Some time ago, I was asked to make a cremation urn for my nephew’s dog. I have turned hollow forms but never turned a cremation urn. I spent the next several days on the web researching the topic. I had to figure out how to be sure the urn was going to have adequate capacity to hold the dog’s ashes as I would be shipping it to Texas. Here is what I learned after a great deal of searching, trial, and error. I hope this will help you avoid all of the research time I spent online.
The volume of cremated ashes, no matter if they are animal or human, is the same. You need one cubic inch of space for every 1 pound of live weight. For example, if the dog weighed 23 pounds, you need a volume capacity in your urn of at least 23 cubic inches. I would allow at least five cubic inches for errors in weight.
The next problem is determining how big your piece of wood needs to be. How do you tell how many cubic inches are inside of your hollowed vessel? This is what I learned.
There are 14.44 cubic inches in 1 cup, so at one cubic inch per pound, you need 1 cup of space for every 14.44 pounds of live weight. I found that rice works great to determine capacity. Buy yourself a 2-pound bag of rice and keep it in your shop for measuring. Take the weight of the animal, for example, a 23-pound dog and divide it by 15(14.44 rounded up) to determine the number of cups, 23/15 = 1.53 cups. I always round up to the nearest cup. Take a baggie and pour 2 cups of rice into it. Squeeze out the air, and now you have something that will give you a pretty good idea of how large a piece of wood you will need to hollow out this much volume. When you are hollowing, and you think you are getting close to the size you need, remove your chuck from the lathe, leaving the urn attached and stand it upright. Pour in the rice. If the rice fits in the urn with some room to spare, you’re there. Remember to leave a little extra room as most people know about how much their pet weighs but probably not exactly. You would not want them to come up short when they transfer the ashes to the urn, so error on the high side. Also, make sure you leave extra space for your lid if it will be glued into the top or bottom of the urn. Make sure you make the hole in the top or bottom of the urn large enough to facilitate the transfer of the ashes without a problem. You do not want a really small hole here.
If you are going to get into making cremation urns and plan to sell them, you should also know that many states require that the lid of a Cremation Urn must be threaded.
There are many ways to accomplish this from threading the urn to creating a threaded insert that you can glue into the neck of the urn. There are many good YouTube videos to help you find the solution that suits you best. You might also ask a club member experienced in threading to make you an insert for your urn or help you to do it.
If you want to try hand chasing threads, I would recommend that you ask your local countertop company for some Corian cut off scraps. Corian is one of the best and easiest materials to chase threads in. Most cabinet shops are more than willing to give you some scraps. You only need a couple of pieces about 4 inches square, depending on the size of the hole in your urn. I would get a couple of spare chunks to practice with. If you do get Corian, cut a circle larger than you need on the band saw and then use hot glue to glue it to a waste block for turning. From that point on, you follow the same instructions you would use for making any insert for a hollow form lid and use the same tools. You will need to buy or borrow thread chasing tools to do this work.
My second recommendation is for you to view the 3 YouTube videos by the Wyoming Woodturner, Sam Angelo.
They are titled
THREAD CHASING: HAND CHASED MALE THREADS IN WOOD PART 1
THREAD CHASING: HAND CUT FEMALE THREADS IN WOOD PART 2
THREAD CHASING: MATING FEMALE AND MALE THREADS
These are excellent videos and will have you chasing threads in no time. One thing to mention is that he has changed his opinion on speed and now recommends that you turn at around 300 to 350 RPM’s when you chase threads.
I hope this helps you out and answers some of those questions you may have.