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What is the purpose of centre drill and how does it work?

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pgrbff	14/08/2011 12:14:16
261 forum posts 31 photos	I assume a centre drill is used to ensure a hole is in the right place? Is it simply because it is short and fat and therefore will not wander when it contacts the material? How deep should one drill with the centre drill, what size should one use in relation to the final hole? Finally is the purpose of the hole left by the centre drill to guide the next drill used?
JasonB	14/08/2011 13:25:38
25215 forum posts 3105 photos 1 articles	The tip has a slightly more pointed geometry which makes it less likely to skid about than a standard jobber drill and as you say being short its more rigid which also helps. When used for starting a drill you do not need to go that deep Again the range of sizes does not matter much when starting a drill as you would normally work up through the sizes if its a big hole. Its more to do with using the hole to accept a centre in the tailstock to support long work, then there are suggested sizes based on the diameter of the work The pilot part of the ctr drill does nor really relate to the following drill bit. If you look at the end of a jobber drill you will see a "flat" area between the two flutes, aim to have the ctr drill hole larger than the width of the flat, this can either be the pilot or part of the "countersink "part of the drill. For your RC work I would say BS1 & BS2 sizes will do all you need If you are just thinking of using a ctr drill to locate holes then think about getting a spotting drill, these work better than centre drills, are more robust and if sized correctly can countersink or chamfer the hole at the same time as locating the drill. 3mm & 6mm will do nicely J Edited By JasonB on 14/08/2011 13:30:34
Andrew Johnston	14/08/2011 13:35:50
7061 forum posts 719 photos	The primary use for a centre drill is to provide a 60° hole for a tailstock centre as a support during turning. The little drill bit on the end gives clearance, and provides an oil reservoir, for the point of the centre. For drilling, especially for holes where placement accuracy is important, the recommendation was to use a centre drill first and then follow up with conventional drills. You are correct in assuming that because the centre drill is short 'n' fat it is less likely to wander. Personally I think that the above advice is a little outdated now. On a flat machined surface I find that four facet ground drills start accurately without needing a pilot hole. If the surface is rough then I start with a carbide spotting drill. These are essentially stub drills, so are stiffer and less likely to wander. Also carbide is about three times stiffer than HSS, which also helps. In my experience you've only got to look at the smaller centre drills and woof, the centre bit breaks. So I try not to use them, except when I need a centre support hole in turning. There's always an exception to the rule; bizarrely I did use a centre drill as a drill recently, as I needed a countersunk hole that was 60° for some odd screws. Regards, Andrew Edit: Blast, typed too slowly! Sorry for the duplication. Edited By Andrew Johnston on 14/08/2011 13:36:45
peter walton	14/08/2011 13:36:29
84 forum posts	I always thought the centre drill was used originally to drill a chamfered pilot hole to accept grease when used with a solid centre when turning between centres! Peter
Chris Trice	14/08/2011 13:37:23
1376 forum posts 10 photos	To answer the specific question, a centre drill is actually designed to drill a conical seat in the end of round bar so it may be turned between centres or one end supported by the tailstock. The pilot section of the drill acts as an oil reservoir and ensures that the very tip of the centre doesn't bottom out in the hole. The cone of the centre drill matches that of a lathe centre. It's adopted use for starting off drills is a secondary use and as JasonB points out, has largely been superceded by spotting drills although old habits die hard. EDIT: That makes three of us typing at the same time. Edited By Chris Trice on 14/08/2011 13:39:03
pgrbff	14/08/2011 14:20:38
261 forum posts 31 photos	Thanks all of you. I couldn't really believe that its main purpose was starting a hole, I'm glad that has been cleared up. When starting a hole, is there a difference between the method adopted on the lathe and that on mill? If I wanted to dril a 3mm to 4mm hole what would the process be and how would it differ from say a 12mm hole? Edited By pgrbff on 14/08/2011 14:21:21 Edited By pgrbff on 14/08/2011 14:22:52
Roger Woollett	14/08/2011 14:44:03
148 forum posts 6 photos	If you look at the point of a twist drill you will see that it has a chisel edge at the centre. On a 12mm drill this is likely to be about 3mm long. Smaller drills will have a proportionately shorter chisel edge. The advice I was given is to make your starter hole bigger than the chisel end. This allows the true cutting edges of the drill to bite and so start your hole in the correct place.
Jon	14/08/2011 15:00:45
1001 forum posts 49 photos	Assume you mean opening up a 3mm hole to 4mm. If between lathe centres just slap the 4mm dril in, it will follow the runout of existing hole if not centred. Mill i usually peck away watching for the deflection, adjust X and Y axis so 3mm drill is over the hole, swap over to 4mm and drill works every time. 12mm hole wouldnt entertain a centre drill just plough straight in. If wanted an accurate hole i would rill undersize poss drill again ot far off size required then ream or bore. 90% of the time i dont use centre drills on lathe and mill. Bear in mind i do a lot of drilling through on irregular shapes, concave and convex surfaces even drilling offset holes overlapping an existing hole together with drilling and tapping round held at 15 degrees. What i have noticed is that flimsy machines when set up perfectly centred tend to deflect to one side when putting a cut on. Aways had to use a centre drill on my ole Myford and chinese supposedly robust driller.
