Machines with ground cast iron surfaces

The new fence arrived today, much better than the others. According to the "straight" edge and feeler gauge method it is cupped .004" in the center and less than that moving towards each end. I assume this is the best one they could find in stock so I think I will call it a day on this matter. As Rob mentioned it would be nice for manufacturers to publish their specs for the machines so there's no questions of what is good enough.

Here is a crazy thought but why not just attach a piece of mdf or that uhmw stuff to the fence to make it flat. For that matter why not a piece of solid maple and then run it over the jointer with it attached to the fence to make it flat and straight.
 
The new fence arrived today, much better than the others. According to the "straight" edge and feeler gauge method it is cupped .004" in the center and less than that moving towards each end. I assume this is the best one they could find in stock so I think I will call it a day on this matter. As Rob mentioned it would be nice for manufacturers to publish their specs for the machines so there's no questions of what is good enough.

I would not count on the straight edge being so straight. You don't know if the fence is cupped or in the straight edge is cupped. My guess it that both are distorted. So if you end up with .004 - I would say you are in the good.

Grizzly did not answer my query about flatness.

Are you having some sort of a problem? If you are there is more than likely something else going on.

What problem are you having?
 
I would not count on the straight edge being so straight. You don't know if the fence is cupped or in the straight edge is cupped. My guess it that both are distorted. So if you end up with .004 - I would say you are in the good.

Grizzly did not answer my query about flatness.

Are you having some sort of a problem? If you are there is more than likely something else going on.

What problem are you having?

I have tried 4 different straight edges. I also compared those edges to each other, I could see no light or gap when comparing them. It was my understanding that if you have at least 3 edges and compare them to each other and they " show" straight then they are. Or are at least straight enough for woodworking purposes. I didn't have a chance to put the new fence on the machine so I don't know if there are problems with the boards that come off it. If I were to skin the fence with Mdf or something I think it would cut down the face jointing width capacity. I don't think the fence will move much behind the head. I'll hopefully have some time tonight or this weekend.
 
I know on my 8" G0586 jointer the fence moves about 3/8" past the cutter head. I have to hold the guard back when moving the fence to get maximum width other wise I would get a rude awakening.
 
I gently ground the new fence on my edge sander then lapped it on sandpaper on the infeed table. It's now less than .002" and probably less than .001" on most of the fence. I'm happy with that.
 
I have received a specification of flatness from Grizzly.

Dear Leo,
Thank you for your email dated January 22, 2014.
A table saw table has an acceptable flatness of 0.015” – 0.020” over the entire table surface. A jointer has an acceptable tolerance for the tables of 0.002” per foot of bed length.
If we may be of any further assistance, please do not hesitate to contact us. You are a valued customer, and we look forward to serving your future woodworking and metalworking needs.
Sincerely,
Vince C.
Technical Service
Grizzly Industrial, Inc.
EN #901
ETC#R

So – the flatness spec of .002 per foot for a jointer surface sould be taken as .002 per linear foot of jointer table surface. Mind you – that does not mean a single line – but it does mean the entire surface on a linear foot. If your jointer table it 6” wide – it means the length plus the width to be within .002. The .002 is to be cumulative along the linear surface – 2 feet would be .004 – and 3 feet would be .006 I would apply the same spec to a table saw fence.

The table saw “surface” of .015-.020 is again the entire surface – not – a single line.

When someone tries to use a “straight edge” that is a single line. That is NOT flatness – that is straightness. It is not a trustworty method of straightness either. BEST you can derive from that is a "relative GUESS" about straightness of a single line. You are not even close to flatness there.

Also – Flatness is never measured in reference to anything. It is relative to itself only. It is flatness – no datum. When using a straightedge – (which is probably not straight) – to try to measure flatness – the not so flat straightedge is the Datum – or reference point and even at that can only represent a single straight line.

I have some of my engineering books at home that describe how to measure flatness, but the process is not within the capabilities of a home hobby shop.

What I am saying here, is that it is not possible for the home hobby shop to measure flatness, unless you have a LARGE surface plate, plan to disassemble you machine, have precision jackstands, and precision measuring instruments. The thing being measured for flatness must be mounted upside down on the jackstands in 3 points, and the “underside”, entire surface needs to be scanned for total indicator deviation.
Anything other than this method is not flatness, and anything other than this does not relate to the machine builders specs.

Here is a simple explanation of flatness
http://www.toolingandproduction.com/features/2007_December/1207_flatness_and.aspx

I have been all over the forums and read hoards of discussion on flatness. I can say with a high level of assurance, there is very low percentage of people that understand what flatness is. Therefore – just because you read it on a forum somewhere does not mean it is correct information.

Even in manufacturing machine shops, not many people understand flatness.
 
I have looked into measuring flatness and saw the jack stand and surface plate method. You are correct I don't have the means to do that. Is it correct to say something can be straight but not flat. For example the length of a cylinder. But it can't be flat without being straight?
 
I have looked into measuring flatness and saw the jack stand and surface plate method. You are correct I don't have the means to do that. Is it correct to say something can be straight but not flat. For example the length of a cylinder. But it can't be flat without being straight?

Dave, yes a straightness spec can have a different specification that flatness. A cylinder has a roundness specification - but straightness is often applied as well - the rod - can be held to straightness. A cylinder would have a "profile of a surface" not flatness - but yes it can have straightness applied. Like Grizzly's answer - the cylinder would have a spec per distance tolerance applied

There are many specifications for tolerance zones to be applied to all different forms of geometry.

FOLKS - sorry if I came across a little overbearing - seems to be a trait of mine.

I was hearing a bunch of talk about wishing manufacturers would clarify the flatness spec on their product.

Well - I am one of those technical people that work in engineering, developing and applying those spec's. I don't make woodworking equipment, but I do make product.

I got the specs from a woodworking tool manufacturer and I tried to explain how the specs are applied. Not many on the forums really understand what flatness is.

Anyway - sorry for bearing down of this. I am not the most diplomatic person. Just tried to help is all.
 
leo i thank you for the explanation many of us probably didnt know the difference till now,, and you did get a answer from the supplier which threw your experience and proper method of asking got a response we were after..
 
Leo, thanks for your expertise. It was a frustrating experience buying a new tool and having to wait over 3 weeks to get a decent replacement fence for it.
 
Leo anything but overbearing. Highly educational and just what the doctor ordered. Its wonderful to have a guy of your abilities explain things like this. Thanks for taking the time and effort.

sent from s4
 
Alan, I am with you on this issue. I enjoy precision but working with wood is a different situation. I have been able to ply the woodworking hobby for lots of years with the tape measure and conventional squares, rules, and reasonably straight straight edges. I have never been too concerned about flatness of tables but I do pretty much require dead on setting of blade to table and fence to table. This position is just me but it has served me well for over 50 years.
 
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