My "take" is that China is at least a decade away from being able to make a good drill bit. The Indians are nearly there (and their prices reflect this). I do have a set of (relatively) cheap Polish-made bits that are what I keep around my shop for "visitors" to use. (You will pry my Chicago-Latrobe drill sets out of my cold, dead hands!)

BK:
I've had this problem making hubs for spoke wheels (5"), I couldn't for the life of me get 6 centres on a piece of round brass, so I used hex, marked, drilled, turned it down to round. 2 of these made a 12 spoke wheel.

John Stevenson:
Centre drills are a throw back to the days of using dead centres in lathes.

They have a small pip at the front to aid starting and breaking off in a part that already has 10 hours thrown at it and a 60 degree cone for the centre to be supported.

The pip's main purpose was to start the drill, provide clearance for the point of the centre and provide a reservoir for white lead as a lubricant.

Unless you are drilling a support hole for a centre forget the centre drills, times have moved on but unfortunately no one has updated any of the books, everything is repeated ad nausuem.

Industry today uses spotting drill which are a very stiff, short fluted drill made for accurately spotting holes. Stub drills do virtually the same thing at a quarter of the cost.

A tip for using centre drills when you have to is to take a new one and grind the short pip back to half it's length.
This will make it stiffer and less liable to snap in a hole, true it will reduce it's life but so will snapping a brand new one in a part.
Seeing as we mostly use revolting centres nowadays that reservoir isn't needed and you only need the bare minimum for clearance.

John S.

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