Using tablesaw to make straight cuts?
I have a confession to make: I can’t rip a straight line on a table saw for the life of me. To be more precise, I can’t make repetitive rip cuts. I’ve been woodworking for a long time. I have tuned my saw with a steel straight edge, feeler gauges, caliper, and dial indicator. I have no burned edges, no chattering saw marks, my feed rate is consistent…everything on the saw is parallel or square. I cut with a Forrest WW set parallel a Bies fence.
I am making a torsion box with MDF, which requires repeated rip cuts to make the grid. I find that every pass I make on the saw with my MDF (for anything longer than…say…2′), the sawn edge (on the cut-off side) develops a concavity, effectively ruining subsequent cuts. So I get one good cut, then subsequent cuts get worse. This is MDF, so there’s no case-hardening, or anything funky like that.
This forces me to cut wide, fix the edge, and recut to final width. Am I the only one with this problem? Every book I’ve read on the topic of table saws never seem to point this out…for sheet goods anyway. I’ve always take it on faith that if my saw is tuned, I can make these types of cuts and get consistent results.
…can I get a witness…?
Replies
Are you using a splitter?
2'? So does that mean that the stock has to begin to clear the fence before the problem arises?
Yes, always with a splitter/knife. I notice the curve always appear at the last 6" of the cut. Subsequent cuts will increase the length of the curvature.
Some of the responses suggest that there is confusion about what "appears" to be happening, i.e., not just why but what.Is it fair to ask if the cut appears to be the result of the material wraping around the far end of the fence, i.e., to the right? Or to the left?If I've got this pictured correctly . . . here's what I'd do: break down the elements. I try a partial algorithm (knowing it's flawed with simplicity).> Operate without splitter/knife
> Clamp the far end of the fence
> Consider trying more than one kind of material
> If that fails, please speak up
> If it's okay, start adding variables: unclamp the fence, and cut
> If ok, replace the splitter/knife. Cut. (I guess this is where the problems will begin. If so I'll postulate further).Please remember kickback causes more personal injuries than do saw cuts. Maybe you'd want to at least rig up a vertical featherboard.Good luck. Consider the fun of overcoming this problem.
Sorry, folks, just can't help it- have to say it.....FORWARD with Altendorf, Lazarri, SCM and all other SLIDING TABLE SAWS.Philip Marcou
Philip:See your point. A slider would be nice to have. But on a rip operation, I assume it'll be done the same way on a slider, i.e., with a rip fence.
Oh sure, if you're going to do it that way, anyone can do it!
Hal
PS: I won't claim that I can move an Altendorf by myself.
you know, for my larger cuts I built support tables with rollers so the material glides easily. this took alot of pressure off the fence and consequently I am not getting off cuts. I also built a larger sacrificial fence so I have edge support on the longer side much better than just the table saw fence. I always cut 1/8 larger than I need and us a flush trim bit to flush the materials once they carcases are assembed. This relieved me of my need to cut perfect on the table saw and since everything is assembled square there are no issues. My set up is completely portable since I travel around my county to build. I do alot of built ins and cabinetry this way. I am starting to build the cherry console cabinet from the projects area also I will be starting soon the craftsman style kitchen cabinets.
Often man-made materials will have this problem. I had a salesman once tell me it was my saw blade. I laughed at him.
You can find out if it is the material by simply cutting a 4 x 8 sheet of whatever right down the middle so you end up with 2 pieces 24 7/16" x 97" (usually MDF or Melamine is oversized if it is industrial quality).
If both pieces have a concave shape on the cut edge, it's the material.
I have decided that this is caused by the material being dried a bit too much at the factory, then once it is back in the real world, the outside edges draw moisture, either in the warehouse, on the train or truck, or maybe even in the shop. But I really think it happens in the warehouse where out of a unit, only the edges are subject to the moisture. So this added moisture vapor causes the material along the edges to expand a bit.
So quit beating yourself up over it. Just rip them oversize, straighten them, and rip them again. That's the only way you can get straight sheets.
Hal
http://www.rivercitywoodworks.com
I don't think it's the material. It appears that the panel was fed with a slight turn during the last 8-6" of the cut. Perhaps I should've included this last bit of detail. The curve invariably occurs during the last 10-6" of the cut.
Maybe you should sight down the edge of the material prior to cutting. I always do this when cutting sheet goods as some of them are not straight from the mill. It's pretty surprising, actually.
Unfortunately most industrial sheet goods are manufactured for the secondary manufacturing businesses out there, which means the use of panel saws, beam saws, and high speed CNC or Point to Point routers. None of these machines index from the edges of the material, so it doesn't matter to most of the users of the material whether the edges are straight or not. You and I use a sheet or two once in a while, these guys cut up units of the stuff every hour.
Being small time users with outdated equipment, we need to figure out ways to use the materials regardless of how they come to us.
