Drilling Holes in Rod on my Lathe

DoctorBill

DoctorBill

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I am an amateur - perhaps what I am asking about is a dumb question to most machinists.

I am experiencing something strange when I drill holes in Threaded Rod to make a Reloading Die (sizer)
using my 9" ENCO Bench Lathe.

I drilled a starter 'guide' hole thru a 1 inch long piece of threaded Rod with a 1/4 inch drill.
The hole measured 0.249", as expected, with my calipers. No problems.

I was going for a 0.318" hole.

So - I put an "O" drill in the chuck and drilled it out. Came out 0.326" - not 0.316" !!!!

I tried AGAIN, but this time I used the smaller 5/16" drill into the 1/4" pre-drilled hole....
it came out 0.324" - NOT 0.313" !!!!

So - I thought my Lathe is out of alignment (Runout). But the Main Chuck lines up
with the Tailstock Chuck (using very small center drills in each). ? ? ? ?

NOW - here's the rub....I tried AGAIN with a piece of threaded rod, again pre-drilled with
a 1/4" drill. THEN I enlarged the hole one letter drill at a time until I got to the
"O" drill... NOW the hole measures 0.318" - What I wanted to begin with !

Does anyone know why the first two holes would come out seven thousandths larger (or more)
than they should have ? I cannot see why that would be. The drills check out in size with
my calipers.

DoctorBill
 
Really tough to get an on size hole using a twist drill. A safer bet is to get it close( but undersize ) with a drill, and then either ream it, or use a boring bar.
 
Just googled 'DIY reamer' and had a bunch of hits. Some use music wire for smaller holes, maybe you could grind a drill bit after chopping off the fluted section ?
 
DoctorBill said:
Money limits me.

DoctorBill
Click to expand...
You say money limits you, so let me ask are your drill bits bought on line, or low cost ones from a big box store? If so they are probably made in china and not very accurately ground. this will cause them to drill oversize.

If you hand grind your drills, check that they are accurately ground on center with correct cutting edges and equally sharp, or get them machine sharpened.

I have even found that drills sharpened with some popular hobby level sharpening tools or jigs are not always correct.

You can help a drill to cut on size in a pilot hole by carefully grinding a radius on the outer tips of the cutting edges, use a radius guage to keep both tips the same. this will act as a poor mans reamer.
 
Doctor Bill
Can you make a D reamer? Need a grinding wheel and a hone and some elbow grease. Low cost using some drill rod or other hardening steels.

www.hobby-machinist.com

Making D Bit Reamers

I have been trying to make some reamers recently and I am having some trouble getting the geometry correct. The most info I have been able to find just says to turn a rod to the right diameter, then cut it in half. I tried that and it didn't go very well the first time. I had trouble with it...
www.hobby-machinist.com www.hobby-machinist.com

Pierre
 
You didn't make any mention of using a spotting or center drill to start your hole. I would first face the end of the threaded rod then use a spotting drill to provide an accurate starting point for your twist drill. The end of a twist drill is not pointed and will tend to wander when trying to start a hole on a flat surface.
Food for thought.
Ted
 
Good quality USA made high speed steel drill bits cost money, no way around it. Even with good drills, "up-drilling" by a couple sizes at a time will give a smoother hole that's closer to the proper size. Consider buying a small collection of reamers also when you can afford it, for the most common sizes.
Use some type of cutting fluid when drilling steel- even old motor oil is better than nothing- your drills will last longer
-Mark
 
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