5 cuts method acceptable accuracy

[semenza]

dood, wood moves more than that.

        I have never thought this was a valid point when it comes to getting pieces accurate  and square.

If all things are equal it will still be a tapered cut when it moves.

A piece that is not square might expand and contract but it still won't be square.

Parts that are not accurate in size and shape still won't fit together correctly.

      It's got nothing to do with wood movement.

Seth

 

[sancho57]

I was looking at it over the total length of the wood being cut. that minusule difference over the length of the piece wouldnt shoujldnt affect anything.

But thats just me, reminds of the guy who was complaining about me saying that I thought it was awesome to rip a 8 ft piece of ply and it being a 1.5 mm difference over the 8ft length.

IMO i thought that was pretty accurate. But some people who are ex engineers and mad scientist think eveything has to be like theiyre building the space shuttle.

Even then the machine parts for the space shuttle had design tolerances. Lot smaller tolerances iof course but still had tolerances +/-

 

[mino]

I was looking at it over the total length of the wood being cut. that minusule difference over the length of the piece wouldnt shoujldnt affect anything.

But thats just me, reminds of the guy who was complaining about me saying that I thought it was awesome to rip a 8 ft piece of ply and it being a 1.5 mm difference over the 8ft length.

IMO i thought that was pretty accurate. But some people who are ex engineers and mad scientist think eveything has to be like theiyre building the space shuttle.

Even then the machine parts for the space shuttle had design tolerances. Lot smaller tolerances iof course but still had tolerances +/-

It is not about being a "mad scientist etc.". It is about doing stuff which is fit for purpose and which is not.

In your case, I would say you were in the wrong. Not because of the (in)accuracy, but because of the situation involving a sheet of ply.

Here is the why I believe so:
1) ply does not move asymetrically, unlike wood
If you make a rip that will be 1.5mm off at the end, once you cross-cut it your shelves, for example, will be too short/wobbly or too wide to even fit into the cabinet. With raw wood, you would have to include a sufficient buffer, of say 1mm, for it to contract/expand. With all-ply piece you do not need to, so the design usualy does not include such a buffer ... This is even more pronounced with laminated MDF or chipboard which are absolutely "static" as far as expand/contract with humidity goes and the designs assume that.

2) a cutting method needs to be as accurate as the MOST DEMANDING use for that method
This means, while it is OK that 80% of the time I do not really care within 0.5mm as I am making rough stuff just bolted together. But those 20% when I need the pieces to be precise, so they fit together means that my cutting process/method/station needs to account for those 20% case.

Granted, at big volume it may be worth it to have a cheapo rough-cut station and a proper precise-cut one. But most people here do not make the volume for that to make sense.

3) The more accurate your original cuts, the LESS WORK you need to spend fiddling/fitting/adjusting.
This is the most universal reason yet most individual reason.
For this reason I precise-cut even rough lumber. Not because it is required. But because when I precise-cut it, it saves my time when putting it together. And these are real savings. For one, having 8 poles each exactly 2407 mm means I can choose any one of them and I KNOW they will fit. No need to cross-check, double-check, cut off a piece here and there. They will just fit, interchangeably.

---
Case in point:
I take my TSC plus rails plus purchased a cordless vac to my lumber yard. They have a formatting saw which is pretty good on cut quality BUT is about 0.5mm off here and there (they guarantee 1mm +/- cuts). That means in practice I cannot use pieces off that saw directly, but only as rough-sized for a follow-up precise cut. That not only wastes material, causes a LOT of dust in shop (cutting a few mm off a ply sheet side is the worst for dust extraction) but in some cases would mean I cannot fit two 600mm+ pieces inside the 1250mm wideness of the material. And I would still have to pay for those formatting cuts ...

With my TSC and a GRS 16 on hand, I can just take the sheet, cut it up as needed for transport, and know those cuts to be straight and square - so they become the final cuts when planned for furniture.

On the orther hand, when buying lumber, I am happy to have them cut the pieces to rough size I need. Whey will anyway need to dry more so I do not mind. Same when buying construction ply for some shop use.

 

[Michael Kellough]

I find when trying to get a very fine increment of movement like moving the rail bracket it is best to set a stop and use a shim like feeler gauges to move the stop before moving the bracket or use a dial indicator.

The increment of movement is so small when chasing the perfect square that going by eye is not an option.

This gives you a gauge so if you move the bracket .010" and it is twice as much as you needed you can use a .005" shim to put it where it needs to be.

I too am a little obsessive when chasing the perfect setup.

That is what's required to move the decimal point to the left.

