Shake, Rattle and Roll part 2

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gtriever

gtriever

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Here we go again. I cured the lathe drilling shake last time by giving the pulleys and belt a good cleaning. This time, not so much. The weird part is, acrylic blanks drill just fine - no shake and a perfect hole. With Wood blanks, however, I have shake like crazy and the entry hole is always oversized. I've cleaned, checked the belt, made sure the blanks were square and not rectangular, and I still can't drill a straight and even hole. What the h-e-double hockey sticks is going on here??
 
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The first thing you need to do is check the alignment of your lathe - does the tailstock line up with the headstock?

But even if the lathe is aligned properly, it is possible for the tailstock to not be centered exactly on the axis of the rotating blank. When that happens, there can be visible wobble in the drill bit, the hole will be elliptical, and if the blank is thin and you are using a very small drill bit, you can end up with the hole going through the side of the blank. DAMHIKT.

On my lathe, there is a very slight amount of backlash in the mounting of the tailstock - the spacing between the bedways is a fraction of a milimeter wider than the protrusion on the bottom of the tailstock, which means that the tailstock can rotate a fraction of a degree until it is locked down. And if the tailstock can rotate slightly, there can be a noticeable shift in the tip of a drill bit - and the longer the bit (and the greater the extension of the tailstock ram), the greater that shift will be. I've compared notes with others who own the same lathe, and they report the same characteristic; I suspect this is typically of less expensive midi-lathes.

The solution is to first flatten the end of the blank, and then use the tip of a skew to cut a dimple in the exact center as the blank is rotating. Then, bring up the tailstock and center the bit in that dimple before locking down the tailstock. That will assure that the bit is exactly on-center with the axis of rotation of the blank.
 
Thanks, Louie. I know I have some play in the tailstock, and I try to compensate for it every time. But - I still can't figure out why it just does it with wood blanks and not acrylic... ???
 
I have a similar problem when the drill is not clearing the chips fast enough. it gets really bad when I am drilling large pepper mills and the hole is packed with wood chips. I found if I keep pressure on the drill bit chuck with my hand so the drill point settles right on center and I back off frequently to clear chips there is a lot less wobble. Using a center drill first also helps this to assure the drill bit does not land off center. Also I have better luck with twist drills than brad point. Ultimately I try to do all drilling on the drill press, its just hard to get the reach for pepper mills.
 
gtriever said:
Here we go again. I cured the lathe drilling shake last time by giving the pulleys and belt a good cleaning. This time, not so much. The weird part is, acrylic blanks drill just fine - no shake and a perfect hole. With Wood blanks, however, I have shake like crazy and the entry hole is always oversized. I've cleaned, checked the belt, made sure the blanks were square and not rectangular, and I still can't drill a straight and even hole. What the h-e-double hockey sticks is going on here??
Click to expand...

When you change to wood, you probably change the bit (first possibility), you may change your jacob's chuck (2nd candidate)--what else changes? speed?

I would look to the changed variables for a solution.
 
Yes, agree with Louie. Tail stock as well as quill play can have an affect on drilling on center. Yes again, use a skew or other tool to make a center for the bit to start in.
You bits can follow the softer grain in the pen blank and lead it off as you start to drill and as you are drilling... causing the wobble/shaking.
Sharp bits are critical...a dull bit requires more pressure to cut and will follow the path of least resistance...softer grain. You are drilling end grain after all.

Putting a dimple in the end of the blank with a skew works for standard twist drill bits. You can bring the tail stock/not up into the dimple to help align the tail stock before locking down and drilling. Start to drill by advancing the bit into the wood slowly so it is as centered as possible.

If using brad point bits make a small starting point for the point of the bit to start in and again, start slowly till the bit is started before progressing.

Hope this helps.
 
monophoto said:
The first thing you need to do is check the alignment of your lathe - does the tailstock line up with the headstock?

But even if the lathe is aligned properly, it is possible for the tailstock to not be centered exactly on the axis of the rotating blank. When that happens, there can be visible wobble in the drill bit, the hole will be elliptical, and if the blank is thin and you are using a very small drill bit, you can end up with the hole going through the side of the blank. DAMHIKT.
Click to expand...

Am I thinking about this wrong? Because this doesn't sound right at all. The hole would be oversized, but not elliptical.
 
ed4copies said:
When you change to wood, you probably change the bit (first possibility), you may change your jacob's chuck (2nd candidate)--what else changes? speed?

I would look to the changed variables for a solution.
Click to expand...

No speed changes, no Jacobs chuck changes, and it does not matter which bit I use... but only with wood. I'm just about to use my precision alignment tool, aka 5-pound hammer, on it. :biggrin:

I'll try all of the suggestions you guys have posted. Something has got to be causing it!! Now I just need to find it.
 
To get the play out of the tailstock and eliminate that as a possible cause, I cobbled together this little jig this morning out of 1/2" plywood. The jig fits to within .005 inches between the ways, and the front was shimmed with layers of tape resulting in less than .01 inch side-to-side movement. We'll see how well it works for drilling later today. Excuse my poor photography (especially on photo 3) - :rolleyes:

Tailstock-Alignment-1_web.jpg Tailstock-Alignment-2_web.jpg Tailstock-Alignment-3_web.jpg
 
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PROBLEM SOLVED - The jig reduced the shake, but didn't get rid of it. Further troubleshooting showed that the bolt on the motor pivot arm had backed out by 1/2 turn - yes, 1/2 turn! And THAT, my friends, solved the shake problem for now.

Learn something new every day. :befuddled:
 
gtriever said:
PROBLEM SOLVED - The jig reduced the shake, but didn't get rid of it. Further troubleshooting showed that the bolt on the motor pivot arm had backed out by 1/2 turn - yes, 1/2 turn! And THAT, my friends, solved the shake problem for now.

Learn something new every day. :befuddled:
Click to expand...

Interesting.....not on acrylics but on wood, that caused your vibration issue.
Glad you figured it out!
 
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