More hollow form questions.

[Dan Forman]

Using an adjustable angle bit holder to hollow the inside of the form, what is the maximum distance you like to have the bit projecting from the holder, using a 3/16" bit? What is the furthest you like to have projecting from the back (opposite of the cutting end. What length bit do you prefer?

What is your preferred technique and tool(s) for hollowing the inside. By this I mean, given that you have a hollowing system, regardless of manufacturer, what do you put on the business end of it for initial hollowing and for smoothing it out after?

I did my fist one last night, but could use any tips you might care to share, especially for the more tricky top sections.

Dan

[Don Geiger]

Hi Dan:

I'll try to answer your questions without writing a disertation.

1) I suggest keeping the distance a bit extends from any holder to an absolute minimum. Keep in mind that to minimize vibration you need to minimize the distance away from the support. Mass needs to be considered too. The bit (in a lot of cases is only 3/16" square) has the least amount of mass of the entire system. Keep it short to minimize vibration and maximize results.

2) The distance the bit extends out the back of the holder will interfere when you remove and re-enter the system through the hole which is done very frequently. Keeping it short will increase the ease of removal and re-entry.

3) For roughing out a piece I suggest using 1/4" square bits for the straight hollowing (near the center) and 3/16" square bits when working off center. The difference is the amount of edge in contact with the wood affects the friction. Friction results in the downward force against the hollowing system. When off center minimizing the force is important. Maintaining a burr on the bits (remember these are just small scrapers) is very important.

For getting a nice smooth interior surface, I suggest going to a tear drop scraper (with a burr). Position the scraper so it has a negative rake angle. When using a captured rest or an articualted system, where everything is held in a horizontal position, I have a holder that attaches to end of my hollowing bar that has a 6 degree declination so my tear drop scraper is automatically at a negative rake angle. I also rotate it so I get a shear scrape angle as well.

I buy 10% Cobalt square bits from machinist's supply company by the box. they are much longer than necessary so I use my grinding wheel to score the bits at the length I want and then put them into a vise a snap them off. I then sharpen both ends. This way I can just flip them around when the burr is worn off. Fortunately, I have square broached holes in some of my bit holders so I don't have to round one end of the bits.

Good luck!

Don Geiger

[Robert McGowen]

Dan pretty much summed it all up. I would add that I don't have any part of the bit sticking out of the back side of the holder. I cut my bits short enough that just the cutting side sticks out from the holder. Having too much bit hanging out the back is a good way to have a larger than expected hole with which to hollow through. :)

[Bernie Weishapl]

Ditto what Don and Robert said. I leave very little sticking out the back. Most times I cut my cutting bits in half.

[Jim Burr]

Longer bit extension = more vibration = shallower cut's :cool:

[Steve Schlumpf]

I will agree that the closer your cutter is to the support, the more stable it is. There are a number of bits, extensions and arms that you can mix and match in order to reach some areas of a turning. While not generally recommended, there are times that a full length 3/16" cutter extended as far as possible in the holder is the only way to reach deep into a shoulder area. Course, you have to take very light cuts with a sharpened cutter to minimize any vibration.

The idea being, you have options as to what tool to use for any given form. Keep in mind that the closer everything is to the support - the less vibration you will experience.

Another thing to consider is the width of cutter. The further away from the tool rest - the smaller diameter of cutter used. Takes a little longer but makes a huge difference in torque/vibration applied to the tool and provides a much smoother cut.

Have fun!

[Dan Forman]

Having too much bit hanging out the back is a good way to have a larger than expected hole with which to hollow through. :)

Yup, that was my experience too. :D

[Dan Forman]

I didn't know it was so easy to trim the bits, so to minimize the length hanging off the back, I had more that I thought was wise projecting from the front of the bit holder, and I really had to watch the chatter. Thanks for all the help, these things are fun, and only half the sanding!

Dan