When you get there (as far as you want to go) it's good idea to save that piece of wood to use a jig to get that relationship back in the future. The trouble with this is that it's a piece of ply that fits under the rail so it doesn't help position the rail, unless you're cutting thinner stuff. If you carefully glue another layer of ply flush with the right side the jig will be easy to use.

 

[Crazyraceguy]

Case in point:
I take my TSC plus rails plus purchased a cordless vac to my lumber yard. They have a formatting saw which is pretty good on cut quality BUT is about 0.5mm off here and there (they guarantee 1mm +/- cuts). That means in practice I cannot use pieces off that saw directly, but only as rough-sized for a follow-up precise cut. That not only wastes material, causes a LOT of dust in shop (cutting a few mm off a ply sheet side is the worst for dust extraction) but in some cases would mean I cannot fit two 600mm+ pieces inside the 1250mm wideness of the material. And I would still have to pay for those formatting cuts ...

With my TSC and a GRS 16 on hand, I can just take the sheet, cut it up as needed for transport, and know those cuts to be straight and square - so they become the final cuts when planned for furniture.

On the orther hand, when buying lumber, I am happy to have them cut the pieces to rough size I need. Whey will anyway need to dry more so I do not mind. Same when buying construction ply for some shop use.

This is my criticism of most of the "YouTube Woodworkers" that is see out there. They all "break down" their ply with a tracksaw (oversize) and then do the final cuts on a tablesaw. I just don't see the point? A good tracksaw can cut as well as a tablesaw can, arguably better than a lot of the contractor saws out there. So why do it twice?
As many of you know, I work in a large cabinet shop, with many ways to cut sheetgoods, but when I use the TS55 to cut something, that is the final cut.

 

[sancho57]

I was looking at it over the total length of the wood being cut. that minusule difference over the length of the piece wouldnt shoujldnt affect anything.

But thats just me, reminds of the guy who was complaining about me saying that I thought it was awesome to rip a 8 ft piece of ply and it being a 1.5 mm difference over the 8ft length.

IMO i thought that was pretty accurate. But some people who are ex engineers and mad scientist think eveything has to be like theiyre building the space shuttle.

Even then the machine parts for the space shuttle had design tolerances. Lot smaller tolerances iof course but still had tolerances +/-

It is not about being a "mad scientist etc.". It is about doing stuff which is fit for purpose and which is not.

In your case, I would say you were in the wrong. Not because of the (in)accuracy, but because of the situation involving a sheet of ply.

Here is the why I believe so:
1) ply does not move asymetrically, unlike wood
If you make a rip that will be 1.5mm off at the end, once you cross-cut it your shelves, for example, will be too short/wobbly or too wide to even fit into the cabinet. With raw wood, you would have to include a sufficient buffer, of say 1mm, for it to contract/expand. With all-ply piece you do not need to, so the design usualy does not include such a buffer ... This is even more pronounced with laminated MDF or chipboard which are absolutely "static" as far as expand/contract with humidity goes and the designs assume that.

2) a cutting method needs to be as accurate as the MOST DEMANDING use for that method
This means, while it is OK that 80% of the time I do not really care within 0.5mm as I am making rough stuff just bolted together. But those 20% when I need the pieces to be precise, so they fit together means that my cutting process/method/station needs to account for those 20% case.

Granted, at big volume it may be worth it to have a cheapo rough-cut station and a proper precise-cut one. But most people here do not make the volume for that to make sense.

3) The more accurate your original cuts, the LESS WORK you need to spend fiddling/fitting/adjusting.
This is the most universal reason yet most individual reason.
For this reason I precise-cut even rough lumber. Not because it is required. But because when I precise-cut it, it saves my time when putting it together. And these are real savings. For one, having 8 poles each exactly 2407 mm means I can choose any one of them and I KNOW they will fit. No need to cross-check, double-check, cut off a piece here and there. They will just fit, interchangeably.

---
Case in point:
I take my TSC plus rails plus purchased a cordless vac to my lumber yard. They have a formatting saw which is pretty good on cut quality BUT is about 0.5mm off here and there (they guarantee 1mm +/- cuts). That means in practice I cannot use pieces off that saw directly, but only as rough-sized for a follow-up precise cut. That not only wastes material, causes a LOT of dust in shop (cutting a few mm off a ply sheet side is the worst for dust extraction) but in some cases would mean I cannot fit two 600mm+ pieces inside the 1250mm wideness of the material. And I would still have to pay for those formatting cuts ...