One way is to simply tack a straight edge to the edge of sheet goods and run that against the fence for your first cut. I usually figure out a cut somewhere near the center of the panel and then cut backwards from there. For instance, if I need a few 8" wide pieces, I would start by cutting 25 inches off for the first cut. Then I would cut backwards, cutting the 8 inch pieces, leaving the factory edge for waste.
Another problem could be that you are allowing the cut to come out less than true, and you simply need to correct it after a few cuts. Anytime you make a lot of cuts from the same sheet, whatever you are doing wrong gets multiplied my the number of cuts you make.
Hal
http://www.rivercitywoodworks.com
If you are breaking sheet-goods down into narrow strips, I normally rip in half, then quarters then go to the narrow parts from there. It is too hard to get a good straight feed when taking narrow rips from the edge of full sheets.
I assume that you are keeping the work against the fence all of the way through the cut, if not, the problem should be obvious. If you are keeping the edge to the fence all of the way through the cut, it can be narrowed down to two things, if the saw is set up as good as you say.
If the blade has a problem, it may cut a wider kerf part of the way through. To check this. Feed a sheet 3/4 of the way through, then turn the saw off, and check the kerf width at the start, and in the middle. However I don't think this is it.
I am betting it is due to the pressure into the fence as you feed that is causing the fence to flex away toward the end of the cut. To see if you can flex the tail end of the fence. or at least how much pressure it takes to. Set the fence up, and lock it right at the edge of the miter slot, then press on the far end, and note how much pressure it takes to flex the far end over a given amount. You can set up a magnetic dial indicator if you want to know in thousandths, but most will move some.
If you now know it will flex some, work on feeding so that the pressure into the fence does not change from one end of the cut to the other. It is common to exert more pressure on the trailing end of the cut, which would cause the concave cut.
I just tested my unifence, and got .001" of deflection for each # of force on the end of the fence.
Keith:I am thinking it is a fence flexing issue. I had used the Unifence and saw this problem. I know the Uni will flex a bit. Now I got a Bies and noticed it took much more force to flex it. Your response gave me an idea: I am going to clamp the far end of the Bies down and see how it affects the cut.
tufenhundel, I would like to hear if this is the cause. In your profile, your remark <Can't believe you'd be interested enough to look here.> actually gives a clue. I have seen some pretty stout men who were descended of some of those mill-towns.At your age, and location, I get a mental image of someone who probably doesn't know their own strength, and probably has formed the habit of starting the cut with a fair amount of pressure into the fence at the beginning of the cut, then shooting it straight down the cut in the middle, then really finishing the cut with too much pressure into the fence toward the end. There is one more thing. If you are feeding with both hands up until that last 8 - 10", if you quit using your left hand, then only use the right hand to finish feeding the cut, this would add to the problem. When all of the feed force is on the right corner, and all of the drag, or resistance is to the left, this would cause the wood to rotate left at that point.I always try to feed at an even pressure and speed with the center of my effort along an imaginary line from the front of the blade to the back of the fence.If your saw is set up as good as you stated at first, and if this is not it, then check your blade very closely. If the tight side is dull, or has a few chipped teeth on that side, or I have seen a different sharpness angle from side to side. all of these will cause one side to cut easier or with more resistance. Dang I can't seem to finish this. One more thought. Make sure that one side of the saw table, or catch-table doesn't have rust or something that causes more drag on the left side. Please don't come back and say that you use a roller stand to catch the outfeed! K
tufenhundel,
I have the same problem. My Bies knock-off has a plastic face and I wonder if sawdust falling behind the face(bulging) could be causing the problem? I've yet to try a short fense to see if that helps...
I hardley ever cut sheet goods, mostly I'm ripping rather thick hardwoods.
I have had similar problems, off and on, and I'm with Keith on this one...changes in pressure of the hands can really mess things up. The 3 things I'd monitor are (a) squareness of the sheet at the start; (b) any flexing in the fence and (c) consistency of pushing right at the end there.
I have a friend who used to work in a cabinet shop. He can feed a 1/2-sheet of plywood as slick as ...well, you get the picture. I can't handle those big sheets, and even with smaller stock often resort to a featherboard to keep consistent pressure toward the fence, consistency that my hands can't seem to provide.
A "No!" vote here on removing the splitter as an experiment.forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
I would go along with HAL J on this and I think its down to the weight of what your using. MDF is very heavy and tough to control. I often find the last few inches want to "twist" and give a curved cut.
With a sheet of MDF I would be using a long stright edge as a guide for my cicular saw. I work alone most of the time and big sheets are tough to control.
devildog,
I'm thinking that Keith is on the right track, and that you are allowing the blade to eat away a bit of the offcut stock once it's free of the piece that's being ripped off. That is, the force you are applying to keep the stock against the fence is being transmitted to the side of the saw blade once the offcut is free. If you apply pressure on your rip cuts with this in mind, I think your problem will be alleviated. Still, you'll probably want to true up the edge every several cuts; if you are hand feeding the stock, variations will occur in your feed rate and pressure, and that will telegraph into irregularities in the sawn edge. This may show up as a taper in the waste side of the stock after repeated rips, for example.
