How do I find center of a hole I want to bore, on an LMS 3960 mill?

[cazclocker]

I guess the answer would be about the same no matter what kind of vertical mil I'm using. But mine is a Little Machine Shop 3960 mini-mill.

If I want to drill, ream or bore out a given hole in cast iron to a larger diameter, is there an approved method of positioning the iron piece with the existing hole dead-center under the mill's spindle? I'm very new at this, so please be patient!

What happened was, I took such a piece (it's about the size of your fist, with a 5/16" hole through it, about 2-1/2" long) and clamped it securely on the mill bed. I eyeballed my center, and went for it. I had two pieces to do - the first one came out perfect - the second one was so off-center it was unusable. I used a HSS drill bit, not a mill. Should I have used a mill? So I'm asking so I can learn, and never do that again.

Thanks,
...Doug

 

[rdean]

I would use an edge finder in the spindle.

Ray

 

[JimDawson]

An edge finder is one good way to do this. Find the edge in the x and y axis. and move to the hole center from there. Sometimes if I don't need to be extremely accurate, I just use a drill bit in the chuck that just fits the hole and keep probing as you move the x and y (with the spindle off) until the bit doesn't hit the edge. Put your finger in the drill bit and you can feel it bump the edge when it's not centered. Rotate the bit 90* and probe again on the other axis.

Another way is to again chuck a pin that is the hole size in the chuck, insert it into the part, now it is aligned with the chuck. Clamp the part down in that position.

Or just use a co-axial indicator.)

 

[Bill C.]

I guess the answer would be about the same no matter what kind of vertical mil I'm using. But mine is a Little Machine Shop 3960 mini-mill.

If I want to drill, ream or bore out a given hole in cast iron to a larger diameter, is there an approved method of positioning the iron piece with the existing hole dead-center under the mill's spindle? I'm very new at this, so please be patient!

What happened was, I took such a piece (it's about the size of your fist, with a 5/16" hole through it, about 2-1/2" long) and clamped it securely on the mill bed. I eyeballed my center, and went for it. I had two pieces to do - the first one came out perfect - the second one was so off-center it was unusable. I used a HSS drill bit, not a mill. Should I have used a mill? So I'm asking so I can learn, and never do that again.

Thanks,
...Doug

I would use a dial indicator and sweep the hole in. There may have been a hard spot in the casting to throw the drill and reamer off.

 

[francist]

Could he also use a DTI mounted point down in the mill chuck and sweep the inside of a bore that way? My Last Word has a round shaft attachment and I've often thought about using it that way but never had the occasion to try. Would work kind of like a co-ax indicator, I guess.

-frank

 

[davidh]

maybe my head is on crooked here but are you going to re-bore part number 3 or try to repair part number 2 that's incorrectly located ?
if its part number 3 then indexing from the orig. hole with the original size drill in the chuck as described is the easiest way. after its in place, change bits to the size you want and drill away.
if, you want a dead straight hole, I'd use a proper sized endmill as the drill because a larger drill will try to follow the old hole.
others may correct me if im not correct. . . .

 

[JimDawson]

I would use a dial indicator and sweep the hole in. There may have been a hard spot in the casting to throw the drill and reamer off.

An endmill would be a good choice in this case, at least to get the hole started, a couple of hole diameters deep at least. Normally a drill bit will try to follow an existing hole.

 

[Ray C]

You could use a conical center finder like this.... Just chuck it up and move the table until it fits in the exiting hole.




Ray

 

[schor]

I'm not sure of the application here, if the hole your trying to enlarge is perfect then you can indicate it in many different ways. If your trying to find the center of a worn out hole and indicate off that surface your not going to get there and need some other face to indicate off.

 

[cazclocker]

Wow, thanks guys. So far the DTI mounted into the spindle makes sense, and so does the cone center. They both make sense to me. To the question "am I going to try to fix piece #2 or just know how to do #3 properly", it's the latter. I've decided to keep part #2 but not use it. I simply wanted to know a good way to proceed should this operation come up again. I also think I should have used a mill instead of a drill, just because as several of you pointed out a drill will try to follow an existing hole. Also, as someone mentioned there could have been a hard spot in the casting to throw the drill off.

I don't have a coaxial indicator - but I do have several DTI's. I see now that I could have mounted a DTI in a collet in my spindle, and slowly rotated the spindle as I adjust the X & Y axes...until the runout is the same all around.