How do I drill small holes straight? (I made a suppressor adapter)

Late to the party but I had some thoughts.
  • Without question, if the hole has to be sized precisely and have straight sides and a clean finish, the best way to do that is to bore it. It's not clear to me from your post but how big is the bore? If it is close to 0.200" ID then a 3/16" solid carbide bar with an insert with a small nose radius will do it, provided the bore is not more than about 1.875" - 2.00" deep. It is also possible that Micro 100 may have a solid carbide boring bar that will do it.
  • If you have a good twist drill, you can follow it with a good reamer. A reamer can straighten a hole that is not out more than 0.005" so it's possible that a reamer might do it. If you try it and it works, the advantage is that you can make a lot of parts just like it faster than with any other method.
  • Similar to a gun drill, D-bits, made from hardened and tempered drill rod, are known to drill straight, precise holes. I haven't tried them myself but I will when the need arises. These are what were used back in the old musket days and sounds like a very good solution. Plus, its cheap! Tons of info on D-bits on the net.
 
strantor said:
If I suspected my lathe was at fault, how would I confirm/fix?
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Step 1 is to dial indicate the outside of your chuck relative to the bed (checking the spindle).

Step 2 is to dial indicate to a drill, or ground bar known to be straight, a test bar, whatever, ( checking the chuck).
If the chuck was 4-jaw - this may be not be relevant.
Find out if there is wobble on round reference stuff put in the chuck.

Step 3. Set up a centre in the chuck, and deliberately turn a fresh point onto it. That will automatically be on the spindle axis. Put another centre in the tailstock, and bring up the tailstock to try and touch the points together.
Use a small steel rule, or metal shim strip, and try and gently grip it between the centre's points.
If it is misaligned even slightly , the strip will tip in a very obvious way.

Step 4. Even the centres are aligned, the axis through the tailstock might be tilted. To check this requires a test bar set into the tailstock, checked with a dial indicator set up on the carriage, and moved along both the side of the test bar, and along the top of the test bar.

So far, this is about the "confirm" part.
The "fix" part depends on what you find. There is tons of stuff on this forum about that. Pretty much all of it involves swallowing hard and reaching for a stiff drink at some stage as you read through it

I would think there are plenty of ways to be breaking drills, or making messed up holes by technique, even if the lathe is perfect!
 
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@strantor I ani't saying/suggesting anything. I'm reading about machinist disscusing the physics of making a perfect drill hole...unless I'm mistaken...and I'm sure I'm not.
As you were gentlemen!
 
Got to thinking about this today. The original question was about making a straight hole. The adapter doesn't seem long enough to be talking gun drills and very long diameter to depth ratios or even wild excursion off track. Threads often make poor mechanisms or features to keep things square to each other. Is the problem that the hole in the middle isn't straight or that the assembly isn't straight with the bore of the barrel? The adapter screws to the outside of the barrel, does it seat square of a shoulder or tighten against the last half thread or something similar? Same with the suppressor, the male thread screws into the tube. The last bit of thread becomes the index point and could pitch the bore alignment. The other thing I was thinking about was how the part was made - it seems plausible that you turned the part around to do the other end. Getting things to get exactly (or a reasonable approximation of) concentric and bore aligned on a 3 jaw for this kind of part would be optimistic so assumed you used a 4 jaw. A gauge pin, range rod, the drill bit you used to make the hole to let you align the bore to the spindle before the outside threads get made may help. Maybe you already have it covered, but just trying to turn the problem around, maybe the issue is on the outside instead of the inside.
 
