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Boring drilling

Not an exciting topic, I know. ;)

I'm a newbie and I want to basically replicate the mounting for my compound in a piece of cast iron. It is 1.250" in diameter and a little over 1/4" deep. Old picture from early stages of cleanup follows:

Atlas618 compoutnd mount.jpg

I can drill through the cast iron but I want to keep as much rigidity as possible. I have a couple of boring heads (HSS cutters) available and I think I can start cutting in the recess with a relatively small starter hole. I think a 1/2" or 5/8" through hole would still leave plenty of meet. I have drills up to 1" if needed.

How would you tackle this?

For bonus points, if you wanted to drill up to 1", what sequence of drill sizes would you use? !/4, 1/2, 3/4, 1"? More steps? Fewer?




Super User
Premium Member
I'll let others chime in on the drilling process...but I love your avatar. Very well done.

Welcome aboard Craig.


Active Member
Premium Member
The number of steps depends on the machine you have awailable to drill holes. If it is large, heavy and rigid with a controlled down feed, I would spot drill, drill 1/4” and then 1”. Bore to size (1.25”).

Drills are meant to remove material fast, so I would use them as much as possible before switching to a boring bar. How close to final size of a drill you use depends on how much oversize the bit makes the hole and how much “meat” you want left over for the boring process. Trial and error will show you how much you are comfortable with.


Ultra Member
Premium Member
I'm assuming that bottom plate is a separate chunk & you are boring a hole straight through (as opposed to its integrated and its a flat bottom essentially counter bored hole).
One set of tools I have really come to like for applications like this is annular cutters. They can take a large diameter slug out which you can then preserve for another project & get you closer to boring bar work without a lot of intermediate swarf. I bought an R8 and MT3 holder for mill & lathe tailstock respectively & just acquired cutters as I needed them, although there are probably better economics in a set. Of course, tools cost money but just mentioning for reference. I haven't used them in CI yet but see no reason why they wouldnt work.

If you are just trying to get this particular job done, progressive drilling is a perfectly good way to rough out stock before boring head. If you don't have the machine oomph you will probably have tp progress up in smaller increments.


Annular cutter...hadn't considered that. Between the arbor and the cutter, I'd be in for about $100 which is a lot for what may be one use. Besides, my tiny Atlas 618 only has a morse taper 1 in the tailstock.

For interest, though, I take it these cut a very precise hole? On size and as round as a reamed hole?

PS the latest KBC flyer landed in my mailbox yesterday. Their annular cutter is double the price: $60 for the 1.25" size.


Ultra Member
Premium Member
Yes, like I say, its probably more for the future reference file. The larger shanks for annular cutters kind of go hand in hand with common sizes on larger machines, more power etc.
They cut what becomes the hole ID maybe a tad cleaner than a drill. The OD of the core looks a bit rough as you can see by my pic, but its just crap grade mild steel. I'd think accuracy as good or better than a drill because the cutter is short, lots of rigidity & no real tendency to walk off center. No, its not a reamed finish. Its pretty nice depending on the material but wont replace a boring head. I seem to recall the bore was within a couple thou of nominal but cant recall which way.

I bought my first ones from KBC (TMX brand). The Amazon are redirects from Accusize, a name which made me suspicious. But they look & perform the same so far. An example of hit & miss offshre tools. Sometimes good value when its outsourced & QC'd properly. And sometimes you get burned with an inferior clone.


Well-Known Member
Annular cutters usually cut on size or a little larger. I'd cut 1/8" smaller size, and bore it for a good finish and making it on size.