Centre drill misalignment

[Sam StonesParticipant @samstones42903]

Having to `make-do’ with a very limited amount of workshop equipment and tools, I was preparing to make a soldering jig to hold a couple of clock parts together while silver soldering them.

I marked out a piece of 8mm thick gauge steel ready to drill some 1/8″ pilot holes. Each hole position was centre-popped ready for picking up with a centre drill. Rather than drilling sideways in the lathe, using the tailstock to align and feed the steel towards the drill, I chose what I thought would be the easier option, ie. to use a friend’s vertical pillar drill.

With eyesight leaving much to be desired, I’ve used a 4″ binocular magnifier for most of my work on the clock. However, judging angles etc. through these devices is less than satisfactory, while my friend seems to pay little attention to securing his machines to the bench. Some of the albeit small equipment tends to wobble or wander a little, requiring special concentration on several things at once. As I began to apply pressure to the almost new centre drill aligned with the first of the centre-pops, the centre drill began to travel sideways. Before I realised what was happening, I had machined a slot into the surface of the jig.

It was about then that I noticed the quill of the drilling machine was tilting to starboard, relative to the support column. Closer inspection revealed that the drill head had not been correctly manoeuvred down into place over the column. Instead, it was being held up by one of the locking screws sitting on the top edge of the column.

Feeling somewhat concerned about machinery drifting across the bench, it was time to leave. I returned home and have since completed the jig using the lathe as a horizontal drilling machine.

Apart from the obvious ones about safety etc. questions arise like :-

How susceptible are centre drills to misalignment?

Is there correlation with the types of metals being drilled, and various other factors within the centre-drilling process?

For someone with time to spare, this might be an interesting exercise if it has not already been established.

 

Regards,

 

Sam

Australia

Edited By Sam Stones on 03/08/2010 01:43:13

[Sam StonesParticipant @samstones42903]

Coping with wander

[BogstandardParticipant @bogstandard]

Sam,

 

I tend not to use centre drills any more, but spotting drills, which have fallen in price dramatically recently to a stage where they are a better substitute for putting a drilling point in.

Centre drills are really for putting the taper into a rod end so that they can be turned between centres.

A centre drill has just a normal drill point on it, so any misalignment can send it skating across the job, just like you found out. Spotting drills actually come to a very sharp point but still act as a stub drill, and as such don’t wander.

 

I can’t give you any links in your antipodean paradise, but I can give you one in our rain soaked little island.

 

Spot Drills 

 

 

Bogs

 

[Sam StonesParticipant @samstones42903]

Hi Bogs,

Thanks for your speedy reply.

Like an ostrich, I must have had my head in the sand all these years. I’ve never heard of a spot drill until now, even though I began serving my time as a toolmaker in 1950. That’s (probably) what comes from spending so much time on a lathe. Come to think of it, no-one in the toolroom used anything else but centring drills, or should that be centre drills?

In my earlier years, I used to be fascinated by one chap working on the radial arm. He was mostly involved in drilling deep holes for steam and water-ways in moulds for plastics and rubber. After marking out and centre-popping the centre positions, he would scribe a circle the same diameter as the hole to be drilled. Around this he would centre-pop all four intersections and use these as a guide to get the OD of the drill into the right position. Although I seem to recall him using a centring drill, he would still nudge the radial arm into position as he aimed to get the main drill centred.

I had a look at your link, and can now appreciate the difference between centre drills and spot drills. Meanwhile, I hope these comments helps others too.

Right now, the weather in paradise (ie. Melbourne), is also very rain-soaked and pretty cold, down to 13C.

Regards,

Sam

(ex Bolton lad)

[BogstandardParticipant @bogstandard]

Sam,

 

Spot drills are used mainly in CNC work, where no centre pop is used, just drill straight in.

 

Hence that is why you most probably haven’t come across them. But they used to be very expensive. I sorted myself out with a full set from 3mm to 8mm for, I think, just over twenty squid, and with well over a couple of years constant use, I haven’t lost one yet, whereas normally, I would have lost at least a few centre drill tips.

 

Because I use DRO’s on my mill, I still do my layout, purely as a double check, but I don’t centre pop any more, as being even a tiny bit out with a pop mark can throw the centre drill or normal drill away from exact centre. The spot drill can be brought into position and drill into the material without any aid at all. Normally I go straight thru with material up to about 4mm thick, over that, I follow down with a normal drill.

 

I wouldn’t worry too much about not knowing about them, I still come across stuff in old books that I haven’t seen or heard about, but are still available even today.

