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How I Set Up and Sharpen Captive Hollowing Tools (Read 2,368 times)
 
Vaughn McMillan
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How I Set Up and Sharpen Captive Hollowing Tools
Apr 4th, 2009 at 5:54am
 
Paul G's post in Multimedia File Viewing and Clickable Links are available for Registered Members only!!  You need to Login or Register raised some questions that I think a lot of us have had at some point about setting up and sharpening the cutting bits on they typical hollowing rig. There are probably a bazillion ways it can be done, but I figured I'd show how I do it, right or wrong. I'm not trying to describe the whole hollowing process, just how I set up and sharpen the cutting bits.

First, I make sure the cutting edge is exactly on the centerline of the piece. I almost always start my hollowing with a Forstner bit to give me room to work and a place for the initial shavings to go. The point of the bit leaves a nice hole that's exactly at centerline, so it makes a handy reference for setting the tool rest height to get the cutter centered. In this picture, I've already hollowed most of this piece, but you can still see a little dot that's the sign last of the drill bit. To the left of the dot, you can see the top edge of the cutter. To the right of the hole, you can see a scratch that I made with the cutter to draw a line, showing the exact plane that it's traveling. (The lathe was turned off, of course.) You can see the line runs straight to the center hole, showing it's lined up just right.

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The cutting tip acts as a small scraper. It's sharpened at the same angle as all my scrapers. It doesn't ride the bevel, but cuts with a burr, just like a scraper. Here's a picture of the cutter touching the surface of the wood. It's the same on the inside, just harder to photograph...

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The question has also come up regarding how far the tool rest should be from the opening of the piece. I usually try to keep it around 1/2" to 3/4" away. I want it as close as reasonably possible, but not so close my hand hits the piece while holding the boring bar. For scale, this is a 1/2" boring bar...

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As I mentioned, I sharpen the cutters the same as my handled scrapers. To make it easier to see in the pictures, I used a 3/8" cutter. The 1/4" cutter is done exactly the same way, it's just smaller.

This poorly-focused picture shows the angle at which the tool rest is set on the grinder...

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I simply sweep the cutter in an arc, following (or forming) the curve I want in the tip. I hold the cutter down close to the grinding wheel, and sweep the other end in arc as I grind. You'll notice the curved scratches in the tool rest on my grinder. That's from sharpening hollowing bits and bowl scrapers. Here are a couple views.

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And one more shot, showing the angle of the grind stays the same as the bit is swept through the radius...

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I didn't take any photographs, but I sharpen the teardrop scrapers in the same way, and at the same angle.

Like I said, this is by no means the only (or best) way to set up and sharpen these bits, but it's what works for me. I hope this helps.  Wink
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« Last Edit: Apr 4th, 2009 at 2:27pm by Vaughn McMillan »  
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Re: How I Set Up and Sharpen Captive Hollowing Too
Reply #1 - Apr 4th, 2009 at 7:33am
 
Thanks Vaughn

I changed the links to images so it is easier to view, hope you don't mind.
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Re: How I Set Up and Sharpen Captive Hollowing Too
Reply #2 - Apr 4th, 2009 at 9:47am
 
Thanks Vaugn,
I do the same thing as you, only I have the tool handle (boring bar)end slightly higher so when I have a catch it doesn't dig in(self feed). The cutting tip is on center cutting.
Great pictures showing how it works! Smiley
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Vaughn McMillan
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Re: How I Set Up and Sharpen Captive Hollowing Too
Reply #3 - Apr 4th, 2009 at 2:15pm
 
Ron Sardo wrote on Apr 4th, 2009 at 7:33am:
Thanks Vaughn

I changed the links to images so it is easier to view, hope you don't mind.


I don't mind at all. I was just trying to keep the page load times fast for the dial up guys. Wink Would it help if I changed them to 600 pixels wide? (I have the smaller pics already on the server.)
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Re: How I Set Up and Sharpen Captive Hollowing Too
Reply #4 - Apr 4th, 2009 at 2:24pm
 
Sure, if that is not a problem.
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Vaughn McMillan
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Re: How I Set Up and Sharpen Captive Hollowing Too
Reply #5 - Apr 4th, 2009 at 2:27pm
 
Ron Sardo wrote on Apr 4th, 2009 at 2:24pm:
Sure, if that is not a problem.


