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

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I find using the spotting drill first is a great aid to getting a straight hole and in all honesty I haven't found its relative point angle to have much effect when using a follow up bit. It seems to me that the conical hole guides the next drill bit regardless of whether it's the centre or flank of the bit that contacts first.

One thing that I do find important is that of clearing the swarf frequently. If you don't do this then I think it's possible for the swarf to buid up and compact on one side of the drill tip, forcing it away from the desired line.

Martin.

I don't think it's been mentioned, but the best way to keep drills accurate is to treat them with kindness

This includes a bit of cutting oil brushed on.

Neil

Candle wax can be good too, if you don't want oil spreading over the part.

Martin.

Thanks Jason, I had not followed the linked subject as far as I thought I had. In practice the point angle of a spotting deal does not seem to matter much except for hard materials. This is something of a relief for me as I became aware of spotting drills through the forum some time ago and thinking I was being smart, I ordered ones with a 120 degree angle (which seem to work very well).

Do you re-sharpen your spotting drills? Presumably this would need a good deal of accuracy, better than off-hand grinding ?

Chas

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The forthright advice that I referenced on on the other thread comes from a supplier of tungsten carbide drills ... I think that source is probably very relevant.

[quote]

Some interesting notes and warnings about [carbide] ‘Spot Drills’ here:
**LINK**

https://www.destinytool.com/spot-drills.html

[/quote]

MichaelG.

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Ref. https://www.model-engineer.co.uk/forums/postings.asp?th=157991&p=4

Edit: inserted the word ‘source’ for additional clarity

Edited By Michael Gilligan on 22/05/2020 14:00:18

Not that conclusive on your link Michael, though I have not read it all.

"The spot drll point angle should be greater than or less than the final drill’s point angle."

Anyidea what they mean by:

Using  a spot drill with an angle less than the drill point angle WILL cause the drill flutes to be "blown off"

Edited By JasonB on 22/05/2020 13:45:03

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Yes ... I believe that they mean if the cutting-edge of the large drill makes first-contact with the edge of the spotted hole, then the carbide will chip severely [and continue to do so if the feed continues]

Clearly this is in the context of what they supply, and its expected usage.

MichaelG.

Edited By Michael Gilligan on 22/05/2020 13:55:37

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Hi, well in my day jobs that I've had, I must have drilled many thousands of holes of various sizes up to about 2" and on one or two pretty rough and or very worn machines. One of the most important things I've found, is the drill needs to be sharpened concentric and symmetrically whatever the angles are. If you are drilling into a centre punch mark, then the centre punch mark should also be symmetrical, i.e. that is it needs to be punched square to the metal, so that any one part is not higher than any other part, (a uniform crater if you like). I've found it makes no real difference weather I use a centre drill or a spot drill to start a hole without a centre punch, but one shouldn't go like a bull at a gate when drilling the hole with a pilot drill or the drill size and yes, some kind of lubricant will always help. I always use a pilot drill/s (unless it's a small hole like up to 3 or 4mm) if I need to be fussy with the hole and drilling square to the metal is also important, drilling holes at an angle where required needs a bit more attention and care. Just about all the drilling machines that I've used in my day jobs, have been large to big and are very ridged and don't flex so you'd notice, but some have had sloppy quills or the bearings have had a bit slack, but that never seem to make a drill wander from a good centre punch mark or pilot hole, although you may get a slightly over size hole, but even slightly bent drills have seldom wandered either, but in the most part of the holes that I've drilled needed good location rather than a true size hole. Don't believe that a follow up drill will not wander off a pilot hole, because they can, especially if the drill point is not concentric or symmetrical. Well that's my experience.

Regards Nick.

Edited By Nicholas Farr on 22/05/2020 14:00:55

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This slightly longer extract might help:

[quote]

Ideally, the spot drill diameter should be about 70% of the size of your final drill diameter. The spot drill point angle should be greater than or less than the final drill’s point angle.

• Spot drilling is not recommended in hardened steels.
• Contacting the corners of the carbide drill before the center point in the opening of the hole could lead to corner chipping.

Using a spot drill with an angle less than the drill point angle WILL cause the drill flutes to be "blown off"

Never use a 90 ̊ Spot drill that will be followed by a 140 ̊ drill point!

[/quote]

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MichaelG.

If that's what they mean (I'm sure it is), then they're saying that you shouldn't open up any hole with any carbide drill, so they're being quite specific.

I've got a small number of carbide drills, but many more in HSS, so I doubt this would be a problem for me. However, I'd still use a carbide drill despite their advice if I didn't have something more suitable.

My spotting drills are cobalt 90 deg and made by Osborn. So far, I've had no problems at all.

Martin.

Edited By blowlamp on 22/05/2020 14:34:09

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Yes, I agree Martin

... we are, of course, perfectly entitled to ignore their assertions when using whatever we have to hand.

MichaelG.

Here's a couple of photos of the pilot point drills I mentioned earlier.

A/k/a "proline bullet drill". Here on eBay. Power Tools Direct Homebase seem to have sets. DeWalt also have a version. You can see how the pilot point projects from the flat-ended cutting edges and is split to give a fine point for starting. The last significant job I did was a transverse on-diameter hole through 2 inch stainless bar, 10mm diameter, done in one go. A 10mm ground steel bar was a tight sliding fit in the hole afterwards. Highly recommended.

Thanks for all of the info guys, and suggestions. With these in mind, I did some googling and found a very useful video which confirms a lot of the theory here, and certainly I have more understanding and confidence to have another go over the weekend.

https://youtu.be/Ot8wPGQW3JI

Hope this is OK to post.

Here you go!

Joe Pie has just put up a looong video about drilling straight holes HERE. Might be useful?

NDIY, Yep, seen it this morning, and very impressive! He did start it with a centre drill (but lets not go there)

Using centre drill, reamer, boring bar, and of course the drill. Very very accurate.