 

Bogs

 

 

[GoCreateParticipant @gocreate]

Hi Sam

Thanks for your post, it’s uncovered some interesting stuff I did not know.

Hi Bogs

Thanks for that very interesting info on spot drills and also the link.

 

I notice there are 120 deg and 90 deg spot drills available, which would be prefered?

 

Thanks

 

Nigel

 

[Sam StonesParticipant @samstones42903]

Hi Bogs & Nigel,

Thank you for your very useful replies. I’m pleased too, that others are also finding your information useful.

I had a feeling there was a CNC connection, especially when spot drills are produced in carbide. I might (eventually) have come across them had I not sold my Taig Micromill.

Your point about a centre pop ` . . being even a tiny bit out . . ‘ etc. is very true.

Wrt the link, Nigel’s question regarding 120 and 90 degrees suggests to me that 120 deg suits the normal drill-tip angle, but where does 90 degrees fit into the picture? Could it be that a 90 deg tip is even more `stable’?

There’s a reference to cobalt and carbide, and I can see a significant price difference. I understand that carbide will have a longer life, but is cobalt just another name for one of the HSS’s which, according to Wikipedia (and a couple of `still connected’ neurones in my brain), sometimes contains a percentage of cobalt?

Interestingly (I trust), the stem and ball of my right hip joint is cobalt chromium. This alloy comes close to matching what we often used, when `drilling’ through hardened steel in those early days, ie. Stellite.

Thanks again,

Sam

[BogstandardParticipant @bogstandard]

Sam,

 

I have no exact reason for the difference in angle other than yours, maybe someone in the CNC big boys world could shed some light on it.

 

Like your suggestion, I use the 120º for starting off normal drill points and for drilling thin sheets. But four facet ground twist drills with their much larger angle and finer point do have a reputation of being able to self start accurately without aid from a centre or spot drill. Even though I have a full set of them, I still like to put a small spot in first.

 

The 90º ones get used when I am drilling a blind hole for tapping, the angle on the end usually matches the point on the end of a tap. Whether that is right, I have no idea, but I haven’t come across any problems yet.

I only have one set of carbide ones, the 3mm ones (that is the size I use the most), the rest are just standard cobalt ones.

 

Cobalt is just another HSS alloy that gives a slightly sharper, tougher and better lifespan than normal HSS.

 

Bogs

[BogstandardParticipant @bogstandard]

Sam,

 

I have no exact reason for the difference in angle other than yours, maybe someone in the CNC big boys world could shed some light on it.

 

Like your suggestion, I use the 120º for starting off normal drill points and for drilling thin sheets. But four facet ground twist drills with their much larger angle and finer point do have a reputation of being able to self start accurately without aid from a centre or spot drill. Even though I have a full set of them, I still like to put a small spot in first.

 

The 90º ones get used when I am drilling a blind hole for tapping, the angle on the end usually matches the point on the end of a tap. Whether that is right, I have no idea, but I haven’t come across any problems yet.

I only have one set of carbide ones, the 3mm ones (that is the size I use the most), the rest are just standard cobalt ones.

 

Cobalt is just another HSS alloy that gives a slightly sharper, tougher and better lifespan than normal HSS.

 

Bogs

[Eric CoxParticipant @ericcox50497]

You say that the centre drill ran off, maybe the centre pop wasn’t big enough. You would give the centre punch a light tap to mark the metal, check to see if it’s in the right place then if it is, give the punch a damn good hit to enforce the centre pop.

[John OlsenParticipant @johnolsen79199]

Hi Guys,

 

I was buying a long series 1/8 inch ball ended slot drill at the weekend, the list had two sorts, high speed steel and cobalt HSS. The latter was more than twice the price. From which I would conclude that it is a superior grade of HSS. Of course the expensive one was the only one is stock. Oh well, at least it did the cut and didn’t bust on me! (They of course had carbide ones as well, but I didn’t even ask the price for them.)

 

They used to call a centre drill a Slocombe, I presume it was either the inventors name or an early brand name. They are actually not the ideal thing for starting a drill, the angle is wrong, and if things go slightly wrong they are more likely to bust the centre drill than carve a slot. I am not sure if that means you were lucky or not, neither result would be very helpful.

 

One old idea used to be a thing like a lathe centre that popped into the taper, but had a four sided pyramid instead of a cone. The edges of the pyramid would carve out a cone shaped depression, presumably the angle of the pyramid would be chosen to suit normal drills. Stub drills will start more reliably than normal series drills, although I wouldn’t rely in it for precision work.

