Gunsmithing Question on Grizzly/Gritters Rods....

[Wannashootit]

When chambering through the spindle, I've always used a correctly fitted gage pin to get me in the ballpark quickly, then direct indicated.
Always wanting to try other ways, just ordered a couple of Grizzly rods figuring I'll give that method a shot and see how it works out.

What I don't "get", is the end of the rod being held in the tailstock, usually via a chuck from what I've seen but I suppose a collet would do the same. Seems to me, that this method aligns the bore to the tailstock- and there's no tailstock I know of that's perfectly aligned to the lathe's axis of rotation. If it were, there'd be no need for a floating holder- in fact, I'd see it as detrimental. Perfectly aligned? Drill, bore and ream with the reamer rigidly held in a collet.

Exaggerating to make the point, I see this as the rod bending around as the bore is gradually dialed in as the tailstock end is rigidly held. When done, the bore is a straight line to the tailstock center. Many tailstocks have wear, usually uneven and tilting downwards unless it's been shimmed. New lathes can be a couple thou high from the factory, to allow for some wear-in.

I must be thinking of this incorrectly, as it would make a lot more sense to me to just let the rod end float and not hold it, but I know it would likely deflect from it's own weight, even IF it were perfectly straight (and I'd bet they're probably not).

Really DON'T want the thread to devolve into the gazillionth thread on "why my way is the best way to chamber or crown"- just hoping that someone can explain why holding the rod end in a chuck makes sense...

 

[A&8's]

Before I indicated with a long reach indicator, I used range rods held in the tailstock. Out of curiosity one day, I indicated with range rod, then check with long reach tip. There was 0 difference. Now, maybe it was a fluke. ??‍

 

[Rubicon Precision]

You are thinking of it incorrectly. The alignment of the holder 11”-12” away is insignificant. You are measuring movement on the rod. The rod is not spinning.

 

[Wannashootit]

^^^
Nah, I get that.
After mulling it some more, I think I've got it visualized now.

Tailstock "alignment" is complicated. Sure, we can put a test bar between centers and indicate vertical and x-axis alignment at whatever location on the bed we want to check. But that only tells a small part of the story, as tailstocks are often worn more at the front resulting in a downward cant of the spindle as it's extended. One of my lathes had this issue- check with a test bar, perfect at the tip of the center with the tailstock spindle retracted. Extend it, and there was .003 drop over the length of the extended spindle. Now, put a 12" ground rod in a collet or chuck- and extend that drop over another foot. Not even close to being in alignment with the center of the headstock's axis of rotation.

Now, setup a blank in the headstock spindle and direct dial it in true. This way, the bore will run true to the spindle's axis of rotation. Nothing to do with a tailstock; forget it's even on the lathe bed.

Then, extend that 12" rod from the tailstock. Doesn't matter if it's out many thou as in the example above- or just a couple- it's still "out" of alignment with what's been dialed in with the headstock. Despite the barrel's bore being dialed in dead-nuts with the headstock spindle, the Grizzly rod WILL move and bend like a two dolla Ho, showing runout as relates to the tailstock spindle's centerline, projected axis.

By holding the rod in the tailstock, seems to me the bore is actually being aligned with the tailstock, and it's axis- and not the headstock spindle's axis. By being able to gimbal the barrel at each end, one is free to set that barrel anywhere in space in that spindle (not necessarily dead center) so that runout is zero when a rod is extended into it from the tailstock.

In theory, seems to me this is what we'd all actually want- knowing that a tailstock is never perfectly aligned at any spot on a lathe bed.

In fact- if I have this right- getting this alignment dead nuts with a Grizzly rod would seem to negate the need for a floating holder.
IF the bore is in perfect alignment with the projected tailstock centerline, boring and then reaming in a rigid holder would seem to have no downside.

Anyway, sorry about the novel.
I'm a guy that needs to understand the theory behind whatever I do... please rip apart my "logic", or nod in agreement if I've "got it" correct.

 

[A&8's]

^^^
Nah, I get that.
After mulling it some more, I think I've got it visualized now.

