Differences between bits

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10 Mar 2021 10:44 - 10 Mar 2021 10:45 #201637 by Jocman
Hi all.
I'm testing some bits before try to realize a job involving Plexy and MDF.

I tried cutting both materials with 2 different bits:
- single flute (spiral) 3 mm
- double flutes (straight) 2 mm

Without considering the tool diameter, I got different results:

Plexy:
- single flute: clean cut, sand paper not (or barely) needed, no problem from the spoilboard's debris
- double flutes: dirty cut (lot of plexy debris such...melted), file and sand paper needed (and very...), spoilboard's debris melted into the plexy...

MDF
- single flute: clean cut, sand paper lightly needed just for the surface
- double flutes: almost clean cut, like single flute.

Beside my personal considerations (from my POV, the single flute spiral has to be preferred with plexy and MDF):
- for what to use the straight double flutes bits?
- would it be better keep using the single flute spiral for plexy and MDF, or better more flutes (2-4)?

As I got even some end mill bits 0.8 - 3 mm (I used them sometimes with PCB with good results), can there be used even for plexy and MDF (or other woods), or could I get some problem (i.e. material overheating - melting or firing - etc)?

Andrea
Last edit: 10 Mar 2021 10:45 by Jocman.

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10 Mar 2021 12:27 - 10 Mar 2021 12:30 #201641 by Aciera
Replied by Aciera on topic Differences between bits

for what to use the straight double flutes bits?

Most tools used in machining metals have right hand flutes so the chips are extracted. For other, mostly non metallic materials, straight or even left hand flutes might be preferable.
Straight flutes can be handy to cut materials that are veneered or laminated on both sides. Laminated or veneered sufaces can be cut very cleanly if the cutting edge enters the material at an angle and cuts from the surface towards the middle of the material as not to lift the top layer off the core ie. a but with a left hand flute. If the cut goes all the way through then a straight flute is a compromise since the left hand flute might cause bad cut quality on the underside of the cut.
Note that there are even V-fluted mill bits, left hand flutes on the shaft side and right hand flutes on the other end.
Single fluted bits are often used for thermoplastic materials to reduce heat while cutting as you have noticed.

Mill bit with positive and negative angled cut
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Last edit: 10 Mar 2021 12:30 by Aciera.
The following user(s) said Thank You: arvidb, Jocman

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10 Mar 2021 23:32 #201755 by andypugh
Replied by andypugh on topic Differences between bits

Beside my personal considerations (from my POV, the single flute spiral has to be preferred with plexy and MDF): - for what to use the straight double flutes bits?


I know almost nothing about this subject, but I do know that other
factors (such as coatings, materials, cutter edge geometry) have as
much or more influence than the number of cuts.

Did you try experimenting with feeds and speeds too?

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11 Mar 2021 14:14 #201814 by Jocman
Replied by Jocman on topic Differences between bits
Thanks Aciera for the explanation. As I'm an hobbystic user, my knowledges are very limited, so it's a very good thing to improve my...science

About Andypugh question: honestly this issue has puzzled me for long time (and still does).
In my ignorance (and fear of destroying bits...) I use to keep the various parameters very low (or, at least, I think very low).
I mean:
- I'm using my spindle (a Kress) set at 1 - 2 (on 5 levels)
- pass: no more than 1 mm (maybe 1.5 if using a 3 mm bit - the max diameter I use)
- speed: right now I'm not sure, but I think is set at about 120mm/min

There are really a lot of variables to deal with.... I.e., I think if I set the spindle at minimun rate, the heat due the rotation should be low; but with a low progress speed, the bit will stay more time on the same spot, meaning the rotation could heat more the surface...

From some point of view, as I'm just an hobby maker, I do not have to deal with time (I understand that for a worker time means money), so I've no...economical damage if a job is performed in 2 hour rather than 1, that's why I tend to keep lower speeds.
But I'm also pretty sure my philosophy is not the best.....

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11 Mar 2021 14:42 #201819 by Aciera
Replied by Aciera on topic Differences between bits

if a job is performed in 2 hour rather than 1, that's why I tend to keep lower speeds.


Getting the feed and spindle speed right is certainly at least as important as using the right tool for the job. If you are getting burn marks and heat issues then its usually a good idea to try and increase the feed rate.

It a good idea to get acquainted with the concept of cutting speed:
www.sistemiklein.com/media/pdf/files/2-%20Cutting%20Speed.pdf

In the table you find "cutting speed" in m/s which is the speed of the cutting edge (ie. determined by spindle speed and tool diameter) and the "feed rate" which, here, is the depth of cut for each cutter blade, so dependent on spindle speed, number of cutter blades and the rate of forward motion of the tool (so what we usually call the feed rate). This is usually somewhat neglected when working with non metal materials as these are more forgiving but it can give you an idea if you are way off the mark. Of course the desired cutting speed depends on the material the tool is made of as well,

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11 Mar 2021 15:10 #201821 by andypugh
Replied by andypugh on topic Differences between bits
For a 3mm solid carbide 1-flute bit in PMMA FSWizard suggests:

23327 rpm
392mm/min

But that is for 8mm DOC and 3mm cut width,

For 1mm cut and 3mm depth it says 670mm/min

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12 Mar 2021 08:46 #201940 by Jocman
Replied by Jocman on topic Differences between bits
Wow, well over my usual settings....
It could be time for me to take some risk :-)

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