Drilling problem

[Robin GrahamParticipant @robingraham42208]

Hi, I have recently acquired a drill press (one of the smaller Axminster bench-mounted jobs) and today used it to drill  8mm through the axis of  a 1.5 inch long piece of 1.25 inch diameter round steel bar.  The procedure I adopted was to turn the bar to diameter in the lathe, centre drill whilst still in the chuck then transfer to the drill press for drilling through. I clamped the bar in a vice, aligned the centre-drilled hole in the workpiece with the drill bit as best I could by eye, tightened the vice to the table and chewed my way through.

The problem is that the resulting bore, whilst spot-on 8mm (as near as I can measure) at the ‘exit hole’, is around 8.5mm at the ‘entry hole’.  Can anyone explain what is happening here?

Thanks in advance, 

Regards, Robin.   

 

[Robin GrahamParticipant @robingraham42208]

[chris stephensParticipant @chrisstephens63393]

Hi Robin,

I may be dense, but why did you not drill the hole in the lathe?

It is likely that as the drill entered the centre drilling it wandered about a bit before it went in. Once in the hole it probably bent its way to the centre, where it continued till it came out the other side.

Sometimes it is better to not to tighten the vice to the table but to let it find its own position. The important point is to hold the work in such a way that if it all goes wrong, you are not left holding any sharp bits. Usually the weight of the vice is enough hold things safely, using the drill you mention. Big drills or sheet metal drilling are a different case!

chris stephens

[mgjParticipant @mgj]

Have you actually checked that table and quill are set square to each other? That’s one quite easy way of getting an oversize entry if the hole is a bit deep. And that it is tight and didn’t tilt in the drilling?

 

If you extend the quill and lock it, clean the table off, then bring it up and  put a setsquare against the quill Tell you straight away.

 

if you don’t have a large enough square, if you set up as large a dia bar in the 4 jaw as you can, CLOCKED TRUE, and turn a recess in the base to leave a thin ring (skim the surface of the ring true of course), that will stand square, and might perhaps save buying a big square for a one off use.  

Edited By meyrick griffith-jones on 09/10/2009 22:11:00

[Robin GrahamParticipant @robingraham42208]

Thanks Chris, you are not being dense at all! I didn’t drill in the lathe (which I knew should work) partly because I was trying out a different method just to see what would happen  and partly beacause I wanted to see what the (cheap) new drill press could do. I have no  background in mechanical engineering, so I’m still experimenting.

Your explanation  of the reason for the ‘conical bore’ makes good sense to me.

Thanks  again, Robin. 

[MikeParticipant @mike89748]

Hi Robin: All of the above advice is good, and I guess it comes from much better engineers than me! However, there are some further tips for drilling true, parallel-sided holes. Firstly, did you push your 8mm drill through in one go, or did you drill a smaller pilot hole first? I’d have been tempted to drill a slightly smaller hole first (say Letter M or N or a near metric equivalent for 8mm), then go through very slowly with an 8mm drill bit and plenty of lubricant.

Also, all “as supplied” drill bits tend to drill slightly over size, because cutting pressure and torque makes the tips of the cutting edges splay slightly outwards. You can get around this by grinding a very slight radius on the tips.

This isn’t my advice – I read it in Model Engineer well over 30 years ago – but it does work. It is very useful if you need an accurate hole but don’t have the right size reamer.

[Jeff DaymanParticipant @jeffdayman43397]

Could be your drill is warped, this will cause the hole to wander.

 

There is one other thing that it could be , that actually happened on a cheap drill press I once had. The chuck was actually slightly wobbly on its’  taper. It worked OK for small drill but any drill with a long overhang wandered about. To check for this, put a long drill in the chuck and push/pull it from side to side at its end. If the assy moves, you have a problem.

 

I fixed it in my case by applying some Loctite bearing retainer compound to the taper. Crude but it worked for a few years, then motor burnt out so drill was finished. 

 

Cheers Jeff

[mgjParticipant @mgj]

But – with respect a reamer will follow a hole. You can make a D bit to dead size out of silver steel – that should straighten a wandered hole – within limits of course!. Or take GHT’s advice which is to bore to near size after drilling, and then ream the last .005″.

 

Which doesn’t help much in our small sizes where you cannot get a boring tool down the hole.  Back to 4 facet sharpening and a tool and cutter grinder of some sort.

 

If it is small and deep and has to be straight and to a size, then I use a 4 facet sharpened drill bit, and a SS d bit down it to finish. (Or build things that are big enough not to have holes that small in them)

[MikeParticipant @mike89748]

Hi Meyrick: The great thing about these forums is that you learn something new every day. I am only vaguely familiar with the concept of four-facet drill grinding, so I would be grateful if you could tell me the angles involved. I really ought to make myself a set of D-bits in the more popular sizes, too. Another excuse to spend all day in the workshop!

