Down-cut Bits or Straight Flute?

[KevinCarr1996 (Kevin Daniel Carr)]

Good Morning, all!

So I just recently had a project where I was cutting .050" deep with a .06" upcut bit. I have been slightly cheap/stubborn in not getting more bits that are downcut (I have already spent a bunch of various size bits, all upcut). Well, this project made me frustrated enough with having the burrs around the whole edge that I decided I’m getting more bits to get proper results…

So, doing some searching, I found these on Amazon:
https://www.amazon.com/HOZLY-Straight-Milling-Cutting-Tungsten/dp/B07YC8XHCL?ref_=ast_sto_dp

Perfect, a variety pack that’s inexpensive! However, I really wanted downcut bits with the thinking that they will have better results in eliminating the burrs. Does anyone have experience of using both downcut and straight flute? Wondering if the results will be the same, or if downcut will produce a little better results?

Also hoping to hear suggestions on if anyone has found either inexpensive downcut bits I can get separately or in a nice variety pack, like what I had linked… just in downcut?

I am looking for sizes of 1/32, 1/16, 1/8, and 1/4" (Metric equivalents are fine, ex. 1.5mm for 1/16")
Also do not want single flute, preferably 2-flute.

Thanks in advance, everyone!

[Julien]

Pretty sure @LiamN will be able to chime in. I can’t find his most relevant posts on the matter right now but I remember his mentioning a straight cutter and what kind of finish he got in plywood with them

[LiamN (Liam Newcombe)]

You rang?

Yep, here’s one that may be relevant;

Baltic birch Plywood

So, I went and found some offcuts of furniture grade 18mm Baltic Birch Ply and did some experiments with some blunt cutters, sharp cutter and a compression cutter. The TLDR is that, if you use sensible feed rates, a good toolpath strategy and are prepared to do a little manual finishing, you can get away with surprisingly blunt upcut cutters. I found the fastest way to tell if the cutter was still sharp was to carefully and gently run my thumb across (not along) the cutting edge near the top …

I see those bits are mostly straight flute which is likely to give you a more shearing of the fibres at the top surface and less pulling out, as you already know a downcut will be mostly shearing the fibres at the top surface.

As your cut is only 1.5mm deep chip clearance is not really a problem, although presumably bottom finish is so a cutter with flat faces on the flutes is likely to do better, looks like these do.

I don’t think flute count is particularly important here, you’re not cutting plastic or metal and the cut is so shallow.

I’ve got a few of the Hozly 3.125mm single flute upcut and they’re surprisingly good for the price. At about $2 each in a multi pack they’re consumables.

There’s also Yonico which has an interesting range of pretty decent cutters like these Amazon.com which I’ve used and been happy with. (Avoid their dovetail cutters, the angle on the box is some sort of unfunny joke).

Have you tried using the flat back of a marking knife to cut the fuzzies down after the cut? It’s a quick and simple post-processing action for when the material and cutter conspire against you.

The usual cautions about sharp cutters, roughing & finishing passes and feedrates apply.

[KevinCarr1996 (Kevin Daniel Carr)]

Thanks for the replies, @Julien and @LiamN.

The type of details I’m trying to cut would be very hard and time consuming to take a knife to. I may take a scotch brite pad or something similar to try and break burrs without removing the detail. I tried running a very small, .005" 45 degree chamfer around the edges, but there was too much loss in detail… (You guys all seem to love using your metric parameters )

I suppose you’re right that the finish of the face will be better with the straight flute than downcut. I may get that pack for the smaller bits, then a downcut or compression for the 1/4" size, so when I want to make sure the walls have a nice surface finish, that will do the job. I just don’t want to start getting chatter from the straight flutes.

If anyone else has input on suggestions of bits, I’ve got open ears

[LiamN (Liam Newcombe)]

@KevinCarr1996

(You guys all seem to love using your metric

For everything but roadspeed and height in feet and inches, I’m British and units-confused, Julien probably measures everything in rational units

@KevinCarr1996

I suppose you’re right that the finish of the face will be better with the straight flute than downcut. I may get that pack for the smaller buts, then a downcut or compression for the 1/4" size, so when I want to make sure the walls have a nice surface finish, that will do the job.

Ah, now that I’m not sure about. I was suggesting that you want a flat-faced cutter to make a clean job of the bottom of the cut. As your cut is so shallow there won’t be much in the way of chip being rammed down under the cutter to mess things up, especially if you run a roughing and then light finishing pass to take the last bit of depth out.

What wood are you using?

There’s a bunch of threads on V-Carving where people suggest various forms of adhesive liner paper and things to prevent tearout on the surface;

V carving in walnut has broken corners, how to fix?

Going to try a combination of smaller DOC and increased IPM for the flat end mill, because I did see that the wood slightly burned on the sides when i was cutting through this 3/4 piece in order to cut out the main shape. Will also try a final pass, let’s see! I really hope this isn’t just the grain because that would make this really really hard for me. I’d have to learn it eventually, but I hope today isn’t that day.

[Julien]

@LiamN

I’m British and units-confused, Julien probably measures everything in rational units

You would think so, but no, I’m very units-confused too

@KevinCarr1996: I did not mention that my go to solution when tearout is at stake is to first run a pocketing pass with an upcut and a 0.5mm radial stock to leave, then come back and shave the remaining material off the walls using a contour toolpath with a downcut endmill, at full depth minus a hair, so that it does not scratch the bottom. This is probably the only use I have for my downcuts, because pocketing with a downcut…well I don’t like it, at all.

[KevinCarr1996 (Kevin Daniel Carr)]

@LiamN

What wood are you using?

Right now, southern yellow pine. That’s what I had laying around and I wanted to test everything out on something cheap.

@LiamN

There’s a bunch of threads on V-Carving where people suggest various forms of adhesive liner paper and things to prevent tearout on the surface;

That’s not a bad idea… I’ve heard that before and have actually done that with plywood on a tablesaw, but for some reason it didn’t cross my mind right now
I use blue painters tape to hold stock down, but that really gums up the bit when it cuts into it. Any suggestions on something to put over the surface to prevent tearout, that won’t gum up the bit?

@Julien

This is probably the only use I have for my downcuts, because pocketing with a downcut…well I don’t like it, at all.

I usually try to avoid using different bits as much as I can to prevent alignment errors, but it’s something to keep in mind.

[LiamN (Liam Newcombe)]

Nothing to recommend myself, but Guy suggested this stuff;

V carving in walnut has broken corners, how to fix?

I have always used the Easy Liner Adhesive Shelf Paper on raw wood. I have never had any residue stay behind. I do use a J Roller when I adhere the shelf paper. You need to peel back an edge and apply across the project because you get air bubbles trapped. I use the J Roller to make sure the paper is well stuck. The shelf paper does not always work for every carving. Some complicated items the paper gets shredded edges and even on some simple carving that are at 90 degrees to the direction…

There’s a bunch more threads on V-Carving where tips to avoid tearout are discussed.