Have we lost the plot as far as tollerances are concerned?

[dicktimber]

After reading various posts I think we should talk about what we ar trying to achieve with this wondrous material called wood.

We are being encouraged to buy machinery that claim to enable us to achieve lower and lower dimensional tolerances.
We have read about digital read outs that can be attached to planer thicknessers etc to achieve thicknesses down to tolerances within .00000000000000mm...you get the gist?

But have we lost the plot, in reality?
And is this the correct way for newcomers to be introduced to woodworking.?

Many of the retired metal work engineers who are on this forum understand that when working with metals, measurements on manufactured component drawings, have evolved as the base material has evolved. Limits and fits standards give precise tolerances for design engineers to adopt and translate into working drawings, letting them state tolerances for transition fits, sliding fits etc.
Engineers have milling machines to rough out while precision grinding allows them to reach minute tolerances.
When it come to woodworking, we seem to be trying more and more to assimilate metal working tolerances to woodworking.
Why?
The guide line for woodworking used to be 1/32"....30 thou.
This was because with a good steel rule and a keen eye, it was deemed the level most woodworkers could easily achieve.
So when someone decided to use a digital engineers vernier caliper for measuring thicknesses, overnight we thought we were building better parts. We could measure down to 0.000000!
And we did.
We added digital readouts, we put micro adjusters on outr router tables, and bragged that the buff told us we could machine surfaces to a tolerance of 0.0000000000!!!!!
Have we not been taken, hook, line and sinker, spending more and more on things that don't work with an unstable material, unlike metal?
If you do the job right and plane your stock rough first, bring it indoors and let it acclimatize for a while....they say you can work it and it, SHOULD remain stable?
Rough cut rails from the above stock, and let them stabilize for a week or so.
Machine up those rails, and then leave them for a while and sods law will say 2 out of 4 will be bent, to some extent.
And that is my point.
They will be more bent than the 0.0000000 digital thingie you took so much time to save up for, that you just machined these parts with.

But do we complain?
NO we just get on and use the parts........
which defeats using a digital do dah in the first place, and a good steel rule and the 1/32" I was taught would maybe have been just as good?

Are we in overkill on tolerances. I think we are because depending how much moisture is in the air, where the timber was cut from the tree, will not make a hapeth of difference to how you measured it.

I am sure Tom Chippie would have a good laugh if he could see us all today....don't you?

 

[Karl]

Clearly a thinly veiled dig at me Dick, so i'll be the first to respond.

First off, I don't use "one of those digital thingies".

Turning to the point I made in the other thread, if you machine a batch of components and there is 0.25mm difference in the thickness, how do you accurately set up the next machine for the next job (in this case creating mortice and tenons). Or are you advocating that each joint should be made individually, rather than production line?

I don't deny that wood moves, but if you thickness to a uniform thickness and then carry out the next job, it doesn't matter if it moves later, as all machining has been done. But trying to create joints in pieces of differing thicknesses makes no sense.

Just my opinion.

Cheers

Karl

 

[chingerspy]

I think certain people will strive for perfection no matter the cost. There is something admirable in that. Maybe because I have an engineering education (not background)

However, with wood it is a living thing it will expand, contract, warp, cup and maybe even rot away. I don't have experience using hardwoods yet, but I think I will probably have some surprises when I do.

I'm not sure I would ever be that worried about measuring my pieces of wood to the nearest 0.01 of a mm. I am having enough fun just getting the plank planed down to the pencil line without the added concentration on precision a micrometer/vernier caliper would involve (and I do own a dvc) I do woodwork for fun and relaxation.

Those that do it for money however may have a different mindset. Maybe they have been bitten by supplying an "inaccurate" piece to a discerning customer and daren't do it again? or maybe it's just their mindset is accuracy or nothing.

I thought it was a bit much during Mastercrafts when the trainees were expected to get things pole lathed or drawknifed to 0.2mm while bodging their first chairs. But then what do I know! It's all opinion.

 

[dicktimber]

Karl
No mate it wasn't a dig at you at all, infact if the boot was on the other foot I would be gooing ape......

One thing that is apparent is, if we didnt know about it, itwould it make any difference?
We couldn't pick it up with a rule....so are we over engineering our measuring?

