Question on spotting drills

[jason867]

Hello all. I'll try to keep this short & concise. I also realize I might be (re)opening a can of worms.

Recently acquired a bridgeport vertical mill with step pulley head. Max rpm is 2720.
I have a nice set of USA made maintenance/mechanics length 135* split-point HSS drill bits that I like to use. I will eventually acquire some stub length bits as well.

I'm aware that spot drilling isn't totally needed with split-point drill bits, but I'd like to get some to use when I feel the need for it.
It's my understanding that I'd want a 140* or more spot drill for use with 135* drill bits (and 120* spots for 118* drills).
It's also my understanding that I should generally avoid carbide mills/drills in my BP, since it usually can't spin them fast enough, and they're fragile/expensive compared to HSS.

My question is, why can't I find any HSS 140* spot drills?
All of the 140* or greater spot drills I see for sale are solid carbide or carbide tipped.
All of the HSS spot drills I see for sale are only 120* or less.

What gives? What should I use?
Also, what diameter spot drills make sense for use with 1/8" through 1/2" drills? I figure I'll step up to anything larger.

Been doing a lot of reading online, but still can't seem to answer these questions.

Thanks in advance.

 

[extropic]

The reason to use a spotting drill is to place a spot (to minimize twist drill wandering). Spot drills are stiff. Carbide spot drills are very stiff. You don't need optimum SFPM to place a spot. Use Carbide.

Diameter? Have them the diameter of the collets you usually use. 1/4" ??? 3/8" ??? 1/2" ???

 

[pontiac428]

This is a topic of differing opinions and rationales. If you spot a wider angle than the bit, the center tracks but the center also has to the cut. Spot narrrower than the bit, then the cutting edges engage first and start the cut. A center drill gives you the best of both and are available in different angles. But that's my opinion.

Here is a thread with some great input on your question by two wise men who are no longer with us:

Twist Drill Angle with Center Drills vs Spotting Drills

I read this message from Bob and it got me thinking. I didn't even recognize the difference in twist drill split point angle (note the beginner's forum!) but when checking my drill bits, I find them to be 135 degree tips. Sigh, and I just ordered a set of 60 degree center drills. Looking at 140...

www.hobby-machinist.com

 

[extropic]

Actually, I'm not sure why you want spot drills on a manual mill at all. Unless you are one of those that like to spot out a hole pattern for a sanity check before actually drilling, you will probably be time and money ahead if you just start with stub drills rather than jobbers length.

I want to do a good job, but I also like to minimize tool changes. Why would I use a spot drill? For highest precision location? The hole will be quite deep and I want the best start?

 

[extropic]

13 pages on spotting drills? I'm not nearly that interested in "learning" that much about reading that many comments on the subject. I quit at reply #3 or 4, where Bob wrote essentially what I believe. KISS

 

[RaisedByWolves]

This is a topic of differing opinions and rationales. If you spot a wider angle than the bit, the center tracks but the center also has to the cut. Spot narrrower than the bit, then the cutting edges engage first and start the cut. A center drill gives you the best of both and are available in different angles. But that's my opinion.

Here is a thread with some great input on your question by two wise men who are no longer with us:

Twist Drill Angle with Center Drills vs Spotting Drills

I read this message from Bob and it got me thinking. I didn't even recognize the difference in twist drill split point angle (note the beginner's forum!) but when checking my drill bits, I find them to be 135 degree tips. Sigh, and I just ordered a set of 60 degree center drills. Looking at 140...

www.hobby-machinist.com

Agreed.

 

[gard]

I think that carbide end mills will work well in your machine in the larger sizes, depending on what material you are cutting. I use 1/2" on steel within those rpms. Sometimes a slower rpm is more pleasant to use, chips are not as hot, don't fly as far with less noise. It's a hobby for me, not production work and I am still learning.
I also tend to use center drills if the stub length drill seem to be wandering, especially with the smaller sizes. The center or spot drill has the advantage of working in the collett which is more accurate than my drill chuck which is needed for uncommon size drill bits.