Interesting test for sure. I believe our pieces of steel are behaving in a similar fashion - allowing for mine being only 3/4 as thick as yours. My deflection was in a horizontal direction, (the "straightedge" placed flat on a piece of 4' x 8' plywood) and not really measured - just eyeballed - but I know a 1/32" when I see it - or pretty close. You make an excellent point, really - all material will have some degree of flex, but my idea of clamping a piece of scrap wood against the opposite side of my 9' "straightedge" is looking pretty acceptable. Thanks for your very helpful additions to this thread.
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Metal Straightedge
- Thread starter Brian26
- Start date
Metal
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Really great addition to this thread. Thanks. I had a funny moment thinking more about this... Assuming some person - perhaps a little like myself, paying huge (for me, $1000 is huge!) dollars for a precision reference edge, then using that to draw a pencil line on a workpiece, then taking it away and using a circular saw freehand to make the cut!
My notion is to do the best job of woodworking that I can. My normal goal in working with wood is to keep everything to within 1/100". When I wear my (less experienced) metalworking hat, I strive for the nearest 1/1000". And there I stop. But a good friend has gone to NAIT and taken their gunsmithing courses, and he tries for 1/10,000" - which needs specialized measuring tools to even see these small differences.
Very interesting information. Not yet having finished my second cup of coffee this morning, I wonder at your statement that "high compressibility" leads to higher stiffness. I would have thought the opposite - the less something compresses, the higher the stiffness? Maybe I will re-read your post after the third coffee kicks in and it will make more sense! Thanks for your polished addition to the discussion. Most appreciated.
Johnwa
Ultra Member
I have a piece 6’ piece of 1”x3” aluminum extrusion made by 8020 that is as straight as I can tell. A piece of 1”x1” has a definite bow of about ⅛” over the 6’ length. Princess Auto sells similar extrusions. They may or may not be as straight but you could likely pull a couple from the rack and see.
A fairly inexpensive and lightweight option is a “screed level”; made from 4x.75 aluminum box extrusion. Although, I have no idea how accurate they are in their straightness or flex, but they are designed for landscape & concrete work, so they should be somewhat stiffer than a standard tracksaw track.
https://www.chtools.ca/level-screed-alum-25m.html
https://www.chtools.ca/level-screed-alum-3m.html
PA has similar screed levels, but quite a bit more expensive.
https://www.chtools.ca/level-screed-alum-25m.html
https://www.chtools.ca/level-screed-alum-3m.html
PA has similar screed levels, but quite a bit more expensive.
I've been enjoying this discussion but staying out of it till now. In general, I admire your goals. I am an amateur precision gunsmith myself and an engineer. I often find myself chasing one tenthousandth. But I also think I overdo that more often than I should.
Wood is obviously not metal. I can't really imagine myself cutting wood to a precision of a 32nd over a length of 8ft. For me, even a foot seems too tall a goal. But then again I'm no high end cabinet maker.
In my case, I make up for my cut precision by taking advantage of the special somewhat magical ability of wood to be easily bent to conform.
Almost all of the finest woodworking I have seen involved the frequent and copious use of joiners, clamps, and fixtures to improve the fit of wood components and compensate for both the limits of the materials themselves and the tools that are used.
The simple absence of available off-the-shelf guides and rails that can support tight tolerance goals suggests to me that it isn't normally done.
@Dabbler's comments are indeed very interesting. I didn't know about these latest material science discoveries either. The guy is a wealth of varied knowledge.
His comment "the stiffest structures which we are referring to here, are a combination of tensile strength and high compressibility." can be read many ways. Sometimes the English language isn't very good at conveying meanings. I think what he meant is "high resistance to compressibility".
But keep in mind that we use the word rigidity in many ways - especially when we talk about machines and machining. As a result, rigidity and strength often get confused - especially by machinists. I'd get my knickers all twisted up in a huge knot if I tried to be too literal about it. It's best to just go with the flow and try to understand what the other guy means instead of his exact words.
And of course every material is different. They all have their own modulus of elasticity (in all its varients), ultimate strengths, density, etc etc.
