Machinery Choices for "Hollow Joints" on Rips and Glue-Ups

A conversation about the machinery and tooling required to create "hollow joints" for strong glue-ups. May 16, 2014

Question (WOODWEB Member) :
I've been trying to learn about glue line rips on a straight line rip saw. I hope to select the right saw for my purposes considering the current marketplace. I've read every related post I found on WOODWEB and thank all of you who have shared your knowledge though there are several unanswered questions. One is whether Diehl really is the only manufacturer whose saws that can intentionally, consistently saw hollow or sprung joints. There's a lot mystery surrounding this issue. Can you set me straight? Is there some basis for the contention that either Eckstrom-Carlson or Mattison built superior machines? Is Diehl best because it has survived?

Diehl guarantees a 5' glue line rip capability on their new saws. I assume that is the maximum length of the sprung joint their saws can cut. Is that correct? I have not found instructions for setting up Diehl's saws to cut sprung joints. Neither have I found any mention of what is required to return them to cutting straight lines. Is it is feasible to rip a few boards with sprung joints for a table top then revert to cutting straight straight cuts? Does a Diehl Straight line rip saw always impart 5' of "spring" to the first 5' of its rips? How about on both sides of the blade? How is that achieved?

What about the preparation of 2' panels? Would some portion of that same 5' curvature be cut into those edges? I realize the sprung joint is just a bit deeper than hypothetical but I've been springing table top glue lines with a hand plane for the last 35 years and believe it does matter. Does a Diehl in good condition spring all joints? If you glued up a 3' wide, 2' long panel of 1/2" wide rippings would there be a pronounced bow to the outer edges?

Distributors of Chinese over-cutting SLRs claim their machines are capable of 16' glue line rips. What does that mean? Is it true just in the sense that your clamps can always bring two of their 16' rips into intimate contact? I've come across the advice to equip a SLR with a good laser. What are the characteristics of a good one? Is there anything in them to wear out? Would a ten year old laser that cost $600 new be a questionable value at $100? How does one recognize a stinker of a laser? I understand about chain diamonds not being worn to flats more than .040 across and about chain grooves and their guides not feeling worn. Can you tell anything about their condition by measuring lateral play?

Forum Responses
(Solid Wood Machining Forum)
From Contributor O:
My experience hands on is with only Diehl saws - four-five of them over the years. I have hearsay about other saws. I will say that Diehl is the only "Old Iron" that has parts and tech support as well as techs that will actually help with their products. Give them a serial number and they will provide history on the saw.

In practice, an SL 55 will produce glue line quality joints for three shifts a day for 20 years or more, with maintenance. The saw is designed to produce blanks for molders or panels by ripping off an edge, and then ripping again for width. There is a slight adjustment for hollow, but this is a one-time thing - not changed from day to day or board to board.

The saws I used had race and chains in good shape and they produced 16' long 4/4 rough blanks to be glued for width almost every day. They were perfect - no rejects and no open ends, kinks, etc. They also were not hollow. There is no reason for them to be hollow - if they were, we would adjust the saw so they were as straight as we could make it.

The notion of a hollow joint is outdated in my opinion. Two straight edges coming together will glue just fine. An SLR is only for production; for the small shop, a joiner is used to produce good edges for gluing. A patternmaker's joiner (like Northfield) can even be adjusted to make the joints hollow if one wants. I will ask why you feel the hollow joint is a must have characteristic. Two straight edges are what is required unless you require a certain curvature to the edge. Then a joiner, of CNC or hand plane is called for.

A ripped edge is satisfactory for millwork glue for width, but a small furniture maker producing small quantities would want the control a joiner gives. A ripsaw will eat a certain amount of material. If miss-fed, it still goes through the saw, and boards can be ruined. Skill must be acquired, usually through two of three units of 4/4, edging for molder blanks. One would certainly not send that rare 8/4 piece of curly Bubinga through a ripsaw and hope for the best. The laser is used for aligning the wood with the cut line in that the laser line represents the cutline. A narrower and brighter beam is preferred, especially with long lumber. It needs to be positioned so that it is bright on the wood at 18' or further. Calibrate it accurately, as the saw will not 'cut right' if the laser is not aligned.

From contributor K:
Below with my post are a few pages from our ripsaw maintenance documentation archives that should enlighten you some to hollow-joints and setting them on Diehl ripsaws. Note: we have plenty more information on file regarding ripsaw set-ups and wear issues should you want me to forward you additional information. I can't tell you from empirical data that hollow-joints are better than straight-joints or vise-versa, but it is obvious that much work has been put into hollow-joint documentation and study, which can only bring me to the assumption that someone along the way has put in a good bit of time and effort into determining this fact, but based upon my experiences in sales with saws in the furniture industry, I question whether the hollow-joint is more ideal than the straight-joint for gluing up panels.
What I do know is hollow-joints are not required to glue up good panels, plain and simple!

