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John Stevenson
01-24-2009, 07:01 PM
Continuing from the earlier thread on making mistakes a couple of posters recommended using spotting drills instead of centre drills.
I do use spotting drill on the mill but never thought to use them on the lathe.

I want to post a couple of pictures to lay the scene on the why of why I got used to using centre drills.

http://www.stevenson-engineers.co.uk/files/tdrill1.jpg

Normal shot using a drill in a drill chuck.

http://www.stevenson-engineers.co.uk/files/tdrill2.jpg

Shot of using a centre drill in a custom holder, note that although this is a staged shot because of the short holder the tailstock is hard up against the saddle and the ram is quite extended.

It is because of this that I made these ages ago on my other lathe with MT4 taper.

http://www.stevenson-engineers.co.uk/files/tdrill3.jpg

As you can see the blank is extended to make up this shortfall.

It's because of this that I have come to rely on centre drills having got the 4 most popular setup this way to save time on machining.

That's my excuse and I'm sticking to it <g>

So today I decided to do something about it.

The problem is the import Morse stubs are too short, shorter in fact than the one in picture 2 so I set to to make a few, an hour later I had the 4 following ones. [ top pic ]

http://www.stevenson-engineers.co.uk/files/tdrill4a.jpg

Four long series MT2 arbors with the earlier sample one I found and the standard import arbor. The pip at the end is to mimic the tang so the self extractor can push it out.

Don't get excited about the 1 hour to do four because I decided to use the little Sieg CNC lathe and the taper wizard, better than all the setting up and working for a living <g>

Bottom pic shows the new spotting drills in the new arbors.

.

wierdscience
01-24-2009, 09:19 PM
Looks good to me,I never really liked center drills ,even for drilling in centers.

That lathe,what make is it? The tailstock casting doesn't extend out over the carrage very much.

BobWarfield
01-24-2009, 10:55 PM
Interesting.

Not sure this would be faster for me than swapping bits in the keyless chuck.

But definitely interesting.

Cheers,

BW

Spin Doctor
01-25-2009, 12:07 AM
I've always found center drills problematic in the lathe for a couple of reasons. One, if the TS and spindle aren't dead nuts then the center drill is already walking. Two, on larger machines the shear power of the machine and the usual lack of a two speed tailstock means it is all to easy to overload the Center Drill resulting in a broken tip. Three, unless care is taken to bring the facing tool to center and then file the face of the part you most likely will have a small tit there. Small CD+tit="OH %$@&". One trick I used to use on stock that was sawn but not faced was to take a center punch* and place it in the drill chuck. With the spindle off run the punch into the work to mark my starting point.

* I made my own out of die punches in the surface grinder with a spindex. Plus for center drill I prefer this type http://www.use-enco.com/CGI/INPDFF?PMPAGE=55&PMCTLG=00. The center is less likely to get dinged up. Important when doing a lot of trning between centers or OD grinding

Dawai
01-25-2009, 02:09 AM
http://www.mytoolstore.com/klein/59007.jpg

Ese are kinda pricy.. but in a tailstock marking center they don't wobble, or break.. put a nice divot dead center.. or close.. too thick to bend off on a tangent.. I have used them in the cnc to center drill holes too.. overkill..
The single or dual flutes don't pull them off on a angle..

Nahh.. none of you real machinists probably have ever saw one? I am a electrician, what do you expect? They were made for punching a hole in light sheet to save the 1st knockout slug pull.. Go directly to the big bolt and larger Ko punch..

John Stevenson
01-25-2009, 06:17 AM
Centre drills are an abortion anyway, I have to use them doing a lot of long supported shaft work but The way we work has evolved over the years but the centre drill has evolved the opposite way.

Rewind 100 years, Tailstock centres were hardened fixed centre, usually lubricated by white lead and run at slow speed.
Revolving centres were basically unheard of, the pilot on the centre drill was there to provide a lubrication pocket for the white lead.

Now fast forward 100 years and the fixed centre is virtually extinct having been replaced by the revolving centre so the pilot pocket isn't needed, as long as the centre can reach the end of the taper.

However have you noticed that the pilots on modern centre drills are getting longer ?
Presumably so you get a few bites at the cherry re-sharpening them ?

That doesn't make economical sense then because of this long pilot you stand more chance of breaking one and scrapping it instead of extending it's life :rolleyes:

At the price they are they should be classed as consumables. I usually buy them in 10's, works out cheaper but I do this for a living.
In the past I have reground all the pilot to half their original length from new, this till allows one or two touch ups before being scrap, I never attempt to do the cutting flutes, times too short.

The current batch I have has never been ground, obviously too busy / forgot / Altzhimers kicked in [ delete as required ]

Why can I never remember how to spell Altzhimers ?

