Drilling Issues

[HBilly1022]

I am having issues when end drilling on the lathe. I use a centering bit and cut to the top of the taper and that always goes well but when I chuck a drill bit in the tail stock and advance it into the center hole it almost always starts to kick to one side and wobble and then cuts off center. I suspect it is due to my poor drill bit sharpening skills but I'm not sure if that is the only problem. Are there some other causes I need to check out or just become better at sharpening bits properly?

 

[Tony Wells]

I sometimes ease the QCTP up to the bit just enough to steady it once it touches to prevent it from getting off to a wobbling start. When a cut is just starting, if the edges are not perfect, and the starting hole is not dead true (two difficult conditions to meet), it can easily grab a little and start an uneven cutting action that is impossible to correct without external measures.

If your bit is long, of course it's going to be more limber and this makes the problem worse. If it is sort of critical that your hole be straight, then start off about 1/32" under and go about 1 diameter deep. Then come back with a stubby boring bar and open the hole up to the drill size, maybe a few thousandths under. This will allow the drill to cut with very little deflection. But when you make contact with the point left by the undersized, and presumably out of true center, go very slow with the feed and it won't generally affect the full size drill much. If you just drill with normal or heavy feed, it will try to follow the pilot point and you will lose at least part of your improved concentricity.

 

[Terrywerm]

There are a few things that can cause what you are describing, but a drill bit that is sharpened unevenly will wreak havoc for you no matter what you do. Incorrectly sharpened drill bits not only cut off center, they also tend to cut a hole that is oversized. There is a good book available about sharpening drill bits and end mills that you might like. It is called 'Tool and Cutter Sharpening' by Harold Hall. It can be purchased for about $20 online and is well worth having. The author also discusses some jigs that you can make that will make your sharpening jobs so much easier and much more accurate.

 

[Franko]

Try drilling the pilot hole a little deeper, using the flange of the pilot bit to chamfer the mouth of the pilot hole slightly larger than your drill bit, so the drill is entering a chamfered start. The drill should settle in and might run true.

If a slight chamfer is unacceptable for your part, you can face it off square after it is drilled.

 

[HBilly1022]

I sometimes ease the QCTP up to the bit just enough to steady it once it touches to prevent it from getting off to a wobbling start. When a cut is just starting, if the edges are not perfect, and the starting hole is not dead true (two difficult conditions to meet), it can easily grab a little and start an uneven cutting action that is impossible to correct without external measures.

If your bit is long, of course it's going to be more limber and this makes the problem worse. If it is sort of critical that your hole be straight, then start off about 1/32" under and go about 1 diameter deep. Then come back with a stubby boring bar and open the hole up to the drill size, maybe a few thousandths under. This will allow the drill to cut with very little deflection. But when you make contact with the point left by the undersized, and presumably out of true center, go very slow with the feed and it won't generally affect the full size drill much. If you just drill with normal or heavy feed, it will try to follow the pilot point and you will lose at least part of your improved concentricity.

Earlier today I read the SB "How to Run a Lathe" book and found a section that described what you are saying about supporting the drill bit as it enters the cut. I will give that a try next time. I also like your idea of using a boring bit to get the hole true but not sure if that is possible with the smaller holes I usually drill. My larger bits (1/2" to 1") aren't good quality and have been abused. Based on my very limited experience, I would prefer to bore a hole to drilling one but not sure what the smallest size I can bore is.

There are a few things that can cause what you are describing, but a drill bit that is sharpened unevenly will wreak havoc for you no matter what you do. Incorrectly sharpened drill bits not only cut off center, they also tend to cut a hole that is oversized. There is a good book available about sharpening drill bits and end mills that you might like. It is called 'Tool and Cutter Sharpening' by Harold Hall. It can be purchased for about $20 online and is well worth having. The author also discusses some jigs that you can make that will make your sharpening jobs so much easier and much more accurate.

I think that is the biggest issue for me. The bits are not sharpened properly and are cutting on one side.

