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A polite question - from a beginner :) Drilling a NONE wandering hole

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Lainchy21/05/2020 13:13:42
271 forum posts
103 photos

Hi group!

Seeing the note RE beginners questions... I have one that troubles me....

When trying to drill a hole, centred in a part, I mark up, edge find in the mill, centre drill..... all good, hole STILL in the middle, but, when drilling, it wanders!

I know there's some reasons why a drill will wander, but haven't worked it out yet.

Any suggestions regarding going up from a centred centre drill, to a bigger hole? Is there some theory other than not trying to poke a 1/2" drill into something that doesn't want to be drilled? I have sometimes managed to get things square again, with a 12mm endmill in one instance, but say I just needed to drill a 3/32 hole, or a number 30, or an 1/8 and get it to remain on the centreline?



Gary Wooding21/05/2020 13:25:50
932 forum posts
236 photos

One reason why a drill will wander is if the cutting flutes asymmetric - usually because of poor sharpening. To minimise the effect it's often beneficial to first drill with a small drill that's just a little bigger than the web of the big drill, which will then be persuaded to follow the path already cut.

Former Member21/05/2020 13:29:17

[This posting has been removed]

JasonB21/05/2020 13:31:30
21999 forum posts
2539 photos
1 articles

Not sure about that Garry as first hole could wander and second drill follow, you may be getting mixed up with advice on getting a drill to cut to size, by doing it with two drills it does not matter if the second is asymmetric about it's point as the point is not being used.

I'll let a few others have a stab at it.

Nick Clarke 321/05/2020 13:39:04
1322 forum posts
53 photos

Well aware of recent posts I am a beginner and have been one for a very long time.

I am an expert in drills wandering too, but recently I have started to use a commercial spotting drill in the same way as Barrie, only 3mm in my case, and it seems to be more successful. I will keep practising.

Former Member21/05/2020 13:54:39

[This posting has been removed]

JasonB21/05/2020 14:04:37
21999 forum posts
2539 photos
1 articles
Posted by Barrie Lever on 21/05/2020 13:54:39:
Posted by JasonB on 21/05/2020 13:31:30:

I'll let a few others have a stab at it.

There you go again.


Sorry if you have a problem Barrie, I thought that I had been posting rather a lot lately and would give others the chance to reply, as you said many answers will be forthcoming, may even offer a few more later myself.

Former Member21/05/2020 14:15:38

[This posting has been removed]

Neil Wyatt21/05/2020 14:42:21
18894 forum posts
734 photos
80 articles

I know it's warm today, but let's keep cool


Lainchy21/05/2020 14:42:28
271 forum posts
103 photos

OK, so, I'd be drilling mild steel... maybe 3/32" - 1/4" thick.

I have been centre drilling these with a BS1 or 2 centre drill, but I have also read that a spotting drill should be used to start, not a centre drill. Perhaps I'll get a 3mm spotting drill to try to improve, so many thanks for that. The drill's I've been using are all still factory fresh, all be them not "high quality" named brands, but not also the cheapo cheapo sets.

I'm also drilling with a Warco WM14 mill, which so far is proving to be great, even if not a huge throat. I do tend to try also to drill with very little of the quill out and the tables/head locked.

I am currently building Juliet II and my concern is mainly for when I need to drill and ream the coupling rods. The Baker valve gear is mostly made, but with some wandering on the levers, so not too critical I hope. on the whole... pretty ok, but just... OK. I really want to improve my drilling accuracy.


Lainchy21/05/2020 14:44:12
271 forum posts
103 photos

There's no secret in drilling a pilot hole smaller than the centre flat on the next drill up?

Martin Connelly21/05/2020 15:06:05
2013 forum posts
214 photos

Ian the issue of spotting drills v centre drills has come up in the past. A lot of old books say use centre drills because spotting drills are a relatively new thing that came into use with CNC. Creating a dimple with a centre pop was no longer feasible in the middle of a machining program. They started off as an expensive option but the price has come down to the point where they are good value for home machinists. They are far stronger than centre drills, I have broken many centre drill tips. They are far stiffer than similar diameter drills as they are not fluted over a multiple of the diameter. The final thing is the angle of the hole they create. The centre drill hole is suited to lathe centres where support against side loads is important so they are steep sided. They are steeper than the angles of a standard drill so the first thing that makes contact with the workpiece is the outer part of the drill cutting edge against the sharp lip. This then digs in and the distance from the centre of rotation is enough to give leverage to push the drill off centre. Spotting drills create a shallow sided hole and the drill point makes first contact where the torque is low due to the fact it is nearer to the centre of rotation. It is also not a sharp edge against a sharp edge so does not grab as much.

