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Apr 24, 2019 - 9:47:47 PM
188 posts since 8/23/2011

Why does the last piece of my blockrim not fit in, when the dimensions of my trapezoids are true to plan?

This is my third blockrim, I think I'm getting better, but still have to fudge the last piece in.

Do you all experience this too?

I am using 10 segments for a 12" rim that is 3/4" thick.

My angles are 18 degrees, or 72/108 depending on how you measure them, with the long side measuring 4 inches.

 

Thanks guys,

Matt


 

Edited by - DiddyBum on 04/24/2019 21:49:16

Apr 25, 2019 - 4:58:24 AM

1403 posts since 6/2/2010
Online Now

my guess is because your angles are not exactly 18 degrees. On my miter saw I can try to set it to any degree I want between 0-45 but there is also some play in those settings.

Apr 25, 2019 - 5:20:46 AM

2526 posts since 2/18/2009

I had the same problem with the miter saw, also the fence often gets a little bent or moved so the reading on the table is no longer accurate. Now I cut segments on the band saw and I have a miter gauge that is set to 10 degrees which is what I use for 18 block rings. I tweaked the angle and kept cutting blocks till I got a perfect circle, and since then I have not moved that miter gauge. It should stay aligned unless I drop it on the floor, and then I'd have to do the whole process over again. The problem is that a very tiny fraction of a degree gets magnified over however many blocks you have in your ring, so it's very sensitive.
Zach

Apr 25, 2019 - 5:33:27 AM

1239 posts since 10/13/2004
Online Now

Matt, here is my procedure for building a block rim.....First, may I ask how you cut the inside of your rim after you have glued all your segments together?.....

The procedure I use will work the same whether you use 6, 8 or 10 pieces, as long as it is an even number.....

I use 6 pieces, but in your case of using 10 this will work the same, so here's what I do......I glue up 1/2 of the segments together...Once I have the two halves glued together I sand at the same time the two exposed edges on a belt sander.....Lay the two halves together (they should fit perfectly together after sanding) without gluing them and mark the inside radius of the rim.....Saw the inside radius out on the bandsaw and then glue the two halves together......Once all of the layers are finished I run each layer through my thickness planer and then glue the layers together.....I usually don't cut the outside radius until all the layers are glued up.....I then bandsaw the outside radius with all the layers glued together........Ready for the lathe now......Works for me, too old to change now....

Clancy
"Mullins" Banjos

Apr 25, 2019 - 6:20:34 AM
Players Union Member

rudy

USA

13885 posts since 3/27/2004

Your angle or segment length is slightly off.  Hypothetically you should be able to get a perfect ring, but in the real world that's seldom possible.  Your example photo is a bit extreme, so you should be able to get your rings closer by modifying your angle or length slightly.  I do my block rims exactly as you show, gluing the segments end to end and fitting the last piece.  By doing it that way you get stress-free butt joints at your segments.  The butt joint end grain glue joint is poor gluing practice, but it's good enough to hold the rings until the adjoining offset layers are added.

Photos of my glue up procedure and a pdf for segment patterns for an 11" or 12" rim can be found HERE.

Here's the segment guide pdf:


Edited by - rudy on 04/25/2019 06:27:50

Apr 25, 2019 - 6:56:18 AM

424 posts since 7/10/2012

I don't know if this is helpful for this discussion, but I found kind of a cool way to get all of the angles identical quickly. I cut all of the blocks as rectangles, clamp them all together really tight, then run the stack against an angled table saw blade one side at a time. Then I lay them out and, if its off a bit, I re-stack and shave a bit off the sides. If I start out a hair longer than I need, I can nail the angle down and then trim up to the length I need.

Then I glue up two halves and sand the ends to line up just like Clancy cause its still off in the end :)

Apr 25, 2019 - 11:19:51 AM
Players Union Member

Helix1

USA

74 posts since 4/17/2019

You got good help up above and you got good answers.

I would ask what are you cutting with? A table saw, compound miter or a chop saw? I cut once and let the saw stop before lifting it.
I'm sure you'll find perfection and it will make it easier for you.

Apr 25, 2019 - 8:00:36 PM

188 posts since 8/23/2011

Thanks everyone, on my first two rims I cut mainly with a chop saw and some handsaw to fit.

On the pictured rim segments I cut them with a table saw.

I will double check all my angles and lengths to make sure they are as close to perfect as I can measure, then I will either make a fudge cut or do the half and half sanding business.

One different plan I have for this banjo is leaving a dry joint and shaping the inside of each segment with my bandsaw before gluing up, its been a task shaping the inside of the rims without the ability to get a bandsaw blade in there.

Apr 25, 2019 - 8:22:13 PM
Players Union Member

rudy

USA

13885 posts since 3/27/2004

quote:
Originally posted by DiddyBum

Thanks everyone, on my first two rims I cut mainly with a chop saw and some handsaw to fit.

On the pictured rim segments I cut them with a table saw.

I will double check all my angles and lengths to make sure they are as close to perfect as I can measure, then I will either make a fudge cut or do the half and half sanding business.

One different plan I have for this banjo is leaving a dry joint and shaping the inside of each segment with my bandsaw before gluing up, its been a task shaping the inside of the rims without the ability to get a bandsaw blade in there.


If you're careful about not breaking the end grain glue joints you can easily saw the entire inside waste out with the band saw before fitting and gluing the last segment.

Apr 25, 2019 - 8:57:52 PM
Players Union Member

rudy

USA

13885 posts since 3/27/2004

If you're using a table saw you can take a lesson from the segmented bowl turners and use a segmenting jig:

Apr 26, 2019 - 2:29:28 PM

11710 posts since 6/29/2005

I feel your pain.

I don't make block rims as a general practice— only once in a blue moon, but  I make wood tone rings, which follow the same general logic.  If you want to get it exact, that requires a setup that is perfect and perfectly reproducible for the next time.  Since my method is not one of mass production, my most important tool in this process is a 12" disk sander, which I use as a cutting and shaping tool with 40 grit— a common technique of pattern-makers and design model-makers.

So for whatever it's worth, here's a picture of cutting segments with a bandsaw and fitting the joints with a disk sander‚— I don't even have a chopsaw / miter saw and don't need one:

Then the segments are arranged ( I usually number them as I fit them), and glued with a web clamp

 

Apr 26, 2019 - 4:15:55 PM

188 posts since 8/23/2011

Thanks for the advice guys. I really like the way you fit those Ken, less waste to if you lay up your pieces in your stock nested. I work with a Lot of waste from the furniture making shop I work at and I have a lot of Spanish cedar stock asking to be utilized.

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