centre and spotting drills

andrewnummelin
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centre and spotting drills

Postby andrewnummelin » Sun May 15, 2016 6:51 pm

Having just done something stupid. and broken my small centre drill as result, I went on line to see about buying a replacement. In so doing I came across a spotting drill and I confess that I had not heard of one of these before and it did seem to offer some potential advantages. A great way to start a hole and prevent a drill wandering.

For those interested there is a description of a spotting drill on Wikipedia at
https://en.wikipedia.org/wiki/Drill_bit#Center_and_spotting_drill_bits
and some good illustrations at
http://www.cutwel.co.uk/hole-making/miscellaneous-hole-making-tools/nc-spotting-drills.

At ExpoEM yesterday (congratulations to SLAG) I asked at Eileen's for a centre drill but they said they could no longer get them! So, two questions:

1. Does anyone have experience with using spotting drills for our type of modelling?
2. Can anyone recommend a good supplier for small centre and spotting drills?
Regards,

Andrew Nummelin

John Palmer
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Re: centre and spotting drills

Postby John Palmer » Sun May 15, 2016 7:21 pm

A couple of suggestions:

http://www.arceurotrade.co.uk/Catalogue/Cutting-Tools/Centre-Drills/Plain-Type - Arceurotrade's page gives the full spec. and they offer BS1 to BS4

http://www.tracytools.com/centre-drills2?keyword=centre%20drill Tracy Tools' page doesn't give the full spec. but it looks like they also offer BS1 to BS4, and at a keener price than Arceurotrade.

3/64ths appears to be the smallest pilot hole size commonly available.

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45609
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Re: centre and spotting drills

Postby 45609 » Sun May 15, 2016 8:01 pm

Drill Services - Horley

Centre and Spotting Drills

You'll find smaller than BS1 pilot diameter here but at a price.

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Winander
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Re: centre and spotting drills

Postby Winander » Sun May 15, 2016 8:12 pm

This post from Terry Bendall is relevant to spotting:
viewtopic.php?f=39&t=4847&p=45588#p45588

Appears to be best to spot a hole with a drill that has a point angle close to that of a normal drill which is 118 degrees.

regards
Richard Hodgson
Organiser Scalefour Virtual Group. Our meeting invitation is here.

Terry Bendall
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Re: centre and spotting drills

Postby Terry Bendall » Mon May 16, 2016 8:03 am

andrewnummelin wrote: In so doing I came across a spotting drill and I confess that I had not heard of one of these before


I have a little saying "Learn something new every day" and it is surprising how often that is true. After getting on for 60 years using metal (yes I did start at about the age of 10, working with my father), I had not heard of or used a spotting drill. Having had a look at the sources Andrew posted I was interested to see that the point angle on most spotting drills is 90 degrees whereas the standard point angle on a twist drill is 118 degrees. I have no idea how that works but presumable it must do.

As Winander says, a good source of tools of all sorts is Chronos Ltd in Dunstable, http://www.chronos.ltd.uk/ and they do centre drills with a 1/8 inch diameter main body. I have however found that there do seem to be different sizes of the actual drilling part of a centre drill for the same size of main body.

Usually a centre drill is used when drilling holes using a lathe but I have occasionally used one in a drilling machine. I have no idea how useful a spotting drill would be but having got this far in lift without feeling the need for such a thing, I can probably survive without trying one.

Terry Bendall

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45609
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Re: centre and spotting drills

Postby 45609 » Mon May 16, 2016 9:15 am

It is worth reading this as I don't think anyone has understood the purpose of a spotting drill yet.

When to use a spotting drill

In short do not use a 90 degree spot drill with a 118 degree angle drill. Use a 120 degree.

Morgan

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Winander
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Re: centre and spotting drills

Postby Winander » Mon May 16, 2016 10:42 am

45609 wrote:It is worth reading this as I don't think anyone has understood the purpose of a spotting drill yet.


Me for one! I read that spotting drills have a tip angle of 120deg, but apart from reading a vague mention of the shoulder of a regular drill causing the drill to wander, couldn't understand the functional difference between a regular drill (tip 118 deg) and a spotting drill (tip 120 deg).

Question is, as we mostly use soft metals and drill tiny holes, do we really need one?

regards
Richard Hodgson
Organiser Scalefour Virtual Group. Our meeting invitation is here.

John Palmer
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Re: centre and spotting drills

Postby John Palmer » Mon May 16, 2016 5:08 pm

That article to which Morgan has posted a link was a real eye-opener to me too, but all makes perfect sense when you think about the tip angles involved. I, for one, am acutely conscious of how prone to deflection small diameter drill bits can be (partly due to the wear in my chucks and in some instances my cack-handed re-sharpening of them), and I think this makes a strong case for their use for the reasons the article sets out. The fact that we may be drilling into relatively soft metal is no guarantee that deflection will not occur, and in my experience the smaller the drill the greater the risk such deflection will take place.

andrewnummelin
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Re: centre and spotting drills

Postby andrewnummelin » Tue May 17, 2016 9:40 am

Thanks to all who have responded with useful and interesting information.

