Look at the drawing attached. I have 24 of these parts to make (in steel), not to mention several others that have the same situation. Right now I am looking at 3 steps: center drill, thru hole drill, then counterbore. I am trying to figure out the quickest way to do this. First question is can I eliminate the center drill step? These will be clearance holes for a 1/4-28 bolt 1.5 inch deep, so how much could the bit walk? Second question: how would you do this, I do not have an automatic tool changer, it is CNC with quick change tooling though. I can either put a part in the vise and do all operations, or I can do the first operation (center drill or thru drill both holes) then change out part, go through all 24 parts, change tooling to next operation, unclamp, clamp, unclamp, clamp, etc. I would think without an automatic tool changer it would be best to do one operation at a time.
Also, Mcmaster sells combination counterbore/drill in screw sizing, any opinions on these? It looks like the overall length is only 1.75 though, so I doubt they would be any use for this.
Thanks for any input!

If you have center cutting drills, no flat in the center you could. but you may need to center drill them, it might be the best way to do it.

I almost never center drill in aluminum or brass, but sometimes in steel, it also depends on the drill bit I am using. At work, a lot of times we dont use center drills when we are using center cutting 'split point' drills.

Jon

I am a bit fuzzy on the "3/8-24 Clearance" hole - does this mean it's threaded for clearance??

Easy part to make - are you certain you need help?

You could probably get away without the centerdrill op - just start a well ground drill slowly. The thicker the central web on the drill, the more it will walk.

Scott

Sorry, the print is wrong, it should say clearance hole for 1/4-28 bolt. It's not that its a hard part to make, its that their are a lot of them, and my time is priceless :) . I don't want to waste time on operations that aren't necessary.

If you can get away with using screw length drills you don't need to spot drill first.

jderou....well first of all you don't have a tolerance chart on the drawing...and since you are specifying 4 decimal places on the hole placement....you'll need to centerdrill ....

When you drill those holes to within a thou let me know how you did it. You may look into using a reamer if your tolerence is that tight.

The tolerance is not that tight, its just not on the drawing. If the outside of the bolt is .25" and the hole is .280 for clearance, then obviously it can be within +/- .015. I will center drill them.

I wouldn't center drill. Use a high-end drill bit. Like the Dormer Tri-na-do. 118* split point, must be new and hold it in a collet and with as little tool protrusion as possible. You should get near perfect holes with proper feed and speed.

If you have alot of parts and the same shape hole you could either have a form tool made or probably find one from some company the makes form tools "acme" and a bunch others out there. This will allow you to use only one tool. I have done this a bunch of times on manual machines as well as cnc machies. I design my own for the cnc that I program. For the record you could use parabolic or three flute carbide drills and they don't need centerdrills.