Sloppy hole after drilling on lathe

[martik777]

Drilling 17/64" in 416, 6061 and resulting hole is ~.007 oversize. Tried 2 diff jobber drills

 

[benmychree]

Drills are not intended to make accurately sized holes, for that, one should ream or bore the hole.

 

[darkzero]

That can be a number of things. Quality of drill bit or bent, uneven length cutting edges, tailstock alignment/tailstock chuck concentricity. You can sort of check this by after drilling a pilot hole, run your drill bit up to the hole with the tail stock & slowly enage the hole, look closely to see if the drill bit deflects.

Even if it doesn't deflect, that won't guarantee anything, still a drill bit doesn't make accurate size holes, it just one of the more efficient/quicker was to make a hole. If you need an accurate sized 17/64 hole, drill undersize then ream or use a boring bar for final dimension.

 

[The Custom Crafter]

There are many considerations. The chuck holding the bit for instance. Starting the hole with a centering bit gives you a good start but does not garenty the tail stock wil lock dead center every time. Same goes for the chuck and bit in the tail stock. than just bump a 17/64 bit sideways while in said chuck and it is now bent. You will not see this bend when the bit does not turn. Happens all the time in my milling machine. I am moving materials around and bump the bit. When youy spin it up slowly the run out is visible. I keep at least 10 of the 17/64 bits in stock. Quality brand name cobalt. They do not cost much if you order 10 at a time. Seams I throw a new bit in the chuck every few days. The old one is still very usable in a hand held drill for less accurate work.

 

[markba633csi]

7 thou is about 2.5x the error I would expect- try drilling a pilot hole first one size smaller then run the final size drill through with cutting fluid or oil
I'm certain the result will be better

 

[martik777]

7 thou is about 2.5x the error I would expect- try drilling a pilot hole first one size smaller then run the final size drill through with cutting fluid or oil
I'm certain the result will be better

Yes, it was better with a1/4" pilot, but the 1/4" pilot hole was only about .001 or .002 " over, still don't know why a 17/64" was so oversized.

 

[Illinoyance]

Is the tailstock quill concentric with the lathe spindle? If not, the hole will be oversized and may also be eccentric.

 

[RaisedByWolves]

Yes, it was better with a1/4" pilot, but the 1/4" pilot hole was only about .001 or .002 " over, still don't know why a 17/64" was so oversized.

New drills?

They may have actually been too sharp.

The bigger the drill bit the more error there will naturally be, but depending on your technique this doesn't sound horrible. Is the hole concentric with the outside of the part on both ends?

 

[martik777]

yes it looks centered, just more play than I usually get with similar size bits

 

[great white]

I’ve always though of drilling on the lathe (or any other machine for that matter) as a “rough” pass. If I just need a hole, thats where I leave it. If I want something precise, I drill undersize and finish with boring or reaming…whichever applies best to the size of precision hole needed.