Insert drilling, off center?

I noticed something about doing this with insert drills in a recent thread but I ain't too clear on it as I never tired it myself yet but, can you actually drill with a bit of an X offset to get a bigger hole (lathe operation) with Insert drills? I know you can rough bore with them a bit, but say you got a .750" nominal drill (korloy or sandvik) and want a .790" hole in one shot with it, without boring it out, could it? seems like it wouldn't have much room to offset before the center insert gets chipped if its not pretty well on center?

Obviously yes, you find out with mistakes . But some Drills are specifically designed to do it. Kennemetal's DFT line does. The Drills with Trigon inserts come pretty far off center, but the ones with Square inserts aren't designed for it.

R

I think most if not all insert drill manufacturers usually list the offset value per drill body on their book/site/catalog. At least sandvik and tungaloy had.

It is different for each body, and needs to be checked before you actually offset anything.

While it's possible, i wouldn't offset or bore in anything else than low volume job, depending on various parameters. If you are using big insert drill and offsetting in not so stable operation, it most likely wanders some while drilling, and after you come out of the hole, it "straightens" itself out, and might chip the peripheral insert.

We used to do it for years, but with high volume stuff we couldn't get decent insert life when offsetting or using it as boring bar afterwards.

Edit (After reading rob's post): Used square with sandvik & seco which seemed to be bad idea, and tungaloy trigon insert where i can't recall if we actually had problems with peripheral insert

If your idexable drill won't drill off-center, then you bought a turd of a tool. A .790 hole with a .750 drill is only .020 off-center

A Sandvik 880 or Seco Perfomax will eat up your off-center drilling and crap out good parts all day long.

Yes to boring as well, but you'll go through outer inserts a little faster. Under normal drilling circumstances, an 880 or Perfomax will use about 2 outer inserts per 1 inner insert. If you are in mild material and the machine is rigid, you could go to the harder grade on the outer insert and get better life for drilling & boring with the outer insert.

I sometimes drill on-center, then redrill off-center a little and use the drill like a boring bar. This doesn't ask too much from the center insert. Makes for a cleaner ID as a bonus.

Regards.

Mike

Just found the offset spec on the drill I'll use for that hole, korloy king drill 0.8mm offset, great, more than I need
I'll give it a go in a couple days, thanks

JJM said:

I think most if not all insert drill manufacturers usually list the offset value per drill body on their book/site/catalog. At least sandvik and tungaloy had.

It is different for each body, and needs to be checked before you actually offset anything.

While it's possible, i wouldn't offset or bore in anything else than low volume job, depending on various parameters. If you are using big insert drill and offsetting in not so stable operation, it most likely wanders some while drilling, and after you come out of the hole, it "straightens" itself out, and might chip the peripheral insert.

We used to do it for years, but with high volume stuff we couldn't get decent insert life when offsetting or using it as boring bar afterwards.

Edit (After reading rob's post): Used square with sandvik & seco which seemed to be bad idea, and tungaloy trigon insert where i can't recall if we actually had problems with peripheral insert

Click to expand...

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Think Snow Eh!
Ox

litlerob1 said:

but the ones with Square inserts aren't designed for it.

R

Click to expand...

Rob

Iscar's DR drills are square and are offsettable to the next size up: ISCAR Cutting Tools - Metal Working Tools - DR-3D-N : 3201323 - DR0625-1875-075-05-3D-N

And, unlike the miserable Sandvik 880, they use the same inner and outer insert, and they have 4 edges instead of 2.

SeymourDumore said:

Rob

Iscar's DR drills are square and are offsettable to the next size up: ISCAR Cutting Tools - Metal Working Tools - DR-3D-N : 3201323 - DR0625-1875-075-05-3D-N

And, unlike the miserable Sandvik 880, they use the same inner and outer insert, and they have 4 edges instead of 2.

Click to expand...

I love the DR Drills from Iscar, been using them for years. I have had problems in the past with the Peripheral insert being stupid though. (different ways). I personally have better luck with them as a Drill then a subsequent Boring Bar, but not in one shot. Maybe I didn't try hard enough. I've always thought the inserts were a little thin, but I do use the snot out of that Drill, very fast, very clean holes. Now looking at the Data it isn't offset much though 1/32" I'm POSITIVE I've done that with the DR's (not intentionally, but done it for sure). The KM DFT style Drills you can offset something like 8-10% of the Tools Diameter. I tried finding Data but failed, you'll just have to take my word for it.

FWIW, I offset insert drills all the time, including square insert ones like the 880 (especially the 880).

Ox, dropping the drill slightly in X is something I do routinely on retract with big drills, like over 75mm. Doesn't usually seem necessary on smaller drills for whatever reason.

Most vendors that sell insert drills also sell eccentric bushings for offsetting the drill when you're using it in a mill. Those work great too.

I also offset practically every time. Every day I'm running some size of Sandvik 880 drill in 316SS on at least one of my machines...with an X offset going in to raw material. In fact, it's rare that I ever drill at X0. I'm drilling with an X offset, many times at or near the manufacturer's max amount.

Go here for an example at how Sandvik defines the max offset amount... 880-D3000C5-03
The max adjustment in X (ADJLX) for this 30mm drill is .044. That means you can program an X.088 (the lathe actually moves .044" from centerline) and drill a 1.269" hole with a 1.181" diameter drill.

I used to take Sandvik's old U drill (now called 881) to even greater X offset amounts. I had a 33mm drill (1.299") and was drilling a 1.5" diameter hole in one shot in 316SS. Sandvik's newer 880 drills do not have as much offset capability as the older U drill.

If you look at the drill ranges in a catalog you will notice the X offset amount gets less and less as you move up the drill size range with a specific sized insert. As soon as the insert size moves up to the next biggest size the max X offset amount jumps up on that size drill.
For example, Sandvik's 20mm drill uses insert size 04 and that drill has max offset of 0.9mm. The 23mm drill uses the same size insert but max offset is 0.5mm. When you move up to a 24mm drill you now need to use insert size 05... and the max X adjustment now jumps up to 1.10mm. That's because the inserts get farther apart on the drill body and can no longer overlap as much until you jump up to the next insert size.

Hopefully I explained that okay...it all makes sense to me. I started doing this in SS, Inconels, and Hastaloy back in the late 90s with Sandvik's U drill.

It worked out pretty decent, had to knock the feed down from .003" to .0024", then it was happy again. Only thing is the bottom flute on the clearance side gets less coolant pressure than the top one due to that gap so chip flow isn't as great but still got'er done.