When is a new 90 degree drill not 90 degrees?

[Bill PhinnParticipant @billphinn90025]

I recently bought a new spot drill (pictured) off ebay.

Can anyone explain the lack of correspondence between the marked tip angle and the actual one?2

[DiogenesParticipant @diogenes]

Used, reground, put back in it’s packet, and sold? ..the box looks a bit ‘handled’..

[Bill PhinnParticipant @billphinn90025]

I think that’s unlikely, Diogenes, though I understand your cynicism; the drill is its full 75mm in length, the tip grind looks factory-like for its evenness and smoothness, there are no signs of use on the shank or the lands, and the seller tells me all the stock of these drills he has have visibly the same tip angle.

82 degree spotting drills do exist (presumably in both LH and RH), but I’d expect them to be marked as such, not marked 90 degrees. I can only assume this was a batch that got marked 90 but was ground to 82 degrees.

It can be used for screw extraction and to countersink for UNC/UNF screws if nothing else.

Now I just want some 140 degree spot drills, which seem to be rare as hen’s teeth in HSS, and ludicrously expensive in carbide.

[not done it yetParticipant @notdoneityet]

I would think that left-handed ‘spot’ drills are not a regular item?

[JasonBModerator @jasonb]

Presto only seem to list 90deg so less likely to be marked wrongly, have you contacted them and asked why?

[Bill PhinnParticipant @billphinn90025]

The seller can’t explain it, but offered a refund, which I declined.

He said the other three he has in stock (“had”, actually, because he appears to have sold another two since lunchtime today) have the same angle as each other and as mine “from the look of it”.

 

[old martParticipant @oldmart]

Apart from several spotting drills in HSS and carbide which are a pretty good 90 degrees, I have a 5 or 6mm carbide with the 140 degree end. It rarely gets used as the shallow angle makes it more of a blind spot drill.

[ChrisLHParticipant @chrislh]

Standard countersunk cap screw heads are listed with 82 degree conical heads so these drills could have originally been designed for that rather than for spotting. I’ve no idea what’s wrong with 90 degrees in either case.

[ChrisLH]

On ChrisLH Said:

Standard countersunk cap screw heads are listed with 82 degree conical heads so these drills could have originally been designed for that rather than for spotting. I’ve no idea what’s wrong with 90 degrees in either case.

I’m not sure what is meant by a countersunk cap screw*, Chris, nor what you mean by standard.

The box says “spot drill”, so presumably the contents were intended to be a spot drill.

And I’m at a loss as to your point in your last sentence.

*Do you mean a socket countersunk screw?

[alecsParticipant @alecs]

Way of the world these days. Lots of factory seconds and back-door jobs for sale on the net. Or could be a manufacturing defect or more likely a packaging defect. The customer is the QC inspector these days.

[DC31kParticipant @dc31k]

I am not sure why anyone thinks the seller might explain this.

As above, an email to Presto themselves costs nothing and might be a better use of a person’s time than endless speculation here.

At worst, it will receive no response.

Perhaps it was a special/custom order from the factory, just shipped in a standard box.

Consider what would be required for all to be correct: a computer system (ERP system) has to have an entire new product added so it can print a different label; the laser etching machine would also need to be re-programmed. This may have been considerably harder (and more costly in human resources) than telling the grinder operative to change the point angle on the next ten that pass through the machine.

When you make multi-hundred ‘same’ things, making a few different ones has unexpected implications.

 

[not done it yetParticipant @notdoneityet]

C’mon guys, think straight.  It is a specialised cutting tool, not the normal spotting drill used by a model engineer.  I expect it is only termed as a spot drill because of it’s length?

Who wants a spot 12mm wide?  (what a waste of 50% of the flutes for normal modellers)

Who needs flutes (that long) on a spotting drill for spotting a dimple?

Who uses a left handed drill for any normal/usual engineering operation (unless the machine is a specialised jobbie)?

 

The only task I use my left handed drills for is broken bolt/stud removal. (be very careful, if buying anything from the ‘claimed’ aerospace surplus cheat at exhibitions, etc – my set included some right handed bits).