What was I writing about ?....................................

Teenage_Machinist
01-25-2009, 01:41 PM
Well, you just about got it.!


I got a revolving center for christmas and it is way better. Never broken a center drill yet though.

nheng
01-25-2009, 01:52 PM
To adapt an unrelated phrase from an electronics article I read last week:

There are two types of machinists, those who have broken center drills and those who will :D

Den

vinito
01-25-2009, 01:54 PM
Why can I never remember how to spell Altzhimers ?

We always call it "Old-Timer's". I forget who said it first.

Oldbrock
01-25-2009, 02:04 PM
When you are nearly there it's called Partstimers. Peter

BadDog
01-25-2009, 02:07 PM
I've wondered the same thing. Other than the rare use of a dead center, I think the only excuse now is get clearance for the (nearly) dead sharp point on many live centers. Of course you could do away with that point, but then what do you do when you need to mount a 1/4 (or less) rod on centers? If the point is small enough for that, then it's not likely to fit in a center mount hole drilled with a big center drill on that 3" shaft. They could make them with an exaggerated web like a spot drill, but that plays the devil with resharpening.

Teenage_Machinist
01-25-2009, 02:11 PM
I say that it should be a pretty sharp splitpoint, and then a slightly blunt center

Alistair Hosie
01-25-2009, 03:42 PM
How did you make the tapers so accurately John they lok great?Alistair

Teenage_Machinist
01-25-2009, 03:54 PM
He said he used CNC.

2mt is short enough that a compound slide may be usable, as told in the Making your own Arbors article.

motorworks
01-25-2009, 05:07 PM
John
Not to steal but I suggest a magic chuck:
I have several set up for various jobs
including gun taps.
I have two size holders, 1 MT#4 and 1 MT#5 so
they fit all 3 manual lathes.
Change over is really quick
One repeat job consists of spot,tap drill,chamfer,and M9-1.25 tap-20Mmm deep
Tool changes and all I can do it just over a minute.
(Never broke a tap yet,,,knock on wooden lathe :)


http://img.photobucket.com/albums/v22/motorworks/pg363-1.gif


http://img.photobucket.com/albums/v22/motorworks/tools4.jpg

John Stevenson
01-25-2009, 05:26 PM
Nice Eddy,
You need a longer stub on the centre drill to save all the winding out, better still someone suggested using spotting drills :rolleyes:

I did have my eyes on a set of these, some company sent a sample over to the importers but he decided not to carry these. In the meanwhile they disappeared.

Alistair, the tapers was done on the little Sieg KC4 CNC lathe, I can do them manually but it seems silly to have this machine that is so easy to program, just fill boxes on screen and press go, not to use it.

The CVA has a taper turning attachment but it's set and I don't like to disturb it as it's set for an ongoing monthly job and the topslide or boring head taper turning attachment is harder to setup then pressing the big green button.

.

oldtiffie
01-25-2009, 06:57 PM
Centre drills are pretty well an anachronism as John S said. In some ways they always were.

As I am firmly in the "old time(rs)" demographic, I can say that a centre drill is not and never was all that it was cracked up to be.

It was supposed to be "self centre-ing" and to some extent it is - but not always and not always accurately either. Its saving grace was an accurate 60 deg counter-sink.

The pilot and the main "counter-sink" have zero back/side rake and other than some other countersinks and "special" drills, are about the only ones that will have such a small rake. Because of the small (zero) rake, the tool will not only get blunt faster, but will require a lot more feed "pressure" and a slower speed than a plain HSS drill with normal back rake that has been ground as a counter-sink.

The points have been made that a "tit" left on a faced surface will tend to throw the centre-drill off-line which in the ideal state will be resisted (and hopefully corrected) by the tail-stock quill - both of which will put undue lateral strain on the centre-drill generally and its pilot or "point" in particular. The transition point between straight pilot and tapered 60 deg is a sharp corner and a considerable "stress-raiser/concentrator" with all the requirements for a sudden "snap" of the pilot. The zero rake centre-drill is a form tool and has the same problems as any other form-tool in the tool-post - screwing tools, radius tools and the like included. If you have to take it slowly and carefully and use lots of cutting oil there, then on that basis there is good reason to take similar care with a centre-drill - for more or less the same reasons.

If I want to use a centre-drill as a pilot, there is no reason why I need the small pilot on the centre-drill at all as the 60 deg part is quite stiff enough to use as a "pilot" or "spotting" drill in its own right, on its own.