I think the best way to think about it might be wood itself, or perhaps better yet spider web. Spider web can be compressed or stretched to extreme limits yet remains an incredibly strong material. Wood is perhaps less obvious but if you think about it, we all know that it stretches and can be compressed but yet it is an incredibly strong material - especially in tension and in bending for its cross-sectional area. I try not to get too wrapped up in the whole matter. Stuff is what it is and it behaves accordingly.
Common sense usually prevails in the end. I'm sure that will be case when you have completed your interesting journey about cutting plywood sheets as accurately as possible.
My own Veritas straight edge & track saw guide is just formed aluminium about 3"x1/4" (I did not measure it). But I use it regularly and have learned to trust it despite the fact that is a two piece tool held together with very short connectors. I vaguely remember seeing a one piece festool guide that was made of extruded aluminium with about a 1/2x6 cross-section.
Wood is obviously not metal. I can't really imagine myself cutting wood to a precision of a 32nd over a length of 8ft. For me, even a foot seems too tall a goal. But then again I'm no high end cabinet maker.
In my case, I make up for my cut precision by taking advantage of the special somewhat magical ability of wood to be easily bent to conform.
Almost all of the finest woodworking I have seen involved the frequent and copious use of joiners, clamps, and fixtures to improve the fit of wood components and compensate for both the limits of the materials themselves and the tools that are used.
The simple absence of available off-the-shelf guides and rails that can support tight tolerance goals suggests to me that it isn't normally done.
@Dabbler's comments are indeed very interesting. I didn't know about these latest material science discoveries either. The guy is a wealth of varied knowledge.
His comment "the stiffest structures which we are referring to here, are a combination of tensile strength and high compressibility." can be read many ways. Sometimes the English language isn't very good at conveying meanings. I think what he meant is "high resistance to compressibility".
But keep in mind that we use the word rigidity in many ways - especially when we talk about machines and machining. As a result, rigidity and strength often get confused - especially by machinists. I'd get my knickers all twisted up in a huge knot if I tried to be too literal about it. It's best to just go with the flow and try to understand what the other guy means instead of his exact words.
And of course every material is different. They all have their own modulus of elasticity (in all its varients), ultimate strengths, density, etc etc.
I think the best way to think about it might be wood itself, or perhaps better yet spider web. Spider web can be compressed or stretched to extreme limits yet remains an incredibly strong material. Wood is perhaps less obvious but if you think about it, we all know that it stretches and can be compressed but yet it is an incredibly strong material - especially in tension and in bending for its cross-sectional area. I try not to get too wrapped up in the whole matter. Stuff is what it is and it behaves accordingly.
Common sense usually prevails in the end. I'm sure that will be case when you have completed your interesting journey about cutting plywood sheets as accurately as possible.
My own Veritas straight edge & track saw guide is just formed aluminium about 3"x1/4" (I did not measure it). But I use it regularly and have learned to trust it despite the fact that is a two piece tool held together with very short connectors. I vaguely remember seeing a one piece festool guide that was made of extruded aluminium with about a 1/2x6 cross-section.
Dabbler
ersatz engineer
If you have size constraints AND you need support for a significant load, you have 2 options: high profile steel tubing, like 1X4, 1X5 or 1X6, depending on your load, or a similar sized steel I beam. If you are using this with a handheld router, then a single 1X4 or 1X3 would suffice if you are careful. A 50 lb load will still produce a slight deflection - around 30 thou or so. but the longer you go the deflection increases geometrically for a given load.
I'm sorry I misspoke. I intended to say High resistance to comressibility, I cannot edit that post now.
EDIT-> I edited your earlier post Dabbler. -JA
This is done in bridge members by embedding high tensile cables into the bottom of the beam, pre-tensioned, and the very resistant-to-comression concrete cross section at the top of the beam, and and what looks like a flimsy concrete separator section. This makes for a member that is VERY resistant to strain (deflection, sort of) under load. Deflection and resonance caused Tacoma narrows bridge disaster, for instance. That is why bridges are now made to deflect as little as is practical. Longer life and more resistance to unusual forces result.
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That reference to cutting with a circular saw without a guide was just my attempt to lighten up the thread a bit. I do use various guides in practice, but the one I use most often needs replacing. I did like my old model track saw and wish I still had it. It was the only affordable solution when I got it but sadly, no longer available. So, I am trying to make or buy something as accurate as before, but am hoping for something more affordable than the latest offerings from the major names - DeWalt, Milwaukee and the like. Milwaukee has a good one, but they don't sell it as a package - you need to shell out for the track separately. If I were making a living at this hobby, I would spring for the Festool model.
