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I assume that both tables (in-feed and out-feed) on a jointer should be (ideally) perfectly flat. Correct? (I am not talking about the elevation difference necessary between the in and out feed tables.) I just bought a used jointer and there is a dip (concave) in the center of the tables. The center of the tables are about 0.007" lower than the outside edges.

I intend to surface them until they are flat, unless I learn that I should not. Shouldn't they be flat?

Ed

Ouch. They should be flat, but they also should be coplanar.If you resurface them but they end up on different planes you will have a real mess on your hands. I bet they were machined too soon after being cast and deformed a bit. The fix could be worse than the problem.

Does the jointer work reasonably well?

not too much experience with jointers here,
but i would think the outfeed table is more important than the infeed.
.007 ain't much.
should be ok with short boards, no?

Having the center of the tables a bit (.007") lower than the edges doesn't seem to me to be very serious. If you have access to a large surface grinder or similar machine shop solution, I suppose you could flatten the tables.
The important thing (to me) is how flat the tables are in the 'lengthwise' direction, since that will have more effect on how straight the board edges will be made by the jointer. If you are getting good results once you have the jointer set up with sharp blades, I wouldn't worry too much about a few small irregularities in the tables- we are working with wood, after all.

Cheers
John

Yes, it works reasonably well, but it would be easy to mark the top surface of each table and take them down that few thousands (.007+/) since I have a granite surface plate larger than the tables. It will be mainly the edges coming down a tad.

Since the in-feed table raises and lowers, I'm not sure what you mean by coplanar, unless you mean when the in-feed is raised up level (same elevation) with the out-feed.

Thanks for the reply, Steve, and also thanks to all the others.

Ed

Ed: You could scrape the tables flat but it is a bit of work. Surface grinder in a machine shop much easier. DON'T lap the tables directly on your granite surface plate,you will ruin the accuracy. The plate should only be used as a referance to check for flatness.Get some mechanics blue to do this checking.
Tom

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Solution : Sell jointer buy new one . .007 is too much out of surface and to be honest , to get that surface ground would most likely cost you more than the machine . Also I think blanchard ground would be better than surface grinding , but then I was a machinist.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

On my 'poitras' jonter the tables are attatched by 6 bolts to another cast iron piece & I am wondering if shimming at say the 4 outside bolts & leaving the 2 middle ones where they are, might bend it down even just a bit to produce a better result. I use soft drink cans as shim stock.
MM

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Mike McNerney

An intersting problem that you need to discuss with the shop that grinds them, if you decide to do that. Almost all use a magnetic chuck to hold the piece. The magnet is so strong that it will warp the piece until enough points on the bottom are in contact with the magnet. Then, they grind it flat, but upon releasing the magnet, it springs back and you have a warp again in the newly ground surface. That may be what happened druing the original manufacture. So, I think that a shop experienced in this will carefully position shims between the magnet and the piece to be machined prior to applying the magnetic force. They probably have other techniques to surface this that I am not aware of. I am sure that it is a common problem but only shops used to dealing with this can do it correctly.

Good replies to your question here.

By "coplanar" I was referring to the fact that it would be easy to make the table flat but have them on different/not parallel planes. That would mean that as wood traveled from "mainly on the infeed table" to "mainly on the outfeed table" it would have to shift. The North-American-made General jointer castings sit in storage for something like a year before they're machined to allow the stresses to equalize, then both tables are machined with the jointer assembled to make sure that they are on the same plane (when the jointer is set for no cut). The tables need to be on parallel planes when the jointer is adjusted ; each table being flat does not mean that the jointer will perform correctly.

As long as you understand that making the tables flat is only part of what you have to do, and you have access to that granite plate, with care you should be able to fix your jointer. Whether you can, or if it's worth it... well...

Good luck

Tom,
Thanks much for the input. Its too late. I lapped the tables last night. I sprayed a very light film of 3M 77 and put 4 9x11 sheets of sandpaper down and lapped the tables. I never touched the granite itself which is 18x24x3" thick. I am struggling to see how I could wear the granite since I didn't touch it directly. Can you please explain this for my understanding?

It was interesting that as I started it was easy to tell where I had sanded and where the tables was untouched. The tables were identical. The patterns were in the same locations and shapes. The tables are not very flat. It was more work than I expected. I started with 100 grit. I finished with 150 grit which is smoother than the original brush finish. It would have been easier starting with 50 grit but I was out of more course grits.

This little 6" Delta variable speed jointer was a $90 Craig's List purchase and it was worth the effort since, after adding new $19 knives today, will be a nice portable addition to my small shop.

Thanks to everyone for all the help.

Ed

Thanks, Steve.

I understand you. Set the in-feed and out-feed tables to the same height and then used my 36" straight edge to check that they were flat all along the 36". They were. And now with flat tables and new knives I have just run it with excellent results. I was facing some mahogany for a laminated neck and two pieces that I planed have a nice "suction" between them when I try to pull the faces apart. The faces feel very smooth.

Thanks again for the help.

Ed

Ed: As long as you did not lap directly on the granite I think things are fine.Worked for years in machines shops connected with the Canadian Navy. We had metal lapping plates to actually lap items with lapping compound. The granite surface tables were only used as a referance or for precision measuring. I just thought you might use lapping compound directly on your surface table. Did not want to see you ruin your table. No doubt your OK. Take care.
Tom

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A person who has never made a mistake has never made anything!!!

Another method to check you plane alignment is use a square on both tables. You don't need to have the 2 tables on the same plane . the squares will butt up perfectly if you are co planer. If your tables do not align in parallel you won't have a good result. Good luck , sometime it is as much fine working the tools as it is working with them.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it