Jointer Problem (Anti-snipe?)
desertmaster
| Posted in Power Tools and Machinery on
I put a fresh set of knives in my jointer recently. Since then, it won’t cut the last 6 to 12 inches of the board. In other words, instad of snipe (cutting too deeply on the trailing edge of the cut), it cuts fine up until it reaches the last several inches, then won’t cut at all. When I first installed the new knives, I made sure the outfeed table was level with them, and both tables appear to be parallel. Before I start fiddling with the tables (and/or knives) I wanted to get some advice. Thanks to anyone who can offer some. Gary
Replies
desertmaster,
If you have a fixed outfeed table , then the knives should be set dead flush with it. If your outfeed table is adjustable then in the raised up position the knives should be set flush. Otherwise there aren't many other areas to adjust.You said you made sure the outfeed table was flush with the knives. If it is adjustable then it may not have been in the full raised position.
good luck dusty
It is likely because the knives are too high - even slightly can do this. Also, make absolutely sure that your infeed and outfeed tables are parallel. If so, then set the infeed table to its highest position and check the knife height at dead center. I realize that the kife height is adjusted to the outfeed table only, however a certain non-parallel situation can cause this.
I'll try to dig up an article I saved that dives deep into these problems and what to do to fix them. It describes the effect of up or down sloping infeed and outfeed tables and if memory serves me, one of these conditions can cause the board to draw away from the knives as you've described.
Let me know if you've solved the problem by adjusting the knives. If not, then I'll look through the article and send along what they say could be the cause / fix.
Good luck,
Tony
Maybe I haven't had enough coffee this morning, but if the jointer is not cutting the last few inches, wouldn't it be because the knives were too low relative to the outfeed table? If you have time, I'd appreciate info on the article you're referring to -- might have it on the shelf somewhere.forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)Another proud member of the "I Rocked With ToolDoc Club" .... :>)
forestgirl,
Yes, the knives are too low. Or, to put it correctly, the outfeed table is too high. Not only is it causing the knives to miss the last few inches of the board, but the mis-adjustment is producing a convex curve to the jointed surface. If the bed is too low, it will cause a concave curve. A slight concave curve is desired by some workes as it produces a slightly sprung joint whose ends are tightly closed before the center closes up and clamping the center very nicely brings everything together.
The technique on an adjustable-height outfeed table is to adjust the height slowly until the jointed edge is perfectly straight as measured with a true straight edge.
In absence of a true straight edge, here is an alternative, which is actually more accurate. Joint two pieces one after the other. Bring the jointed edges together. Only when they are both perfectly straight will they come together exactly all along their length. Otherwise they will both either be convex or concave and the "test" will exaggerate the discrepancy by a factor of 2.
Rich
Well, I better reread the article (it's from 96' or something) to verify my thinking. Here was my rationale, however:
High knives essentially might create a tapering effect on the board. Just like you'd get when you only joint part of a table leg to taper it. One indicator would be that the missed portioin of the board gets longer and longer as it's jointed and re-jointed.
Another possible cause is the outfeed table sloping down from the cutterhead. This would essentially lift the trailing end of the board away from the knives.
Here's another thing I just thought of. Actually, I can't believe that this wasn't my first thought since I've seen it many times:
If the board is convex (bellying toward the table) near the middle, the jointer can (usually will) create a straight line that's at an angle tapering away from the board. The line starts from the leading edge and is essentially tangent to the belly of the board. As an exaggerated example, pcture a badly arched board that was run on the jointer, convex side down. Only a portion of the board would contact the knives, and many passes would be required to find a true, full length line. Typically, the board is pushed down in the front first as it's fed into the knives, which is why the angled line begins at the leading edge and stops at the belly.
Test this by jointing a concave-side down board and run it a few times. If it begins to miss at the end, then this isn't the issue. If it joints true, then the problems not the jointer, but the board. I avoid jointing a board convex side down because it will almost always cause "anti-snipe".
Found out today that I'm traveling to Japan for a week, so I might not be able to find the magazine until I return - I'll try to find it if I have any time, though.
