Firstly, I’m hardly a “seasoned” woodworker, so there may be some basics I’m somehow missing….
Today, I tried to make a cutting guide for ripping sheet materials to sizes that are more easily manipulated on a tablesaw. The plan was to cut a piece of 1/2 inch plywood (G1S), 3 1/2″ wide x 8 ft. long, run the edge over the jointer, then attach a piece of hardboard to it and use my circular saw against the edge of the plywood to cut he guide to size, so to speak. Should be pretty straight-forward, I would think, assuming that plywood would retain its form/line after jointing (??).
So, after cutting two such pieces and running them through the jointer (GI, 6″), one would expect that the two jointed edges would lie perfectly in line with each other. But they weren’t… 🙁 The two straight edges (presumed) were out by about 1/4 inch from one end to the other. So clearly something is awry here…
Any ideas, suggestions, advice, would be very much appreciated.
thanks,
Ted
Edited 12/7/2008 3:06 am ET by old baldy
Edited 12/7/2008 3:07 am ET by old baldy
Replies
Ted,
I'm not 100% sure I understand your problem, but as I see it you're basically asking why your jointer doesn't give you a perfectly straight edge, right?
There have been lots of articles and threads about jointer adjustments. It all comes down to a few principles: the outfeed table must be absolutely in plane with the cutter height, the infeed table must be absolutely parallel to that, and your technique has to transfer pressure correctly as the board goes over the cutter. Check out these 3 things methodically and you can't go wrong. If I've misunderstood your problem I apologize for the lecture.
BTW, plywood is murder on jointer knives.
David Ring
http://www.touchwood.co.il/?id=1&lang=e
David,Thanks for your reply.I think you are right on the money with respect to interpreting my question: why don't the jointed edges of the plywood come out perfectly straight? Yes, I am pretty much of a novice at fine (read: precision) woodworking, although I do have some experience with less demanding stuff like framing, and so on... (with apologies to our friends in Breaktime). I have done the best I can to ensure that my jointer is properly set up (aligned end to end). One test that I need to do (preliminary tests showed this to be OK) is to joint a couple of solid lengths of wood (not plywood) to make sure that the jointer itself is not the problem (will do that in the morning). I've done my best to ensure that the infeed and outfeed tables are properly aligned, but my lack of experience may have resulted in some misalignment. So much to learn, so little time (I was a biologist before retirement :) )TedBTW, plywood is murder on jointer knives.So I've discovered... do you have any explanation why his should be so...?
"BTW, plywood is murder on jointer knives.
So I've discovered... do you have any explanation why his should be so...?"I think it's mostly because you're planing so much adhesive, interlaced with endgrain. You can virtually get a wave pattern on the knives where the glue lines are.David Ring
http://www.touchwood.co.il/?id=1&lang=e
First, jointing plywood is never going to be a fun nor accurate thing to do. There can be huge differences in density along a piece of plywood, including outright voids that can fake out a joingter and distort the cut it makes. Second is the technique-sensitive aspect of the jointer itself. Combine the varying density of the plywood with pressure in the wrong place during the cut and bad things can result, including non straight edges.
I have no idea if you are new to the jointer or not but have a couple stories/videos on the jointer if that might help. See the lnks below for those if they might help.
http://www.newwoodworker.com/basic/usejntr.html
http://www.newwoodworker.com/jntrtaprs.html
Tom Hintz
Because there is always more to learn!
Tom,Thanks for your reply. Yes, I am very new to the jointer and I do indeed have a lot to learn. There is only so much theory that can be extracted from books, web sites, and so on, and eventually, one has to put the theory into practice and learn by doing. The links you provided add to this theory. I will explore them more thoroughly in the morning when my head is less heavy! :)As for using plywood, my hope was to make a good guide economically, but perhaps it's a "penny wise-pound foolish" situation. Maybe I will have to use a length of hardwood instead, and use a planer to reduce its thickness, after jointing....Ted
Ted,
"Doing" is always the final step to learning something, particularly in woodworking where touch and technique can be so important.
The idea of using hardwood is legit. Making a perfectly straight edge is tough to without having a perfectly straight edge to make it! I have lucked oput and found a very straight edge on factory cut ply that only needed a touch with a rasp here and there to get it string-straight over an 8-foot length. I still have that guide and guard it whenever someone new is in the shop....Tom Hintz
Because there is always more to learn!