GL said:
Got to thinking about this today. The original question was about making a straight hole. The adapter doesn't seem long enough to be talking gun drills and very long diameter to depth ratios or even wild excursion off track. Threads often make poor mechanisms or features to keep things square to each other. Is the problem that the hole in the middle isn't straight or that the assembly isn't straight with the bore of the barrel? The adapter screws to the outside of the barrel, does it seat square of a shoulder or tighten against the last half thread or something similar? Same with the suppressor, the male thread screws into the tube. The last bit of thread becomes the index point and could pitch the bore alignment. The other thing I was thinking about was how the part was made - it seems plausible that you turned the part around to do the other end. Getting things to get exactly (or a reasonable approximation of) concentric and bore aligned on a 3 jaw for this kind of part would be optimistic so assumed you used a 4 jaw. A gauge pin, range rod, the drill bit you used to make the hole to let you align the bore to the spindle before the outside threads get made may help. Maybe you already have it covered, but just trying to turn the problem around, maybe the issue is on the outside instead of the inside.
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Ok I see your point. The adapter registers against a shoulder turned on the barrel, and the suppressor registers against a shoulder turned on the adapter. The threads are not used for alignment. Or at least not the primary method of alignment. But the inner threads and outer threads of the adapter do need to be concentric.

I think you may have touched on a flaw in my thought process. See, my original plan was to drill out the blank, thread the barrel-side, thread it onto the barrel, and from then on treat the barrel+ adapter as one piece. Put the barrel back in the lathe with the adapter installed on it, between centers, and machine the OD and threads of the adapter so as to ensure that the suppressor would be concentric to and square with the barrel. That's why it mattered that the hole be straight.

After so many failures I attacked it a different way. I machined the adapter 100% separately from the barrel. I turned a section of 1" bar stock down to .750", cut it off, and put it in a MT3 draw collet in my headstock. There I drilled it with an end mill almost half way (deep as I could go), turned it around, and drilled it from the other end with the same end mill, almost meeting in the middle, and finished the bore off with my best twist drill. Then proceeded to do my turning and threading. It worked. But since it wasn't my original plan, it felt like a compromise, a last resort that just wasn't "right." I've been obsessing over how to do it "right" ever since. But maybe the first idea isn't the right idea. Maybe I did it "right" in the end, and I should just leave well enough alone. "Right" is whatever works?

But it still bugs the heck out of me that I can't drill a 1/4" hole through 2" of stock without veering off course by an amount painfully obvious to the naked eye.
 

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Did you try and see where the problem exist on the bad parts? Chuck back up and center, then take a .0001 gauge and see at the end of the hole is centered. Then run the tip of indicator as deep as you can in hole and see if the readings match.
 
I wasn't wrong; there is a gunsmithing subforum. I just didn't look hard enough.
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My Craftsman lathe drills holes like that. Test bar showed the spindle centerline ran uphill and away. Have not fixed it yet. Fixed problem by getting a new PM1340GT. Now can mess with old lathe without not having a lathe. Not an option for some, but it was time to upgrade anyway. See threads above for how to check ( graham-xrf and mikey) as a start point. Remember Rule 1: never let inanimate objects kick you butt. I share your pain, good luck with the search.
 
strantor said:
But it still bugs the heck out of me that I can't drill a 1/4" hole through 2" of stock without veering off course by an amount painfully obvious to the naked eye.
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Reading Connelly "Machine Tool Reconditioning", (which you can get on this site), I see Sec.14.16 page 105 mentions a cause why a machine will no longer bore a straight hole. Hmm.. the text did say "bore" as opposed to "drill", so I may be wrong about mentioning this.

The lathe may have developed a "wind" where the line of the ways are no longer parallel to each other, this, apparently by the bed having stood for a period not level, or under some stress.

A lathe bed is a big chunk of strong steel, and I would have thought the ways acquiring a "twist" sounds unlikely, but I recall a post on this site mentioning how a lathe bed casting can "take a set" for being out of level, and how it can take time to unbend when again set up level. To check this, you do need a machinist's precision level, and possibly more bits to check it properly. Across the tops of inverted V ways may be OK because they are often a convenient unworn surface.

I don't say this is definitely the cause of the hole going off centre like in your picture, but it is is worth a check if you have not already been there.
 
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