Thursday, September 20, 2007

Cutting plastic -

Buzz: - Hey Skippy - this one sounds pretty easy:

"We are currently cutting a (approx. .040 wall) PVC profile extrusion with a diamond saw blade. (Very messy) Has anyone got any suggestions how we can make an accurate and clean cut with a minimum of dust? We only do about 500 cuts per day."

Skippy: Fortunately, cutting PVC stock materials is a rather common activity in the plastic business, and a good number of saw blade manufacturers have already solved this problem. Your results will improve if you have access to several pieces of information and or tools - how you approach may well be dictated by the arbor size and speed of the motor (and speed reduction?) available to you.

a) the saw blade design - several saw blade manufacturers produce a 'bi or triple tooth' style design - the saw blades have carbide inserted teeth that are welded on to the blade body - some are 'spade like - more pointed' and some are 'clean-out like' more blunted by slightly wider - in this way, the blade progressively nibbles into the material and then cleans out the sides of the cut. Blades having teeth slightly wider than the blade body are preferable.

b) the foot per minute (fpm) of the teeth moving through the material is important versus the size of your part, the wall thickness of any unsupported walls and the support given to the part while the cut is going on - saws with variable speed motors are best - often with multiple pulleys and belt reductions etc to be able to change the final arbor rotation speed - simple calculations in combination with different saw blade diameters should enable you to get into the right ranges -

c) just going from memory for rigid pvc, 4500-9000 fpm saw blade speed (sweet spot might be around 6000-9000 fpm if the product has about a .040-.050 wall typical) with a 80-100 tooth bi or triple tooth design blade with a slight NEGATIVE RAKE (5 degrees) angle on the teeth in a 10-14" diameter blade will suffice - so depending on the arbor speed(s) available, the arbor size dictating the build of the blade, the diameter to put you into the right speed range, you should have good success. Additionally, these blades will cut many thousands of cuts before needing resharpening, and can be resharpened generally a couple times. I don't sell for any saw people, but an example catalog can be found at

which contains a general catalog – “friends in the business" use blades similar to those described on pages 10 and 11 –

Buzz: Thanks for the 'quick answer' but aren’t their any other cutting methods that are less messy – like ni-chrome hot wire or spinning knives etc?

Skippy: Well, the best solution was one I saw was published almost 30 years ago and had to do with the spinning knife (which I'll get to in a minute) - if you do in fact feel compelled to move away from currently inexpensive sawing methods -

a) First, the 'ni-chrome wire solution' - unless you buy an off the shelf product (source included), you are going to have to mess around a bit with the ni-chrome and a variable DC power source - amperage/heat/tensioning will be an issue - requires the wire or heated knife to be hot enough to part the part but cool enough not to get the sides gooey for the wall thickness - be prepared for some trial and error - and by all means provide some ventilation for the hcl gas that comes from PVC as it burns and degrades - there are heated knives that fabricators use for cutting sheet - not too wild about them myself - examples here: - look under "Heated Knives"

b) if you decide to go with a spinning knife style cold blade 'cutter', some info -

You'll need some 'bushings' that approximate the shape of the extrusion to keep it from deforming during the impact/torque of the cutting and shearing action. As the knife is rotated about an axis (like any single tooth on a saw blade or the shearing action not unlike a paper cutter) you are looking for either two support surfaces - one on either side of the rotating knife path, or one to shear against with the cutting action not unlike a desktop slicing paper cutter style movement. We always accomplished making cutter bushings with a wire EDM machine to burn a hole the shape of the extrusion through the center axis of a 2 1/2" to 3" steel rod 6-8" in length, then part the rod in two making to pancake shaped disks and ground the faces that the knife would pass between perpendicular to the outside diameter (except for turning a slight chamfer on the circumferences for a 'entry' for the knife.)

An inexpensive way to do this is to take a couple of 2-3" long 'rings' of steel or other round metal material and convert it to bushings with a cental hole shaped like the extrusion in the following manner:

Turn the ring on it's side so that when you stand the profile up in the center of the ring, there is space between the part and the inside ring diameter - it is this air space that needs to be filled -
Do a little analysis to find one of the LARGEST parts in terms of overall size and thickness, and then add a layer of masking tape to all surfaces which approximates a part just a little bit bigger. You might want to then spray a little silicone spray on the outside of the tape to increase it's release characteristics. Suspend or stand the part (say 2 1/2 times the length of the ring) up in the center of the ring with a portion of the taped part held parallel in the inside central axis of the ring and fill the void with ordinary automotive body filler and hardener. Hold every thing square until the body filler sets and hardens. If this is a SET of bushings, align another ring on top of the first ring by aligning their outside diameters, and fill the second ring with body filler and hardener. Once all is set, pull out the taped profile and then use a grinder or sander etc to dress the 'face' perpendicular to the central axis and outside diameter (if you did one bushing) or the two opposing 'faces' of the bushing pair that a knife can now pass between. If you have done this, you will end up with a bushing pair that once held in alignment (line up as you set up with a part or two on the inside to align) will give you very nice support faces for a knife to pass between and good support on the part. You also will find that if you heat the part slightly (say to about 120-130 degrees F, the part will cut with much less torque requirements and still be under the approximately 160 degree heat deformation temperature of the material.

Now of course, all you need is a hot enough knife or a cold one moving with enough inertia or slicing torque to "cut" the part while it is at rest - and that's a whole different discussion.

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