 

There is an interesting problem if you use buttons to set out some holes, then want to open them out. There is of course a screw hole already near the location you want, the one you used to hold the button in place, and it will make the drill run out. A possible answer to this one is to start the actual hole with a slot drill rather than trying to drill with a normal drill. Probably if you are using buttons in the first place, you are planning to bore the hole eventually, but if it is to be a small size then runout in  the initial hole can be a real problem because it may easily take the hole past the intended limit of the final hole. Slot drills are not necessarily all that happy doing a straight down plunge cut, but will usually do it without running out to the existing small hole. * Of course you only have to go deep enough to eradicate the tapped hole that held the button on. That technique only applies when the work is rigidly attached to a machine table  or chuck of course

 

* They are supposed to be able to do a plunge cut, but sometime kick up a fuss doing it. I think sharpness and machine rigidity are factors here. 

 

regards

John

[Simon ThomsonParticipant @simonthomson54574]

Hi everyone,

 

i’ve been following the post from the forum and decided to speak to the guy at the link you put up earlier for http://www.engineeringsupplies.co.uk ….this is what i’ve discovered…

 

HSS drills or cutters etc are just normal high speed steel. They should cut plastics, wood, mild steel etc.

 

Cobalt drills and cutters are a mix of high speed steel with an added cobalt content (Cobalt being an element in the periodic table). Cobalt items (usually marked HSCo, Co or HSSE) usually have about 5-8% added to the mix of HSS. The benefits of cobalt is that it is inherently “Tough” and has better wearing properties in comparison to HSS. For this reason Cobalt cutters and drills have a much longer lifespan than an HSS and will drill through stainlesses and other materials with a high tensile strength. Comparitively an HSS used on stainless etc will burnout within seconds and usually results in workhardening the material.

 

Carbide: is a different thing all together. Carbide is made from powder and is pressed then sintered then ground. Carbide is fairly similar to cobalt but alot “harder” which means it wears even less and can be run at higher speeds. Because of the added hardness carbide is more prone to chipping. Carbide is usually used for stainlesses, duplexes, inconells etc.

However, aparently there are more than one grade. Think of it like being made of grains of sand – the higher the grade the smaller grains of carbide – K10 being big grains and K50 being microscopic – the stronger the tool which means less wear and less chipping. This also reflects on the price K10 being really cheap and K50 being more expensive.

 

According to the nice guy i spoke to, a cobalt spot drill is usually more than sufficient and alot cheaper for most jobs. Also a 90degree angle is most popular as its alot sharper and more accurate.

However, (and i thought this was really interesting and would save me a tonne of time) they do coated jobber drills which are self centering meaning i can drill the holes i want without needing to spot it first. I’m only doing mild steels anyway so i don’t need the cobalt ones so a pack of 10 HSS is quite a good deal. I think they sell cobalt self centering but i ran out of paper when i was writing most of the info down for the forum.

 

Si

[BogstandardParticipant @bogstandard]

Si,

 

Some good info there.

 

The 90º point issue does now seem logical, as a normal centre pop is the same angle for drilling.

 

 

Bogs

[Lawrie Alush-JaggsParticipant @lawriealush-jaggs50843]

Hi Sam

Try the following links for spotting drills

http://www.suttontools.com.au/Catalogue_Australia_HighPerformance

http://www2.blackwoods.com.au/infoBANKproduct.aspx?SG=2000165&S=4084741&G=7722300&P=7799999

Lawrie

[chris stephensParticipant @chrisstephens63393]

Hi Guys ,

Just to add some other thoughts to thread, I recently bought some “spotting” drills at a SMEE rummage sale. They were home made from old 2 flute slot mills and ground as four facet drills. They are really quite good, being made of “the good stuff” and short enough to be rigid.

I don’t know who made them but I shall be making some more if ever the need arises.

chriStephens 

[Sam StonesParticipant @samstones42903]

Hi Everyone,

This has turned out to be a most interesting and unexpected series of comments and ideas so, having received so much help from various ME postings, I thought it important to try to close this thread with a photograph in my CLOCK album and a few final words.

Just to recap, I have a very limited range of equipment with which to finish a skeleton clock. The majority of the original machining for this clock was achieved some 30+ years ago using a Myford ML7 and a dividing attachment. The plate work and wheel piercing was achieved by hand fretting, filing and polishing etc.