Clive Hartland	14/08/2011 15:13:24
2929 forum posts 41 photos	There are two types of centre drill, one with the straight conical form and the other has a curve in the that position. The second type is used as a support when turning a piece of metal that is off center to the chuck center. The curve allows contact all round the hole even though the tail stock is set over. Most all lathe workers now own a running center, which no longer require grease or oil lube when working. Clive
JasonB	14/08/2011 17:34:28
25215 forum posts 3105 photos 1 articles	As you are asking about ctr drills I'll assume you want to drill a new hole around 3 to 4mm dia say a clearance hole for a screw In the lathe first take a light facing cut across the bar as a rough surface will make the drill wander. Put the BS1 ctr drill in the drill chuck bring up the tailstock and then feed in the ctr drill until the full width of the pilot (small dia) is cutting then withdraw. Replace the ctr drill with your chosen drill bit, bring up the tailstock and again feed in the drill. Don't keep just feeding it in,every so often wind it back to clear the swarf, the thinner the drill and deeper the hole the more you will need to do this. A bit of cutting fluid helps on steel or in your case aluminium is more likely so I just use a small kiddies paintbrush to dab on a bit of parafin or WD40. If doing the same with the 3mm spotting drill then just feed that in until the dimple is about 2mm accross and then carry on as above. For a 12mm Hole it depends a bit on the power & rigidity of your machine but I would use the BS2 ctr drill in the same way as above or the 6mm spotting drill to give about a 3mm dimple. Rather than drill all in one go I would use say 6mm followed by 10mm and finally your 12mm. The speed will likely need reducing as the drill dia goes up. In the mill it really depends on how you have placed the hole. If it was a punch mark that you have located with a centre finder then the punch dimple takes the place of the ctr/spot drill so just start with your drill bit. If you have placed the mill spindle over where you want the hole by locating the edge of the work and then winding the table to teh correct position then treat it as drilling in the lathe starting with a ctr or spotting drill. If the hole were being tapped then I would use the spotting drill to form a cone shaped hole just a bit larger than the thread so for say a M4 tapped hole I would use the 6mm spotting drill until it had made a depression about 4.5-5mm across, drill tapping size and then tap without moving the mill or removing the work from the lateh chuck. The reason for going deeper with the spotter is that when tapping particularly in softer metals the tap throws up a burr around the hole and the pre chamfering stopps this affecting the surface. J J Edited By JasonB on 14/08/2011 17:37:18
JasonB	14/08/2011 17:45:55
25215 forum posts 3105 photos 1 articles	Just thought You may want to have a look at the MIT videos, they give a good basic explanation of how to use the mill and lathe. Sit down, blow them up full screen and come back with any queries after about 10hrs EDIT rather than David's link above they are easier to view straight from MIT here look at number 8 about 33mins in Edited By JasonB on 14/08/2011 18:20:28
Chris Trice	14/08/2011 22:39:34
1376 forum posts 10 photos	Something worth serious consideration is a set of split point stub drills (sometimes referred to as four facet drills). These are much stiffer than standard drills and self centring. I've got a set I use all the time on my mill. No need to use either a centre drill or a spot drill. I guarantee if you buy a set, you'll use them 90% of the time. I only use the standard drills for deep drilling and often I'll start with a stub drill first.
Roderick Jenkins	15/08/2011 08:39:59
2376 forum posts 800 photos	I have a centre drill mounted in its own 2MT arbour and start all holes from the lathe tailstock with it. I would, however, echo Chris's comment on stub drills, even without the split point. For both accuracy and headroom (on the mill), I find them superior. Unfortunately, our usual suppliers don't seem to offer sets of stub drills cheers, Rod
John Stevenson	15/08/2011 08:57:00
5068 forum posts 3 photos	+1 for the stub drills. As an aside with centre drills, as others have said, their historic use was for a lathe centre and to hold the lubricant. As things have moved on most of us use revolting centres now instead of dead centres things change. The most frequent accident with centre drills is that the pip breaks off but if you study the geometry of them that pip is far longer than it needs to be to come to a point. Probably to hold the now non existent lubrication ? Working on the premise that a broken one will have less life than a resharpened one I grind all mine down from new to about 1/2 the pip length, they still work but I now have less chance of breaking one and so get longer life out of them. It's called Inverse Theory John S.
pgrbff	15/08/2011 09:19:42
261 forum posts 31 photos	Too much to take in!!! Thank you all. I managed in the end but the link to MIT MIT Videos doesn't work for me, maybe Firefox? The difference between stub drills and spotting drills seems subtle, but I'd like to know what it is. Many thanks again. Edited By pgrbff on 15/08/2011 09:28:45
John Stevenson	15/08/2011 09:25:28
5068 forum posts 3 photos	Posted by pgrbff on 15/08/2011 09:19:42: The difference between stub drills and spotting drills seems subtle, but I'd like to what it is. Many thanks again. Price Spotting drills are 4X the price of stub drills. John S.
pgrbff	15/08/2011 09:42:33
261 forum posts 31 photos	So to drill 4mm through hole, shallow, not tapped, simple use a 4mm stub drill? 4mm tapped hole, mark with 3mm stub, drill out to 3.3mm and tap on mill turning by hand? I'm very fussy by nature so I welcome any pointers/criticism/alternatives. I must say my biggest concern with this whole thing is that the mill is correctly set up, square not sloppy etc. Its a Bridgeprt and my back is in such poor conddition just turning the handles to move the table is painfull.
John Stevenson	15/08/2011 10:10:54
5068 forum posts 3 photos	Personally I'd drill the 3.3 with a stub drill, one less tool change. If it's a deep hole then it would have to be changed to a jobbers drill but you can easily go 3/4" / 20mm with a stub drill if you clear the chips regularly. John S
pgrbff	15/08/2011 10:13:58
261 forum posts 31 photos	I have a standard 3.3 drill, but if I can find a single stub I'll do that.
chris stephens	15/08/2011 12:16:51
1049 forum posts 1 photos	Or you can junk your drill chuck and replace it with an ER collet chuck, then you can have as much or as little of your drill poking out the end as you like. Instant stub drills. I know the purists are now having apoplexy, but who cares. chriStephens

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