making a 90 degree cut with a miter saw


I've posted this to both rec.crafts.metalworking and to rec.woodworking because I am actually cutting plastic--and I suspect that members of both groups will have experience that might answer this question.
I have a miter saw (or chop saw, as some people call it) that I have been using to cut both UHMW polyethylene and Delrin rod into 3" cylinder. In the smaller sizes (2 3/8" diameter and smaller), this works pretty well. I end up with close enough to a right angle that I can put these into my lathe and face the ends to a perfect 90 degree angle.
I'm now trying to cut larger rods (2.75" and 3.25" diameter), and the results are so far from 90 degree angles that I can't lock the lathe chuck down firmly enough to hold the workpiece in place. I can see the miter saw blade bending as I cut the cylinders off the rod.
What am I doing wrong?
Am I using the wrong tool for cutting rods this thick?
Am I putting too much pressure on the blade, and I need to go a lot slower?
Help!
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The three things that cause a circular saw blade to wander are 1) too much pressure, which could be caused by 2) a dull blade, and 3) not enough 'set' on the teeth.
If your blade has gummed up with plastic, but is otherwise sharp, clean it, and inspect to make sure the teeth really are wider than the thickness of the blade body. On plastic, it takes a bit more set to make a clean cut, because softened plastic tends to fill up the cut.
Cut easy, keep the work cool (by applying water if necessary), and cut sharp.
Another possibility is that the saw frame is bending (or a pin is loose) allowing the blade to sit square until cutting pressure is applied, then allowing it to skew off vertical. This is not likely unless the saw is old or has been abused.
LLoyd
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snipped-for-privacy@claytoncramer.com (in snipped-for-privacy@g44g2000cwa.googlegroups.com) said:
| I've posted this to both rec.crafts.metalworking and to | rec.woodworking because I am actually cutting plastic--and I | suspect that members of both groups will have experience that might | answer this question. | | I have a miter saw (or chop saw, as some people call it) that I have | been using to cut both UHMW polyethylene and Delrin rod into 3" | cylinder. In the smaller sizes (2 3/8" diameter and smaller), this | works pretty well. I end up with close enough to a right angle | that I can put these into my lathe and face the ends to a perfect | 90 degree angle. | | I'm now trying to cut larger rods (2.75" and 3.25" diameter), and | the results are so far from 90 degree angles that I can't lock the | lathe chuck down firmly enough to hold the workpiece in place. I | can see the miter saw blade bending as I cut the cylinders off the | rod. | | What am I doing wrong?
The miter saw blade shouldn't be bending. Is the workpiece shifting as you cut?
| Am I using the wrong tool for cutting rods this thick?
Not really. I usually cut UHMWPE on my table saw and Delrin rod on a horizontal bandsaw; but the chop saw should work.
| Am I putting too much pressure on the blade, and I need to go a lot | slower?
Hmm. Could be a problem. Don't attempt to push the blade through the material - let the saw do the work. If the blade doesn't cut without pressure, it's probably time to get it sharpened.
-- Morris Dovey DeSoto Solar DeSoto, Iowa USA http://www.iedu.com/DeSoto
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First thing I would try is a new blade. Sounds like the one you are using is dull on one side. Also check to see if the "set" of the blade is equal from side to side. There are washers available to stiffen the blade if you have the clearance.

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Very common problem when using a thin kerf blade and/or the material is slick and subsequently moves in the process of cutting.
Since you are cutting Delrin and UHMW (building an airplane?) you should use a better, standard kerf blade designed to cut acrylics/plastics and clamp the material.
Dave
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Would have to agree with Dave regarding the blade being the problem. It's my opinion that the use of thin-kerf blades can often be counter productive, because the material saved by the narrower cut is more than offset by spoilage of material due to blade deflection and off-square cuts. Judging from inventory at HD and Lowes, however, it would appear that the majority of customers prefer thin-kerf blades. Perhaps square cuts are not important when one is doing rough construction.

use
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TEF wrote:

There is a problem with long stringy chips that are carried fully around the blade and back into the cut. They tend to roll into little "fuzzballs" which bind and push the blade off path. I suspect the sound your saw is making is kind of like a "zing...zing..zing" as balls climb and pass around the blade....the blade guard makes this worse as the chips build up in there and are grabbed in clumps also.
And....even with a good saw and blade, a miter saw gets kind of scary when cutting the larger diameters. Things are so slippery that everything tends to move on you. Definately a bit on the dangerous side. Basically, you have to go fast to keep heat from building but going fast means higher pressures trying to move the part being cut. Also, the rigidity of most miter saws is not quite high enough to keep from bouncing under the pressures and speed needed. 2", no problem. 2-1/2", not bad but a little weird. 3", hold on and pray. 4", hold on, pray, and count your fingers at the end of the day.
Personally, I'd say your best bet is to just figure the cuts are crooked and face both ends on the lathe. your chuck should be able to get a decent grab on the cylinder to be fairly accurate. All of these plastic rod materials are NOT round to begin with. If you truly need a round cylinder, you are going to have to buy oversized stock (comes that way) and face the cylinder also.
I'm not sure what you mean by the ends being so far off 90 degrees that you can't chuck it...using a smaller lathe maybe?
That being said, I also cut it with a miter saw using a thicker blade designed for non-ferrous metals. I still have all my fingers although it's been close sometimes. When time is not critical, I use the horizontal band saw as it's just a safer method. For high volume production, I'd use a jump saw with compressed air brushes, hudraulic clamping, and one heck of a vacuum system to get the strings out of the way.
Ya want some real fun, try doin 6 inchers where you have to turn the stock and re-cut. It works but your day is hell.
Koz
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Lose the thin kerf blade and get something with thicker saw plate. Figure on a kerf of 0.125 or near. Triple chip with eased edges on the flat teeth will give you a pretty melt free cut.
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I mentioned that I used a horizontal bandsaw to cut Delrin rod - then thought you might enjoy seeing an accessory I built to make the job even easier. I've posted a photo to news:alt.binaries.pictures.woodworking of my replacement for the movable vise jaw.
-- Morris Dovey DeSoto Solar DeSoto, Iowa USA http://www.iedu.com/DeSoto
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