Tailstock "alignment" is complicated. Sure, we can put a test bar between centers and indicate vertical and x-axis alignment at whatever location on the bed we want to check. But that only tells a small part of the story, as tailstocks are often worn more at the front resulting in a downward cant of the spindle as it's extended. One of my lathes had this issue- check with a test bar, perfect at the tip of the center with the tailstock spindle retracted. Extend it, and there was .003 drop over the length of the extended spindle. Now, put a 12" ground rod in a collet or chuck- and extend that drop over another foot. Not even close to being in alignment with the center of the headstock's axis of rotation.

Now, setup a blank in the headstock spindle and direct dial it in true. This way, the bore will run true to the spindle's axis of rotation. Nothing to do with a tailstock; forget it's even on the lathe bed.

Then, extend that 12" rod from the tailstock. Doesn't matter if it's out many thou as in the example above- or just a couple- it's still "out" of alignment with what's been dialed in with the headstock. Despite the barrel's bore being dialed in dead-nuts with the headstock spindle, the Grizzly rod WILL move and bend like a two dolla Ho, showing runout as relates to the tailstock spindle's centerline, projected axis.

By holding the rod in the tailstock, seems to me the bore is actually being aligned with the tailstock, and it's axis- and not the headstock spindle's axis. By being able to gimbal the barrel at each end, one is free to set that barrel anywhere in space in that spindle (not necessarily dead center) so that runout is zero when a rod is extended into it from the tailstock.

In theory, seems to me this is what we'd all actually want- knowing that a tailstock is never perfectly aligned at any spot on a lathe bed.

In fact- if I have this right- getting this alignment dead nuts with a Grizzly rod would seem to negate the need for a floating holder.
IF the bore is in perfect alignment with the projected tailstock centerline, boring and then reaming in a rigid holder would seem to have no downside.

Anyway, sorry about the novel.
I'm a guy that needs to understand the theory behind whatever I do... please rip apart my "logic", or nod in agreement if I've "got it" correct.

You're indicating, or at least I do, about .25-.50" from the barrel, not 12" away. So, the flex (assuming your tail stock is out) has no bearing on the pilot wobbling. Then when inserted 2-3", the pilot is still stabilize the section of rod from bearing to indicator. "Ideally" we would like to see zero-zero at end, as well as when inserted 3" in. However, when checking at, say, end...1" in, then 3" in, regardless of the dial reading, as long as each point has a run out of the same (at it's respective point),

Tail stock wear and way wear is the reasoning behind using a floating reamer holder. If, by chance, the wear point coincide with the 2-3" of stroke (start to finish of chamber), the floating holder compensates. With a rod, and indicating .25" from barrel, you may not see a difference on the dial, due to rod flex...even thou your TS may be .005" out of kilter. This is why, despite indicating dead nuts with a rod, it is best to use a floating holder.

 

[Rubicon Precision]

Quick bored in hotel room sketch...
Dashed line is the spindle centerline. To the right is a misaligned tailstock holding the "Grizzly Rod". Top two sketches are with the bore misaligned, 180 degrees apart. Bottom is the bore aligned to the spindle centerline.
Again, the tailstock alignment has nothing to do with this method. I don't even have a tailstock on the lathe I chamber on but still use a similar method.

 

[Danny1788]

it is because the rod flex’s and the bore bushing doesn’t. The misalignment from the tail stock is a constant. So what you are trying to do is use the bushing as your contact tip and the rod will be in a constant flex correct me if I’m wrong...

I have used range rods, and grizzly rods and long stem intrepid indicators. Fastest for me is rough in with a range rod then use the long stem to get the bore running perfectly straight. JGS Floating reamer holder takes care of the rest. Don’t over think it.

 

[Easy_E]

Correct me if I'm wrong the only issue would be with a tail stock not square the indicator won't share the same zero at the throat and breach ? This is what I fight on my setup there's about .001 I can't get out with the rod so I switch to the .005 long stem and finish up.

 

[Rubicon Precision]

Correct me if I'm wrong the only issue would be with a tail stock not square the indicator won't share the same zero at the throat and breach ? This is what I fight on my setup there's about .001 I can't get out with the rod so I switch to the .005 long stem and finish up.

You’re never going to have the same indicator “zero” at two points in the barrel with this method. You don’t need to. All you are measuring is movement.