Need help. Cutting a straight clean line. - Woodworking Talk - Woodworkers Forum
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post #1 of 29 Old 12-02-2006, 09:44 PM Thread Starter
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Need help. Cutting a straight clean line.

Never did wood work before (cut junk plywood, and 2x4s but only for function not for looks).

I need to cut some plywood, and MDF straight and as clean as possible.


All I can use is my dad's cheap circular saw and the old blade it came with.



Should I replace the blade? And what type/brand for a clean cut?

What kind of guide should I be using? What is a shoot board? How do I make one?
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post #2 of 29 Old 12-02-2006, 11:13 PM
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If your only option is a circular saw then you will need a good straight board the length of the plywood to guide the saw so you can get a straight cut. A couple of clamps also. You will need to measure from the edge of the blade to the side of the base and set the board that distance from where you need to cut. I would get at least a 1x4 for a straight edge.

As far as a blade get something with more teeth. The more teeth you have the smoother the cut will be. For sure get something with carbide teeth.

Hope this helps, Dave.

Do one thing at a time, do it well, then move on.
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post #3 of 29 Old 12-03-2006, 10:57 AM
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Also set your blade depth to about 1" below the sheet. You can set it to just below like 1/8" but this is not good because, if you are using a blade with too many teeth (for instance on a 7 1/4" circular saw 40 teeth is too many for ripping) you can leave burn marks on your edge because heat builds up with so many teeth always in the cut.
On a circular saw, 18 - 24 teeth is fine for ripping. A 40 tooth blade on a 7 1/4" blade is for crosscuts.
It is relative. If I have my 36 tooth 16" blade on my table saw it rips just fine, but the teeth aren't building up so much heat either. My 80 tooth 16" blade would be smoking used for ripping anything with any appreciable density and thickness.
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post #4 of 29 Old 12-03-2006, 11:49 AM Thread Starter
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Quote:
Originally Posted by TexasTimbers View Post
Also set your blade depth to about 1" below the sheet. You can set it to just below like 1/8" but this is not good because, if you are using a blade with too many teeth (for instance on a 7 1/4" circular saw 40 teeth is too many for ripping) you can leave burn marks on your edge because heat builds up with so many teeth always in the cut.
On a circular saw, 18 - 24 teeth is fine for ripping. A 40 tooth blade on a 7 1/4" blade is for crosscuts.
It is relative. If I have my 36 tooth 16" blade on my table saw it rips just fine, but the teeth aren't building up so much heat either. My 80 tooth 16" blade would be smoking used for ripping anything with any appreciable density and thickness.
I thought Dave answered all my question but now i'm confused.

So if I set the blade 1" below the wood I will not get burn marks?

But if I have more teeth, can't I feed it slower to keep it from burning, will a 18-24 teeth give a really clean cut (has to be flush against another piece I'm going to cut, and you will see where they join)?
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post #5 of 29 Old 12-03-2006, 12:27 PM
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Burn marks are relative to blade speed, feed speed, sharpness of blade and blade depth. The wrong combination of any of these will cause burn marks. If you have a sharp blade and feed at a reasonable rate of speed you should not have any burning.

I agree with Tex on the blade depth. It's not the safest thing but when the blade sticks farther below the workpeice it will not build up as much heat which will cause burning.

I hope this cleared it up.

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post #6 of 29 Old 12-03-2006, 02:52 PM Thread Starter
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Just to be clear,

slower feed = less heat

right?
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post #7 of 29 Old 12-03-2006, 04:28 PM
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As a general rule yes. But too slow can cause exsesive heat too. there so many variables, as Dave mentioned.
Your best option is to get that saw busy and don't be afraid to learn the hard way. Experience is the best teacher and after you have the basics, which you do have, the next step is to jump in.

If you have some scrap wood of the same species as you are going to be using, then make some test cuts with the blade you have and see what happens. If you have a little tear out, adjust your depth of cut one way and then the other until you get the result you like. If you have a hard time keeping the plate of the saw against your straight edge, play around with your speed (the speed at which you push the saw-same thing as feed rate) and also concentrate on oblique pressure against the saw handle as you also appoly forward pressure so the saw doesn't "yaw". If it does, you will not get a straight cut.
just keep playing with it and before you know it you will discover things we didn't mention and you will want to share them with us.
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post #8 of 29 Old 12-03-2006, 04:39 PM Thread Starter
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It just dawned on my, I haven't bought the wood, yet but I'm going to get it at Home Depot.

