Replies

1. 
   JohnWW | Sep 09, 2010 03:05pm | #1

   Jeff,

   Just to be clear: if you joint two boards one after the other and place them together the edges touch in the middle and gap at the ends?  Do the boards straighten out some as you gradually raise the outfeed table?    

   A few other questions:

   What is the machine's model #?

   Are you sure that the blades are still sharp?

   How long is the machine end to end?

   Does the outfeed table use a crank to adjust the height or is it a lever type adjuster?

   What are the dimensions and wood species of the boards you are trying to joint?

   It sounds like you are taking the right approach I not sure at this point as to why you are having a problem.

1. 
   DogwoodDesign | Sep 09, 2010 05:33pm | #2

   John,

   Thanks for the reply, here's some answers to your questions.

   Yes, if I joint two boards one after the other, they touch in the middle, gap at the ends.  Just the opposite of what I desired in this case, the assembly of a desktop.

   The jointer is a Jet JJ-8CS, and I've just replaced the knives. Just as a side note, what's the tolerance for each knife relative to the others? I'm shooting for .003, that seems to be as good as I can get.  I'm using a Oneway Mulitgauge to measure off the outfeed table.

   The jointer is 66" end to end, uses a crank on dovetailed ways.  There are also 3 set screws to tighten in addition to the lock knob.

   I'm using cherry for this top, they are about 66" long, varying widths from about 6" to just under 8"

   I now have some MDF to make a test bar, I'll let you know the results of that.

   Thanks,

   Jeff

1. 
   DogwoodDesign | Sep 10, 2010 11:07am | #3

   Test bar results...

   Okay, I built a test bar that I feel is accurate, and it tells me that the tables are coplanar to each other.  I checked from back to front, there does not seem to be any variation or twist either.

   So I'm at a loss here, hopefully you'll have a "magic" trick to help!

   thanks,

   Jeff

2. 
   DogwoodDesign | Sep 11, 2010 09:58pm | #4

   One other thought I'm having, what if I were to shim the ways on the upper ends, in effect, dropping the ends of the tables.  Because of the convex edges I'm getting, would that make sense?  Just a thought...

   thanks,

   Jeff

1. 
   JohnWW | Sep 13, 2010 04:06pm | #5

   Jeff Shimming the beds will

   Jeff

   Shimming the beds will just add a new layer of problems.

   First thing to try is to start out with the edges of the stock concave, once they are convex it is very hard to feed them through the machine without rocking them and perpetuating the problem.  

   Do your testing to adjust the outfeed table height with a pair of boards about 24" to 30" long.  Don't try to make the adjustment with the 66" boards you will be using to make the table top, they are too hard to handle as explained below. 

   Unless handled carefully, the overhang of a 66" long board on a machine the size of your Jet will make the stock hard to hold down tightly at the extreme ends of the cut leading to curved edge.

   Although it probably isn't the cause of your problem of not getting straight edges, you should be able to set the knives to considerably better than .003" difference.  Using a dial indicator is doing it the hard way.  Get or make a magnetic jig to set the knives.  With a magnetic jig, and the knife locking screws working smoothly, you should be able to get the heights to exactly the same for all three knives without much trouble.  Also the knives are set to the height of the outfeed table only after the outfeed table is set to a specified height slightly greater than the height of the cutter head.  This height and how to set it should be found in your machine's manual.

   The line of set screws that adjust the tightness of the dovetail gibs on each side of the machine are set to give a smooth sliding fit but with no play for the tables, if you have loosened them for some reason or if they are not properly adjusted, the machine will not cut straight.  Normally they are set once and then only checked and readjusted once every few years on a lightly used machine.  The one screw that serves as a lock won't effect anything as long as the set screws to either side are adjusted properly.

   Any chance you live in New England and I could see the machine?

1. 
   DogwoodDesign | Sep 17, 2010 11:45am | #6

   Thanks John, sorry for the delay...

   I'll run your ideas and see what I get.  As far as the knife setting jig, my issue with using that setup is that the springs under the knives push up too hard and cause trouble.  Any reason to keep them in place, as i would be lifting the knives as opposed to pushing them?

   As far as the dovetail gibs, I reset them according to your article, anything else to watch out for there?

   And no, I live in Oregon, near Portland. 

   thanks again,

   Jeff

2. 
   JohnWW | Sep 20, 2010 12:41pm | #7

   Toss the Springs

   Jeff,

   If you are using a magnetic jig, there is no need to keep the springs or any other type of height adjuster.   To keep the knives from shifting as you tighten them in place, make sure all of the surfaces are smooth on the knife gibs, the locking bolt heads, and the inside surfaces of the slots.

   To adjust the sliding dovetail ways, you want to start with adjusting screw closest to the head and get it snug so the table slides with just a bit of resistance from the screw's pressure, then adjust the other screws so they add slightly to the pressure on the gibs. 

   John W.