I think we are and it is causing concernes that we can ill afford to spend time worrying abou when we have to make stuff.

Good salesmen, marketing,...and Wizzer, (just a joke.....) selling unnecessary stuff maybe

And btw I have a 260 jet!!!!!!!!!!!!

No offence meant so keep smiling!!!!!!!

 

[Karl]

OK Dick - no offence taken then.

But my point is this - if some of your workpieces are oversized by 0.25mm, then that is going to affect the fit and finish of the M&T joints. If, for arguements sake, the tenons are cut on the TS, and each cheek is cut by reference off the opposing face (ie the workpiece is reversed in the tenoning jig to cut each cheek), then the tenon will vary in thickness by 0.25mm from piece one (19mm) to piece fifty (19.25mm). Agree?

Cheers

Karl

 

[Richard Findley]

I've got to agree with Dick on this one. Far too many engineers trying to do woodwork in thou's. As for 0.25mm, that's what abrasive is for!!! using a material that can move as much as wood and expecting tolerances of much less than 1mm - let alone 0.25 or less is pretty optamistic to say the least!!

Just my thoughts,

Richard

Ps, I've not seen this other thread either so I promise I'm not picking on you Karl!!

 

[les chicken]

My hobby has always been woodwork, very basic in the early years but more complex now.

But after forty odd years as a mechanical engineer, it is impossible to get out of the habit of using the tolerances as used in engineering.

It drives SWMBO nuts when I am annoyed something does not look right, she says well how far is out. When I reply about 1/2 a millimeter, the answer is usually "you are sad who is going to notice that" my answer is usually ME.

??????

Les

 

[SNight]

Could you just use expanding foam to close up the 0.25mm gap in your tennon?

 

[Woody Alan]

I don't think it's over the top at all. when working with hardwoods it's always been the case to work to a few thou'. Agreed the specific measurement may not be precisely true as long as it is true in it's relationship with adjoining pieces. what would be the point in scribing as line if it's not worked to. There is no reason not to aim for perfection the difficulty comes in expecting it. There are some people who will always be able to acheive higher standards than others, that doesn't diminish the acheivement of those less able. I work on a simple premise now, to avoid driving myself nuts. Aim for perfection, don't expect to acheive it but the sum of all errors will be an acceptable end result. Aim for any less and you may not be happy with the end result. As a footnote I think in the joinery world due to fibre compression etc, by neccessity softwoods require a little more tolerance allowance than hardwood. To summarise yes hardwoods need to be worked close to the same engineering levels as metal IMVHO. I haven't seen the other thread so no idea what that is about.

Cheers Alan

 

[dicktimber]

Kark
Wood glue would swell wood and fill the gap.....and glue it together.

Lets look ot the other side of this.

We know what a loose joint, or undersize part is......we can feel or see it..
But what is a tight joint?....and what realy is a loose fit in wood?

If you are working to these sort of limits on thickness then you should in theory have a tolerance.
If you make both parts the same size, they won't fit together.
So what should you use as a useable plus or minus tolerance.

I reckon that the 10 thou you are talking about would be the lowest you would need to be able to take into consideration climate, movement, twisting etc, and maybe even more. There has to be a gap for glue......at any rate. Maybe 10thou isn't enough atr all?

That's why I consider we are over engineering the dimensions we are trying to achieve, making us worry about nothing.

 

[Aled Dafis]

My hobby has always been woodwork, very basic in the early years but more complex now.

But after forty odd years as a mechanical engineer, it is impossible to get out of the habit of using the tolerances as used in engineering.

It drives SWMBO nuts when I am annoyed something does not look right, she says well how far is out. When I reply about 1/2 a millimeter, the answer is usually "you are sad who is going to notice that" my answer is usually ME.

??????

Les

Same here Les, but my wife is just as bad if not worse, she sees things that even I think are OK.

Cheers

Aled

 

[Richard Findley]

I work on a simple premise now... Aim for perfection, don't expect to acheive it

I'll drink to that!! Always aim high and do the best job possible!!!