Otherwise, we wouldn’t see “Mattison style” ripaws with a single chain or gang ripsaws for that matter, ripping glue-lines for panels. I might be convinced, (with concrete empirical data) that the hollow-joint set-up is better for gluing panels at or under a given length, but I question whether it actually hinders gluing panel lengths longer than the ideal set-up length, hence I hypothesize that it may actually be a poorer set-up if the saw is to be used to rip a variety of different lengths. In my mind the straight-line set-up is a better overall set-up because it minimizes the variables as well as minimizing wear forces etc. on the saw and its components. I am sure there are some folks out there that can add to this debate and would love to hear from them, but, for what it is worth you have my opinion based upon over 15 years of practical experiences.

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From Gene Wengert, forum technical advisor:
I used to see a lot of hollow glue joints, but in the past decades it is rare. I believe that these joints were desired when the moisture content from the kiln was a bit too high for the atmosphere in the manufacturing plant (most common in the winter). This meant that after ripping, the ends would dry and shrink a small amount. So, if there was a delay between ripping and gluing, the hollow cut would actually result in a straight joint at the time of gluing. The hollow joint also meant that only one clamp (located mid length) was needed in an ideal situation, versus having several clamps, including one very close to each end. Today we seem to have better MC coming from the kilns, recirculating air in dust systems so that plants run about 34% RH, and less time delay between ripping and gluing. This means a hollow joint is not so critical.

From Gene Wengert, forum technical advisor:
To the original questioner: I just want to make sure that in your research, you noted that the saw blade itself (especially the sides of the teeth, which is the part of the blade that actually prepares the surface that will be sawn) is super critical. Some saw shops do a good job of side grinding.

From the original questioner:
Thank you all so much. I feel privileged to hear the experiences of each of you who responded to my questions. Your insights are invaluable, especially in my isolation. No one around here even knows what a straight-line ripsaw is - I should have been clearer about what I want an SLR for. One of the few things I've profitably made in my one-man shop is mixed North American species end grain laminations, mostly cutting boards. The idea is the patterning and in practice almost each board is individual. The straight line, laser guided first cut is appealing because so much of my lesser-species lumber comes in small, irregular pieces like Russian olive, Osage orange, and apple - an awfully high percentage of waste but also an opportunity to do something not usually seen, a virtue when marketing through art galleries. The mix and repetition of colors and grain patterns are important and composition takes place as I work. I often use rips as narrow as 3/32" in the mix while laying up the long grain blanks. In trying to envision what an SLR can do, one of the questions is how narrow can I make high quality, relatively short (16"- 42") rips in thicknesses between 3/4 and 10/4?

From contributor M:
It sounds like you need a Festool track saw for the straight-line cut followed by a good cabinet saw with an easily adjusted Biesmeyer type fence. You don't need an SLR if it’s sitting idle 90% of the time.

From the original questioner:
That's an interesting remark with strong appeal. I don't actually want to own a straight line rip saw, I just want to put a glueline edge on thousands of pieces of live-edged stock and to make tens of thousands of fairly short, parallel-edged, glue line rips, a few of them quite narrow. You're right, I'd use an SLR less than 10% of the time. I have a Powermatic 72 with a Biesemeyer fence and a power feed. I have the smaller Festool skilsaw and some track. I've become resentful of Festool for their expensive kits to do all of the things their machines are supposedly engineered to do and their lack of documentation.

Festool's lack of documentation really chaps my cheeks. I'm not especially stupid but I've never figured out what some of Festool's little parts are for. The worst of that is I'm missing out on some of the capabilities. Without that understanding those parts ought to be in a coffee can in the back room rather than a in a "sustainer", the $100 plastic box that came free with the saw. I've used the saw infrequently because it is a pain to take out, set up and put away for one cut. That cut, however, will be as good as from any of my stationary machines. Trying to use several short sections of Festool's guide track are also a pain with their easily-lost proprietary set screws. Also, multiple splices in a guide rail simply cost accuracy. Having vented, I'm beginning to understand how that could work: a shop-built straightening bench and one long section of track.

From Contributor O:
An SLR is designed for long blanks - not 16". I have used one for parts down to 48" long but not less, though I think some are rated for shorter parts. The edging coming off the blank is often at risk of kick back. The smaller and odder the edging, the more likely it is a threat, in my opinion. These saws have been known to pin an edging through an overhead door 30' behind the saw. If I were in your position I would use a band saw with an aggressive blade to edge with, perhaps with a laser line, and then a simple proper jointer.

From contributor M:
Looks like you're already there. Just get a vacuum and a longer fence. I have a fence that's about 10' long with no splices, and they make a longer one. I hear you about the Festool, but keep it in perspective: your old Porter Cable skill saw didn't come with a "how to" booklet either. Supposedly, there is a FS forum for free advice, and there's a small aftermarket for support info and products, but you should use your sales rep. They can't change the price, so they have to compete somewhere. Find a rep that knows the tools. Ditch the SLR idea until you're ready to hire someone to run it for a full shift.