Best wishes,

Mike

[chris stephensParticipant @chrisstephens63393]

Hi Mike,

Don’t be tempted to think others are better engineers, just because they have itchy typing fingers. If you can get the results you want, you are as good as anybody, you may not be as fast as some but this is just a hobby. 

You are right about these Forums, and the day you think you know it all is the day men in white coats come to take you away.

Always listen to others, if the advice sounds good try it yourself. If it doesn’t work for you, move on and try something else.

Regarding four facet drill grinding, perhaps our esteemed editor could be persuaded to reprint DAG Brown’s articles in the next “Best of ME”. I think it would qualify.

chris stephens

[Robin GrahamParticipant @robingraham42208]

Many thanks all for your useful comments.  I had in fact set the table as square as I could left-to-right but there is a residual error of about 0.1mm in 100mm (ie about 0.06 degrees) fore-and-aft which I imagine can be corrected only by shimming somehow.

I tried the test for play which Jeff suggested and found that there is indeed significant ‘wobble’ – I could ‘scribe a line’ nearly 1mm long with the tip of the 8mm drill bit by grasping the chuck and pulling on it.  I then tried replacing the chuck and arbour (hope that’s the right word – the thingy with B16/MT2 tapers on it) supplied with the press with those which  came with the lathe.  The play was reduced by an order of magnitude which suggests that the problem is with the chuck and/or arbour.

The drills I am using are from a cheap (about 7 quid) set of 19 which I bought years ago from Screwfix, so that might not be helping either.  My general experience of tooling so far is that it’s very much ‘you gets what you pay for’. Which prompts my next question… is it worth shelling out say 50 pounds for a set of Dormers?  

Thanks again for all your suggestions and general advice on making holes in metal,

Regards, Robin. 

[mgjParticipant @mgj]

Mike – the angles, like any drill grinding are not critical. However you set, like any other drill a half of 118deg = 59deg (or 90-59= 31 depending on how the grinder is set), because one is going to grind the 2 halves separately.If you want a drill for sheet metal, then you’d set a different 1/2 angle of course.

 

Then in “pitch”  you set the secondary clearance. In the Quorn book I think it says about 25 deg. I do 20, simply because in upside down mode I can only get 20. Anyway, enough to ensure the heel clears.You grind both tip faces till you have cleaned all up, and then a quick clean at the same infeed.

 

Then set the primary clearance of 10deg., and grind that. The trick is to grind only so much so that the back edges of the primary lands intersect – exactly – its quite easy to do by eye. What that does is give you the effect of two chisels edge to edge, and exactly cenred. Importantly, at the join there is a tiny point, so in effect you have a built in centre drill and thats what makes them run so true. And when sharp, they cut so fast that in brass you have to blunten them – because unlike a commercial drill bit, there is no chisel point – which isn’t a chisel point, its a graunching point. (Zero rake for brass is the technical term, but blunten them is what you do!!)

 

Sorry but I don’t have a pic for you, but I think Harold Halls book on tool sharpening has the detail and also a method of making the holder to do the job. (The same holder also does endmills and slot drils as well)  Its worth doing, because they are far better and sharper than brand new ones and particularly good for small lathe owners where power is at a premium.

 

I don’t know the articles Chris is talking about,  but I’m sure they’d be perfecty good too.

[Rob ManleyParticipant @robmanley79788]

Is having a decent set of drills in a workshop a must – yes.  I bought two sets from J&L, both hertel and going up from 1mm – 6 and 6mm-10 in 0.1mm increments.  They are the first thing i bought after i got the lathe because i could not do without them.  The metric set allow me to get as close to any size hole before reaming / getting that magical 75% engagement for tapping etc. It also means that i can go up in 4 thou steps if a hole needs opening out + use any imperial drill that comes in between.  

 

I don’t think ive used an imperial drill under 3/8″ for a very long time and all my work is done in imperial measurement.   When a drill becomes chipped or blunt through excessive use – such as my ‘opening up’ favourites, they get replaced as my equipment is not accurate enough to re-sharpen them to cut true and to size.  Its not expensive and it happens very infrequently.  Plus, as the flutes are tapered the further down you go, the smaller the diameter of the drill. 

 

I have a chart i printed off the computer similar to the one in the zeus book but much more complete with all the number size drills, letter drills, metric drills, standard imperial and standard metric sizes then their imperial decimal equivlent and its the most used document in the workshop.  I think it was printed in the ME. 

