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Machining PVC Block

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This might actually be my first "I need help" post......

I have a block of PVC that's 3" x 6" x 3/4" thick.
I need to reduce the thickness to 11/16" (0.688 +/-0.005)

I have a Grizzly combo machine with enough travel and a mill vise.
Here are the cutting tools at my disposal:
-1/2" Endmill
- boring head and a variety of carbide bars

I'm thinking of using the boring head and my smallest boring bar as a fly cutter (which, BTW, I've never done with any material) taking 2-3 passes per cut depth to keep vibration to a minimum.

Anyone got advice on RPM and Depth of cut?
Other suggestions?
 

Comments

#2
I think either would work, but the cutter has to be sharp with a good shear angle to get a decent finish. A dull cutter just kind of mushes and melts its way through the plastic. As far as speed, run slower than you think you should, unless you can crank the feed handle at about 40 IPM.

A wood router bit would be my first choice, they have a cutting geometry that is pretty well suited to plastics.
 
#3
Thanks, Jim. I do have wood router bits. Hadn't thought of using them.
 
#4
I have used a fly cutter to machine nylon and pvc with great success using a HS cutter a 1/16" minimum radius point with positive rake. It is a 2" diameter with the tool bit set in a 45 degree fly cutter. Flooded with wax base coolant. 100 to 250rpm. Feed rate slow to get and excellent finish. Depth of cut ? till it cuts nice. Stay away from the carbide, you need a lot of positive rake in the tool.
 
#5
Use flood coolant if possible, I often turn 16"+ CPVC rounds and it improves the finish a good deal, dish washing soap diluted in water sprayed on with a hand spray bottle works well, dish soap and water also works well on aluminum cast jig plate if you are not plowing through it when roughing.
MIC 6 and similar materials.
 
#6
This would be the perfect excuse opportunity to buy a fly cutter.;)
 
#7
With what you have. I d go boring head but I Think it would be a god time to make your own flycutter. Easy peasy and another shop tool. Just grind your tool so there is a leading angled edge that shears the cut and material supports the cut to not blowout on the edge. Consider clamping pressure. I d get close then reclamp with minimal pressure than finish to square and not be affected by clamping pressure to distort.
 
#8
V. Sharp HSS edge cuts best. As others have said, avoid carbide. Biggest issue is how you hold it. Clamp it in such a way that won’t alter the machined dimensions when you release the clamping force.


Mal
AKA The Felsted Skiver
 
#9
Thank you, all, for the ideas. I'm off for a vacation, so I'll see what I can concoct after I get back.
 
#10
V. Sharp HSS edge cuts best. As others have said, avoid carbide.
I get excellent surface finishes on CPVC with carbide insert tooling, chip control is a problem however, I do not see how HSS tools would solve this.
Please explain.
 
#11
It’s just that most carbide inserts are made with a radius on the cutting edge to reduce the risk of chipping. Even the sharpest ones when ground to an edge have a radius because they consist of sintered particles (OK, I know we’re talking microscopic now). HSS can be sharpened to a keener edge because it’s homogeneous. I find sharpness and angles are more more important than hardness for plastics.

Mal
 
#12
It’s just that most carbide inserts are made with a radius on the cutting edge to reduce the risk of chipping. Even the sharpest ones when ground to an edge have a radius because they consist of sintered particles (OK, I know we’re talking microscopic now). HSS can be sharpened to a keener edge because it’s homogeneous. I find sharpness and angles are more more important than hardness for plastics.

Mal
How do you control chip formation?
This is by far the largest hurdle to overcome.
 
#13
I find sharpness is more important than angle, but interestingly a slight negative angle helps control in softer plastics: the cutting edge doesn’t cut until some pressure is applied to the tool. Try it if you have too much ‘bite’ in soft materials.


Sent from my iPhone using Tapatalk
 
#14
V. Sharp HSS edge cuts best. As others have said, avoid carbide. Biggest issue is how you hold it. Clamp it in such a way that won’t alter the machined dimensions when you release the clamping force.
It’s just that most carbide inserts are made with a radius on the cutting edge to reduce the risk of chipping. Even the sharpest ones when ground to an edge have a radius because they consist of sintered particles (OK, I know we’re talking microscopic now). HSS can be sharpened to a keener edge because it’s homogeneous. I find sharpness and angles are more more important than hardness for plastics.
I find sharpness is more important than angle, but interestingly a slight negative angle helps control in softer plastics: the cutting edge doesn’t cut until some pressure is applied to the tool. Try it if you have too much ‘bite’ in soft materials.
@MalR, I've been trying to sort out what you're trying to say here. I assume you are speaking of a HSS tool used in a flycutter, right?

In the first quote above, are you referring to the part being held in a vise?

In the second quote, I'm not entirely sure that I agree. An AK insert is ground pretty sharp and it has very positive rake angles. While a HSS tool can be honed sharper than an AK insert, they're pretty close. I've been fly cutting Delrin for a long time with AK inserts and find them to work well for me. I don't cut a lot of PVC, though, so that might make a difference.

In the third quote, can you clarify which angle on the tool is negative? I assume you mean negative side and back rake but am not sure. Negative side and back rake on a HSS tool tends to plough and create a lot of heat, which really impairs finishes but maybe you're speaking of something else being negative?

Not trying to give you any heat here. Just trying to understand what you're saying. My experience with plastics suggests that positive rake tooling with sharp edges, like an AK insert or a good HSS tool, cuts well with slower speeds and higher feeds. I have never had any success with negative rake tooling on plastics, which is why I'm curious.
 
#15
Sorry, it’s difficult to be precise and brief:)
The angle I was talking about is the clearance angle, which applies to fly cutters, mills and drills. I’m not trying to make a big deal out of it - if you’ve got an approach that works for you, great, go for it. If I get the chance over the next few weeks I’ll run some tests and take some photos but I’m quite busy at the moment so it won’t be quick.
As for inserts, you’ll probably find ones designed for aluminium work best (sharper and better polished). And uncoated should be better than coated since surface coatings increase the radius of the cutting edge. I found this out when I tested blades for cutting leather - the TiN coated blades were significantly worse than uncoated (https://www.felstedskiver.com/razor_blade_test_report.pdf)


Sent from my iPhone using Tapatalk
 
#16
The angle I was talking about is the clearance angle, which applies to fly cutters, mills and drills.

As for inserts, you’ll probably find ones designed for aluminium work best (sharper and better polished.
Okay, thanks. I suspect we just have some confusion about terminology, that's all. The clearance angle on a flycutting or turning tool is called the Relief angle and a negative relief angle will not allow the tool to cut. You may be thinking about the rake angles, which can be negative. Interesting because I've not been successful using negative rake tooling on plastics. Might be I have to take another look at this.

Yes, the AK inserts I referred to are intended to be used on aluminum. They are uncoated and have a sharp ground edge and very positive rake angles.

EDIT: I should add that I did not mean to challenge you in any way. I was simply confused and did not understand what you were saying. Tool geometry can be very confusing. The terminology is precise and clear but not all of us use the terms in the same way, which is why I queried you. No need for tests or photos unless you feel it is necessary to further clarify.
 
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