Marking out / drilling holes in precisley the correct place !

Hi All , Having had to mark out several holes in plate , bar e.t.c. since i started a few models i'm finding it very hard to get the drilled holes in exactly the correct place. The problem is centre punching the hole in exactly the correct place ! I mark out the metal with a very sharp scriber then centre punch using a good sharp punch and even using a magnifying glass to try to ensure the punch mark is precisley on the marked out centre and then centre drill and drill but i always find the hole to be out by 10 thou or perhaps more. How do you guys get the hole spot on. Any advice / echnique much appreciated

Ta Chris.

First thing is that you should start with a dot punch not a centre punch, this has a sharper point (60degrees included angle) that will locate into your scribed line, give it a light tap and inspect the mark, if its a bit off the punch can be tilted and tapped again to pull the mark over. Repeat this until the dot punch mark is as good as you can get it and then follow up with the centre punch.

If you look at the end of a drill you will see a small flat area at the point before the two flutes spread out, you need to ensure the width across your centre punched mark is wider that the flat bit so the two sloping cutting edges of teh drill engage into the punch mark.

Obviously you can't put a massive punch mark if drilling say a 12mm hole so punch to suit say 5mm, drill that then open up to say 9mm followed by the 12mm drill at a slower speed.

J

I start most of my accurate drilling in the milling machine and by using the micrometer graduated collars on the Mill means the dimensions are (almost) always spot on, and though I will mark out all holes with a scribe I never punch them but centre drill.

I dont always complete the drilling in the Mill as the drill press is quicker (and for me more sensitive) but having already centre drilled the hole it is relatively easy to keep to dimension.

If you havent got the use of a mill (or cross vice in the drill press) you could have a look ay optical punch's they will certainly improve accuracy.

regards

Paul

Here's my personal method, which may or may not be applicable depending on what equipment is available.

For precision parts I never mark out or centre pop hole locations. All holes are drilled straight off on the vertical mill using the DRO. I find that four facet drills start more accurately without a centre pop on any flat surface, machined or cold drawn. For surfaces that are not flat I either mill a flat first, or start with a carbide spot drill.

I only mark out and centre pop for sheet metal work where everything will be done by hand, or for DIY bodgery.

Regards,

Andrew

Chris

There are, as you will find here, many approaches. The one I use for really accurate hole location is not to mark out at all, but to find the edge(s) of the metal with an edge finder and to located the holes by co-ordinate drilling. I have a 3-axis DRO on my milling machine, which is all I ever use for metal drilling, and this method locates the holes within 10 microns or so.

It also helps to start the holes with a centre drill; since these are so much stiffer that a normal drill with the same point diameter, so there is less chance of it wandering. To be really anal, follow it up with a stub drill (about half the length, so about 8 times stiffer). Make sure the drills are sharp, with even lips, and don't let the flutes clog up.

Before I used the DRO approach I used an optical punch, which was pretty accurate, but not as good as the above method. Any method relying on locating holes via a punch mark cannot of course be better than the initial marking out; the best method for this for most home users is probably to use a surface plate and height gauge.

David

Edited By David Littlewood on 01/06/2012 19:26:25

Hi, I am a real newbie to this and I don't have a mill (yet!) only a lathe and a drill press. One trick I have used which I found in one of my many machining books was to turn some thick discs on the lathe to a precise diameter with a hole in the middle slightly smaller than the holes you want. With a straight edge and these thick discs (I think the term might be "buttons" they can be laid on the workpiece against a straightedge with the discs touching. The distance between the centres of the discs will be exactly the two radii of the buttons. I clamp this up and put in my drill press and use the discs as a guide. I have got this accurate to a thou or two on the couple of times I have had to do it.

Not a great explanation I'm afraid but I hope it makes sense.

Well Yes I use the DRO on the mill now but as marking out questions like this are basic beginners thing I assumed Chris was starting out and did not have a mill with or without DRO.

J

Chris,

The 60 deg dot punch is sound advice but on occasion I have added a scribed square box at the hole diameter so that you have a witness of the hole and if you just get the drill point to about 3/4 of your size and check you will see if the drill has wandered off.

My solution to correcting this would be to gouge a couple of grooves down the side of the hole that I was on with a sharp diamond point chisel, the grooves being on the side to which you need to pull the drill towards. When the drill is repesented to the job it will tend to cut heavy on the side with the grooves for a few moments and draw the centre over. It might need a bit of practice to judge the amount but it can save scrapping a cherished bit.

