Jointer bed tolerences

[allan kuntz]

By looking at the attached photo. It is the out feed table. The in feed table is dead flat. If this not a big problem do I set knives from where the bed
levels out. Is the head without the knives to be flush with the out feed table or down a bit. If it is down how far323098
thanks
Al

[Mike Chalmers]

I would advise reviewing the Wood Whisperer's video. His point of view is that a couple of thousands is a big deal (I agree). It is not the cutter head that needs to be aligned with the out feed table, it is the blades.

http://www.thewoodwhisperer.com/videos/jointer-setup/?as=jointer%20setup&mode=posts&ap=1&apt=video (http://www.thewoodwhisperer.com/videos/jointer-setup/?as=jointer%20setup&mode=posts&ap=1&apt=video)

[David Kumm]

I'm not sure what you are showing. Knives are generally referenced to the outfeed table. If the bed is not flat by .008 over that short span, it is not anywhere within normal tolerance, even for a new machine. Old jointers were spec'd to be more like .002 over 8' although that good isn't necessary. Very few jointers allow for the cutterhead to be adjusted relative to the tables. Straight knife heads can compensate with the knives but spiral need to be parallel as the knives have no adjustment. The old Porter jointers are the only ones that I know of that had an elevation adjustment on the end of the head to bring it into alignment with the tables. Does the bed actually have a hump along the length or does it just droop? Droop can be leveled. Dave

[allan kuntz]

Dave the picture is of the outfeed table. The line in the center is just to show the measurements. The end 20" is flat and the closer to the head the more the wear
Thanks
Al

[Chris Barrett]

The cutter head isn't flush with the outfeed table, the top of the knives are.

[Kevin Jenness]

By your description, the outfeed table is not very flat. Another issue is whether it is coplanar with the infeed table. With a .008" dip over 9" I would not expect good performance without having the outfeed table machined or ground flat at a cost of some hundreds of dollars. The machine may or may not be worth the effort.

That said, try it as is. Set the knives in the head parallel with and at the same height as the lip of the outfeed table. Joint two boards the same thickness and 30" long and lay the jointed edges side by side. If there is no gap between, leave it alone. If the gap is concave (boards are touching at the ends), raise the outfeed table .001" and try again. And again. If the gap is convex (boards touch in the middle), lower the outfeed table. An accurate straightedge and a dial indicator are very useful here.

I suspect that the boards will initially establish a 9" long flat surface at the near end of the outfeed table and then rock on the transition to the farther end,making consistent results difficult to achieve but the proof is in the pudding. If not you will have to assess whether the machine is worth putting some money into to make it right.

[glenn bradley]

If you caught the Wood Whisperer video you probably have a good idea of what is supposed to be happening in the relationship between the beds and the cutterhead. He is showing a parallelogram bed jointer which adjusts differently than a dovetail way jointer. Knowing what you have would help. In general:



Align the outfeed table's height so that the long dimension centerline is even with the height of the cutterhead (not the knives).
Adjust the short dimension of the outfeed table so it is even with the height (at Top Dead Center) of the cuttehead along its axis.

Different machines handle this differently. I do not favor adjusting the knives to compensate for an out of line cutterhead or table.


Raise both tables above the cutters
Align the surfaces of the tables so that they are coplaner.
Align your fence so that it is at the desired angle to the tables (generally 90* during setup).
Set the height of the outfeed to that it is level with the knives when they are at TDC.
Set the height of the infeed table for your depth of cut.


The jointer is a very simple machine but, with simplicity comes the ability to be out of whack due a failure in one of the simple requirements.

[ian maybury]

Hi Allan. Another here that's been through the mill on table flatness - in my case on a planer thicknesser. I ended up hand scraping my tables flat to better than about 0.001in all over - although once in situ it's possible they deflect a bit under their own weight.

It seems like your outfeed table is essentially bowed up at the ends, with one half more or less flat. 0.008in is a long way out of flat and seems likely to need sorting out of the machine is joint consistently. It's not necessarily out of spec though as at least some of the makers like to cover themselves by specifying an outrageously wide (and totally impractical) permitted out of flatness tolerance. Presumably for the obvious reason…..

It can be hard to predict exactly what the consequence of out of flatness will be, as quite a lot depends on the length of the piece being planed, and how the machine is set up and to some degree technique. Yours seems to be pretty regular across the width of the table - mine was not. So my cut varied depending on line on which the work went through too.

The effect of out of flatness depends not just on the vertical error (humps/hollows), but also on the distance over which it occurs (the steepness of the slope) and where its positioned. Before or after the knives tends to be the worst. My experience (the result of careful testing once my tables were accurately flat) was that raising or lowering the end of the outfeed table by about 0.003in was enough to produce a just detectable deviation from a straight cut. Prior to flattening I had a dip in the centre of my infeed tables of about 0.005in in the last 9in or so before the knives, and that was enough to completely screw up the ability of the machine to produce consistently straight jointed stock.

It in the end depends on what you want or need off the machine - but if it's consistently jointed stock with no peculiarities (especially the option to run predictably straight material) then more than a couple of thou out of flat or coplanarity can make it very tough to achieve. There's a lot gets talked about jointer set up and technique, but in the end once the tables are flat these machines start working properly. Just set the tables coplanar, and dial in the knife height. No hocus pocus or magic technique required...

I guess whatever way it's set the lip of your outfeed table has to be more or less the correct height below the tips of the knives. If too high pieces will collide with it and drag or if really bad actually be stopped. If too low it'll cut a bit concave, and maybe slap if really bad. You have choice in terms of how you set the tilt - how much you raise or lower the outfeed end by while maintaining a functional knife height. It looks like going high (presuming your numbers are heights above datum) by the right amount could deliver a surface almost flat /coplanar with the infeed for 12 - 15in past the knives, but once the front of the piece reached the second half of the outfeed table it'd likely be deflected upwards by the tilt. Causing a straight jointed 12 - 15in or so, but convex rounding in the piece past that length. A very short piece might be joint straight.

You could lower the out feed end, but in that case the effect might be to cause the piece to follow the resulting downslope for a similar distance after it leaves the knives (causing a concave cut for the first 15in or so - but its onset might be delayed by weighting whatever is still resting on the infeed table) - but it presumably would be deflected up again in the second half of the table and if so adjusted it might then cut straight for the reamining length of the piece....