Best Regards,
Tony
Forestgirl,
Sorry - I got so caught-up in the anti-snipe question, that I missed answering your question re: the magazine. It's American Woodworker, from either 96 or 98 if I recall, and at the top in bold yellow letters it says something like "8-inch Jointers Tested". The article right after the jointer review is about diagnosing and fine-tuning jointers. I saved it because I know how finicky these babies can be - well, truth be known, I'm a bit of a pack rat, too.
Best Regards,
Tony Z
First, make sure that you tightened the bolts which keep the outfeed from moving if you changed it. If it flexes down on the ends, this would cause this.
Next find a good strait piece of wood. Place it on the outfeed table with a few inches sticking out over the cutter head, and turn it by hand. Each knife should just slightly touch and move the board toward the infeed side no more than 1/4".
If everything else looks good, consider your feed method. If you are new to the jointer, you should crown the board up, so that the end get trimmed while the middle is missed. Little by little some is nibbled off the ends, until the meet in the middle in a straight line. Don't press the bow out of the board in the middle to get it to cut before it is time., this is counter -productive since it will spring back whenever you release the pressure.
If you are feeding with the crown down, you should not change the downward pressure as you feed it across the cutter-head. If you are applying too much pressure toward the outer end of the outfeed table without keeping enough pressure over the cutter-head, you could get the results you have described with a well set-up jointer.
I just lent the book out of coarse, but in David Charlesworths Furniture Making Techinques vol. 1, I THINK that he recommends that the knives be set .001" higher than the outfeed table. This is to account for the peaks and valleys left by the rotating cutterhead. This is why you see the installation method of finding top dead center and then measuring how far a piece of wood will be pulled off the outfeed table when the cutter head is turned. I won't have the book back until next week but I am sure that there was a recent article in Fine Woodworking about setting your knives. Good luck.
The article people have been mentioning in this thread regarding Jointer tune-up is in FWW #142, June 2000 written by John White. It's an excellent article I've referenced a bunch of times that gives advice that works.
There are more old drunkards than old doctors. Ben Franklin
As others have said, the knives need to be a gnats whisker above the outfeed table, otherwise the stock will ride up over them and create the problems you describe. A very easy method to set knives is to use a 1/8" x 1" x 12" steel blade from a combination square. Place the ruler on the outfeed table, up on edge and align the 8" mark with the throat opening, long end towards the outfeed side. Make sure the machine is unplugged. Rotate the cutter head slowly by hand. The tip of the knife will grab the ruler and move it ahead. You are looking for a movement of 1/8". Check each knife on the ends and in the middle. This is a method that has been used in every pro shop I have worked in. Simple and effective. Return the ruler to the 8" mark each time.
Operator technique can also have a big part to play. Put pressure on the infeed table for about 1/3 the length of the stock. This will give you a reference cut that you want to keep flat to the outfeed table. Once you have started to get a reference cut, walk ahead and put the pressure on the outfeed table only to finish the cut. If you are trying to straighten a board, you never want pressure on both tables at the same time.
Beat it to fit / Paint it to match
Invest in John White's book "Care and Repair of Shop Machines". It covers the jointer; knives, tables, problems, maintenace and adjustments. Covers BS, TS, planer as well; great resource for the shop. Money/time well spent!
Regards,
Mack
"WISH IN ONE HAND, #### IN THE OTHER AND SEE WHICH FILLS UP FIRST"
Man, what an abundance of intelligent, thoughtful replies. Thanks to each one who took time to write.
I fixed it; here's what I did: this morning I rechecked the setting of the knives, by using the "pencil mark on the straight stick" method. I apparently had not been precise enough when I originally reset the freshly sharpened knives because when I roatated the cutter head, on one knife the stick, didn't advance at all. On the next knife the stick only advanced about 1/8". I looked in my owner's manual and found that I had made notes from the last time, a year ago, I had replaced the knives. At that time according to my notes, I had set all the knves so they advanced the stick 1/4". So I reset all 3 knives to this 1/4" setting and now it cuts like it should, i.e., straight, except it cuts a slight snipe whenever I forget to use correct downward hand pressure (the procedure for which was so aptly explained in one of the replies).
Thanks again. I am always amazed by number of folks willing to share their expertise in solving problems. Gary
desertmaster,Good to hear all is well. If you hadn't had the snipe problem caused by the overall low knife setting, the uneven setting would have caused poor cutting in general. I don't think you ever mentioned whether your jointer has an adjustable out-feedtable.Rich
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