Bob D

Would an optical centre punch overcome the problem

Holes in a straight row can be drilled on a drilling machine, parallel to an edge by clamping a strip to the table and keeping the job against it sliding the job along to each hole pitch. This method can be enhanced if it is possible to set a stop at the end of the job to allow measurement in a longitudinal direction using a rule, digital calliper or distance blocks. It may be necessary to clamp the job at each hole position. Using this method requires only the first hole to be accurately located. Colin

Matt.

I think your talking about toolmaker's buttons, these are set using slip blocks aka machinist's blocks using an anglle plate for verticial refference and the surface plate for horizontial ref.

To make sure that the drill follows your centre dotting, fully establish the location by using a suitable centre drill.

Hi. John.

I wouldn't think most beginners would know what a spot drill is, let alone possess one. The idea of a centre drill is that the body is larger than the pilot making it much more stable and able to fully establish the centre dot than using a flexing small drill bit. The resulting cone is then a positive location for the following drill.

Eric asks whether an optical punch would overcome this problem, has anyone seen an optical punch that is really that accurate?

Perhaps I am not looking in the right place, but all I have seen have been troubled with the optical part not been truly centred (try rotating the optical part) they all seem to wander about.

Good old DRO every time for me, at least the results are repeatable and you can drill holes that line up every time with any matching part.

Edited By KWIL on 02/06/2012 14:28:47

I've got one of the Veritas Optical punches and the cross hairs seem quite accurate but they do all suffer slightly from parallax so you need to move your head about a bit to average out what you see.

The advantage of a spot drill ocer a centre drill is they have a finer point so will skid about less than a centre drill, I've gone over to using them now for starting holes. The centre drills really only come out when I want to support the work with a centre.

Some drills available these days are of pretty awful quality with oversize and off centre flats that often don't locate properly in even a deep centre punch making it virtually impossible to drill an accurate hole. I have recently purchased metric and imperial sets of unspecified but presumably Chinese origin that fall into this category, some can be corrected by re-grinding but many are not even straight. No problems with my diminishing stock of 30+ year old Dormers though.

The problem for a beginner though is knowing how to identify bad drills, and obtaining quality drills seems to be getting harder for us all.

Ian.

This may not be a proper engineer's method but, with a limited amount of equipment, it works for me when using the drilling machine. After dot-punching the location, put a very small (say 2.5mm or even smaller) drill in the chuck, and lower it to the work with the machine switched off. If the position is slightly wrong, you can actually see the drill bit flex slightly. View it from two angles, one 90 degrees from the other, and you should be able to get on the spot as near as dammit.

Of course, this assumes your small drill bit is straight and accurately ground in the first place, but I keep one set of drill bits only for use in the lathe or the drilling machine. Drill bits easily get bent when used in hand-held drills. I would never start a hole on a punch dot location with anything as rigid as a centre drill - but that's just me.

As I suggested, a pretty crude method, but reasonably accurate if you don't have more sophisticated machinery.

Mike,

If you are going to follow that method, I suggest it may be even better to mark a cross at the hole location (using whatever method you use for marking out) and then just position the centre drill on it by eye, make a very light dot with the drill turned on, and adjust until the conical hole made by the centre drill exactly bisects both horizontal and vertical lines. No punch hole needed (and indeed it wouold hinder). The chisel point of the centre drill will easily cope with cutting into the side of the conical hole as you make adjustments, provided that the hole is smaller than the drill's conical point, and the eye is very good at aligning things symmetrically. I have used this method many times, though these days I only do it for non-critical holes and use the DRO approach (explained above) for critical ones.

David

If you are going to do it like that you may as well use a wobbler, centre finder or sticky pin.

Ian I get my Dormers from MSC/J&L the A002 Jobbers and A022 Stub length are both very nice to use and of the split point end which makes them almost self locating.

Mine is called "OnMark" made in California - I've had it best part of 15 years. I use the DRO method on the mill when convenient but if I want to do it by hand with any accuracy I lay out, using a vernier height gauge where I can, followed by the optical punch (light tap) then recheck optically. Finally a regular centre puch. Always been spot on as far as I can tell.

The comment on parallax is interesting. My optical punch can be used like a microscrope .... that is you can put your eye close to the magnifier, which is what I always do and I don't have a parallax problem. I've seen others since where you couldn't do that (the eye needed to be a foot or more away) and when I expressed surprise was told that that's how these devices normally work. I have one of these other types but never use it - I find it difficult.

This is exactly what I do whenever I mark out and drill holes that I want to line up precisely. Has always worked for me and have even had holes as small as 1/16" that have been marked out and drilled separately, line up satisfactory without any problems.

Don't have any spot drills though, just use a normal drill bit, but do also use an eye loupe to line up on the pop mark when I need to be really fussy.

Regards Nick.

Edited By Nicholas Farr on 02/06/2012 19:16:31