Drilling a parallel 1/2" hole

I was pottering about in the workshop today and decided to make a tail stock die holder.

I mounted a 11/2" diameter piece of EN8 steel in a four jaw chuck and set it up to 1 thou TIR.

Faced the end off.

Centre drilled the end.

Drilled it through with several pilot drills.

Drilled it to 1/64th of full size.

Reamed it to Full size if 1/2".

Now the problem, If I spin the finished item on it's central axis, The end that had been away from the Tailstock has a 8 thou run out, should I have drilled it from both ends prior to putting the reamer through it. ?

How long was the hole. Better to use 3 or 4 flute core drills to open holes out as 2 flute will wander.

The hole was nearly 3" long.

Not sure from your description if the 3" piece was internal to the chuck jaws or outside. If outside the jaws, your error would suggest that you did not clock both ends of the visible piece.

You could make a mandrel on which to mount your 3" piece, then turn the outside to be parallel to your reamed hole.

At 6x diameter the hole too deep to drill accurately. As Spurry says mount on a mandrel and re-finish the od. I also wonder why the hole needs to be reamed all the way through. If the hole is for a holding spindle, fit the spindle and mount the assembly in a chuck to bore the die bore.

It seems like you started off with a fairly small drill which would be very prone to wander over 3" the next drills would just follow the first. you might have got a better result drilling with the 31/64" drill only. For best results drill 15/32", bore out to 31/64" or .490" also not easy over 3", then ream.

Chris Gunn

I use the biggest drill I can to start a long hole specifically so that it is as rigid as possible to minimise the chance of wander. I have a number of "pilot point" drills that have are rather like 2-flute slot drills at the end except for a short split-point bit of reduced diameter to start the drill off in the right plave. They need no more than a minimum centre drill to start and drill very fast and true. So for a 1/2" hole I'd start with an 11mm drill so I only had 1.7mm for the second drill to remove.

Given that the 1/2" is presumably for the guide rod that the holder will slide on, you could bore it out to say 13mm and get a piece of 13mm silver steel which I think is a standard size, rather than use 1/2". Boring will endure that the hole is concentric to tha lathe axis.

Spurry The majority of the 3 inches were located within the chuck. As I started off with 3/16" pilot drill this where to problem started. As for reaming the total hole. It seemed easier and more accurate than using a smallish boring bar. Next time I will start off with a near full size drill.

Well, I can drill a 21 thou drill to 7/16th depth in the lathe which is 20 times drill diameter to depth with it being perfectly accurate and true...

Something is wrong with Peter's set up. I dont know what it is.

My homemade tailstock dieholders have a 5/8" register for 2MT.

Cheers,

Julian

Indian made 'budget' 4 jaw by any chance?.....checked the jaws for parallel internal surfaces?

Should have been bored clean, hence concentric with OD, prior to reaming, or finish bored.

Trying to drill, starting with a flimsy, flexible, drill is flawed. One must start with a rigid cutter such as a lathe centre drill. Further, the tail stock will never be perfectly centred vertically and horizontally (even if well within the machine specs). Starting the hole from an off-centre point will invariably cause some misalighnment of the hole. It may well be straight, but not necessarily parallel over the distance the OP expecting.

One of the reasons why model engineers bore cylinders. One of those very good reasons is to achieve parallelism.

Another ploy by those same people is to straighten pilot holes by drilling with a stiff/rigid end mill and not a twist drill (which will tend to follow the pilot hole).

'Nuff said?

+1 for Chris Gunn's comments.

Your smaller pilot drills, even though started with a centre drill (a spotting drill is better), will wander unless they are sharp, you clear the flutes often and don't use too much pressure.

Jon

To be able to drill straight and true through shafts this useful publication
from Camden books is worth looking at. "Making rifle barrels",it is about
drilling long straight holes ,some of it is about using gun drilling which would
probably be beyond the capability of most model engineers workshops but
there are sections giving advice on conventional drilling which result in
drilling straight holes.

I have used these methods for some of the examples shown here in the second photo.
The shaft seen here in the lathe is 8 5/8 inches long and drilled through
at 14 mm (.551 in) .Using 2 extended drills a smaller diameter pilot drill
has a bored hole formed to be a close fit at least 5x drill diameter and the hole
drilled at least another 8 to 10 x drill diameter ,the 14 mm drill size is also
bored a close fit and drilled to open the hole leaving at least 5 x depth
of the smaller diameter so the hole depth can be advanced.By changing drills
the hole is drilled right through.As can be seen in the photo a good supply of
neat cutting oil helps by cooling and lubricating and washing the drill and hole
of all swarf ,the drills should be withdrawn often to to clear the swarf and avoid
crowding the flutes.Two similar shafts were made one came out on centre the
other disappointingly was .002 inch off centre but probably within the accuracy
of the 3 jaw chuck. At full depth the hole is only 16 x the drill diameter.Using the
hole as the reference the shafts were re-centred and the finished from there.

The third photo shows one of 3 internal grinding spindles at 6 5/8 inches long
and drilled through at 7.3mm dia ,the hole depth is 22 x diameter .As can be seen
the shaft is held in the 4 jaw and supported by the fixed steady using a similar
sequence as previously the holes were drilled ,one as before was on true centre
the other .002 inch off ,the photo shows the last part of the sequence the shafts
having been referenced from the hole re-centred and rough turned to true up
to turn the 1 morse taper in the nose as in the photo .
The drills are sharpened as 4 facet drills ensuring the cutting edges are the same
each side.
The moral of the tale is if you start off with true hole the drill will most likely
follow it through.

John

SOP is to drill the hole well undersize to quickly remove metal in a rough manner. Then bore the hole to slightly undersize to ensure the hole is parallel and concentric. Then finish to size with a reamer. For a 1/2" hole, drilling to 7/16 or 15/32" then boring to 5 to 10 thou undersize should work OK.

If you don't bore the hole, the reamer will follow the wobbly drilled hole, as you have found out.

What sort of boring tool would you be using for this type of procedure?

For a hole less than 1/2" I use a tool like aD-bit ground from HSS

For 1/2" and above I just use one of several small standard boring bars.

... and yet, said tailstock will form the reference axis for the tool when its finished and being used to cut threads.

I have serious doubts about the emphasis in this thread on perfection of the hole size/straightness for something as prosaic as a die-holder. For everyday threading, a tiny amount of wobble isn't going to be noticeable (look at the average tap wrench). If significant accuracy of a thread is required on occasion, then cut it.

Thanks Neil.

Pete

+1 on this, but I grind the tool something like a D bit but for a 1/2" hole I would probbly use a piece of 3/8" or maybe 5/16" round HSS. It is then held so the shank is at an angle of say 5 or 10 degrees from parallel with the lathe main axis. Thus it is cutting as a boring tool on the front lefthand corner (as viewed from the tailstock) rather than drilling as a D bit.

Probably with a 1/2" hole almost 3" deep there will be a bit of chattering going on. Just take light cuts at teh finish, running the tool through job at the same setting several times until no more cutting is taking place. The reamer will take care of any chatter marks. All that matters at the boring stage is that the hole is basically concentric and straight; surface finish not important.