I can have them cut it for me. I had them cut MDF for me, and it was really clean and straight.

But now I'm going to be getting plywood, shouldn't be a problem right?
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post #9 of 29 Old 12-03-2006, 11:57 PM
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HD isn't very accurate with their panel saw cuts, so if you need accuracy, you'll have to go back and fix it. Also I don't think they rip plywood -- only crosscut. That may just be my local store, though.
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post #10 of 29 Old 12-04-2006, 11:42 AM Thread Starter
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All the HD heres are new, and they have a cutting device that holds the entire piece of plywood almost vertical to the ground.
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post #11 of 29 Old 12-04-2006, 11:58 AM
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I think that's the panel saw you're seeing. You drag a piece of plywood or MDF (or any other sheet good) into it. The wood stays stationary and the saw moves up/down or right/left to make the cut.

They probably use the same panel saw for MDF as for plywood, etc. MDF can dull blades quite quickly, and a dull blade can result in a pretty nasty cut in plywood. You can ask for a sample of waste from a recent cut to get a good idea of what you'll get.

I have them use the panel saw to chop the wood down to a more managable size, then use the table saw or circular saw with a sharp, high tooth-count blade to clean it up. I rarely see perfect measured and clean cuts (or perfect right angles, for that matter) when it comes off the panel saw.

Another idea would be to ask them to change the blade for you before they make the cut. They might look at you as if you're crazy, but you never know! It'd be a good test of customer service!
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post #12 of 29 Old 12-04-2006, 12:02 PM
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Oh -- also ask them to dust off the bottom of the panel saw before they make the cut. If there's sawdust built up, the edge of the sheet won't be flat against the bottom of the panel saw, and that could further throw off the cut.

A good way to see if the saw is cutting accurately is again to ask for a peice of waste from a recent cut, or grab a tape measure and wait around for someone else to have a sheet cut, then measure the width of the boards at both ends after the cut. If they're equal, then the cut is accurate.
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post #13 of 29 Old 12-08-2006, 08:40 PM
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Straight line cutting.

Hi,
I'd advise you to make yourself a straight line jig for your circle saw by buying one of those 8' 6" aluminum straight edges that can found at any "big box stores". Then get some 1/2" plywood that is about 10" wide and 8' long.
Stretch a string nice and tight along one edge of the plywood and screw down the aluminum straight edge along this string keeping it as straight as you can. Next make a 'sled' that will slide along the straight edge supported on both sides of the straight edge. This will keep the saw from wandering. The sled has to be wide enough to be able to mount your circle saw so the edge is snug against the blade.
Also, pick up some 1/2" x 2' x 8' Styrofoam sheets to put under the material you are going to cut. (Your saw blade can cut into this Styrofoam without damage to your sawhorses or whatever.)
Okay, after your saw is mounted on the sled cut the edge of your plywood so it's even with the edge of your saw blade. This edge will now be perfectly straight because the sled will slide along the aluminum straight edge.
VERY IMPORTANT!!!! Your blade guard is being held up by the sled so you have NO BLADE GUARD!!!! BE very observant and careful!!!
Now set your wood on top of the Styrofoam then the jig on top with the edge exactly where you want to cut. Set the saw on the straight edge and carefully move the saw along the edge.
I've got two of these jigs made, one is 8'6" long and the other is 16' 6" long and I use them for straight line ripping of oak boards to straighten them out before going through the gang saw.
Again, BE EXTREMELY CAREFUL because of the absence of the blade guard!!!
If you want some pictures send me a personal message and I can send you some digital pictues.
Jim
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post #14 of 29 Old 02-17-2008, 11:30 AM
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If you are going to use a circular saw... the more teeth the better. And use a straight edge as a saw guide. Also... scribe the cut line with a knife. This will stop chipping. Rick

Never... I mean always... never mind Rick
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post #15 of 29 Old 02-17-2008, 12:34 PM
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Just a few suggestions to go along with all the info so far. When using a circular saw, the good face of what you're cutting should be down, for a clean cut to the face. Depending on the thickness of the plywood you're cutting, will determine the tooth count. If you're cutting 3/4" hardwood plywood, you will get an easier cut, but not necessarily as smooth with a tooth count less than 40T. A carbide tooth blade will give the best cut.