I got around this by buying a length of 40 x 20 mm aluminium box section. It was just about perfectly straight and the 21' length has yielded a set of straight edges (10, 5, 3 1/2 and 2 1/2 ft) for a total outlay of 25 euros. The 10' one is better than 1mm in 10'.Your mileage may vary but you can check a couple of lengths ahainst each other in the store.
I use a spiral bit to edge joint everything. And yes, finding the first 'straight edge' is a challenge. I got lucky with a square JointAbility (second fence!), but I was ready to visit a machine shop.
What's the total length of your joiner table, three feet? Even if your joiner tables are perfectly co-planer, it's very difficult to "nail" a perfectly straight edge on a piece of eight foot stock.
Using additional support on the infeed and outfeed can help, but if either one is even slightly "off", the potential error grows as you pass over the cutters. Things usually get worse as you vary the pressure when you move your hands or adjust your stance.
Plywood is murder on joiner knives. The glue and endgrain are very hard and will quickly "ding" the knife edges. Last spring, I ran several pieces of 1/2" ply over my 6" joiner and I now have a bad spot on it. Luckily, I had the fence mostly closed so the bad spot is at the end of the knives, but replacement is on the winter maintenance schedule.
Finally, even if you somehow make a "perfect" saw guide, you probably won't get glue up quality joints. Very few circular saws are capable of making highly accurate cuts, and your movements as you run down the guide will add little variations.
I use my saw guide to rough cut my sheet stock ~1/4" oversize, then clean things up on the table saw.
Dave,The jointer is a General International with a 56 inch table. As best as I can determine, the tables are pretty much co-planar, and the fence is square to the table.The original intention was to be able to get a decently straight cut on sheet goods before finishing the job on the table saw, hence the "home-made" guide project. I realize there are commercial guides available for this purpose (e.g., Lee Valley and others), but with a limited budget, there is only so much a person can buy....My question had to do with the fact that two separate pieces of jointed plywood did not lie together very well when the two jointed faces were matched up. I tried the same procedure with two lengths of 1 x 4 today, and had no problem, so the apparent conclusion is that one cannot use plywood the way I had initially thought. Maybe at the hands of a more experienced person than myself the plywood could be machined OK, but I'm not there yet. The positive thing I've learned is to keep plywood away from the jointer! :)Thanks for your reply, and those of others as well! Ted
I totally agree with keeping plywood away from a joiner. - lolI would still bet that the table length has a lot to do with the problem, however. Your 56" table has ~28" on either side of the knives so when you start an 8' (96") workpiece, you have almost 5.5' of it hanging in space. When you finish the pass, you have the same amount hanging off of the outfeed table.Throw in the hand movements and feet shuffling needed to do an 8' piece, and ........ - lol
Ted,Don't be too discouraged. Actually, what has happened is not that bad. More on that below.But there are two pretty firm realities in what you went through.The first one is, no matter how a joiner is set up, its beds MUST be co-parallel. If not, nothing else is possible regarding adjustment.Second, jointing plywood is murder on the joiner knives.As you learn to use a joiner, you get real selective about what you are willing to run over those knives! Newer machines with "instant" blade changing capability reduce some of that anxiety, but the knives are still expensive. Plywood has lots of abrasive material in it, principally the glue used in its manufacture. Also the inner plys are not the cleanest of material.As far as the the joiner not producing a straight cut on the plywood. I'm not so sure it was due to varying density of material as has been suggested above. I think the material was simply very sensitive to the joiner's adjustment. And long enough relative to your joiner bed to show a curved cut.How was the cut not straight? Were the board edges concave, so that the joint gapped open in the middle, while the ends of the boards came together?If that happened, that's called a "sprung joint." It's actually a desirable condition if kept to about 1/8" over several feet of joint length. It allows clamping with fewer clamps and more assurance of joint contact over the length of the joint. Clamps are required along the whole joint, but when the middle clamps close the joint there, you are assured that the ends also are closed.An earlier post said the correct height of the outfeed table is exactly level with the knives (at the highest point of their arc). That's basically correct. However, it is very hard to actually measure that relationship. A much easier and dead accurate way to do it is to examine the kind of joint the machine produces.You adjust the knives to the table with approximate precision (as close as possible without being obsessive about it), then joint two boards and place their jointed edges together. If the outfeed table is too high, they will be convex to each other - never wanted. The table is then lowered in tiny amounts and the boards are run through with each adjustment. At a certain point, the faces will be dead parallel, touching all along the length of the joint. Lower the table just slightly more, and the slightly sprung joint will appear. Most people adjust for perfectly parallel or slightly sprung, depending on the need.Rich
Edited 12/7/2008 7:24 pm ET by Rich14
Rich,Thanks for your reply and insight.The problem with the two pieces of plywood was that, when the jointed edges were placed together, there was up to a 1/4 inch separation between the two at one end (while holding the other end and the middle together). I imagine that means the edge(s) were slightly convex. I can't say for certain that the tables of the jointer are "perfectly" aligned, but they are not out that badly either. When I repeated the experiment today with solid 1 x 4s, there was virtually no gap at any point along their length. In any event, the question is now moot since the apparent solution is to use a solid hardwood board rather than the less costly plywood. Also learned is to keep the plywood away from the jointer! :) Ah well, when one is learning on one's own, "misadventures" are part of the "fun"...Cheers,Ted
old baldy ,
The possibility exists that the factory edge of the plywood was not straight to begin with .I cut a lot of sheet goods and I do see bad factory edges and ends .It is also possible the plywood had too much moisture or an unstable core.
