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Center drills

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Jeremy Smith 209/04/2020 03:19:25
45 forum posts
7 photos

i noticed that there are several sizes of center drills whichyou can purchase. What designates the use of certain sizes when drilling the end of a bar, for use with a dead center while turning? They are all 60 degrees, but they come in different sizes.

Which sizes should I be purchasing for my myford ml10?

Steviegtr09/04/2020 03:24:59
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1153 forum posts
94 photos

Basically the one that fits your centre you will be using. I have a very small centre not used yet ,so I would use the smallest centre drill I have which is,(I have no idea but the small section is around 1.5mm) The 2nd one I have is around 2.5mm ,which I use the most. Somenone will be along shortly to give you better advice.

Steve.

Emgee09/04/2020 03:30:08
1442 forum posts
212 photos

The pilot hole size is the determining factor, BS1 is small so suits shaft sizes 1/4" up. If you are carrying out heavy machining on larger diameters you will need to use the centre drill that offers the best support.
If using a smaller centre drill do make sure there is no pip and that the tailstock drill chuck is true in all directions or you will lose the pilot drill.

Emgee

JasonB09/04/2020 06:58:37
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17821 forum posts
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The main governing factor is the size of the part you are working on, I'f I'm making say a valve that only has a 3mm dia stem then I will use a BS0. (and still support it with a big centre) only going in to form the smallest 60deg chamfer. If on the other hand its a 300mm length of 50mm bar then I will use a larger one.

I prefer a spotting drill to start drill bits but as Emgee says the size of the "flat" on the drill should be less than the ctr hole.

Edited By JasonB on 09/04/2020 07:26:53

John Hilton09/04/2020 08:35:32
112 forum posts
22 photos

Jeremy,

After using some pretty large lathes (Harrison, Smart & Brown) I downsized to a Myford ML10 like you have.

I can honestly say you have a brilliant lathe there. Most capable, if you can hold the work you can machine it!

I wouldn't worry too much about center drill sizes. I use anything from very small 1mm to ones which only just fit the tailstock chuck - about 13mm stock. The lathe is fine with all of them.

Yesterday I drilled some steel with a 1" drill held in the tailstock. No trouble.

I tend not to use the 4 way tool post, preferring the original single tool clamp, but this is mostly so I can continue to use the tools left over from the bigger lathes I once had.

My best addition is the rear goalpost for parting/grooving. The Myford one is a bit big, but I made a thinner one which is just as good and leaves you with more room.

John

Mick B109/04/2020 09:47:34
1540 forum posts
83 photos
Posted by JasonB on 09/04/2020 06:58:37:

The main governing factor is the size of the part you are working on, I'f I'm making say a valve that only has a 3mm dia stem then I will use a BS0. (and still support it with a big centre) only going in to form the smallest 60deg chamfer. If on the other hand its a 300mm length of 50mm bar then I will use a larger one.

Edited By JasonB on 09/04/2020 07:26:53

+1 for that. I'm not sure how the pilot size is defined for BS centre drills. I've got some unbranded ones where it looks to be thinner and longer than on some Dormer examples I have, and therefore at greater risk of breakage.

A more fragile pilot puts the emphasis on chuck and tailstock concentricity in the very small sizes, so that's something to be aware of. However, provided the centre drill isn't being overstressed, it cuts all over - on the sides as well as the point, unlike a standard drill. That means you can also use it *with great care* as a tiny slot drill - I've found that useful on several occasions.

Thor09/04/2020 10:31:38
1209 forum posts
37 photos

Hi Jeremy,

Know Your Lathe from Boxford has a table showing Centre Holes for 5 to 100mm dia shafts:

centredrilltoworkdia.jpg

Thor

Edited By Thor on 09/04/2020 10:32:36

Bill Davies 209/04/2020 11:08:16
180 forum posts
10 photos

I find I use a No.3 for most jobs, which matches the table provided by Thor. Looking at very large items turned on YouTube, I note that the centre sizes are quite small relative to the work. I wonder what the failure mode is.

I have never seen a centre point shear off, which is unsurprising given that centres are made from higher strength steel than the average workpiece. The ones I made as an apprentice were case hardened but I would expect commerical ones are through-hardened. Equally, it is difficult for a centre to 'tear out' of the work, although I have seen that happen, e.g., when a jam occurs.

Bill

Nigel McBurney 109/04/2020 11:08:54
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694 forum posts
3 photos

if you are model making also consider the appearance of the finished job,for example a crankshaft with centre holes in each end,full size cranks had centre holes,so a model should have the same and should if possible be in proportion ,ok you could after finishing the crank face off a bit of the end to reduce the diameter of the tapered hole, though the short length of parallel hole formed by the front of the centre drill can then look too large.Also when using the smallest centre drills ,the parallel point can seize up when lubricating with soluble oil,to save the drill point breaking its best to use a better lubricant,just a drop of the fluid type ROCOL works well.Theres no cutting rake on these drills so withdraw the drill a couple of times to clear the swarf and relubricate. Also if the centre drill is frequently used for centering prior to drilling,the drill point gets blunt,so it gets resharpened and can be sharpened a number of times without problem,but do not use drills like this on jobs where the centre hole is to be used with a centre,as the point of the centre can bottom in the shallow parallel hole and not engage with the tapered hole.

mechman4809/04/2020 15:26:09
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2627 forum posts
407 photos

...'I have never seen a centre point shear off, which is unsurprising given that centres'...

I have at least 3 small 0 size centre drills that have broken pilot points...?

George.

JasonB09/04/2020 15:28:34
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17821 forum posts
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I think bill was referring to the point of the dead ctr in the tailstock not the drill's pilot.

Mick B109/04/2020 15:37:52
1540 forum posts
83 photos
Posted by JasonB on 09/04/2020 15:28:34:

I think bill was referring to the point of the dead ctr in the tailstock not the drill's pilot.

Ah. Yes, I think burnt one out once, probably in the 70s, probably using sheeps' tallow as lubricant.

Maybe it's a thing you only do once - or most of us...

blush

mechman4809/04/2020 16:30:53
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2627 forum posts
407 photos

Mea culpa; have to admit I've never had one of those, nor a live centre burn out on me

George.

old mart09/04/2020 16:35:37
1512 forum posts
136 photos

I have some Dormers which look like centre drills, but have 90 degree instead of 60 degree tapers. I prefer these for starting a drilled hole and save the 60 degree ones for actually using with a centre. For drilling, spotting drills are very good and even stiffer than centre drills. Be careful choosing sizes, there are centre drills out there which are too big to fit in a 13mm chuck.

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