Where it became necessary to drill accurate holes for arbors and the like, the ML7 became a horizontal drilling machine with parts (eg. the frames and wheels), thrust against the drill using the tailstock in various forms. This is a rather cumbersome, almost three-handed method, but had to suffice, pending the purchase of a more suitable drilling machine.

I must state that for the whole time of drilling these holes and many while building other jobs requiring a fair degree of accuracy, I never once experienced trouble with using a centre drill to pick up on centre-popped marking out. It was only after discovering that the quill of a friends bench drill was crooked because the head was incorrectly fitted, that I finished up with the result shown in the photograph. The workpiece was to be a simple jig to hold a couple of brass parts in line while silver soldering. I have since started afresh with another chunk of gauge steel, and will introduce these bits in another (clock) thread.

To get an idea of scale in the photograph, the smallest divisions on the rule are half millimetres, and the semicircle has a radius of 5mm. The metal is a piece of gauge steel 25mm wide. This stuff is a high carbon steel, and can be a bit rough to work with, but cleans up well. Being a high carbon steel it can also be hardened and tempered as necessary. The curious colour is simply from using a felt tip marker pen which normally shows up black on paper. You can see the degree of centre-drill drift which, because of my less than perfect eyesight, I missed before it was too late. I would assess that the drill quill was inclined to the work-table at about 3 to 5 degrees, but since the drill head wasn’t properly secured, what was actually happening is anybodies guess.

To respond to Eric Cox’s thread, my method for marking out centres begins with the smallest indent, usually from the point of a scriber. Progressively, the size of the indent (centre-pop) is increased. I usually start with the `softest’ setting of an automatic centre punch and, if necessary, I increase the impact to its maximum. Of course, this also relates to the hardness of the metal. If this impact doesn’t produce a deep enough recess, which it generally does for the type of work I’ve done in the past, and now includes the fairly fine clock stuff, I would then progress to a hefty wallop with a hammer and hand punch. Incidentally, my first job as a toolroom apprentice was learning how to handle letter and number punches, before progressing onto numbering moulds for ID purposes.

To wind up this thread, I could say that perhaps I’ve become too accustomed to getting such good results from centre drills, especially from my extensive use of various lathes. I would say that when centre-drilling into a `faced-off’ workpiece in the lathe, if the tailstock is accurate, everything else automatically lines up. Perhaps one day soon, I’ll treat myself to a spot drill.

Thanks to everyone, especially for the postings.

Sam

[John Wood1Participant @johnwood1]

It’s interesting to note the problems with centre drills, I always thought these were the best things to use so have put down past errors to my own poor techniques and have, in fact, been trying out different methods without centre drills.

 

Never used spotting drills but, now I know about them I will try and locate some to try out.  I understand that a normal twist drill sharpened as a four-facet drill is much better for locating directly into a centre pop. I have just completed such a drill sharpening jig for my Warden cutter grinder but havn’t tried it out yet.  Just a thought.

 

John

[David Clark 13Participant @davidclark13]

Hi There

You can also use stub drills.

If you are making a largish reamed hole, run a stub drill through, follow with a slot drill or end mill and the hole will be dead true.

Open up as normal then.

regards David

 

[Sub MandrelParticipant @submandrel]

One old idea used to be a thing like a lathe centre that popped into the taper, but had a four sided pyramid instead of a cone.

I have one of these – and it has a really good use. Imagine you have a long piece of work in the chuck, its a bit unstable, but you don’t want the bother of setting up a steady or it isn’t round, yet you need to centre it? Just make a centre pop and put the tip of the tailstock-mounted four-facet centre in i. Apply some cutting oil, switch on and feed in until you have your hole, swap the centre for a normal running or fixed one, and “Bogs’ your Uncle” (sorry Bogs).

Neil

[Anonymous]

I hate centre drills! You’ve only got to look at the smaller ones and the centre bit goes ping. I use them only when I absolutely have to, in order to be able to use tailstock support on the lathe.

 

For general drilling I use Dormer 4 facet drills. On cold drawn, or machined, flat surfaces they do not need a centre drill to be used first. This applies both to manual and CNC mills. Can’t say I’ve ever actually measured the errors involved. But if I drill and tap a set of M6 holes, and then drill 6.1mm matching clearance holes in the mating part they fit together with no problems.

 

On round, or rough, surfaces even the 4 facet drills will wander. In these cases I use 90° spot drills first. These are similar to stub drills. However, mine are carbide and are thus much stiffer than a normal drill.