Thanks everyone for weighing in on this matter. It is always fascinating to learn what our members have to contribute to these exchanges.
Six O Two
(Marco)
I hear ya. Tracksaws are expensive. I had the same thought and looked into figuring out a cheaper way to make myself a tracksaw which was a step up from a plywood fence, and this is where I ended up.
(Note: not much metalwork here, other than welding and modding the quick-release clamp to fit the extrusion's slots)
Someone upthread mentioned 80/20 extrusion. I ended going with Misumi, a competitor extrusion to save a few bucks (total extrusion & hardware costs were ~$140). I settled on an 20mmx80mm extrusion which I felt would do a good job resisting lateral forces that might try to bend the straightedge, while still being low profile and less expensive. Many extrusion companies have online calculators where you can figure out deflection based on load, extrusion and length. i bought one ~4' length, and one ~5' length. I mostly use the 5' length for crosscutting plywood sheets and connect the two extrusions with the hardware shown in the bottom of the picture if I'm ripping a full sheet.
I ended up keying the sliding part to the track so that it can't be lifted off, but in hindsight that's probably not necessary.
It works really well, but setting up cuts is a little fiddly because of the 150mm offset from the extrusion to the blade cut line. If I'm being honest with myself, I'm not sure I'm that far ahead the ~$300 WEN tracksaw, but it was a fun little project.
BMW Rider
Super User
I used my circular saw on a shop made guide track for years to rip down sheet goods into manageable sizes for finish cutting on the table saw. I finally upgraded to a Makita track saw a couple years ago and the difference is incredible. Much easier to use, has excellent dust collection with the shop vac, nice clean and accurate cuts. The plunge feature of the track saw is so much nicer than the circular saw where the cut must be started off the workpiece. Still, the guides I made still hang on the shop wall and worked well enough for much less cost since I already had the saw.
I did the same recently. Cuts like the best you ever got off your table saw, or better. Its not just the easy, straight cut, its the rubber support for the wood where the blade exits so you don't get tearout
I have a small Inca table saw, really nice to have given I have next to no space, but entirely useless for big sheets. Even if I had a fancier saw, I can't really get a 4x8 sheet into the small basement room where the woodworking stuff is. The track saw address that and, unless you have a large sliding table, is imo better at cutting large sheets. With a centipede table and sheet of foam insulation, I could do the cut list in the driveway
Former Member
Guest
Keg tools makes a 4 pc guide and carrier for your saw, works extremely well when set up correctly. More than adequate enough cabinet making. How do I know I use one and love it. Homedepot sell 8ft section one.
Wood working to 0.001" good luck with that, for most getting down to 0.0005 is an honest challenge, yet doable with care and effort in machining.
Down to 0.0001" good luck unless you really know what you are doing taking temperature compensation into consideration while machining and measurement, even on small objects.
Want to see your gunsmiths head explode???? Ask him what temperature is material is at when he measures it? Does he check and is it always at the same temperature? If not has he temperature correct for the materials and measurement tooling?
That will get him thinking and cause the explosion has his mind overloads.
Wood working to 0.001" good luck with that, for most getting down to 0.0005 is an honest challenge, yet doable with care and effort in machining.
Down to 0.0001" good luck unless you really know what you are doing taking temperature compensation into consideration while machining and measurement, even on small objects.
Want to see your gunsmiths head explode???? Ask him what temperature is material is at when he measures it? Does he check and is it always at the same temperature? If not has he temperature correct for the materials and measurement tooling?
That will get him thinking and cause the explosion has his mind overloads.
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Want to see your gunsmiths head explode???? Ask him what temperature is material is at when he measures it? Does he check and is it always at the same temperature? If not has he temperature correct for the materials and measurement tooling?
That will get him thinking and cause the explosion has his mind overloads.
You have to be kidding. The good gunsmiths live their lives understanding the implications of simple stuff like ambient temperature and also the implications of temperature changes caused by machining in different metals. They also know when to ignore it and/or how to compensate for it. Same goes for competitive shooters. I think you need a new gunsmith if you think yours would have explosions in his mind over it.
Perhaps you should have picked a different example......
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