the best way to get a hole on center is as William points out but with the following additions; drill the hole undersize, then bore the hole close to size- this will get the hole running true to the axis of rotation, and then ream the hole to the exact size you want.
Deep Drilling In aluminum block
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It has been mentioned a couple of times that a rotating workpiece (i.e. drilling in the lathe) is less likely to produce a wandering hole.
Can anyone suggest how this could be true?
To my mind, the drill and workpiece rotate relative to each other - it doesn't matter which one is 'stationary' to the observers frame of reference.
Can anyone suggest how this could be true?
To my mind, the drill and workpiece rotate relative to each other - it doesn't matter which one is 'stationary' to the observers frame of reference.
If the drill or the milling machine and the lathe have the same hardness and precision.., I think the result is the same .
I have been hearing that all my life and agree there is no difference - however when drilling a long hole in say a milling machine, the tiniest tramming error produces a large error at depth - but that's an accuracy issue rather than something related to the physics of machining but I would suspect is the source of such a "myth".
Gun drill machines generally rotate both the component and the drill but I suspect that's more to do with assisting swarf clearance (coolant is also pumped down the gun-drill to flush swarf).
As I have mentioned elsewhere on this forum that there can be a corkscrew to the grain structure down the centre of barstock - particularly spun-cast cast iron which can create dreadful drill wandering problems.
Regards, Ken I
mcostello
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What I have heard is that when the part revolves the drill will try to center itself up on the axis of the part. Makes sense. When the drill turns it will center itself up on it's axis. Gundrilling uses the best of both methods.
But, if the hole centering isn't correct, it's because the tailstock isn't accurate and includes the drilling method, then you have a non-straight hole.
One thing that is hardly talked about is speed and feed (I'm also researching it on the internet).
All CNC machines can drill , boring ..1 holes with high precision on fixed workpiece , which proves that with a manual drilling / milling machine and lathe with the same hardness , precision... then the result is the same and depends on your making and use.
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DON'T confuse a 20,000 pound CNC mill with what you or I have at home (I have a 3500 pound mill with 6 HP spindle, it will drill straight for a while).
If you rotate the drill, any error becomes a misalignment along the axis of the DRILL'S ROTATON, and you go off center.
If you rotate the work, any misalignment becaomes an error along the axis of the WORK"S ROTATION and as the work rotates that error moves around in the work and therefor has a decent chance of averaging out and you stay on track. Of course this presumes that you made the desired hole center the actual axis of rotation of the work.
I'm not confused
With a CNC machine, Spindle has the ability to move in many directions like the video below
With a manual milling machine it is easier , the spindle part is usually fixed or fixed after adjustment and it will be exactly where it is , and if the machine body is rigid / strong enough and the parts are Its other is correct and make it right then borehole is correct for need
It doesn't matter to me fixed workpiece or rotating workpiece , because I have done both , depending on my needs , capabilities .... of machines , tools ... and I will choose which method is the best with workpiece
With a CNC machine, Spindle has the ability to move in many directions like the video below
With a manual milling machine it is easier , the spindle part is usually fixed or fixed after adjustment and it will be exactly where it is , and if the machine body is rigid / strong enough and the parts are Its other is correct and make it right then borehole is correct for need
It doesn't matter to me fixed workpiece or rotating workpiece , because I have done both , depending on my needs , capabilities .... of machines , tools ... and I will choose which method is the best with workpiece
The can of worms rule "the only way to reclose an opened can of worms is to use a bigger can"
I also forgot to mention that gun drilling normally has a whole bunch of intermediate guides for the long drill which can suffer from "crippling deformation" (as per Euler - Rankine) which will cause it to wander - again by rotating both you stand a better chance of evening out the errors introduced by gravity, drilling forces and the Euler - Rankine deflection crippling of a "column".
Also the shanks of gun drills are typically smaller in diameter than the hole and are long and slender enough to buckle (ER rules again) and therefore must be supported at intervals.
When drilling deep hole as I showed above, as you would for one off's and hobbyist projects - you drill and bore using jobber length drills - followed by a long series followed by ultra long / extended drills which are adequately guided by the short(er) prior hole depths etc. etc..
So I still stand by my "no difference" comments - gun drilling being an esoteric exception.
Regards, Ken I
I also forgot to mention that gun drilling normally has a whole bunch of intermediate guides for the long drill which can suffer from "crippling deformation" (as per Euler - Rankine) which will cause it to wander - again by rotating both you stand a better chance of evening out the errors introduced by gravity, drilling forces and the Euler - Rankine deflection crippling of a "column".
Also the shanks of gun drills are typically smaller in diameter than the hole and are long and slender enough to buckle (ER rules again) and therefore must be supported at intervals.
When drilling deep hole as I showed above, as you would for one off's and hobbyist projects - you drill and bore using jobber length drills - followed by a long series followed by ultra long / extended drills which are adequately guided by the short(er) prior hole depths etc. etc..
So I still stand by my "no difference" comments - gun drilling being an esoteric exception.
Regards, Ken I
One of the things missed in the quotes is the drill sharpness! If the two cutting lands are not of equal length or identical angle then the drill bit will not drill an on size or straight hole, always slightly oversize, unless it is only taking a final cut after a slightly undersize drill is used first. If both cutting lands are not cutting equally then the drill will have a tendency to wander particularly in smaller diameter drills with less stiffness. So for accurate and straight holes make sure the drill is correctly sharpened i.e. angle equal and of equal length John
All the above are true, but if you really want a straight hole then I believe one of the best ways to obtain this is to follow all the above mentioned advice but stop by drilling the hole undersize. Then bore the hole to the size you want or if you are looking for an exact size hole bore the hole to accept a reamer and then ream the hole to the final size. The boring will make sure that the hole is now true to the center of the lathe and straighten out any run out. The reaming will bring the hole to the exact size you want, assuming the reamer is the correct size. It takes longer to explain than to do and you wind up with a truly on center hole that is starlight and on center.
I've always drilled smaller, then either finish drilled size or reamer
Indeed - re prior two comments.....
Rule #1 - never trust a drilled hole (where possible)
Rule #2 - bore before reaming (where possible)
As in this sequence from my 6-Shooter Elbow Engine :-
This posted over 10 years ago.
Regards, Ken I
Rule #1 - never trust a drilled hole (where possible)
Rule #2 - bore before reaming (where possible)
As in this sequence from my 6-Shooter Elbow Engine :-
This posted over 10 years ago.
Regards, Ken I
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