Drilling! - small holes on the mill and centre drills

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Lew_Merrick_PE:

--- Quote from: John Stevenson on July 18, 2010, 01:48:40 PM ---
--- Quote from: Lew_Merrick_PE on July 18, 2010, 12:38:59 PM ---1) Center drills are about 3X as stiff as a spotting drill because less material is removed for the flutes.
--- End quote ---

So why don't you see CNC machining centres using them? Less material removed isn't correct, if you take to pip that always breaks off then it's very weak in comparison to the main body. a spotting drill is the larger diameter all the way.

--- Quote --- 2) Here in the U.S. the "cost equation" for center drills versus spotting drills is reversed. A good quality spotting drill costs slightly more than double the cost of a good quality center drill.
--- End quote ---

Same here but now compare a stub drill to a spotting drill, hardly any difference and a fraction of the price of both.
--- End quote ---

CNC machining centers have (A) a more rigid head and (B) higher speeds than manual mills. Except for #0 and #1 centerdrills (which should only be used in precision small hole set-ups anyway), I have never broken the "pip" on a centerdrill. The "pip" has an advantage (as I see it) in starting holes on contoured or round surfaces. I will be ordering some new centerdrills in the fairly near future as my #2 centerdrill has now been reground too many times and my #4 centerdrill is rapidly approaching that condition. I bought the ones I am referring to in 2004 and use them for several dozen holes each week.

A complete set of Keo (#1-#5) centerdrills runs me (about) US$5. Purchasing one individually runs me about US$2. A Latrobe or OSG spotting drill (1/4") runs a bit over US$6 as of the last flyer I have from Enco. My 1950's vintage Lisle drill grinder has centerdrill sharpening attachments.

I do prototype and new product development work. I no longer earn my living "making chips," though I often build the first few units of anything I design. I was making parts for an ultrasonic (medical) diagnostic and treatment head today. Sometime later this week I will be making flow control valves for a blood filtering unit. Early last week I was making components for a switchblade tool for Force Recon Marines Explosive Ordinance Disposal teams. My equipment is old manual machines.

One of the projects I worked back in the mid-1980's was broken cutter detection for DNC and CNC machines. Conventional means are too slow given the rotational inertia of spindles on such equipment. Breaking "pips" is a real problem there. Whereas we could identify a dull (or broken) cutter down to (about) 1/4 inch in a 15 HP spindle acoustically, such systems were (A) expensive and (B) not being made by the "politically connected" companies. Instead, the GE and ITT systems that monitor spindle current made it to the market. They are worthless for any cutter will less than 5% of the rotational inertia of the spindle head itself. This is why you rarely see them commercially. (The paper on broken cutter detection and adaptive speed control was presented at the 1986 Winter Annual Meeting of the SME and ASME -- with authorship credited to myself and Richard P. Martell, PE. This work was done under the auspices of the USAF and NASA "Factory of the Future" program.)

Dean W:
Surly your connection with NASA, USAF, Rambo, Die Hard & Her Majesty's Secret Service will budget you $3 for a new KEO 1/4" spotting drill. That's how much they are running at Enco right now. Secret agents and classified defense workers are sure to get a discount, too!
Carry on.

raynerd:
Here comes one of my dumb questions but I have to ask since I`ve learnt a lot from this thread. Rob - you sent a link for a 6mm spotting drill. Can I just clarify that you would have a set of these for each size hole? I`ve lost the plot a little as it seems that John is using them to cut the hole directly...say I wanted a 5mm hole and quite a depth, would I need a 5mm spotting drill to start the hole and then move onto my regular 5mm drill bit? Or say my hole was 5.5mm, would I use a 5mm spotting drill to "spot" the hole and then go in with my regular 5.5mm drill or whatever to finish it?

I suppose I`m asking are these just used to spot the hole and I only need a couple of sizes or are you using them to drill through?

Chris

kwackers:
I've used centre drills for years with no problems - it's quite possible that spotting drills are better, but then as amateurs you quite often end up with a growing centre drill population that's been 'handed on'.
Of course having the means to sharpen the tips is pretty useful...

With regards dividing plates - I've seen people go to extraordinary lengths to drill the holes as accurately as they can.
But imo it's worth remembering that when used with a device with a worm (rotary tables etc) the accuracy increases with the worm ratio.
For example drill your hole in your plate 1 deg off, then for a standard 90:1 rotary table the error in use will be 1/90th of a degree.

Lew_Merrick_PE:

--- Quote from: Dean W on July 19, 2010, 02:21:43 AM ---Surly your connection with NASA, USAF, Rambo, Die Hard & Her Majesty's Secret Service will budget you $3 for a new KEO 1/4" spotting drill. That's how much they are running at Enco right now. Secret agents and classified defense workers are sure to get a discount, too!
Carry on.

--- End quote ---

Dean, The KEO spotting drills are (cam relieved) conically pointed twist drills. I do not like them and prefer a good old centerdrill. (One of the design considerations back when centerdrills were first formally defined as part of the American-British-Canadian (or "ABC") Industrial Council as we coordinated interchangeability for WWI was to allow the "pip" to keep a standard twist drill's chisel-point center from engaging until after the drill was 1.5 diameters deep -- the same rule used for drill bushings -- so it was properly guided by the lips of the drill.) Were I to purchase a spotting drill, it would be a Latrobe or OSG product. They are what I specify for CNC processes when I wear my manufacturing engineer hat.

If nothing changes the schedule, there is a Mars Lander that will be launched in a bit more than a month from today. The fuel pump for the vector (steering) thrusters that will guide it from here to Mars are something I developed back in 2006-7. I am not allowed to discuss them in any detail as the work was done under the "International Treaty on Arms Reduction" (ITAR) regulations. This is total BS as any competent engineer could have designed such a pump given the boundary conditions and operational requirements of the pump, but those are the conditions under which I took the project. This is common when dealing with NASA.

The USAF/NASA "Factory of the Future" program has built most of the major manufacturing facilities around the U.S. over the 1963-1996 period. These are the projects that created DNC and CNC machine tools. These are the projects that created CMM's, touch probes, and adaptive control of machine tools. The laser-augmented theodolites (they are called something like "total station system" commercially now) used by nearly every surveyor today were developed on a USAF/NBS (Nation Bureau of Standards) program in the early-1980's. They have a resolution of .002 inches and an accuracy of .005 inches over a 150 metre range. They were originally developed to align AWACS aircraft antennae. This project was "classified" for almost two years after we completed it. Another, totally idiotic project upon which I worked in the mid-1970's is still "classified" -- even though I can point you at magazine articles detailing it completely!

What if you really are a rocket scientist and you still don't understand it?

(My senior thesis in Industrial Technology was to summarize the reports from the ABC Industrial Council.)

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