If I go to Home Depot and pick out a straight router bit, I find that it has zero helix angle; the cutting edges are perfectly parallel to the bit axis.
Why the difference? Why do router bits, generally used for woodworking, not have any helix angle on them?
What happens if I take a metalworking mill bit (with a generous helix) and use it in my router for woodworking?
I have heard this is because a helical bit will tend more to tear out wood fibers instead of cutting through them cleanly. Also for hand held routers the helical shape may tend to pull the bit in different directions though that tends to happen in wood anyway because of it’s ‘inconsistent consistency’.
As it happens, I’m interested in grooving HDPE with my router. Sorry for not being more specific in my OP; it hadn’t occurred to me that the differing properties of plastic and wood could be a factor. With HDPE I currently experience some degree of self-feed (and resultant rough cut) using a conventional straight-flute wood routing bit, so I was wondering whether a four-fluted metalworking mill bit w/helix might give me better results.
Well if you had mentioned that it would have made a little bit of difference because I’ve had to mill HDPE. Weird results with different cutters. Freezing the material helps to make it harder and prevent melting. For cutting straight channels I found a carbide ripping blade on my table saw worked best. Helical cutters put up a lot of resistance for some reason, but I found wood router bits to leave a rough surface. Even though I though the bits were moving slowly through the material you could see the vertical lines along the side of the cut like it was moving through incrementally. Good lubrication is important for any method. I did try one of those rotary burr type things but it ranged from no better to worse. I’d like to know what you end up with, it’s in my plans (that I’ll never get to) to do some more of that kind of thing because I have a lot of the material left over and lot of things undone. Good luck with this and I’d like to hear how things work out.
How about a spiral flute router bit designed for plastics:
A end mill will cut fine. Not sure what the safe max RPM is though.
If you have a variable speed option, try the slowest speed.
Machining plastics such as HDPE, LDPE, UHMW can be challenging. It can be hard to get a good finish, break the chip, and/or hold size. Delrin/acetal is easier if that is an option for your next project.
I’m using a router table with a fence configured such that the cutting action forces my workpiece against that fence as I feed my workpiece into the bit. I can get good results with HDPE using conventional (two-flute, no helix) wood router bits if I feed at a modest rate. However, this is paid work so it’s to my advantage to keep things moving along, and the challenge comes when trying to increase the feed rate. There’s a point at which if I try to feed faster, it develops a self-feed and starts pulling my workpiece through too fast; I suspect the heavy cut is making my router wobble in its mount, and combined with the high feed rate, I end up with a crappy surface finish. This is why I wanted to try four-flute helical metalworking mill bits: smaller, more frequent cuts, and maybe less tendency to self-feed.
Just got done with 80 pcs. of UHMW PE. Used a “zero flute” carbide cutter (it actually has just one flute, lots of chip clearance area) and it is RAZOR sharp. The one I used was 3/8" diameter with a length of cut of over 1". It wasn’t cheap, IIRC about $60 or so, maybe less. The part looked like a Maltese Cross, about 100 mm across, with 14.88 mm wide slots. The material was 19 mm thick. This cutter is designed for soft plastics like UHMW and cut like butter with this cutter. We used 5000 rpm because that’s as fast as the CNC would spin, but the recommended speed was even higher.
(edit) Not sure if I’d recommend this cutter for a hand fed router. It’d probably pull the work out of your grip.
I have found that a single-flute, down-spiral cutter works the best for polycarbonate and similar plastics.
Like so. You must be vigilant about clearing the chips with compressed air, or the plastic will melt.
Four flute cutters have less room for the chip to clear especially at the feed rates of a router. Four fluted end mills are typically used on steel. They are stronger because they aren’t fluted as deep but this leaves less chip clearance.
Two flute typically are used for aluminum and are what I use for plastics on the CNC mill because we aren’t going to spring for special tooling for the occasional plastic job. They will generate uplift in machining the work is clamped and the machine tools are very rigid.
Did you look at the single flute router bits in my and Beowulf’s links. They are designed for plastics. The tooling companies usually have this stuff figured out.
I went to the AWFS convention in Las Vegas a few years back. They had a display of bits at one of the CNC machinery exhibits. Big sign, “Don’t touch, very sharp.” I wanted to know how sharp, so, like a dummy, I touched. And Of course, I got cut. Woodworking bits are very sharp compared to metal ones. They are also air cooled.
The shavings from the metal could cut into the objects metal… ( . the object is that being milled ).
The shavings from wood don’t do damage to the objects wood.
Also note wood drills have the helix to propell the shavings out of the hole, so as to not cause a fire or burn out the drill …
I’ve seen machining operations that had problems with aluminium welding up to cutting tools. The solution was a higher helix on the drill, just to move the matieral away from the cutting edge faster.