Could you measure out and mark the center and punch or hand drill a tiny starter divet?
Then do it on the drill press?
Then do it on the drill press?
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One piece black walnut crotch headless 6 string
- Thread starter Jeff Siddall
- Start date
What are the chances the knobs will fit in a socket? Could put a touch of hot glue to secure them. Chuck a socket extension arm in the drill press?
Yeah, I could do that. It would be easy for sure but never perfectly concentric. Maybe good enough for a knob though!
A variant that should be more precise is to make a jig by securely clamping a block to the table, drilling a 19 mm hole in it, then insert the knob upside down in the hole, then chuck a 6 mm bit and drill the hole in the bottom.
I had thought about that too but figured the chances of it fitting tight enough to stay centered but not so tight that it would damage the knob were near zero. But maybe if I wrapped the knob in the right number of turns of masking tape...
I posted photos of my knob building process somewhere but I don't remember where and I'm on my phone right now which makes it annoying to search. I basically start with a block of wood, drill the wide recess (to allow the knob to sit down over the nut on the pot) using a Forster bit. Then put a hole saw in the drill press and bring it down just enough to cut a groove with the saw, allowing the pilot bit to center on the Forster bit's center. Then I swap in a 1/4" bit and drill the holes for the shafts to the correct depth, following the hole from the hole saw pilot. Then I put the hole saw back in with no pilot and cut the knob out of the block by following the groove the hole saw made in the earlier step. This way everything ends up concentric. It's a lot of steps but if you make a bunch at once out of one block of wood you can buzz through them quickly.
Also - I know threaded inserts for knob grub screws are popular but I've only ever just cut threads into the hardwood, soaked in CA, and moved on with life. I probably have a dozen instruments out in the wild with wood knobs on them and no complaints (yet!) about thread failures.
No, absolutely not! The problem is just that a CNC cuts from the top down, so you can readily cut the outside of the knob. But the hole for the pot shaft needs to be drilled from the bottom up, which you can't do.
Yes, you could, but then there is a problem of alignment. With the hole on the bottom of the blank, how do you know precisely where the center of that hole is so you can make the outside of the knob concentric around it?
I also realize I probably didn't explain my problem that well initially: I want the center of the pot shaft to be aligned with the center of the knob to less than tenths of a mm (a few thousandths of an inch). More precise than I could be with measuring and marking by hand. Perhaps my true problem is I am trying to be more precise than I need to be!
Have the CNC drill the hole as a step, then remove said knob blank and replace with another scrap piece, have the CNC cut a post the same size as your pot shaft/hole to be used as an alignment dowel as the next step, place the knob blank on the alignment dowel and then have the CNC cut the knob as the final step. The CNC would know where center is because it used that center to carve the alignment dowel.
Make a jig to hold the workpiece upside down while you cut the hole, then flip it onto another part of the jig with a dowel pin that keys into the hole? Then you could program a pause to allow you to flip the workpiece and you'd know the exact positioning for each step.
I am curious why you want to be that precise. I could see the argument if you were going to set the knobs into counterbores in the body with very little clearance. (Somebody posted a picture like that recently in another thread, can't remember which one.) You probably could see an eccentricity of a few tenths in a radial gap of a mm.
Deviations caused by sanding and finishing could also be on the order of tenths of a mm, possibly making machining accuracy somewhat moot. You might have to settle for making more knobs than you need and choosing the best ones at the end.
Have the CNC drill the hole as a step, then remove said knob blank and replace with another scrap piece, have the CNC cut a post the same size as your pot shaft/hole to be used as an alignment dowel as the next step, place the knob blank on the alignment dowel and then have the CNC cut the knob as the final step. The CNC would know where center is because it used that center to carve the alignment dowel.
Funny that you both said basically the same thing at the same time!
I agree with your ideas, essentially the CNC version of the drill press thing @RichterScale and I were discussing back in post #63. But the additional problem with the CNC is clamping. Everything needs to be held way tighter than you would expect, and there is no easy way to make a dowel hold tight enough while carving, and then still be able to easily release it after.
I did consider some ways to handle the clamping but then logistical problems come up. For example. I could make the jig large enough to clamp it down around the perimeter and still allow the stock to be clamped to the jig. But that requires extra stock thickness to prevent carving through the stock. Then the hole needs to go through that extra thickness also. My CNC is too small to hold any kind of normal drill bit, which leaves using an end mill and helical cuts. But the flutes on most small end mills aren't long enough to drill a deep hole through extra thick stock.
Gaah!
I think drilling the jig on a drill press and then drilling the pot hole without moving the jig might be the best, least-effort way forward.
Yes, counterbores are one of the things I have planned. I find it pretty easy to see motion even on the order of fractions of a mm as you turn a knob. I am probably more particular about that than most people, but I hate knobs that "orbit" the pot. If I can see it moving from side to side it is too eccentric. However, I am thinking that using some of the jig ideas discussed here might get me to the point where the eccentricity can't be seen. Even if it's not perfect it may be good enough.
We have a saying at work, "perfect is the enemy of good enough." The perfect solution that never comes is worse than the solution that is good enough and ready to go.
I was just thinking about this again while walking the dogs because I have no life. When you tighten the grub screw it will push the shaft off-centre within the hole by an amount equal to the radial clearance. The hole would have to be off-centre in the other direction to get the knob truly concentric to the shaft. I wouldn't stake my life on it, but I'd bet that's also worth a few tenths of a mm.
That might actually be the best solution. Or three, because with two you can only centre it in one plane.Then a slight oversized hole with 2 grub screws to center the knob with
I figure with a D shaft or a split shaft 2 would work (provided your grub screws were precisely centered on the knob
). For a solid round shaft you'd need 3.
Hey, stop it. I wouldn't say that. Maybe a little nuts, but there's nothing wrong with that.
However, I did have to deduct a point for spelling.
That's OK, all the extra U's are worth a point each, so I'll still come out ahead in the long run.
