For my recent engine I had to cut 25mm off a 100mm tube of perspex. I didn't want to make a 4" mandrel so I figured I'd have a go at parting it.
I chucked the tube up in my 3-jaw on the outside of the jaws - very gently though because even light pressure deforms the tube. The far end I just held with the tailstock center using a piece of flat wood with a dimple in it.
I had tried parting first - that was no good because to hold it tight enough caused it to deform too much, resulting in a broken tube end. What I did found that worked very well was this:
Grind a piece of HSS to make a very thin parting tool - the same thickness as a hacksaw blade. Use this very gently to score around the tube a continuous cut - just a mm deep or so as a guide for the next step. Now take a 14tpi hacksaw blade and hold it by hand on the underside of your slowly-spinning tube with the teeth upwards and pointing towards you - I used 100rpm for mine. You have to use a light pressure and move the blade in and out gently as the teeth fill with material and clean it up often. When you are nearly done the blade will break through in one spot with an audible crack and a small tug on the blade - this is time to stop, remove the tube, cut gently around the parting using a junior hacksaw with a fine-tooth blade. All that's left is to clean up the burr with a file or sandpaper.
I only tried this twice and both times got a perfect slice of perspex tube. There were no dramas at all and the hacksaw blade followed the contours perfectly.
I chucked the tube up in my 3-jaw on the outside of the jaws - very gently though because even light pressure deforms the tube. The far end I just held with the tailstock center using a piece of flat wood with a dimple in it.
I had tried parting first - that was no good because to hold it tight enough caused it to deform too much, resulting in a broken tube end. What I did found that worked very well was this:
Grind a piece of HSS to make a very thin parting tool - the same thickness as a hacksaw blade. Use this very gently to score around the tube a continuous cut - just a mm deep or so as a guide for the next step. Now take a 14tpi hacksaw blade and hold it by hand on the underside of your slowly-spinning tube with the teeth upwards and pointing towards you - I used 100rpm for mine. You have to use a light pressure and move the blade in and out gently as the teeth fill with material and clean it up often. When you are nearly done the blade will break through in one spot with an audible crack and a small tug on the blade - this is time to stop, remove the tube, cut gently around the parting using a junior hacksaw with a fine-tooth blade. All that's left is to clean up the burr with a file or sandpaper.
I only tried this twice and both times got a perfect slice of perspex tube. There were no dramas at all and the hacksaw blade followed the contours perfectly.