With my TSC and a GRS 16 on hand, I can just take the sheet, cut it up as needed for transport, and know those cuts to be straight and square - so they become the final cuts when planned for furniture.

On the orther hand, when buying lumber, I am happy to have them cut the pieces to rough size I need. Whey will anyway need to dry more so I do not mind. Same when buying construction ply for some shop use.

hmmmm is Noah still building the ark?

Oh Ham you cant use that piece of planking, Its .0000000000000000004 out of square go cut another one...

 

[jgrout]

If I chased out an odd .05 mm on every cut I ever made I would never accomplish anything....

Case in point: I had a friend who asked me to come to his shop for a show and tell. When I get there he complained about the fit of dogs in his MFT tops. He said that in some holes the dogs fit perfectly where he had to actually reach under the top to push from the bottom as well as pull from the top to remove the dog. yet in others he could drop the dog in and it would be so lose you could see movement when wiggled. He had a variety of dogs from different manufacturers who all claimed fits +/-about a half a mm, I so with calipers in hand we started documenting the diameters of the dogs. Wonder of wonders all the dogs supplied by  companies met the tolerances but some of cheaper ones (not mentioning any names) could be off by as much as -1.5 mm which explained only half the issue. I then took two dogs that met the specs and fit them in every dog hole on the bench. The results were not surprising, the ones that rarely if ever were used were nice and tight, whereas the ones used extensively were noticeably looser due to use.

Moral of the story amounts to if you want perfection you should never use any tool in any situation if you want 100% accuracy, just using it will eventually cause inaccuracies 

 

[mino]

...
Moral of the story amounts to if you want perfection you should never use any tool in any situation if you want 100% accuracy, just using it will eventually cause inaccuracies

Short reply:
Disagree. There is a difference in perfection and efficiency.

To get efficiency, one needs repeatability. To get repeatability one needs a certain level of precision. Without it, every single piece made needs to be custom-fit and measured. That costs time and time costs money.

The correct answer is: It depends, aka: Seek the precision required by the job. But no more.

For some work, being 2mm (0.1") off is no big deal.

For some work, one will be absolutely fine being 0.5mm (0.02") off.

For some work, the same 0.5mm diversion will mean the piece is unusable and must be thrown away (if too small) or re-cut (if bigger)

 

[GregorHochschild]

The correct answer is: It depends, aka: Seek the precision required by the job. But no more.

For some work, being 2mm (0.1") off is no big deal.

For some work, one will be absolutely fine being 0.5mm (0.02") off.

For some work, the same 0.5mm diversion will mean the piece is unusable and must be thrown away (if too small) or re-cut (if bigger)

Completely agree with this. Required accuracy depends on the context. But it would still be great to get more concrete than that.

For example, what do people think is the required accuracy for the following jobs (measured in terms of mm per m):

- Joinery in fine woodworking
- Kitchen cabinet construction (face frames)
- Kitchen cabinet construction (frameless with consistent reveals)
...

Of course, requirements still differ a lot but focusing on specific tasks would make the discussion more useful to answer the original question about acceptable accuracy.

 

[Michael Kellough]

If I chased out an odd .05 mm on every cut I ever made I would never accomplish anything....

Case in point: I had a friend who asked me to come to his shop for a show and tell. When I get there he complained about the fit of dogs in his MFT tops. He said that in some holes the dogs fit perfectly where he had to actually reach under the top to push from the bottom as well as pull from the top to remove the dog. yet in others he could drop the dog in and it would be so lose you could see movement when wiggled. He had a variety of dogs from different manufacturers who all claimed fits +/-about a half a mm, I so with calipers in hand we started documenting the diameters of the dogs. Wonder of wonders all the dogs supplied by  companies met the tolerances but some of cheaper ones (not mentioning any names) could be off by as much as -1.5 mm which explained only half the issue. I then took two dogs that met the specs and fit them in every dog hole on the bench. The results were not surprising, the ones that rarely if ever were used were nice and tight, whereas the ones used extensively were noticeably looser due to use.

Moral of the story amounts to if you want perfection you should never use any tool in any situation if you want 100% accuracy, just using it will eventually cause inaccuracies

I disagree with the “moral of the story”. The lesson should be that you cant depend on a hot pressed mixture of wood powder and glue to keep it’s tolerance long term. MDF is a pretty good material but it’s not phenolic, even if you do manage to drill the pattern of holes perfectly.

 

[sancho57]

...
Moral of the story amounts to if you want perfection you should never use any tool in any situation if you want 100% accuracy, just using it will eventually cause inaccuracies

Short reply:
Disagree. There is a difference in perfection and efficiency.