I have some spotting drills and use them when drilling both on the mill and in the lathe tailstock. I keep my centre drills for what they were designed for which is creating a centre hole for a lathe centre to go in. I haven't broken any for a while now.

Martin C

Oops, put centre instead of spotting in the last paragraph.

Edited By Martin Connelly on 21/05/2020 15:08:29

Lainchy21/05/2020 15:08:43
271 forum posts
103 photos

Thanks Martin. I'll order one and see how I get on with it. Many thanks. Fingers crossed.

John Baron21/05/2020 15:11:10
513 forum posts
194 photos

Hi Lainchy, Guys,

There are a multitude of reasons why a hole doesn't get drilled where you want and expect it to be. Drilling a small pilot hole isn't always the answer. For hole starting a good centre punch helps.

The most common reason for holes drifting off, particularly deep ones, is as already been mentioned is rigidity. Not necessarily in the machine, but the drill. Applying too much pressure to make the drill cut will cause it to flex. Only a few thou, but enough to cause a hole that starts off perfectly to then end up some distance from the place you expected it to come out.

Just another thing, drills whether cheap or dear are not always ground well. I've spent a lot of time learning how to grind drills properly, and built machines to do it. If you have a choice of drill, choose four facet ground ones. A bit more expensive but not a lot more.

Lainchy21/05/2020 15:27:44
271 forum posts
103 photos

Thanks John, I'll bear this in mind also, I hadn't thought about the pressure, but will remember that.

John Haine21/05/2020 15:55:03
4420 forum posts
262 photos

The ideal drill would have a split point to start drilling accurately and be very rigid. I have a number of "Pilotpoint" drills that have an end like a slot drill (but not the sharpened flutes), with something like a small ordinary split drill point at the centre. So for example a 10 mm drill will have say a 4 mm pilot drill formed in its centre. These are excellent, they start accurately and drill to size, and being very rigid (because they're big) they don't wander. I suspect a lot of problems arise because people start with a small drill which isn't stiff enough to keep straight, and the resulting pilot hole leads all the later drills astray.

Former Member21/05/2020 16:14:15

[This posting has been removed]

roy entwistle21/05/2020 16:15:15
1458 forum posts

On 21st OP posted "OK, so, I'd be drilling mild steel... maybe 3/32" - 1/4" thick."

I wouldn't expect a drill to wander much through that thickness, or am I missing something smiley


Lainchy21/05/2020 16:18:15
271 forum posts
103 photos

Perhaps it's not the right terminology Roy, but it definitely doesn't stay on centre anyway.

JasonB21/05/2020 16:36:57
21999 forum posts
2539 photos
1 articles

A few thoughts.

You mention marking out if this is a heavy scribed line and even more so if followed up by a dot & ctr punch mark then if that is not in exactly the same place as what is likely to be a more accurately located position from edge finding and use of handwheel & dro then if there is any play in the machine it will pull the drill over as it starts and its downhill from there. Either locate the punch mark on the mill by ctr finding or what I tend to do is just use the mill to get the location from edges the DRO makes this even easier, a simple sharpie mark is enough as a double check you have the hole in the right place.

As said above I just keep ctr drills for when I need the 60deg cone otherwise it's a dimple with a spot drill.

Good sharp drills help a lot, as you need to replace them get good ones at least in the common sizes ideally with split or four facet ends these seem to cut easier which means you are putting less force into the quill lever that may flex the machine. I don't bother to try and sharpen small ones.

On the smaller bench top machines I would tend not to drill larger than 6mm straight off anything larger use pilot first, not too small as thin drills can wander more. Again less force needed to drive a 12mm drill into say a 6mm then 9mm hole so less strain/flex on machine.

In the desire to get zero tram the column base is often shimmed which can work very well however the factory may have set the machine up for good all round use, you also need to take into account if the vertical Z movement is in line with your now shimmed column or perpendicular to the table. Like wise is the quill moving perpendicular to the table too. You may need to come to a good all round compromise as if one of these two is out then the further up the head goes to accomodate a long drill the further from the spot drilled position it will be in X&Y. Same with quill the further it gets extended the more it becomes out of line.

With the above in mind I'm also a fan of stub length drills, again in the common sizes and split point, they save having to risk errors by reducing head movement and they also save you having to keep cranking up the head to get long drills in.

Use your ER collet holder where you can as its probably truer than the average drill chuck and shorter too.

Make sure your work is tapped down flat in the vice particularly if using a drill vice as the moving jaw can lift so you are starting your drill on a slope which it will want to wander down and the hole won't be perpendicular to the surface so it may actually be straight but as it was drilled at an angle will seem wonkey

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