Pre-reamer Drill Size(s) for 7x16 Mini-lathe

[MaverickNH]

I need to put a 1/2” hole 3/4” into a chucked/faced aluminum rod. I’ll press-fit a matching bronze rod with pin into the hole to make a hammer head.

I have 1/2” over/on/under reamers and 1/64” drill set, as well as center drills. I’ve found charts with recommended pre-drill/reamer combinations, with 31/64” recommended for a 1/2” reamer.

I figure going right from a center drill to a 31/64” drill might be a bit much for my mini-lathe. What intermediate drills are recommended?

BRET

 

[WobblyHand]

Probably about 1/16" less diameter than your final drill. Larger drills will wander less. If you start with a 1/8" drill the hole will wander off center more. Succeeding drills tend to follow the previous hole. So as big a size as you can, then a finish drill 1/64" under the final reamed size.

 

[RickKr]

I regard reamers as "sizing" tools only, so I like to get close to finish size so the reamer has only a little bit to take out. Because drills tend to wander, whenever I can, if I'm reaming a hole, I prefer to bore it to get it true, usually very soon before I ream. This becomes more difficult the smaller and longer the hole.

There isn't much room for chips in the flutes, particularly with smaller sizes. Chip build up can affect the how they cut and the finish. I like to get close to finish size so the reamer has only a little bit to take out, 0.010 - 0.020", maybe 0.030" on larger reamers.

 

[MaverickNH]

I see my smallest HSS boring bar is 0.32” width at the tip. Using such a tool, what’s the smallest hole I could reasonably expect to use the tool to make? The tool would physically fit in a 3/8” (0.3750”) drilled hole, but there must be a practical limit.

Should I drill & ream for a 1/2” hole, or drill, bore & ream?

BRET

 

[RickKr]

I see my smallest HSS boring bar is 0.32” width at the tip. Using such a tool, what’s the smallest hole I could reasonably expect to use the tool to make? The tool would physically fit in a 3/8” (0.3750”) drilled hole, but there must be a practical limit.

Should I drill & ream for a 1/2” hole, or drill, bore & ream?

BRET

To be safe, don't go smaller than the width of the widest part of our boring bar, and make sure it is lined up straight.

For what you are doing, a hammer, wander is likely not an issue, so you could easily drill and ream. For something more critical, drill-bore-ream.

 

[markba633csi]

Start with perhaps a 3/16" drill, then a 5/16" and a 3/8". Then you might could jump to the 31/64" then ream

 

[Smithdoor]

I need to put a 1/2” hole 3/4” into a chucked/faced aluminum rod. I’ll press-fit a matching bronze rod with pin into the hole to make a hammer head.

I have 1/2” over/on/under reamers and 1/64” drill set, as well as center drills. I’ve found charts with recommended pre-drill/reamer combinations, with 31/64” recommended for a 1/2” reamer.

I figure going right from a center drill to a 31/64” drill might be a bit much for my mini-lathe. What intermediate drills are recommended?

BRET

I bore before reading.
Sometimes the drill did not drill straight and boring fixs that problem.

Dave

 

[WobblyHand]

If it has to be straight, then you bore it. If it only needs to be pretty good, the drill with a a big drill, then one slightly larger to clean it up. The second drill should be 1/64" less than the final reamed value. Using lots of drills, like 4 or more steps, is not good for having a straight hole. The first hole wanders, because it is narrow compared to the hole depth. The subsequent drills will follow the first hole.

You will have the straightest hole (without boring) using my technique compared to slowly stepping up sizes.

Even a mini-lathe can drill 1/2" hole in stock - I've done it. Just keep the pressure on so you are cutting a chip continuously, and let off quickly, and start cutting continuous again. That way there won't be any work hardening. Use cutting oil and clear the chips. Don't let the flutes become packed.

Drilling a straight narrow hole can be difficult. The length divided by the diameter gives some measure of the difficulty. A ratio over 10:1 is fairly hard to keep straight, especially for beginners. Over 25:1 is in the territory of gun drills. Small diameter holes drilled deep are really hard to keep straight. (They have high L/D) If you really feel the need to step up, I'd go with a 3/8" drill minimum, then the 15/32". But if it was me, I'd use 7/16", then 15/32", then ream to 0.5000. Of course, you can obviously do as you please, but this is what I learned, and it works well for me.