Regards,
Ray
I am beginning to think it's feed pressure too. I had clamped the Bies down this morning, and still has problems. As much as I hated to do it, I had to run each piece through my jointer to quickly keep moving. Part of the problem with feed consistency is the need to switch to a push stick at the end of the cut. No way around this, I guess.Semper fi.
tufenhundel
I have the same problem and my solution was simple..
A really long fence.
In the case of a 4x8 sheet you need a 16 foot long fence.
I clamp a 4x4 angle aluminum making sure it's square (mines 24 feet long did I mention I work with really long stuff)
I have infeed rollers and outfeed rollers.. not those flimsey tippy things they sell, but the things that roll packages in warehouses. set on sawhorses. (serious saw horses made with 4x4's)
Nice solid fence to lean against and not have it flex and alter my cuts..
frenchy:I tried to do something like this with a 72" level...but I guess it's not long enough, or rigid enough. Unfortunately I don't have a good source for 16' angle. Thanks.
About straight edges and longer fences . . .While setting tile, I purchased a set of aluminum "L" straight edges for $55 (from a tile store or HDepot on occasion). I use them often when hand planing, checking the trueness of long boards, even clamped to a cabinet case to correct a plywood bow. Regarding the TS, I have screwed one such straight edge to a contractor's saw fence with good results. Although awkward, such a straight edge can be used as a false edge (against the fence) while ripping the opposite bowed edge if a jointer is not available.I like the GripTite setup (including the long fence which I have rabbeted in flush to a sacrificial board). So what? Given a splitter and vertical feather boards, you can start early with a long push stick (the thickness of your stock, laid flat on the table) before the near edge gets to the fence if you wish. This is well illustrated on the GripTite site.IMO, jim
Edited 3/22/2007 4:10 pm ET by InMyOpinion
Have you checked the straightness of the rip fence? If it is concave, and many of them are, you will get exactly the problem you are experiencing.
John White, Shop Manager, Fine Woodworking Magazine
John
How can a short 18 inches or so of fence before the blade be expected to keep a sheet 8 feet long straight? Long fences! that's the answer!
You're asking the wrong guy, I don't like short fences.
John W.
John, I use the so called 'short' style fence for ripping, but I'm European, work in Europe with European saws, and so it's normal practice over here to work like that.
I say the fence is "so called 'short'" because European fences are not short. They are as long as the American style fixed fences.
The European style fences have an American copy, the Delta Unifence. It has an extruded aluminiun fence that is attached to the body of the locking mechanism on a clamping bar locked with thumbscrews or kip levers. Slacken the thumbscrews, slide the fence to where you want it fore or aft and lock it in place again.
For ripping, these fences are set so that front end of the fence doesn't extend any further forward than half the diameter of the blade, and usually less if the cut is not as deep as the blade can cut. With the fence set like this there is a lot of fence to support material on the infeed end of the action. Vitally important, and who needs to support the wood or board against a rip fence after it's cut anyway?
This works extremely well for ripping both sheet goods and solid wood. I guess I've got used to the technique over the last thirty odd years of using European style rip and dimension saws.
The truth is I would never use one of those fixed US style fences (that extend all the way past the blade to the drop-off end of the saw for ripping of any sort) especially coupled with the US prediliction for removing all the guards. Now there is true unadulterated lunacy, along with an unhealthy desire to get injured all rolled into one, ha, ha.
I know you already know all this. I just wanted to dispel the false idea that European 'short' fences are short. They're not. Slainte.
Richard Jones Furniture
Edited 3/22/2007 5:47 pm by SgianDubh
John:I am going to put my 4' straight edge against the fence and see. Thanks.
Hey, Tuf,
I have the same problem. I am not saying that just to make you feel good either. Other posters have given reasons as well as good suggestions for a solution. The way(s) that I deal with the problem on sheet goods is to cut the sheet in about half then smaller pieces to start. Next, I cut to near size before cutting to finished size. I don't do this because my equipment is not accurate or because I can't feed material evenly. I do it to avoid wasting material and time. A small piece of stock can be cut much more accurately than can a large piece just because it is easier to control. You also need to be certain that you have straight edges on the material. I follow much the same procedure for the same reason with solid wood. Good luck!
Cadiddlehopper
Yeah, that's what I do...having to sneak up on the final dimension. PITA, for sure. When I'm in the shop, my goal is the end product, not for pleasure in the actual work, so having to do this is tedious.
tufenhundel
BG is probably close to the cause...saw dust accumulating as you cut and eventually pushing the work away from the fence. A small check at the saw table level, cut into a sacrificial fence, will take care of this problem. JL
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