[JasonBModerator @jasonb]

Very common in industry and sometimes in my workshop to use a spotting drill larger than the hole. If you read Prestos description they can be used for camfering. So if you go in deep enough with the spot drill you can pre chamfer the edge of the hole at the same tool setting without needing to go back after drilling with a typical CSK bit to add the chamfer/deburr.

A12mm spot would be right for M10 clearance or tapping size hole and yes I do use that size on my models although usually one of the finer pitches.

All my spot drills have a different grind to similar length stub drills so it is the geometry that makes it a spot not the length.

[JAParticipant @ja]

ebay: enough said. No further explanation required.

JA

[Nicholas FarrParticipant @nicholasfarr14254]

Hi NDIY, you need to be careful who you are calling a cheat in public. I’ve often bought things from that Aerospace Surplus chap, and have never found anything wrong with what he has sold to me.

Regards Nick.

[Michael GilliganParticipant @michaelgilligan61133]

Probably just noise in the context of this discussion, but this is a handy reference:

https://severancetools.com/resources/countersink-angle-guide.html

MichaelG.

[peak4Participant @peak4]

Regarding the direction of rotation; whilst I’ve never owned or used one, but aren’t some gang drilling machines set up to use opposite rotation on alternate drill bits?

This allows closer spacing of spindles without intermediate gearing.

Bill

[alecsParticipant @alecs]

Old mass-production mechanically automated screw machines used reverse direction spindles requiring reverse tooling, including drills.

[Chris CourtneyParticipant @chriscourtney72250]

As both ChrisLH and Michael have tried to point out, 82 degrees is an American ANSI standard for countersunk bolts (eg ANSI/ASME B18.5-2012) so this cutter is a standard countersink bit. I don’t know why it is wrongly labelled, but I would guess somebody has been sorting out tool bits and put it in the wrong box.

[ChrisLHParticipant @chrislh]

Bill P

Yes, I do mean Socket Countersunk Screw as described in your reference. The Unbrako on-line catalogue calls them “Flat Head Socket Screws” in the inch sizes and shows 82 degrees as the included angle of the conical head. Metric screws are called “Metric Socket Flat Head Cap Screws” and they have 90 degree conical heads.

My last sentence questions why not choose the obvious 90 and it seems that some people did.

[Chris Courtney]

On Chris Courtney Said:

…  I would guess somebody has been sorting out tool bits and put it in the wrong box.

It’s a bit worse than that, Chris  … look at the markings on the shank

🙁

MichaelG.

[ChrisLHParticipant @chrislh]

And the point angle of my “VG” 6.0 x 90 HSS Co spot drill measures 90 degrees as near as I can tell.

[Chris CourtneyParticipant @chriscourtney72250]

I’d missed the shank engraving Michael. So I guess it was either mixed up during manufacture or it has been reground.

[Bill PhinnParticipant @billphinn90025]

Some interesting discussion here.

Bill and Alec, I wasn’t aware there was such a thing as gang drilling machines with alternating spindle directions. Useful to know.

Chris Courtney says: “As both ChrisLH and Michael have tried to point out, 82 degrees is an American ANSI standard for countersunk bolts”.

Chris, I’m puzzled by your response here; I thought I’d satisfactorily covered that very point in my second post in this thread.

ChrisLH says: “Yes, I do mean Socket Countersunk Screw as described in your reference. The Unbrako on-line catalogue calls them “Flat Head Socket Screws” in the inch sizes and shows 82 degrees as the included angle of the conical head. Metric screws are called “Metric Socket Flat Head Cap Screws” and they have 90 degree conical heads..”

Again, given what I’d already said, Chris, I’m mystified why you felt I needed to be told this.

ChrisLH says: “My last sentence questions why not choose the obvious 90 and it seems that some people did”.

Sorry, Chris, I’ve no wish to be difficult, but I’m none the wiser what your point is here.

Below are a few photos of the tip. If you can detect faceting anywhere on the drill point, there is none; it is a trick of the light. Also, there was not a trace of dirt or swarf anywhere in the box or on the drill. I’ve seen plenty of meddled with drills being passed off as new; this isn’t one of them.

[ChrisLHParticipant @chrislh]

Bill P

OK apologies, I missed your second post so I have just repeated what you already knew.

I am still curious as to why 82 degrees was chosen by the Americans but perhaps this is considered excessive subject drift.

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