Accurate lathe drilling

[Reggleston]

Yesterday while drilling the end of a drill extender for an engine project I Center Drilled the work piece and then drilled with the diameter drill that I wanted to hold in the extended length shaft. I didn't CD enough diameter to fully accept the tip of the drill size I was using and noticed the drill wandered a bit before fully cutting with the end result being the hole was oversize and a sloppy fit for the #21 .159 drill I wanted the .272 dia. drill rod to accept.

Redoing the CD with additional depth to just accept the drill tip yielded a much better and tighter fit hole to give a good fit to my drill extended to allow drilling a 8+ inch hole position on a vertical steam engine project I am working on.
Probably not new information but something I find use full for accurate drilling of small sized holes in the lathe.
I find this method of building drill length extenders economical as well as providing a more rigid shaft than a long length drill.
Keep chip cutting!

[Stuart]

IMHO
a drill is at best a roughing tool , not on size or straight

To do what you are doing drill undersized , bore to true the hole up but leve under size by a wee bit then ream for a good fit


The problem is worse if it’s not a new drill , a centre drill is the wrong tool to start a drill ( it’s for drilling centres ) use a stub length spotting drill with the same tip angle as the drill ie 118 deg.


Yes you tip for making long drill is great I have done it many times , don’t try it with a numbe 80 bit though ( dont ask how I know)

[zeeprogrammer]

I found this useful and it clarified some terms for me.

I find it odd though that center drills seem ubiquitous when it comes to starting a hole.
Even 'The Home Machinist's Handbook', as well as various construction manuals I had for models use a center drill.

Are center drills simply easier to obtain or less expensive than spotting drills?

A very quick search indicates center drills are quite a bit less expensive.

I also see that the selection of 118 deg spotting drills is 3rd at Enco. That is, the selection for 90 and 120 deg spotting drills is massive in comparison.
61 for 118 deg. 556 for 90 deg. and 434 for 120 deg.

How much does it matter if the depth of hole is not that much? For example, 1/8" (3mm) to a depth of 3/4" (19mm)?

[EDIT] A further question...
To what depth (or diameter) is a spotting drill used? If diameter...to the diameter of the intended drill bit? Or something less?

[john mills]

if you try to use centre drills in a CNC machining centre you will quickly find out .use spotting drills ,i have used only 90   degree .when i started to use spotting drills nearly 40 years ago i had to grind them then now you can buy them .i was  usually machining hot work tool steels centre drills just didn't work just like the teacher told us
when i  did a certificate course in cnc programing at night school soon found out when i tried at work .Ground up 90 deg  from very short drill , have only used spotting drills since.

Only need to drill bigger than the chisel point on the drill.

[Stuart]

Zee

Have a look here

<a href="https://www.youtube.com/watch?v=Ot8wPGQW3JI" target="_blank">http://www.youtube.com/watch?v=Ot8wPGQW3JI</a>

Ok he is OCD about precision but his content is good

Stuart

[Roger B]

I had fun with this during my fuel injection work. It is mostly documented on here:

http://www.modelenginemaker.com/index.php/topic,6562.0.html

I have made extension drill holders for 0.35mm and 0.5mm drills for my fuel injector nozzles

[zeeprogrammer]

Thanks Stuart.  That explained a lot and is much appreciated.

The 1st eight minutes compared a center drill, a 90 deg spotting drill and a 120 deg spotting drill.
It was a good explanation of why a center drill can be problematic (although you can still use it if you only go to the very tip depth).
He didn't mention (as far as I can tell) using a 118 deg spotting drill but his opinion was the 120 deg was best for spotting holes.
The 90 deg was good for countersinking.

So I've been doing it wrong all this time. I'm hoping this explains why I've had such trouble getting pistons, head covers, and cylinders to line up.
I had been using a center drill to a depth where the 'countersink' could be seen. 

Thanks Roger. It seems I remember that but I'll go back through.

[Mcgyver]

So I've been doing it wrong all this time.

How are you doing it?  (I didn't watch the video - you're a better man than I if you can get through 27 minutes of centre/spot video lol)

In few sentences, spot drills are preferred for starting holes.  Don't drill a hole, just make conical shape in the surface.  That's what the drill starts in - if you make a hole, with SP or CD then the drill can grab on the edge, you a get a bit of chatter and possibly the three lobed start.      Using a centre drill is perfectly fine IF you just use the end to make the cone shape, however you need large ones to make a good size cone.  I prefer 118 degrees, 90 or 82 fail to achieve the objective, a cone the drill fits in, so the drill doesn't catch on the lip.  I get the 120 argument, but imo its gilding the lily.  We're drilling here, its not precision work, on the angle or slightly obtuse imo won't matter.  Centre drills have been used to start holes billions of times: it works.  The spot drill just has a few features making it slightly the better tool for the job.

To the OP, we've waded into spot drills.  That mostly affects the accuracy of where you hole is, ie where it starts, not its dia.  yes the drill can bounce around the lip a bit, but its momentary.  Drill size is determined by the grind on the drill.  Lousing grind and the drill will drill oversized.