If I really do want an accurate centre for use on a tail-stock centre - "live" or "dead", then I will either use a small boring bar in my tool-post, set my compound/top slide off by 30 degree and bore it out, so that I know it is a true as my lather and my set-up. I can also grind an old centre-drill so that the 60 degree-forming edges have 5>10 degree side rake, put it in a holder in my tool-post and use it as a form tool in the tool-post. It is quite easy to grind the side rake - just do it "by eye" on the side or front face of a pedestal drill or use a "Dremel" or air-driven "die-grinder". "Touching-up" is just as easy.

I am also amazed that people will pretty well expect that any drill - including centre and spotting drills - will "wander off" on a drill press or a mill spindle but just ignore it when the job is spinning and the drill is stationery in a lathe.

The same applies to all that I hear about how important it is to "split" and relieve cutting edges etc. etc. and how special tools are needed for HSS drills and drilling in everything but with centre-drills - why is that?

This is one of those things where everybody seems to do it the same way "because it always been done that way" and/or "that's what I was taught" etc. etc. without questioning it.

I question just about everything to see why it is done as it is and whether there is any other way and if there is a better way. There isn't always, but it happens often enough to make the effort and the satisfaction well worthwhile.

I am continually amazed that some think that a drill chuck in a MT in a tail-stock quill is more accurate than one - perhaps the same one - in a MT in a pedestal drill or a mill spindle.

When I use a centre-drill - yes I do still use them - I rarely break one - I have the tail-stock quill extended as far as I can so that the centre-drill can "float" and is pushed but not constrained by the tail-stock quill as it will "find its own level" and will self-centre.

The point has been made that shorter pilots are better and that is probably true. I sharpen mine and it works OK.

I hear a lot about how important it is to correctly sharpen twist drills - why doesn't it apply to centre-drills? And should it? I suspect that there are a lot of blunt centre-drills "out there" that are just put away and re-used "as-is" without conscious thought as to the consequences of using them when blunt. I suspect that in a lot of cases, end-milling cutters are in the same category.

"Dead" centres, while certainly in decline and replaced by "live" centres, are far from extinct. They are used extensively on cylindrical and Tool & Cutter grinders. They are used in the head-stock of a lathe when turning "between centres".

"Live" centres don't solve all the problems either. Next time you use one, bore the centre in a job to true it up, turn a plain section on it as well - both while still in the chuck so as to ensure that the 60 deg internal taper and the plain external parts are concentric. Now put your live centre into you tail-stock, run the job slowly and put a good dial indicator, first onto the body of the live centre and then onto the plain turned section of your job. The indicator should show zero deflection and if it does all should be OK, but if either or both show any deflection, then some of your assumptions may be incorrect.

Now repeat the excercise with using a centre drill (no boring of the 60 degree taper) and see if you get the same results.

There's a lot to think about with the common old centre drill that everybody seems to take for granted.

Perhaps the surprise is that it performs as well as it does more in spite of the use (and abuse?) it gets from the machine and the operator than because of them.

small.planes
01-26-2009, 04:46 AM
Center Drills are a lazy solution. The original method of putting a center in a bar (or other thing) was to drill with a 60 degree drill, and pilot with a smaller drill to give the point clearance + provide a pocket of lubricate, thus a 2 stage job. Center drills were invented to reduce this to a single op, and as with most advances, speed up production.
They were never (AFAIK) designed to 'self center'.
As for a drill being more accurate in a lathe than the mill, well, it is.
When you drill with a revolving spindle the drill can walk, and the self centering effect is small, as the axis of rotation is in the drill. When the drill is stationary and the work revolves the axis of the hole is in the center of the hole, always. As the drill tries to walk it would be forced to orbit, and as a result the centering effect is stronger. I can look the maths up if you really want, its in Chapmans Workshop Technology part 2.

Dave

jackary
01-26-2009, 10:36 AM
Just like many others I have also broken centre drills with disastrous results and did not manage to fix it with John S's ingenuity. I think the most common cause in my case was that the centre drill was not on centre good enough. So I fitted a dial guage in the chuck and rotated it round the centre drill body as shown in the picture to measure the eccentricity. I have a Chipmaster lathe which has a No 3 MT but most of my tooling is for No 2 MT so I have made an eccentric No2 to No 3 MT sleeve (about .015" eccentric) which can be rotated to get the correct centre height and is then locked in position. The tailstock offset is then adjusted to get the tailstock centre in line. I did this after correcting the tailstock barrel to be parallel to the lathe axis because it was sloping down slightly due to wear. It is 50 years old. So I now can used No2MT chucks etc.
and they are on centre, so hopefully I will break less centre drills!
Regards
Alan
http://s107.photobucket.com/albums/m287/jackary2003/?action=tageditmany

ahidley
01-26-2009, 02:26 PM
http://en.wikipedia.org/wiki/Drill_bit#Center_drill_and_spotting_drill