Done deal... Wink
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Re: How I Set Up and Sharpen Captive Hollowing Too
Reply #6 - Apr 4th, 2009 at 2:55pm
 
you make it look so easy.  thanks for the fabulous tips.  will have to try this someday. smiley=thumbsup.gif smiley=thumbsup.gif smiley=dankk2.gif smiley=dankk2.gif smiley=dankk2.gif smiley=dankk2.gif smiley=dankk2.gif smiley=dankk2.gif smiley=dankk2.gif smiley=dankk2.gif
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Re: How I Set Up and Sharpen Captive Hollowing Too
Reply #7 - Apr 4th, 2009 at 4:02pm
 
Vaughn,

Thank you for taking the time and effort for doing this..  It will really help...   not just me but for others...

I know that I am not alone in the mysteries of sharpening....   I am getting there --  need to practice..

Paul
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Re: How I Set Up and Sharpen Captive Hollowing Too
Reply #8 - Apr 4th, 2009 at 6:45pm
 
Question for you guys already hollowing..

It looks like Vaughn sharpens his to about a 45° angle. In the link Wizzy gave this one  Multimedia File Viewing and Clickable Links are available for Registered Members only!!  You need to Login or Register Stan from Elbo tool he has his grinding table set at 7° below horizontal. What would be the good or bad points between either degree of angle? their both using the burr & no bevel. Could it be that with Vaughn's angle there is no way he would have the bottom of the cutter touch the wood & with Stan's it's a possibility?
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Re: How I Set Up and Sharpen Captive Hollowing Too
Reply #9 - Apr 4th, 2009 at 9:29pm
 
Chuck, I think that Stan is simply taking out some insurance.  Just making sure there is no bruising by the heal of the tool.
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Re: How I Set Up and Sharpen Captive Hollowing Too
Reply #10 - Apr 4th, 2009 at 9:49pm
 
These bits are scrapers, so while a 45° bevel is technically sharper, it doesn't help you here because you are presenting the bit to the wood at roughly 90° instead of riding the bevel.
As you mentioned, it is important to have adequate relief so the bottom of the cutter stays clear of the wood.

However, the less relief you have, the longer the bit will stay sharp.
Maybe I should rephrase that and say "The more material you have supporting the edge from underneath, the more durable the edge will be."

For me, 45° is "sharper" than optimal and will cause the blade to dull quicker than it needs to. I generally run 60-70° on my 3/16" bits. I grind the 1/4" bits at 75-80° and grind a secondary bevel at the bottom (like Stan shows in the video).
In a typical HF, the place where the bit is most likely the rub is either at the rim or the base depending on how small of a rim you prefer.
Say the smallest opening you expect to hollow through is a 3/4" hole. You can use a penny (3/4" diameter) to lay your bit on and see what angle would provide the needed relief. You will also want this relief to wrap around the point.
Stan seems to prefer larger HF's so he doesn't need much relief.
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Re: How I Set Up and Sharpen Captive Hollowing Too
Reply #11 - Apr 4th, 2009 at 10:00pm
 
Chuck Beland wrote on Apr 4th, 2009 at 6:45pm:
It looks like Vaughn sharpens his to about a 45° angle.


I would disagree with this. While the tool platform looks to be close to 45°, the actual bit looks to be ground at around 65°. The relation between the platform angle and the bevel angle changes depending on how low or high the bit contacts the wheel (relative to the grinder arbor).
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Re: How I Set Up and Sharpen Captive Hollowing Too
Reply #12 - Apr 5th, 2009 at 4:38am
 
Chuck Beland wrote on Apr 4th, 2009 at 6:45pm:
Question for you guys already hollowing..

It looks like Vaughn sharpens his to about a 45° angle. In the link Wizzy gave this one  Multimedia File Viewing and Clickable Links are available for Registered Members only!!  You need to Login or Register Stan from Elbo tool he has his grinding table set at 7° below horizontal. What would be the good or bad points between either degree of angle? their both using the burr & no bevel. Could it be that with Vaughn's angle there is no way he would have the bottom of the cutter touch the wood & with Stan's it's a possibility?


I just checked, and mine are between 65º and 70º as measured by my little plastic Tormek angle checker thingie. Closer to 70º. That's about 20º off horizontal. Keep in mind that the number of degrees below horizontal will differ, depending on how high the tool rest is, and if the tool is hitting above or below the centerline of the grinding wheel. 

I'd say you should try a few different angles and see which ones seem to work best for you. You may find you want to change grinds for different kinds of wood, or that "one grind fits all". If you're already comfortable using a handled scraper, you might start your hollowing bits at that same angle. Then you can tweak it up or down as you see fit. The only right way is whatever feels best to you.  Wink
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Re: How I Set Up and Sharpen Captive Hollowing Too
Reply #13 - Apr 5th, 2009 at 5:44am
 
Kurt, Vaughn
Thanks for the lesson now I understand it better.
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