Details:
There are times when one turns an object such as a large platter (or bowl) where because of the size (or shape), it is not possible to use the calipers we have on hand. One quick method is to chuck a pencil in a drill press (sharpened end into the chuck), then place a flat piece of wood on the bed of the drill press and position the object to be measured under the pencil .
Measurement method
Place a piece of paper in the area to be measured and bring the chuck (pencil) down until it lightly touches the paper. Now with this light pressure maintained, adjust the stop nut on the drill press so that when it is bottomed out, you can just feel slight resistance when moving the paper.Remove the item being measured and again bring the pencil down until the stop nut bottoms out, while holding a tape measure (or other convenient device) against the gap and read the length of the gap, in example shown, we read 5/8” This is your thickness of the item measured from the extreme bottom to the top in the area where you held the paper. For items that have a (designed) profile on the bottom resulting in thickness variation, one would have to repeat this process as many times as necessary to get a good understanding of the thickness profile.
Considerations
The paper thickness is in the order of .005”, so does not contribute appreciably to error.Don’t forget to allow for any undercuts underneath the object being measured to get the true thickness of the bottom. One could also use a small dowel instead of a pencil, however I find that the rubber tip of the pencil is less likely to leave marks if too much pressure is applied accidentally.
Details:
When selecting green wood (or any wood), one must ensure that the piece is sound and has no defects hidden within. Crotch wood (place where two limbs meet) is usually selected because of the “flame” pattern that is displayed when the part is turned cross axis as in a bowl. The part is oriented so that the inside material becomes the bottom of the bowl, so that in this manner, the flame pattern remains in the bottom after the bowl is finish turned.
Safety considerations
It is especially necessary to be very careful with this type of wood because a fault line can be present along the intersection where the two limbs meet or the material might have a rotten spot where water got in over the years. The other possibility is that the bark actually grows and meets inside resulting in a very weak bond. The parts can separate during turning a sever injury can result personally. The attached photographs show a 12” part that was turned and a major part of the blank separated and was propelled across the shop, luckily with no major consequence.
Recommendation
I would discard any wood that shows signs of cracks or bark inclusions, and even if you have a considered “suitable” piece stop, frequently during turning to recheck the integrity of the blank.
I recently invested in an adapter that makes it possible to mount a chuck on the tailstock side of your lathe. The feature of this is that if you are turning a bowl and need to remount it from the other side (to finish the bottom while using jumbo jaws for instance), you can do this by clamping down on the 2nd chuck (jumbo jaws) before removing from the first. This method allows you to maintain the trueness of the centers and will ensure that the outside and inside of the bowl are on center and concentric (read uniform wall thickness)
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Other Uses
In turning small hollow forms such as vases, I usually like to part off the part right on the lathe (without leaving a spigot) so that the part doesn’t require a lot of post operations such as hand carving and sanding. The issue with this is sometimes the fibres right in the center of the wood decide to pull off abruptly and the vase can go flying across the shop if you weren’t quite ready to catch it with your right hand. The other aspect of this is that I like to use a skew to cut the bottom after making an opening with the parting tool, this requires 2 hands to apply sufficient cutting pressure.
I found that if you use the adapter in the tailstock and mount the jumbo jaws, then clamp around the lip of the vase, you can part the vase from the waste block without any danger of losing the vase. Just make sure the jumbo jaws securely capture the lip of the vase (some designs might not make this possible).
Once you part off the vase, don’t be surprised when the vase continues to freewheel after the lathe is turned off , it took me by surprise the first time.
I started making hollow forms up to 10” high in July/06 and at that time was using the ‘Stronghold” chuck to hold the work. The wood was green and I found myself losing the part off the lathe quite often during hollow-forming. Needless to say, this was not a good learning curve! In order to overcome these “flying” rough hollow forms, I made sure that the chuck was tightened each time I shut the lathe down for cleaning out debris.
Details: Left top, 4” and 6” faceplates with #12X1” screws shown, right top are 2.5” and 4” faceplates from LeeValley using #10 X 1 ¼” screws.Bottom left shows that the #12 screws are slightly loose in the 6” faceplate while the #10 screws in the 4” aluminum faceplate are snug. Bottom right shows the color code on the heads for matching to the faceplates.
I like to wear a full face shield when turning to keep chips off my face and also to prevent against a mishap with a flying piece of bark or shattering of a turning (or crazy glue in the eyes). Your choice of face protection may be different however shield cleaning methods would be similar.What I found was that in order to keep the shield clean, I was wiping it regularly with a dry paper towel in my shop, and within 6 months, the shield was so scratched (with fine markings) that I had to purchase a replacement.