Circular Saw - Straight Line?

[drillbit]

Just bought a circular saw and I've been trying to cut some 9mm mdf to size. Saw is DeWalt, with an 18tooth DeWalt blade.

I'm clamping a straight edge to the mdf, and then running the saw against the guide. But even so, my cut is not perfect. The general effect is straight, but up close it tends to have a slghtly jagged result - maybe up to 0.5mm in places, and certainly noticeable when you hold the resulting edge against the flat of another mdf piece. You can't see daylight or anything, but the join is still not consistent all the way along.

I was kind of expecting the circular saw to produce a 'perfect' straight edge, so I wonder if I am just expecting too much? Or am I doing it wrong?

 

[wobblycogs]

How stiff is your straight edge? When I first got my circular saw I found I tended to push it hard into the straight edge to try and ensure the cut was straight. Unfortunately I caused a little flexing in the straight edge and ended up with a cut like you are describing. It wasn't a constant bow since I moved with the cut and changed the pressure on the straight edge.

You might also be feeding it too fast and causing some vibration or flexing in the blade. Alternatively your blade might not be seated quite right but I'd have thought you'd feel that when the machine was free running. Also ensure that your blade only protrudes a little through the bottom of the MDF which will give a cleaner cut and stop the blade snatching at the work piece so much. Make sure the work piece is well supported on both sides of the cut - I use some long battens for support and cut up the middle of them.

It's certainly possible to get straight cuts free hand but you might like to consider building a track. I built one for my saw (it's detailed on here somewhere, there are a couple of others as well) and I can now get cuts that are very straight and fairly easily make parallel cuts which I found hard with just a straight edge.

 

[Roughcut]

I made a simple circular saw straight edge out of a straight length of timber and some 9mm plywood.
I screwed the timber batten on to the plywood and then run the circular saw down the plywood while using the batten as a straight edge.
The result is a guide with the edge of the plywood set at the exact correct width for your particular circular saw.
It's very easy to use as I just line up the edge of the plywood with the line I have marked on the material being cut.
Same principle as shown in this video: http://www.youtube.com/watch?v=CH5dW-QcgeI

 

[Steve Maskery]

Also, 18 teeth for 9mm stock is a bit coarse. That sounds like a rip blade to me.
S

 

[MARK.B.]

Also, 18 teeth for 9mm stock is a bit coarse. That sounds like a rip blade to me.
S

+1 more teeth per inch should give you a much cleaner cut,dont forget to let the saw do the work ,go steady and try avoid pausing part way through the cut .

 

[drillbit]

Thanks for the responses. Sounds like a finer blade might be in order.

I thought I was technically 'ripping' since I am cutting along the length of the MDF, but I suppose in something with no grain, maybe there is no real such thing...?

 

[Eric The Viking]

Absolutely.

I run a 48-tooth blade in the Makita 6000 rail saw for that porpoise. It burns* but cuts straight.

E.

*you can't win on that one. If you go for the slight toe-in Festool suggest, the 'waste' will scuff/burn. If parallel, it doesn't cut so well and the geometry is off.

 

[Sheffield Tony]

I run a 48-tooth blade in the Makita 6000 rail saw for that porpoise. It burns*

My God I bet that stinks. perhaps it's just me - I can't stand seafood.

 

[Eric The Viking]

My God I bet that stinks. perhaps it's just me - I can't stand seafood.

Lemon juice. Lots of lemon juice - hides anything.

 

[Grahamshed]

You get meat and fish bandsaw blades, why not circular ?
Mind you blades for specific fish seems a little OTT

 

[wobblycogs]

You get meat and fish bandsaw blades,

I really don't think a blade made out of meat or fish would work. How would you get a good edge on one? I think I'll stick with steel for now.

 

[Eric The Viking]

Well you could have one of these:

But if you made one out of a porpoise, I think it might need a cetationary engine of some sort to power it.

I'll get me coat...

 

[Jacob]

Sawboard is the answer. Just google 'sawboard'. There's one here and there's loads more variations.

PS It's what Roughcut is describing above. It can be very accurate and clean cutting even with a coarse blade.

 

[Stormer1940]

Also, 18 teeth for 9mm stock is a bit coarse. That sounds like a rip blade to me.
S

I was just about to say try a fine tooth finishing blade... CMT do a good one...

 

[Jacob]

In Roughcut's link Bob Schmidt calls it a 'door board' - he probably thinks he invented it but it's the same thing as a saw board as used by many.
NB it really does work very well.

 

[Roughcut]

In Roughcut's link Bob Schmidt calls it a 'door board' - he probably thinks he invented it but it's the same thing as a saw board as used by many.
NB it really does work very well.

Yes I can't understand why he calls that jig a "Door Board" as Saw Board sounds more self-explanatory.
Before I bought a circular saw I used to rough cut mdf or ply boards with a hand saw and then trim to exact width/length using a similar jig as the saw board but made for my router with a 1/2" cutter.
It leaves a lovely clean finish to the edge of the material.