Try drilling the pilot hole a little deeper, using the flange of the pilot bit to chamfer the mouth of the pilot hole slightly larger than your drill bit, so the drill is entering a chamfered start. The drill should settle in and might run true.

If a slight chamfer is unacceptable for your part, you can face it off square after it is drilled.

The drill sizes that are giving me the most grief are above 1/2" diameter. Those are the ones I tried to sharpen by hand. The smaller ones I use a drill doctor. Now that I think about your comment ...... I better check and see if I'm using the right sized center bit for the hole I'm drilling. That could be another cause of my problems.

I just watched a video Tubalcain made of sharpening drill bits and I'm going to give that a try tomorrow.

Wow this machining stuff is WAAAAAAAYYYYY more complex than wood working but I'm enjoying the challenge and learning something new.

 

[Bob Korves]

Marc L'Ecuyer, That Lazy Machinist, has a great recent YouTube video about the smartest way to drill an accurate sized hole in the correct position. He was using a mill in the video, but the principle is the same with a lathe.

The hole is started with a center or spotting drill, to initially locate it, then hogged out with a drill, or stepped open with multiple drills, stopping safely under the finish hole size. Drills are for removing material, and will not help much with achieving the proper size or the correct location. Drills are roughing tools. BTW, core drills are better at opening up existing holes more accurately due to their multiple flutes. There are lots of vintage ones laying around getting rusty. Pick up a few if you get a chance.

Then, the hole is bored to establish location. The boring operation is stopped slightly under the desired finished size. Boring is a very good way to put (or move) a round hole into an accurate location, but it is a really fussy way to get the exact finish diameter you want. It is devilishly easy to go over your finish size.

Finally, the hole is reamed to give the finished diameter. The reamer follows existing holes well, even when you don't want it to. A reamer easily finishes holes to very accurate diameter and a nice surface finish when properly used.

That approach makes perfect sense. Each cutting tool is doing what it does best. Yes, it does take several operations and requires more tooling, but when you want a round, accurate hole in the correct location, it is an excellent method to consider.

 

[HBilly1022]

Thanks Bob. Lots of new info there.

I started watching his drill bit sharpening video last night but quit because I couldn't stand the music he was playing. Guess I'll turn down the volume until I get past that part and then watch the whole thing and then watch his drilling vid.

Man this machining stuff is complicated. Who would have thought that drilling a simple hole could be sooooooooo complex, lol. I guess it's only complex if you want the precise diameter and in precisely the right location.

 

[Bob Korves]

(snip)
Man this machining stuff is complicated. Who would have thought that drilling a simple hole could be sooooooooo complex, lol. I guess it's only complex if you want the precise diameter and in precisely the right location.

Absolutely! If you just need a hole, then just make a hole and be done with it. Make it oversize enough to accommodate any small errors of placement and size. Punching holes out on a drill press is common shop work, repeated by the millions every day. Only when it gets fussy do you need to pay close attention. It is a shame to ruin a precision project with lots of hours invested in it by screwing up the last hole. The language can get salty and metal can fly!

 

[janvanruth]

You chuck up bits larger than 1/2 inch?

 

[Bob Korves]

You chuck up bits larger than 1/2 inch?

Don't know who you are asking, but I use larger than 1/2" drill bits in chucks when necessary, but only on easier materials and in gentle steps. I will not use my precision keyless chucks for the job. I have a Jacobs 3/4" chuck on a 3MT taper, and that can handle some heavier work. Sometimes you just need to use a Silver and Deming drill to get the job done, and if you are careful and don't push the possibilities it works OK. Watch out for heavy work in chrome moly and stainless steels unless you have a really solid setup and enough power to handle it. A lathe dog around the shank of the drill, set screw tightened onto a ground flat spot on the shank, and tail resting on the compound, helps to keep the setup from spinning in the tail stock or the chuck.

I have picked up a good assortment of MT and S&D drills over the years and can usually get it done without scaring myself or hurting anything :-|