To get efficiency, one needs repeatability. To get repeatability one needs a certain level of precision. Without it, every single piece made needs to be custom-fit and measured. That costs time and time costs money.

The correct answer is: It depends, aka: Seek the precision required by the job. But no more.

For some work, being 2mm (0.1") off is no big deal.

For some work, one will be absolutely fine being 0.5mm (0.02") off.

For some work, the same 0.5mm diversion will mean the piece is unusable and must be thrown away (if too small) or re-cut (if bigger)

well said.  try for te best accuracy as I can. But depending on what Im building I dont stress over being out here and there. There are some things that need absolute accuracies. In my experiance they are far an inbetween. As long as its fits Im good.

 

[jgrout]

pretty much everyone missed the point:

I have yet to see a phenolic MFT top so that is a moot point. If someone did make one it would somehow manage to be off "someplace"

I have worked for years to 1/32" accuracy consistently (.08 mm) on anything larger than a jewelry box and I can assure you that if you want to get something built that is good enough and efficient enough. As a point of reference my MFT using 5 cut yielded a differential of .010 over 30" on that particular day. I am sure one can do better but can you do so "efficiently and consistently?" Heck even a slightly used blade hitting a void or a knot could throw a monkey wrench into that scenario

Yes MDF does wear easier than say Baltic Birch (what my MFT top is made from) but to assume that it was not going to wear was my friend's fatal flaw , even I expect to replace tops when they wear out. As for my friend he was an engineer in the aerospace fields and had great knowledge in accuracy but you don't use much wood or fibrous sheet goods in space. his expectations way over reached the materials provided.

The one thing he or I did not expect however was sloppy manufacturing tolerances from the dogs. The lesson here is if you purchase a set of dogs you better check them ASAP after you receive them. It does matter.

Given the parameters of accuracy required my only thought would be to either sleeve the dog holes (possible with great effort) or to use a much more stable substrate. That is unrealistic IMHO.

I am happy just to use my tools, without the brainpower required to chase down an odd .5 mm. 

 

[smorgasbord]

Just a note that once you get to the levels discussed here, repeatability is the larger concern. If you were to repeat the test process say 5 times, what would the standard deviation be? I submit it's likely that the deviation will be larger than the "error" you're measuring.

 

[mino]

Just a note that once you get to the levels discussed here, repeatability is the larger concern. If you were to repeat the test process say 5 times, what would the standard deviation be? I submit it's likely that the deviation will be larger than the "error" you're measuring.

TLDR:
Actually, no. Most of the errors seen here are systemic in nature and it is worth it to check it once in a while exactly for that reason.

I have spent a LOT measuring the rails and checking all tollerances last year when I (wanted to) use Makita rails in our community shop. It did not work out. The Makita rails had about 2x the inaccuracy of the Festools ones and it was too much for my tastes. it would mean I could not rely on them the way I rely on the FS/2 ones.

The thing I learned is this:
The FS/rails can give you about 0.1mm/m accuracy when used properly, 0.2mm/m when used carelessly. Trying to go beyond that gets prohibitively expensive fast (time, equipment) and is not worth it.

Long stories:
--------------------------
At same, that is on the edge of "acceptable" for some special cases (snug-fit laminated particleboard shelfs etc.) so it is critical to use a cut workflow where compound errors are avoided. That is why I also recommend a Parallel Guides system to anyone who gets a Tracksaw. That is an essential tool for truly coplanar long rips where the accuracy is the most critical.

Since doign that research and KNOWING how accurate my kit is, I have never needed to come back to it so far. I just KNOW how to order the cuts and what accuracy should I expect and I "DESIGN FOR" that. Case closed for me.

To those "what for all that accuracy" who wrote on this thread most have their own ways to achive the accuracy they need. Either by experience or by digging in in the past like I did.
Please, do not put down new members who come hear asking on HOW TO get the accuracy they need and like this thread asking WHAT IS the accuracy they need.

Telling them "0.5mm/m accuracy is GOOD ENOUGH" is simply FALSE. It is false because it cannot be said such universally, and YOU know it. They do not, so many will take that as a Gospel and get burned hard.
Even give up on woodworking as they will not be able to get their simple/cheap chipboard pieces fit and their SO will make herself heard. I have a friend who ended up in that position 5yrs ago. Only after I have shown him where the problem was, was he able to get back into the hobby - he absolutely needed a "perfect" piece to show the SO that he was not wasting family money. Yes, that perfect piece were simple laminated chipboard shelfs for an existing cabinet. Something he could not get from bix box store as they just did not care for that last 0.5mm/m accuracy.