I got to where I did for a number of reasons:
- my inability to accurately mark the position for a small hole (on a good day getting witin a quarter of a mm would be a real success)
- difficulty of "centre punching" the marked position accurately (I know an optical punch would help but I don't have one)
- the tendency for very small drills (say 0.5mm and below) to wander (or go for a walk if the surface to be drilled is not flat)
- having some centre drills (for their intended use on a lathe)
- being fortunate enough to have a small milling machine.
With the milling machine it's easy to locate a position to 0.01mm so I tried using a centre drill in it to carefully mark the positions of a series of holes that had to be accurately located relative to each other. Depending on size, the holes were finished with a drill in the milling machine or separately if very small. This worked very well indeed. But don't do what I did, and move the mill table before the drill has been completely lifted out of it hole.....

Before making my initial post on this topic I did a little searching on the internet and found lots of discussion about the use of spotting drills, which angle to use, use or not use, use for other purposes like deburring and countersinking, etc. Opinions varied widely depending on people's own experience but all I found were about (small scale) production environments and almost certainly about holes bigger than 1mm. I was hoping to come across some reported experience in our sort of environment of needing a handful of holes.

One link I came across may be useful to some as it illustrates well why it is recommended to use a 120 degree spotting drill for a 118 degree (non carbide) ordinary drill. You will probably need to enlarge the screen to be able to see the diagrams properly.
http://pdf.directindustry.com/pdf/ingersoll-cutting-tools/hole-making-catalog/17869-11078-_93.html

I must now order a new drill (or drills) as I need to make a couple of parts with 14 accurately placed holes. I'll be reporting on this in the "On My Workbench" forum.
Regards,

Andrew Nummelin

Terry Bendall
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Re: centre and spotting drills

Postby Terry Bendall » Wed May 18, 2016 7:18 am

45609 wrote:It is worth reading this as I don't think anyone has understood the purpose of a spotting drill yet.


Thanks for the reference Morgan. Very interesting.

Winander wrote:Question is, as we mostly use soft metals and drill tiny holes, do we really need one?


In my view, probably not. :)

John Palmer wrote:partly due to the wear in my chucks and in some instances my cack-handed re-sharpening of them


Sharpening of twist drills is a challenge especially in the smaller sizes that we would use. I have successfully sharpened very small drills (below 1.0mm dia. and on occasions down to 0.3mm) after they have broken but it is more by luck than judgement. The standard point angle of a drill is 118 degrees although it you are being really fussy it can be changed to up to 140 degrees for drilling thin sheet metal. To give an accurate hole both sides of the tip of the drill need to be exactly the same angle and there needs to be a clearance angle which slopes back and down from the cutting edge - when looking at the drill with the tip uppermost.

To sharpen correctly you rally need a proper tool and cutter grinder but a reasonable substitute is a double ended high speed grinder and a twist drill grinding jig. I have yet to see one of these that will hold drills smaller than 3mm dia. Drills can be sharpened free hand and I have often done so but it is an acquired skill.

andrewnummelin wrote: my inability to accurately mark the position for a small hole


Use a nice sharp scriber and mark a cross on the metal where the hole is to be made. The intersection of the two lines is where the centre punch should be placed.

andrewnummelin wrote:difficulty of "centre punching" the marked position accurately


Use a nice sharp centre punch, lean it over at an angle of about 60 degrees to the horizontal and gently position the point at the intersection of the two lines. If the punch is sharp you should be able to feel when the punch is at the intersection. Keep the point in the correct position and move it upright. Use a hammer to make one light tap on the end of the punch and then one slightly heavier tap. Do the centre punching on a block of metal - steel for preference, but if your vice had a flat surface on the rear use that.

andrewnummelin wrote:the tendency for very small drills (say 0.5mm and below) to wander


Using a centre punch should prevent that. It will obviously need to be a very small punch dot.

andrewnummelin wrote:With the milling machine it's easy to locate a position to 0.01mm so I tried using a centre drill in it to carefully mark the positions of a series of holes that had to be accurately located relative to each other


This is a very good idea if you have the machine.

andrewnummelin wrote:Depending on size, the holes were finished with a drill in the milling machine or separately if very small.


The chuck on my milling machine has worn a bit and will no longer gip a drill bit smaller than about 0.7mm What I did was to buy a small pin chuck cut off part of the top end and hold that in the chuck of the milling machine. The small drill is then held in the pin chuck. Some of those who stopped by my demonstration at Scalefour North saw this happen. Using this method I have successfully drilled 0.3mm dia. holes in the machine.

Terry Bendall


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