 

I would suggest that you buy yourself a small and larger metric set from a decent brand and keep them in good condition – you’ll wonder how you survived without them…. my 2p

 

[MikeParticipant @mike89748]

Hi everybody:

Thanks to Meyrick for his very useful comments, and Chris for his support. This morning I have found a picture of a four-facet drill on a website – http://www.gadgetbuilder.com/DrillSharp.html if anyone else is interested.

Best wishes,

Mike

[chris stephensParticipant @chrisstephens63393]

Hi Mike,

The Gadgetbuilder drillsharp stuff is based on DAG Brown’s ME articles, if memory serves.

Another source is Prof. Chaddock’s book on building the Quorn. I think his work is what inspired the use of Four Facet drills in model engineering, previously it was mainly the preserve of industry.

chris stephens

[Geoff TheasbyParticipant @geofftheasby]

Hi,

 

I bought a cheap set of drills at 0.1 mm increments, and so far I’ve found two which were not sharpened properly.   One jammed immovably in a brass bush, and had to be thrown away.   The other I have tried to sharpen, but it is now a 2nd-rate drill, not regarded as accurate.

 

The trouble is, trying to build up a collection of tools is very expensive for the beginner.   I now need a set of parallels!

 

Regards

Geoff

[mgjParticipant @mgj]

The trouble with 4 facet sharpening is that its expensive, and now everyone on CNC uses specialist short spotting drills and that sort of thing. I much prefer stub drills for almost all drilling – they don’t wander.

 

As you say Chris – that’s where I got into 4 facet sharpening. However Harold Halls more basic jig and sharpening system would be better for most I think. The Quorn allows you to make your own reamers and spiral this and thats and spiral flute taps etc etc. But there is a lot of work and expense for facilites which most wouldn’t use. ( I built all the accessories and most remain pristine, brand new and unused from 15 years ago! Its only worth making them at the time to ensure that centre holes are all absolutely the same distance apart)

 

The sharpening process is pretty easy. The only problem with  these things is that you need some sort of degree scale – beyond the 45-90 -45 marks – for many designs I think. They are not difficult to do, but you do need a dividing head, or perhaps a rotary table. On that note, if anyone needs a blank divided and graduated, let me know – as long as it comes on mandrel ready to mount. To pop a blank into an arbor and hold in a collet to knock out 360 divisions is no great trouble, so long as it doesn’t become a flood.

 

As for spending ones money on a set of Dormer or Osborn drills. Well a set of drills going up by .1mm is a good thing to have, but as model engineers do we need quality? If one has sharpening faciliites I’m not sure one does, unless one is getting into drilling very tough stuff. If I had to choose, I’d rather put my money into a good quality chuck and taper.

 

Geoff, I may be teaching my Granny, but if a drill jams in a brass/bronze bush, almost certainly it was too sharp and it pulled forwards. Zero rake for brass and copper and bronze if you don’t want jam ups.Sure it could have been some other problem, but probably it was used out of the box? (Yes, being properly blunt generates rubbing and overheating but some kind of coolant or RTD solves that)

Edited By meyrick griffith-jones on 11/10/2009 13:45:25

[Rob ManleyParticipant @robmanley79788]

I understand what you are saying but i always believe that if you are going to buy something – make sure it’s the best quality you can afford and its not normally that much more than the poorer stuff (poor uni student so cant spend loads either – violin to follow 😀 ). I have found that cheap stuff is generally rubbish and gets thrown away pretty quickly – making it a total waste of money.  £44 on a set from 1-6mm is nothing and will last longer than i will.  When drilling brass / copper / bronze i always tend to lightly close the clamp on the quill/tailstock barrel to prevent it snatching but on lathe tools +1 on meyrick zaro rake.    

 

Re Geoff: the poor drill set strikes again!  Not the most expensive but if its good enough for industry and its a well recognised name then its good enough for us:  http://www.mscjlindustrial.co.uk/DAA-25050H/SEARCH:KEYWORD/product.html  Oh and i can recommend these for parallels – expensive maybe BUT you will never need another set – ever.

http://www.mscjlindustrial.co.uk/JLP-21040E/SEARCH:KEYWORD/product.html

Edited By Rob Manley on 11/10/2009 14:11:57

[David Clark 13Participant @davidclark13]

Hi There

I don’t have a budget for paid for articles.

regards David

 

[Robin GrahamParticipant @robingraham42208]

Thanks again for all your advice – though some of it was way over my head.(4-facet sharpening. Eh? So much interesting stuff to learn!) I am now convinced that a decent set of drill bits is a necessary investment, and shall present my Case for Support to the to the purse-holder for urgent consideration.

Regards,Robin

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