 

By the way I do 99.9% of my drilling on the vertical mill; I almost never use the drilling machine.

 

Regards,

 

Andrew

[Sam StonesParticipant @samstones42903]

Posted by

Andrew Johnston on 17/09/2010

I hate centre drills! You’ve only got to look at the smaller ones and the centre bit goes ping.

Hi Andrew,

I’m inclined to agree with you, having on a (very) few occasions had to dig out the tip of a centre drill from a workpiece. However, over the past 60 years of my engineering career both practical and academic, I have either been too blind or too stubborn to notice the alternatives. I have to admit to never having `discovered’ spot drills until quite recently, thanks to you kind people who have tugged at my reins with your postings.

Like me, when you know no better, you become quite skilled at getting the best results from a particular piece of equipment. For me, centre drills have burned their place into my brain, and thus became my automatic choice. Considering the amount and range of specialised equipment I have built over the years, it now seems quite remarkable how I’ve managed to get away with using nothing else for locating a `marked-out’ hole position.

Getting the most out of a piece of equipment has reminded me of working on the apprentice’s lathe back in the early 50’s. Driven from a 10HP electric motor via an overhead counter-shaft (running the length of the shop), the lathe had a three-step pulley and when it was running, it had about 5 yards of flat belt flying past your left ear.

It was a cow of a machine!

My main tasks were turning ejector pins for plastics moulds, and much larger aluminium shrink-pins (used for controlling the diameter of hard rubber textile bobbins during their steam curing process). Applying more than a ten thou cut, the long thin ejector pins were likely to leap over the tool bit, and the tail-stock never seemed to stay on centre no matter how many times it was re-adjusted.

Changing speeds was also a skill in itself. Aided by a five foot long `spare’ wooden lever which was slightly notched at one end, the out-of-reach (overhead) end of the flat belt had to be jabbed from below to flick it from one pulley step to the next. Then, down at lathe level, the belt was give a swift slap with a flat hand to get it to jump onto the lathe pulley. I suspect that the entire workshop (even the factory) would be closed down under current `work-practices’ regulations.

As you can imagine, I learned quite a bit about the maintenance of flat-belt drives, especially when one broke or the coupling clasp or pin wore through. By the way, via the counter-shaft, the 10HP motor also drove a horizontal power saw, a surface grinder, and another lathe larger than the apprentice’s machine. There were times when the surface grinder would slow down if we were all simultaneously drawing torque from the system. I suspect that the main drive belt from the motor (about a four inch wide flat leather affair) was slipping. 

Another use for the 10HP motor, since it made a fair bit of noise, was as a `Start Work’ signal first thing in the morning, after morning tea-break, and also after lunch.

In closing, does anyone know the name of the steel which we referred to as BND? This was a tough steel which was later hardened, tempered, and then ground to size.

Happy centring.

Regards,

Sam

[Ian S CParticipant @iansc]

Sam, you had that sort of workshop as an apprentice, back in the early 60swhen I was at Technical Collage in Dunedin (NZ), our workshop was run the same way, only the Colchester Chipmaster had its own motor, everything else ran on the line shafts, mills shaper drill press grinders and lathes, we all survived, and as far as I know no serious injury ever happened in the 80 odd yearsthat workshop was there. The workshop was originally powered by a steam engine as far as I know. Ian S C

[Sam StonesParticipant @samstones42903]

Nice to know about your experiences, Ian.

I can’t remember whether it was before or after my RAF National Service stint, that the company where I was serving my time, actually motorised individual machines. However, by the time I was back in the toolroom and close to getting my ONC Mech. Eng. I was operating one of their brand new Super 7’s, with lots of Myford add-ons. Not forgetting those centre drills of course.

A couple of years later I was in the DO, the place where I had set my mind to be at age thirteen.

 

It’s amazing where this centre-drill posting has taken us!?

 

Sam

[John OlsenParticipant @johnolsen79199]

We are building a workshop with line shafting and a bunch of old machines at MOTAT here in Auckland. The biggest lathe has its own motor, but there are several others that will be on the shaft, plus a Denbigh mill and an interesting old shaper that has a very short stroke…seems it might have been a special purpose machine, maybe for slotting out links or something.

 

MOTAT = Museum of Transport and Technology.

 

regards

John

[adyParticipant @ady]

I just use an old carbide tipped centre, and ground the very tip into a half centre so it can cut a centre spot in metal.

 

Seems to work fine.

[Author]

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