You could make several types of jigs for making a curved edge straight this is a jig I use for straight lining lumber but there is no reason it could not be used for strips of plywood as well .
dusty
Do you attach the lumber to the jig? Hate to sound stupid on a public forum, but I must ask.John
If you were using a skill saw you need to clamp it.
On the TS that jig just sits on top of the stock to be straightened and rides against the fence with the warp or hook or curved side towards the fence.
dusty
From your description, it sounds like the edges are not parallel with each other. A jointer doesn't cut parallel. You need a planer for that. Jointer knives are in the bed of the jointer. Therefore, it cuts only in relation to the tables and fence. A planer cuts an opposing face parallel with the one you place on its table. If you take a piece of rough stock and joint all 4 faces, you will not get a perfect parallel, square piece. Imagine taking a tapered leg from a table. You can run all 4 faces on the jointer until you make a tooth pick, but the faces will never be parallel with each other.
To get what you need, finish ripping it on the table saw with the factory edge against the rip fence. The two edges will then be parallel. To make a cutting jig like you are trying to, simply use the factory edge as the straight edge. The other edge doesn't really matter.
Here is an article about preparing stock.
http://www.taunton.com/finewoodworking/FWNPDF/011102074.pdf
Ted..
Sorry ahead of time. I tend to go off the deep end sometimes. What I type sometimes is not what I was thinking of.
I tried to make a cutting guide for ripping sheet materials to sizes that are more easily manipulated on a tablesaw. .....
I am not sure why you would need 'joint' the plywood. From my point of view you are only making a guide for your 'skill saw'.
All you need is something that you know is straight and true that will guide the edge of your 'skill saw and have a 'bed' that you saw rides on. The bed will be cut to the exact width of where you blade cuts.
I have two. One with 1X1 inch square aluminum tubing as the saw guide. The other has a ripped (one edge) 2X4 as the saw guide. Both have a guide and base 9 feet long.
EDIT: BOTH work equally well. Could have saved the cost of the aluminum!
The base material is tempered hardboard. 1/8 or 1/4 inch thick. I forget.. I'd look, but it is COLD out in my shop these days... And frankly. I HATE THE COLD! SO not going to look.
Anyway, The 2X4 is glued to the base material and the so called fancy one with aluminum square tubing is glued with Epoxy.
You get a length of whatever you want for the guide and for the base. Hopefully something longer than you will ever need to cut. Can be a issue to store it safely out of the way when not using it.
The base material should be of a width that will at least support the full with of your 'skill saw base plate' before cutting it.
Once it it is assembled and the glue is dry get two sheets of that dense foam insulation and lay it on the floor or place of you choosing for something to cut you guide.
NOTE: Be aware that if you change to a blade that is thicker or thinner that what you cut your guide with initially your guide will no longer be the same. Useable, but you need to correct for the difference or make one for each type of blade you use.
Set you blade depth to just cut through the base material thickness and into the rigid foam. Not so deep you saw blade tries to cut into the cement floor!
Cut the base it's full length with your saw. Be sure to keep the saw tight to you guide.
DONE!
The edge (cut) of the base that you just cut can be placed at your measurement marks on a new panel to be cut. Your saw blade edge IS the same as the base material you cut at....
Make sense?
Have fun!
By the way, SAVE that foam.. You can use it for many , many cuts.. Now you have a bigger problem.. Where do I put all of this stuff?
Edited 12/9/2008 7:19 pm by WillGeorge
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