Drill accuracy

[just bill]

I've decided to make my own core swaging and core seating dies since they are straight through dies. My curiosity is, are drilling and lapping accurate enough or do I really need reamers to finish the holes before lapping and polishing. I know helical drills are way more accurately then standard drills, however the straight through dies don't need to be as perfectly found as point forming dies, or do they?

[M-Tecs]

Generally the process is drill, reamer heat treat than lap. Can it be done with just a drill? Yes, but you will have a rougher finish and possibly more material to lap. It really comes down to how you are lapping them.

[garandsrus]

Are you using a lathe? You probably won’t find a drill the diameter you want, and probably not even close. For example, if you were making a .308 die, the closest drill is .302. There is a .358 drill, but that is too large to get a die that size. The next drill down is .348, which is pretty small.

When I make similar dies, I drill and the use a boring bar to get within 2 or 3 thousandths of the final diameter. I sand them using sand paper and a split brass rod to get to the final diameter.

For the top and bottom punches, drill rod the size you need works very well. You would need to attach it to a suitable top and bottom piece of the punch.

I am not sure you need to heat treat the die body for this type of die. There isn’t much wear. The punches take the brunt of the pressure and the die body is relatively big. Heat treating a point form die would be important because there is a lot of motion and pressure inside the die.

Have fun!

[Scrounge]

Are you using a lathe? You probably won’t find a drill the diameter you want, and probably not even close. For example, if you were making a .308 die, the closest drill is .302. There is a .358 drill, but that is too large to get a die that size. The next drill down is .348, which is pretty small.

When I make similar dies, I drill and the use a boring bar to get within 2 or 3 thousandths of the final diameter. I sand them using sand paper and a split brass rod to get to the final diameter.

For the top and bottom punches, drill rod the size you need works very well. You would need to attach it to a suitable top and bottom piece of the punch.

I am not sure you need to heat treat the die body for this type of die. There isn’t much wear. The punches take the brunt of the pressure and the die body is relatively big. Heat treating a point form die would be important because there is a lot of motion and pressure inside the die.

Have fun!

If you have a good set of metric drills, 7.8mm is .3071. IIRC, my set of metrics skips that one, but I've got a relatively cheap set from Amazon for the metrics. A fuller set is on my bucket list. It's closer if you really want to go straight to lapping.

[M-Tecs]

Drills generally drill 3% to 5% oversize. Hand stoning a small radius on the outside cutting edge and land will improve both finish and tolerance if done properly.

http://www.carbidedepot.com/formulas-drills-troubleshoot.htm#irregularholesize

http://neme-s.org/2005%20May%20Meeting/drills.pdf

[country gent]

The drill needs to be properly sharpened to cut to size different angles or off center web will cut over sized. The big issue with most twist drills is the dont always cut a truly round hole but a 3 sided hole and the reamers finish and removed this. A properly pilot drilled the drilled to size hole with a well sharpened dill will leave a good finish and can be lapped out to final size. But instead of lapping .001-.002 you will be lapping .004-.005 or more. A lot more time.

If your using a lathe to make these a holder can be made for a 1/4" die grinder in the tool post to grind the hole also with a mounted point. Here still .003-.005 is a lot to remove. In you die drill and bore the die to rough size then back bore .010 over. leave the sizing portion 1/2-3/4 long (what ever is needed) this way only the short section requires finishing.

[rbuck351]

I have found that most drills do drill several thousands over size but if you drill first with a drill one size small and then with the size you need they cut very close to size. You can vary the finished drilled hole by several thou by by predrilling one , two or three sizes under and then finish drilling with the size you need. The holes still need polished. I bought two cheap sets of adjustable reamers, one 1/4" to 3/8" the other 3/8" to 15/32. After drilling I ream them to about .0005 under then polish with wet or dry 600 or finer sand paper on a split dowel. It makes nice round holes as smooth as you want but it's still not fast.

[CastingFool]

How many guys know you can make a twist drill cut oversize holes? All you need to do is hand sharpen the drill bit, making sure the web is slightly off center, that makes one side cut slightly wider than the other.

[DAVE A]

I agree that using a boring bar is the best way to guarantee a round hole , I use a boring bar to get close and then a reamer but I use a barrel lap (Acro Lap ) after heat treat for the final finish.

[country gent]

Actually it takes a lot more practice and skill to hand sharpen a drill to cut to size. A drill gauge used when sharpening keeps the angles the same and web centered making it easier. New dosnt always mean its right. a test hole in a piece of scrap or the above gauge tells the story.

General makes a very good and economical gauge for sharpening drills by hand. Or one can be made with a hacksaw file and scale from 1/8" thick X 2" gauge stock. These go a long ways towards accurate cutting drills.

[midnight]

I drill at least .005 undersize, usually more. It pays to have a lot of metric drill sizes to fill in gaps. I get most of my drill bits & reamers at machinery auctions. You can never have too many. I will ream after drilling to get a couple thousanths under and then barrel lap with 280, 600, & 1200 grit clover using a "go" & a "no go" pin gauge. The die must be tested by making a finished product with it so you know how much springback you are getting before you make your hole too big.

Bob

[bangerjim]

For accurate holes in anything (not just gun stuff) I drill (or bore with a boring head) on my milling machine a slightly undersized hole. They I use adjustable (powered) reamers to gently and carefully come to the size I want, using my sets of plug gauges to know exactly where I am.

Depending on the hole size, I many times go to the final diameter with only the micrometer boring head (+carbide cutter) in the mill.

Using just a drill in a wobbly quill drill press will give you a rather rough finish and will probably significantly oversize. But give it a try! You might hit is on target!

banger

[just bill]

Thanks, I guess at my level of lathe expertise a ream and laping would be the best for me. I used a parabolic drill on my core mold and it came out very good, so I was thinking about using one for the core forming die and a little polishing,however steel reacts differently than aluminum. I have to order ream.
Bill