Question about WTP

[Jim Grieco]

Hello,

Recently I needed to make 4 rings for a project I am working on. All 4 rings needed to be 8" OD and 3" ID. When creating them in WTP, this is what I entered for the OD & ID. When I built the rings after using the cut list from WTP. I then measured the ID of the rings and they all were at 4" not 3".
Usually I am not that critical with measurements in a design so this was the first time I actually measured the rings. Cannot say whether it is always like this or not.
Anyway, what could I have possibly done wrong since I entered the specific ID when creating the rings?

Thanks

 

[mfisher]

Hi Jim.
Would you provide a bit more information.
- How many segments per ring.
- what did your summary sheet say the SEL was for the rings.

 

[Lloyd Johnson]

The formulas to calculate the diameters of inside and outside diameters are well-known trigonometry formulas based on a ‘Unit Circle.’ With the number of segments, a segment edge length is calculated that will make a segmented ring that if you put it on your lathe and remove the minimum amount of wood to make the outside diameter smooth, it will be the diameter calculated by the software.

The inside diameter is then calculated based on the Board Width and again, if you remove just enough wood from the inside to make it smooth, the inside diameter will be as calculated.

The formulas in Woodturner PRO haven’t been changed in 23 years and they are correct.

If your ring has a different inside diameter than calculated, the board width you used was narrower than what was called for in the software.

If you click on the Ring View or the Segment View, you will see a calculation that only shows on those two screens. It shows the segment edge length of the inside (short side) of the segment. On the Ring View, you will have to scroll down to see a box with ALL calculations available. If you cut a test segment, you can print out either the ring view or the segment view and it will be the exact size that the segment should be (this may not be 100% correct depending on your printer). You can lay your segment on it. It is easier, though, just to measure the short edge with a micrometer and compare it to the calculation.

I hope that helps.

Lloyd

 

[Jim Grieco]

The formulas to calculate the diameters of inside and outside diameters are well-known trigonometry formulas based on a ‘Unit Circle.’ With the number of segments, a segment edge length is calculated that will make a segmented ring that if you put it on your lathe and remove the minimum amount of wood to make the outside diameter smooth, it will be the diameter calculated by the software.

The inside diameter is then calculated based on the Board Width and again, if you remove just enough wood from the inside to make it smooth, the inside diameter will be as calculated.

The formulas in Woodturner PRO haven’t been changed in 23 years and they are correct.

If your ring has a different inside diameter than calculated, the board width you used was narrower than what was called for in the software.

If you click on the Ring View or the Segment View, you will see a calculation that only shows on those two screens. It shows the segment edge length of the inside (short side) of the segment. If you cut a test segment, you can print out either the ring view or the segment view and it will be the exact size that the segment should be (this may not be 100% correct depending on your printer). You can lay your segment on it. It is easier, though, just to measure the short edge with a micrometer and compare it to the calculation.

I hope that helps.

Lloyd

Thank you Lloyd. I am not trying to blame your software. As I stated, I entered the info and followed the info. So the only think I can think of is I entered something incorrectly but this is how it looks.

 

[Jim Grieco]

Hi Jim.
Would you provide a bit more information.
- How many segments per ring.
- what did your summary sheet say the SEL was for the rings.

Mike, please see my reply to Lloyd. I am just trying to figure out what I did wrong with the info. I followed it as stated.

 

[Lloyd Johnson]

Thank you Lloyd. I am not trying to blame your software. As I stated, I entered the info and followed the info. So the only think I can think of is I entered something incorrectly but this is how it looks.

I didn’t see any blame on your part, Jim. You are just looking for answers and the software could always be the culprit. I’m very comfortable saying that if actual is ever different than calculated, the problem is with how the stock was prepared or cut. The beauty of the ring and segment views is that it shows exactly what the ring or segment will look like if the stock and cuts were made identical to the calculations.

If you measure the width of the board you cut the segments from, it will be closer to 2.25” wide, not the 2.55” specified. This will only be true if the outside diameter of the ring is 8” which means that your SEL was correct. If it isn’t 8”, the inside diameter won‘t be correct because it is based on the outside diameter.

If your board was 2.55” with an outside diameter of 8”, let me know and I’ll hop on SketchUp and in about 20 seconds I can verify it without going into my shop. I’m on the road now or I would do that now.

 

[Jim Grieco]

I didn’t see any blame on your part, Jim. You are just looking for answers and the software could always be the culprit. I’m very comfortable saying that if actual is ever different than calculated, the problem is with how the stock was prepared or cut. The beauty of the ring and segment views is that it shows exactly what the ring or segment will look like if the stock and cuts were made identical to the calculations.

If you measure the width of the board you cut the segments from, it will be closer to 2.25” wide, not the 2.55” specified. This will only be true if the outside diameter of the ring is 8” which means that your SEL was correct. If it isn’t 8”, the inside diameter won‘t be correct because it is based on the outside diameter.

If your board was 2.55” with an outside diameter of 8”, let me know and I’ll hop on SketchUp and in about 20 seconds I can verify it without going into my shop. I’m on the road now or I would do that now.

I remeasured the segments and I am off on the board width. Not sure how that happened. My bad and very sorry to bother you with questions.
Thanks
Jim

 

[mfisher]

I remeasured the segments and I am off on the board width. Not sure how that happened. My bad and very sorry to bother you with questions.
Thanks
Jim

Asking questions is never a bother.

As Lloyd pointed out board width and SEL are important in having rings come out with the dimensions we want.
I personally tend to overboard by adding .125 to .25 to the OD and ID of the ring. It means turning more away on the lathe to desired dimensions.
I always cut a few segments to check the SEL is at spec or at least within reason.


As I got more confident in my build it is now down to .125 fudge factor.

Have a good one. Hope your project comes out ok.

 

[Lloyd Johnson]

I’m glad we had this discussion. I could have answered that your board width was wrong but the explanations of how the calculations define the bowl will give you and all others that read this thread an better understanding of the software and segmenting.

 

[Wayne Miller]

Jim is working one of my designs. It is my donut vase which I have made about 30 times so I know how it should work. I have done this demo both at AAW andd Segment Sympoia.
He also asked me if I could seee an issue. I sent him my WRPro dims. I'm glad to see a resolution. While being exact to 3 decimal places is not necessary, with board width and SEL you must be as close as possible. I'm happy that he has a resolution. Lessson learned for all.
Wayne