No matter how many times I measure and re-measure before I make a cut I can't seem to cut a straight line . the blade starts to wobble and ruins the cut . . What # xacto blade is best for cutting stryene. .
thanking you in advance
SK2

thanks stavka43
im using a small metal t -square to guide the knife ,I don't cut all the way through ..when i try to scribe some how the line turnes into a curve , then I try to correct the first cut now it's snaps in two places . what am i doing wrong

I clamp a metal ruler to the plastic right at the line.
My first draw over the line is with the Back of a #11 blade. I slightly angle the handle over the rule so it makes contact. If there is any movement it goes up against the ruler and not over the clear plastic.
I then go back over with either the front or back of the blade and then only go 1/2 way through. Then I snap it off.

For long lines I usually scribe the width with a pair of "engineer's"dividers (one point is flattened to run along the edge).......this is then cut deeper with a heavy "Exacto " blade I keep for cutting --I NEVER use a #11 or thin long blade -seen them snap and the point fly up and go anywhere
I also aquired a plastic cutter with a heavy hook type blade (like a tile cutter)that can cut and leave an angle on the piece
Using the rule 6"-12"-------as said in other posts ---if you are not sure clamp the rule/sheet to table edge--on my bench there is a hole about 6" from front edge where I can put a pin into throught he little hole in my rule [that locks one end .....clamp on the other ]--you can then scratch the first line with a pin--divider leg --back of a knife ---then use 2-3 cuts to separate --------try to be "in line "with the cut (like using a saw)when "pulling" the blade along
REMEMBER---WHEN you snap the plastic ,it will "LEAVE A RIDGE"on the cut edge ("Z" shape )and will have to be filed off if "butt"-joining
Practise on some scrap
aye
BARV
(GOSMG)

Try using embossing tape made by Dynamo for their label makers like you used in school as a kid. The stuff that goes into the old fashioned label makers. Use those to stick the tape along the line you want to scribe.

I went out and purchased a .080 (2mm) thick plastic sheet, and glued .080 (2mm) x .250 (6mm) strips on the edges using a carpenters square so it will be exactly square. I then place my plastic to be cut in the corner so that the peice to be used is in the corner. If the straight edge or square being used slips, the "bad" cut is on the non used peice. For angle cuts, use the long edge to hold the longest flat side.

Although not obvious, some plastic does have a grain to it. Try turning your peice 90 degrees. New plastic sheet is rarely square. A slighlty used blade is often better than a brand new one.

I use this for making identical straight cuts. I would not be without one. In fact this is my second one I have bought in the past three years. I got this from Micro Mark.

Here is a direct link to Micro-Mark http://www.micromark.com/

Dave, you've saved the day once again.

Now that is a very handy piece of kit Dave... I'm going to go and find it's equivalent over here!

My versions of "straight" would make put an architect into instant heart attack.

I'll second the duplicutter Dave convinced me to get one and it's great for duplicate cuts....... the Old straight edge & knofe still work for longer cuts..... I've learned to scribe Slowly.....

thanks for all the great info .i was not using the tip of the blade . i was holding the blade flat or flush with the plastic like when your cutting bread, i think that's what caused the blade wobble as i put pressure on the blade.i got more control by just using the tip, but i am going to buy a duplicutter myself if the boss say's it's ok :-) :-) . scratchbuilding is fun or should i say a fun challange .i got a little frustrared at first cause i could't get it right .but the advice you all have given me .gave me the encouragement to finish what i started as i have no real world measurements i had to come up with what i thought was right by comparison to the1/35 scale figures i have ,it kinda got ruff at times . but i have been ( i forgot who said it ) bitten by the bug .again thanks for all the advice you all put me back on track . i will post pictures a.s.a.p.
thanks again
SK2