As for a straight edge, you could get by with something simple as a piece of 1/4" plywood or masonite. The thicker your straightedge is the more clearance you need for the motor to clear over it. It should be wide enough to get your clamps out of the way of the saw passing. The straightedge can actually be used on either side of the baseplate, but your measurements should be made to which side of the tooth you are cutting to.

For setting up under the ply, make sure both pieces have the same support, so when you cut, there is no "falling" of either piece. Something like 2 x 4's work pretty good. When setting the depth of the blade, the gullet (gap between the teeth) of the blade should be clear from the bottom, to allow debris clearance while cutting.

Once you start your cut, don't stop. IOW, make sure you are completely set up and ready to go. Make sure you have your cord/extension cord laid out safely from the cut, and you have enough cord to get the saw to go all the way. I know this sounds very elementary, but it can be an overlooked problem.

The HD's I've been to do a fair job on vertical cuts. For horizontal cuts, they push the material through the saw (made stationary), and use kerf keepers (blade thickness spacers) to position the top piece during and after the cut. The quality and accuracy of their cuts depend on who is on duty that day.






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post #16 of 29 Old 02-17-2008, 03:52 PM
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Wow! 14 posts on how to cut a straight line you got to love this site!
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post #17 of 29 Old 02-17-2008, 06:41 PM
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All the advises above are very good to follow.

I just want to add a few small technical things and a few pictures.

First, as it was already said, you shall need a straight edge that can be anything from an expensive aluminum or just a wide (4"~6") Plywood, MDF or Masonite.

Make yourself 2 "Off-set plates". how? I don't know how but look at this post...very simple
http://www.woodworkingtalk.com/showthread.php?t=3132

When you measure, use always the same measuring tape or rule.

If you have seen a bullet hole on a concrete wall or alike, the side that the bullet entered has very "clean" hole but at the outlet side, the hole is chipped-out and larger.

That's exactly what happens when the blade teeth are attacking the wood...the teeth "entry side" has a clean cut but the teeth "outlet side" is chipped out.

When you are cutting with circular saw it means that the lower (under) side of the board will have a clean cut and the upper side will be chipped out.

If you don't care how one side will look, just keep the side that will be visible (the good side) under but if you want both sides to have a "clean" cut....

You can use the "Scoring blade" method....set your blade to around 1/16" below the base plate and make a cut (a score) backward i.e. starting from the far side and pulling the saw backward. This will leave a clean cut (score) on the upper surface of the plywood.

Now, set the blade to cut the full thickness of the plywood and cut the normal way forward. If your blade is at 90 to the base, it should leave nice and clean cut on both sides.

And now to the pics...





























Last edited by niki; 02-17-2008 at 06:45 PM.
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post #18 of 29 Old 02-18-2008, 11:52 AM
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Hi niki
That is a very interesting dust collector you have on your skilsaw. Would you mind posting some close up pictures of it? I would like to try it out on mine.

Thanks

Gerry
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post #19 of 29 Old 02-18-2008, 11:58 AM
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I think the subject has been quite well covered, but I just thought I would throw out another suggestion to help control chipping on the top side of the piece you are cutting.Lay down a strip of masking tape along the line you are cutting, so it is centered over the cut. Mark your cut line on the masking tape. The tape will help limit the chipping. Two cautions:
1 Use only low adhesion masking tape, usually called painters tape, and usually green in colour.
2 Don't leave it on any longer than you need it. Some masking tapes can bond themselves to material really well, especially if exposed to weather.

Gerry
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post #20 of 29 Old 02-18-2008, 04:23 PM
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Quote:
Originally Posted by metomeya View Post
Just to be clear,

slower feed = less heat

right?
Nope, that's backwards. Having too many teeth will mostly result in slowing the cut and that builds heat. Cutting sheet goods is not like cutting solid wood so you don't need to worry about ripping vs. crosscutting. All of the cuts are more akin to crosscuts because you will be making fine chips. Go with a 40 to 60 tooth thin kerf blade and set the blade to just clear the material.
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