Spotting depth question

Is there a rule of thumb or formula regarding how deep to spot drill with regards to the size drill you will be using? I know to use a spot drill with the same included angle as the drill point but am unsure on depths. Most of the vertical CNC programmers where I work use a 90 degree spot and to a .050" depth, regardless of drill size or tip geometry. Thanks for any help (question relates to vertical machining centers).

Greg

cpm10v said:

Is there a rule of thumb or formula regarding how deep to spot drill with regards to the size drill you will be using? I know to use a spot drill with the same included angle as the drill point but am unsure on depths. Most of the vertical CNC programmers where I work use a 90 degree spot and to a .050" depth, regardless of drill size or tip geometry. Thanks for any help (question relates to vertical machining centers).

Greg

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Always use a spotting drill with an included angle less than the drill. This way the outside edges of the drill will start making a "drill bushing." As the drill center doesn't cut very well, the drill bushing will help keep the drill on location as it penetrates the work.

So the spot hole needs to be larger in diameter than the drill.

Feed is also important. If the feed is too slow relative to the RPM, the drill may chatter as it enters the "spot cone." If the drill chatters it will probably go off location.

Chip load per flute for a standard two flute drill should be near 1.5% of the drill diameter.

Ex: .500" diameter drill, the chip load per flute is .0075". Counting two flutes makes it .015" per revolution. Counting rpm 600 the feed will be 9" per minute.

Regards,

Stan-

If I'm using 118 degree final drills I like using 118 degree spotting drills. I know a lot of people like using 90 degree drills but I think this prematurely wears out the final drills as they first contact the material just on the outer edges, and I've heard this is even more significant when using carbide drills.

With a 118 degree spot the depth calculation is easy:

Depth = Drill Width * 0.3

So for a .250" hole, your spot drilling depth is 0.075 .

If you decide to go with 90 spot drills, the spot depth is 1/2 the drill width.

I use the Keo HSS spot drills which are often on sale from www.use-enco.com, they work well and last a long time.

Regarding hole location accuracy with the different types of spot drills, I remember a post where somebody did a test comparing 90, 118 spots and lathe center drills for the locational accuracy of the final drilled hole. It turns out they all worked well and all about the same, although an advantage of the 118 drill is that its quickest since it drills the least depth to make the hole the width of the final drill.

Paul T.

Bear in mind that you can sometimes kill two birds with one stone by using your spotter to form a chamfer if one is called for on the print. In that case, your depth has to be whatever gives the right chamfer diameter. Also, by using screw-machine length drills, you can often skip spotting.

One caution when using a spot drill of more acute angle then the incoming drill is:

If you are using carbide drills, it is best to use a spot drill of the same angle, and if not available, use one of a more obtuse angle than the subsequent drill. Carbide drill tips (often 135 to 140 degrees included angle) are much more likely to grab and chip off.

I have used a lot of spotter drills that were 120 degree angle, they worked fine, I typically spotted .05 deep for most things, using 3/8 for all but tiny holes, then I used 1/4 spotter drills. The pre made spotter drills are stub length and do not have any clearance on the od like a normal drill, the od is just od ground....probably so you can collet them up very short if wanted/needed.

It is easy to FORGET they have no OD back away, and need to pop a hole in something and use the spotter and have it work not so well, scratch head for a bit....then REMEMBER hehe.

I'm thinking the ones I used were Garr.

You can get away with the 90 degree bit in some materials, but eventually you run into a material where the 118 drill shows you how hard you are being on it by starting just the tips into the 90 degree spot.

Bill