Help cutting a Birch Ply 2.5/3D form

[andym]

Hi, I am hoping that someone might be able to offer some advice with a difficult job I have on.

At our workshop we have a PRSAlpha with the 4HP spindle and extra height in the Z axis. We have been using it to cut a lot of 3D work in Foam, Poly and MDF and have been pushing the capabilities of the machine.

We have just been asked to cut a sample 3D form in Birch Ply. This is a sample for a much bigger job, so if the clients want to use Ply I have to get to grips with techniques I will need while cutting the sample.

We have laminated some Birch Ply together to create a block that is 300mm x 300mm x 150mm and I am now having trouble with bits, feed rates and spindle speeds. We are wanting to hog out the waste wood in a roughing path with a 1/2" flat end mill and then finish with a 1/2' ballnose. Both bits need to be 4" long for clearance.

I have had a couple of frightening and noisy events that have made me sit down and try to understand chiploads etc. We were using an upcut spiral thinking that would be OK for chip clearance, until part way through the job the bit decided it was a drill and tried to lift the block up, in reality dragging the spindle down, OUCH!. Cutting Foam and Poly this has not been an issue but Birch Ply is a different ball game.

The problem is I seem to keep reading info about chip loads and feed rates that vary so much that I am now confused. I've read info saying 15 - 19000 rpm @ 1.67ips (we tried that and ended up with a very hot bit) then I tried to find chiploads for Birch Ply and use the calculator to get feeds and speeds and was coming up with results like 7000rpm @ 9ips.

Can you push a bit through Birch Ply at 9ips? Seems counter intuitive I am more worried about damaging the spindle than the bits

Does anyone have any suggestions as to what bit to use for the roughing path and any definitive chiploads for Birch Ply?
We were trying to cut each pass of the roughing path at 1/2" is this too much/little?

Sorry about the long post, thought I should explain as much as I could.

I look forward to any help, Cheers.

Andy

[harryball]

I cut 3/4" birch ply on a PRT Alpha with a 3/8" compression bit, 6ips and 14k RPM. The actual chipload will vary by the bit depending on geometry. I'm using a decent quality ply. I have cut oak ply that was much denser at 4ips and 12k RPM with the same bit.

An upcut bit will pull up on your material and try to remove it from the table. I greatly prefer compression bits or downspiral if pocketing.

/RB

[beacon14]

4" long bits are a different animal than what most people use to cut 3/4" plywood. The usual techniques won't work as well since there is so much chance for bit deflection at that length. You got a free ride with the foam as it presents so little resistance to the cutting forces. I think you will have to take much lighter cuts to prevent the bit from deflecting.

[zeykr]

Would slicing it be an option? Create a couple locator holes to make it easy to align and re-assemble.

[andym]

I think downspiral bit will be the way to go, not sure if I will be able to find a compression bit that is 4" long but I will do some research.

I will create a roughing path to cut a little shallower each pass, hopefully reducing deflection.

Ken, I can see how that would make life a lot easier unfortunately due to the shape we are cutting using that method is too difficult. You would not believe what this sample is for.

If it all works out we will be having to make a 17meter long 3.5 meter high wall with a dog leg half way along, that is made up of compound curves, fin shaped shelves and deep hollows. Possibly in Ply.

When I feel a bit more confident about what we are doing I may post up an image of the job. I am sure to cries of "Are you mad and?" and "It can't be done"

Thanks for the suggestions, keep them coming, all the info I can get is of great help.

[robtown]

I have an older PRT with a router so I may be off base here, but I doubt you can cut through birch ply at 9ips. I'm guessing your block to be approx 30" x 30" x 12" +/-

Can you adjust your toolpath strategy to clear out from the outside in?

You're removing too much at once if it's trying to lift a block that large up. Either that or you're wedging your bit into a crevass and it's grabbing on both sides.

If you can remove material from outside in, you might have better luck.

Another idea is can you rough cut your birch ply parts before you glue them together, reducing the "hogging out" of the waste?

[upsman]

Robert Ball, my son is going to be cutting out a display out of 1/2 or 3/4 plywood for his work. Can you tell me what 3/8" compression bit you use? I use Onsrul bits a list can be found at

http://www.toolsxp.com/onrbbtbypartno.asp

(Note the page to a little time to load)

There are a few 3/8 compression bit on that page. They start at 60-120.

Thank You
Tim

[beacon14]

300mm x 300mm x 150mm

that's about 12" x 12" x 6" thick.

[andym]

HI all,

Just a quick update. managed to cut the block using a 1/2" downward spiral traveling at 5ips and 7500 rps.

Thanks for all the help.