Machining PVC Block

[MalR]

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

 

[P. Waller]

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.

 

[MalR]

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.


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[mikey]

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.

 

[MalR]

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)


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[mikey]

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.