Richard

 

[wizer]

I'm not intelligent enough to fully understand your point dick (or maybe I just don't care). But I also work on the premise that if my machines and tools are setup to as higher tolerance as I can possibly achieve, then I am removing some of the silly person (human) factor. I also would like to point out that the digital read-out on my Jet planer (that seems to have sparked this debate) was not bought by me, it was a gift from a very kind person. I do own a set of digital callipers, but at something like £6, what's the problem?? When thicknessing I use the digital read out to get in the ball park and then I use the callipers to get me where I want to be. But if I'm shooting for 15mm and the callipers read 15.67mm, then that's good enough if the timber is properly surfaced on both sides. I do use the planer to take the merest slither off with the final pass and that's to reduce the amount of sanding or, more often, smoothing.

 

[jeffinfrance]

hi all,

i'm gonna have to get stuck in on this one.

reason......i've just spent £17000 plus on a new felder cf741 professional(professional, thats a laugh) and i've just given up trying to plane something after checking the levels and clearances of the infeed outfeed tables. guess what, both tables and the cutter block are all out of true.

if a machine manufacturer can't even get the basics right, what chance have they of getting 0.00000000001 on anything.

by the way....after reading the instruction manual, i found that the machine has a working temp tolerance of +10 to +40. i rang head office to find out why....you'll love this....all the fancy moters they put on will burn out if the machine is too cold. if i had been more stupid and not chose a manual machine with good old fashioned handles, my workshop would have been shut for four months of the year.

nice one felder!

if anyone is interested in the full sorry tale of my very unhappy experience with felder, i will be posting soon.

all the best

ps: i'm quite happy with 0.25mm tolerance genarally, always cut my m/t's a bit tighter and sand if nescessary.

jeff

 

[Oryxdesign]

what's a thou?

 

[wizer]

what's a thou?

It's how much you earned today.... :lol:

 

[Oryxdesign]

what's a thou?

It's how much you earned today.... :lol:

Yep, Sarah pays me a thousand Yen per day or four thousand Yen a week if I work Saturday. It's lucky she's an accountant I can never work it out :lol:

 

[matt]

I like the idea of a digital thingie to set the angle of the TS blade (or bandsaw table) - but I haven't bought one yet.

I've got 3 steel rules, a sharp pencil, and occasionally a blade. Sometimes I cut scraps of timber to dimension if I need to repeat something a lot and or clamp chocks of wood to machines to butt the timber up to to repeat cuts.

Measure, trial cut, adjust if necessary, and away I go.

 

[would not]

Having been an engineer for most of my working life i thought I'd chip in on this one.
The reason engineers work to tolerances was originally thought up so parts could be mass produced and any of them could be then fitted together, before this every part was a one off and needed to be mated to the next. This meant any part that broke had to be custom made and could not be bought off the shelf.

When woodworking machines were made to mass produce items of furniture etc then again tolerances had to be introduced.

If you are using machines to make an product from wood then working to a tolerance would negate the use of hand tools after machining to obtain a good fit.

I can't see anything wrong in using fine tolerances with machines as there is some skill involved using machines to cut any material, however I still think the true skill lies in the skillfull use of hand tools.

Don't be fooled into thinking that once you buy a machine any monkey could produce a wooden item, having spent years working on all types of machines including computer controlled machines I have found that they need careful setting up and all have their own quirks which you need to find to get a better product.

As for a rule vs digital vernier....that's just progress that enables you to do the job easier, would you throw your cordless screwdriver away and always use an ordinary screwdriver?

I can't really see any issue with the old or modern approach, it's just a matter of preference. There is also some satisfaction in making a better product.

 

[promhandicam]

In some instances the tolerances that are needed can be relatively large -for example a pegged mortice and tenon in a timber framed building. A suitable measuring implement to make this joint might be a traditional carpenters rule or a tape measure. In other areas the tolerances have to be much more precise and a digital vernier calliper is the ideal measuring tool - setting up a dovetail cutter in a router for use in a dovetail jig for instance. The advantage of a measuring tool like a vernier calliper is that it is relatively straight forward to repeatedly machine a component to the same dimensions. I've been making quite a number of frame and panel doors recently and if I could only get to within 0.25mm on each piece I machined I would end up spending the next month sanding all the components and a) the end result would be very poor and b) I'd go out of business because I wouldn't be able to pass on the cost to get everything to fit to the customer.

IMHO, the important thing is being able to decide when you need a fine tolerance and when you don't and then using the most appropriate tools / method to achieve the desired results.

Steve