I just recently did a large ‘S’ with laminated birch plywood (3/4" thick) and had terrible tear away that I could not fix (expensive mess up these days). I have been reading and there are several saying a compression bit works well for this type of plywood (I was using #201 from Carbide menu - 1/4" upcut). I am looking for advice on how to run the compression bit on my Shapeoko 5 Pro with standard spindle. Any direction on ipm, speed or depth would be very helpful to not get tear out. The bit is coming in next week so it will be brand new and sharp. There just seems to be a lot of literature about depths for the compression bit. Thanks everyone!
With the SPro5, you could probably run a new compression bit the full thickness of the plywood - at 5000mm/min!
I wouldn’t just because it’s likely to not be as dimensionality accurate as doing a few passes.
The key issue with running the first pass of a compression bit is to ensure that the downcut portion is cutting the top layer of the plywood, usually that means at least 3/16" DoC. I’ve seen some compression bits with a smaller up-cut portion at the end - 1/10" - but usually it’s a good 1/8th".
As always, start on the conservative side & push it up as you feel more confident in how it’s chewing up your plywood!
The only compression bits I have run “successfully” are the Cadence Jenny bits. They are different because they don’t require a deep depth of cut. My go-to bit for cutting out shapes is the Mini Jenny 1/8th. My only complaint about compression bits is the horrible noise they make, not sure what it is about compression bits but the noise is very different and I find it very annoying.
However, I think you would be pleased with any downcut bit if your main goal is to avoid tear out on the top of your material… bottom tear out is naturally avoided due to your spoil board.
You are going to be limited by the lack of power from a trim router.
Bits like the Jenny bit have a very short up cut section to help with this.
You can do 1/4” doc and still cut clean with it where a traditional compression bit needs to be 3/8” to 1/2” deep.
I can’t help you with feeds and speeds. I haven’t use a trim router in several years. I would run full depth at around 200 ipm with my 2.2kw spindle with a 20 degree ramp.
Make your initial cut a downcut profile with DOC enough to get through the top plywood layer (at least.) I’ve even used a Groovee Jenny downcut Vbit (with a slight outside offset profile) to do this.
Then come back with your upcut profile down to the bottom.
Unless you are doing some kind of production work, you don’t really need a compression bit.
Thanks guys. I actually cancelled the compression and got the down cut. Any recommendations on feeds and speed and how low to go on the down cut on the plywood? I see at least the top layer mentioned but thought I would ask.
I had a great experience using oracle masking and a 1/4-inch down cut end mill from IDC to cut my first hole on 3/4-inch Birch plywood. The hole was precisely cut at the correct spot, and there was no tear-out on either side of the board. The speeds and feed data base on IDC’s phone app made it easy to achieve professional results.