On the (in)accuracy aspects, what I have researched when diggin in last Y:
--------------------------------------------------------
In case of dog-based setup, the dogs simply "give" a bit.
In case of the MFT hinge, both sides of the hinge have some play, small, but do. And you NEED to calibrate them for correct position to begin so a 5-cut check is a must.
In case of a GRS+rail, the back of the rail is not (absolutely) coplanar with the guiding ridge, there is some variance.
In case of any fixture, there will be compoung errors from the measuring instruments.
As mentioned, the rail itself has a manufacturing tolerance (very very small, but it is there and limits on how precise you can get, that is the 0.1mm/m or so I mentioned, I had a class0 straight edge borrowed to check this when having issues with my Makita rails which were 0.2-0.3/m off).

Overall, these things are why it makes sense to check, the 5-cut being the most accessible way, avoiding a need for an expensive engineer's square.

All these is why the GRS16 PE is such a wonderful design. It has no moving parts and the only error with it is the GRS manuf. tolerance (which is easy to fine-tune) and the rail edge tolerance. Both are generally very very small and static in time.

The only improvement on the rail + TSO GRS setup that can be done is making the rail more straight - taking a Class 0 straight edge and sanding the guiding ridge and the outer edge for absolute accuracy with it. That way one can get that last 0.1mm/m "bend" out of the cut line. That is a $500 excercise though to get a straight edge more accurate than the FS/2 is outa factory... THAT would be on the OCD syndrome worth a doctor check, indeed.

And lastly, the rail also dynamically bends in the 0.05mm-0.1mm /m range when placed on the piece. This is why the longer rails should not be hanged "on the side" as it can make such bends permanent.

This dynamic bending is made worse by excellent the anti-slip strips when the rail is being positioned - they are so strong that they can hold the dynamic force in, keeping the rail bent during cut. Lifting the rail when adjusting is a way to limit this.

ADD:
It is often missed, but a "0.1mm/m" inaccuracy ref. rail being dynamically bent is a distance-base radius, which means about:
0.03 mm (0.001") off on a 500mm piece
0.1 mm (0.004") off on a 1000mm piece
0.3 mm (0.01") off on a 2000mm piece
0.5 mm (0.02") off on a 3000mm piece => if referenced on two points 0.5m from the edges, this becomes 0.1mm in the centre (e.g with a PG use)
etc.

This is why using a PG system is so desirable for rips and long cuts. Square-base systems are absolutely fine for cross-cuts and shorter cuts up to 5' or so as there the dynamic bending is minimal so the angle-accuracy rules instead.

 

[tsmi243]

Just a note that once you get to the levels discussed here, repeatability is the larger concern. If you were to repeat the test process say 5 times, what would the standard deviation be? I submit it's likely that the deviation will be larger than the "error" you're measuring.

With stock MFT rail mounts, you're absolutely right.  The Dashboard parts are a lot better. 

 

[bertv]

Quote:
The thing I learned is this:The FS/rails can give you about 0.1mm/m accuracy when used properly, 0.2mm/m when used carelessly. Trying to go beyond that gets prohibitively expensive fast (time, equipment) and is not worth it.[/I]

Case in point: the 'cursor' on the parallel guides, where you have to eyeball a one millimeter line across a mm-ish divide against a one third mm raised line with slopes in the same colour… Gluing a needle in place is considered a necessary update.

 

[woodbutcherbower]

The entire thread can be solved in 10 minutes using a tiny smear of colour-matched 2-pack filler and some 240-grit Abranet.

 

[mino]

The entire thread can be solved in 10 minutes using a tiny smear of colour-matched 2-pack filler and some 240-grit Abranet.

Try that with a laminated chipboard fixed shelf .. as is usual in life - it depends.

 

[semenza]

The entire thread can be solved in 10 minutes using a tiny smear of colour-matched 2-pack filler and some 240-grit Abranet.

    [huh]  What's that got to do with a cut being tapered / out of square?  [huh]

      I think the accuracy that the OP has achieved is perfectly fine. But if it was off more it is not something that can be fixed with filler.  The pieces have to fit together and be square within a realistic tolerance.

Seth

 

[tsmi243]

The pieces have to fit together and be square within a realistic tolerance.

Seth

The final answer, and from an actual pro, too.

I'm not a pro at this, but I'm in a related field that puts me in a lot of kitchens- paying attention to how flush the cabinet frames are in high-end homes has put things in perspective for me.  There's things about your own work that will drive you crazy, but that you wouldn't even notice in somebody else's house.