Vulcanite Drilling Speeds and Physics

[osiris01]

G'day folks

I have a question that has been rattling around this empty head for a while and I have been unable to find any relevant info that will help.

I drill vulcanite rod on a pillar drill but rather than chuck the bit, I chuck the rod and have a chucked jig on the table holding the bit stationery. As I'm sure lathers will know, this acts to pull the bit into the center of rotation and thus centers fairly impressively. Now I'm making a few assumptions about the physics at play that I'm not qualified to make, but my understanding is the spinning rod is subject to centripetal/centrifugle (can't remember which) force which effectively pushes the rod away from the center of rotation. When the bit makes contact with the rod, an equal and opposite force pulls it into the centre of rotation. Assuming this is correct, the greater the force acting on the rod (i.e. the faster the rod is rotated), the greater the force acting on the bit. And therefore, in theory, the more RPMs, the greater the benefit of this
centering action. But of course, you can't just crank the drill into overdrive without melting the bit into the vulcanite.

My primary objective is to establish a spin rate that maximizes the centering without creating too much heat. I appreciate that it is a lot more complicated than that since bit diameter,feed speed, technique etc. all need to be taken into account, but I'm looking for a ballpark figure - a starting point. So my question is: what is the general consensus on RPMs when drilling vulcanite rod?

But I'm also interested to know if this centering is something that you pro lathers consider when establishing chuck RPMs, or do you just calculate speed based on material and bit diameter and consider any centering an added bonus. I have done some experiementation and have found that you can crank it up fairly high without burning the rubber if your technique is sound, and that there is very little difference in cut quality between a hole drilled at 400 RPMs and a hole drilled at 1300 RPMs.However, I'm (figuratively) burning through a heck of a lot of rod.

I apologise if I have articulated this question in a completely arse-about-face fashion, but from a centering perspective, it seems that faster is better if you can keep the heat down, but I can't help feeling that I am missing something major. (I use sharp 9/64 split-point ti coated Dormer bits.)

Many thanks folks.

Geoff

 

[georged]

You are massively oversimplifying the situation, I think.

Here is the approach that MUST be used. (Ask any established pipemaker. They disagree about many things, but not this.)

 

[osiris01]

This is a hobby George and I just asked for help.

I wasn't asking anyone to calculate it; I asked "What is the general consensus on RPMs when drilling vulcanite rod?". I also mentioned "ballpark" and "starting point". My musings on which forces caused centering, were just that. I came here for advice, not to be told I am too halfwitted to drill a stem.

 

[georged]

.

 

[jhowell]

"Rifle drilling" (spinning the work-piece against a stationary bit) has been used for centuries. It is naturally self-centering. Speed differential has relatively little effect on the process. Better to concentrate on the most effective cutting speed (effective surface feet per second obtained by the bit) and most effective feed rate... I think more benefit can be obtained by optimizing the grind on the cutting angle and profile of the bit (see: Holtzer - The Anatomy of a Cutting Edge)

 

[doug535]

And "most" pipe makers use a tapered Fuller bit to drill from the tenon side. If you haven't already, check out pipemakersforum.com lots of info there.

 

[mingc]

Geoff, I don''t know what georged was thinking, though I can hazard a guess (Shrodinger Equation!). And I'm no pipe maker, or any other kind of maker, but I think you might be overthinking it a wee bit.

 

[jhowell]

Geoff, I don''t know what georged was thinking, though I can hazard a guess (Shrodinger Equation!). And I'm no pipe maker, or any other kind of maker, but I think you might be overthinking it a wee bit.

Agree

 

[elessar]

There is no rpm/centering relationship that I know of. Slower rpm is more forgiving. Spinning faster won't help centering. Centering is achieved with proper cutting geometry (even grind) and good drilling alignment. Set your speed based on the material. You will probably have to just get a feel for it. 500rpm seems like a good start but slower won't hurt. Peck drill so as not to clog up the bit.

You can rifle drill spinning either the bit or stock. It is the bit geometry of the gun drill that does the magic. They also pump pressurized oil through the gun drill. Probably easier to do that with a stationary bit. I have gun drilled with both scenarios.

 

[osiris01]

Thanks for all your comments folks. I would normally try to reply to each, but it is late in the UK and my bed awaits.

But in general, yes I do overthink things, but it is more a natural curiosity than anything else - if I don't know why something is happening it drives me nuts even if it is totally irrelevant. It's interesting that there seems little relationship between RPMs and centering, and that may explain why my experiments were less than conclusive. I don't actually have a problem with centering, but I'm not particularly happy with the smoothness of the bore and I can't work out whether it is because my RPMs are either too high or too low, or my feed rate too fast. I'd not really considered bit geometry other than a split point seems to lock on to center more successfully. The reason for posing the question as I did is because I had assumed a relationship between speed and centering and didn't won't to lose the centering at low speeds. But to sum up the comments here, it seems I have overestimated the importance of speed and need to concentrate more on feed technique and swat up on bit geometry.

Regarding full tapered bits - I would have to import from the US. Not a show stopper, but I use tapered points to knock out shoulders and provide a good center for the 1.2 mm I use at the slot end (I do all the drilling from the tenon end), and I'm happier with the results than I expected, so they have dropped down the must-haves for a while.

Thanks again for all your comments - much appreciated. And thanks to George: I now know what I need to consider next time I drill a hole through an orbiting satellite .

 

[jhowell]

I don't actually have a problem with centering, but I'm not particularly happy with the smoothness of the bore

Drill slightly undersize and them ream to desired size - OR - I lap my stems first with valve grinding compound then with Flitz (tm) on a pipe cleaner...

 

[osiris01]

Drill slightly undersize and them ream to desired size - OR - I lap my stems first with valve grinding compound then with Flitz (tm) on a pipe cleaner...

Hmm, hadn't thought of reaming - another something for the US shopping list. I had tried Tripoli on a cleaner but either I'm doing something wrong, or Tripoli doesn't have enough cut. Valve grinding compound sounds fairly hardcore so I might hunt for something like that. With that said, I drilled a couple of short sections today but slowed everything right down. Drill speed at 400 and a very slow, lub'ed feed, and when I split the rod the results were much better than I had been getting (apart from my mitre saw gasping its last half way through). I suspect I had been generating too much heat - going too fast thinking that I'd lose centering on slower RPMs and feeds. I think, as with most things, practice and technique are the keys, and I very much appreciate the leg-up from the folk here. Thanks again.