Plywood Settings

While I have had my PRTAlpha with Spindle for a while, I am just getting started learning the ins and outs of various operations. I have searched the Forum (as well as others online) and would appreciate some sage advice, peanut gallery comments,guidance, help, etc., with my current project.

I am cutting a bookcase as a gift out of pre-finished 0.75" China Birch. I have .25" compression bits on order. This will be my first use of compression bits and have the following questions:

- Full depth cutting or multiple cuts at progressively deeper settings?
- Conventional or Climb Cutting?
- Speed recommendations for the 2.2HP spindle
- Compression bit or ?? for the rabbets and adjustable shelf holes?

Quality of cut is important. I know much could be done on the table saw, but I really want to use the Bot if for no other reason that to say I did :)

Thanks in advance.

Roy

Roy;

With a 2.2HP spindle you can cut in one pass all day long (it has more real power than a 3.5hp Porter Cable router). If you are experiencing deflection make 2 passes - the first pass as a climb cut at about 0.65" with 0.020" offset and then a conventional finishing pass at full depth.

With climb cutting you may get some fuzzies in the cut - go with conventional.

Again with the spindle you have lots of power so 4 - 6" / sec at 20K should not be a problem providing your setup is stiff enough (see above).

A compression bit is OK for rabbets providing the down cut section of the bit is in the board. If it isn't, use a mortise compression bit. Drilling with a compression bit will cause burning on the inside of the hole unless you use a peck strategy and a light bite, do not drill all the way though with a compression bit or down-cut bit unless you are planning on starting a fire.

Guy,

Thanks for the info. I was pretty sure that I have more than enough power to do what I want, but I am trying to get the best cut possible and to minimize the "fuzzies" Are there any recommended settings that can minimize the chipping/splintering on the top and bottom veneers? This pre-finished birch plywood seems pretty brittle.

Thanks

Roy

To limit the fuzzies make sure you are always using a sharp bit. The compression bit cuts shearing both the upper and lower veneers into the core to minimize splintering. It will take a wee bit of playing with the feeds and speeds to get the best cut as every machine is different, but you have a starting point.

Roy,
I have the same setup as yours. Maybe I'm overly cautious, but I typically run 3 passes on .75" material.

I make such shallow dados that I will go to the trouble of using a down cut strictly for rabbets and dados. Since converting to 5mm system for shelf pin, drawer slide and hinge mount holes, I'll use a 5mm bit from a line boring machine for those - fits in a 10mm collet. Lots of tool changing, but the results are exceptionally clean.

I've always thought it seems low, but my math typically calls for 12,000 rpm at that 4-5 ips feed rate. Works well, and it generates very uniform granular chips.

Make sure your first pass is at least .30 deep, as first .25" tip of the compression bit is up-cut. My formula for cutouts with a compression bit:
1st pass .300 Climb cut
2nd pass .650 Climb cut
3rd pass .755 Conventional cut (new tool path )

If finish is that critical and this is your first run at it, You may want to consider this gentler approach. I see good results with a 3/8" bit with more aggressive parameters, but it's not the machine as much as it is the bit deflecting when running a .250 diameter. Maybe I'm crazy, but I swear I can see that extra hair being shaved off as it makes the final conventional pass. Anyhow, the results are always super clean.

(Note to self: go ahead and try Guy's approach - maybe I'm wasting time running so many passes? :o)

best of luck on your project!

jeff

Roy, Shopbot put out a video on cutting plywood and selection only last week. Goes for an hour toooooooo!

http://youtu.be/XteEyLYvtXA

Roy:
On the subject of Chinese prefin birch:
There are several manufacturers over there and all of them sell their product through US based wholesale distributors (not anyone you can buy from) and they resell it to the suppliers we deal with. How it looks, what is inside it, and the "quality" of the veneers is all over the place. None of our suppliers get to pick and choose. They can only call up and order "X" units of unfinished, pre-fin one side, or prefin two sides. As a result, every time you get some it may cut entirely different than the last batch. One batch may lay flat and look great, and the next time you order from your supplier it could look like sh-t and bow like a surfboard. Although for years I built tons of commercial cabinets out of the stuff due to the price point, now that I've redone my biz to do more custom work I may eliminate it from my product list. If a project is only taking a couple of sheets the dollar savings is not that much, and if you are doing a "large" custom kitchen, for example, there should still be enough money in the doors, drawers, trim, manufacturing, and installation to do the job with domestic and get more consistency. Frankly, I'm pretty close to building my stain grade casework the way I used to. Same finish in and out. If they don't want to pay for it let them find the "cheap" guy and get the same shlock darn near everyone else is building.

Roy and Dave,

We refuse to cut Chinese Ply at any price. I agree with Dave the quality is to inconsistent and of course according to the client it is always our fault that the cut is crappy.

Also after seeing the damage done by a 3 flute spiral cutter hitting a stray piece of steel in the ply, I decided it was not worth it.

Stick with local ply or European or even Chilean much better choices.

Regards,
David

Quote:

---

Originally Posted by wrkwatchr

While I have had my PRTAlpha with Spindle for a while, I am just getting started learning the ins and outs of various operations. I have searched the Forum (as well as others online) and would appreciate some sage advice, peanut gallery comments,guidance, help, etc., with my current project.

I am cutting a bookcase as a gift out of pre-finished 0.75" China Birch. I have .25" compression bits on order. This will be my first use of compression bits and have the following questions:

- Full depth cutting or multiple cuts at progressively deeper settings?
- Conventional or Climb Cutting?
- Speed recommendations for the 2.2HP spindle
- Compression bit or ?? for the rabbets and adjustable shelf holes?

Quality of cut is important. I know much could be done on the table saw, but I really want to use the Bot if for no other reason that to say I did :)

Thanks in advance.

Roy

---

Roy, I cut tons of prefinished American Maple plywood and have learned a way to gives you cuts that are ready to be edge banded. Cut climb direction leaving a .032" skin. Then make a conventional cut cutting .032" in to the spoil board. For 3/4" I can cut all the way leaving the skin in 1 pass using a 1/4" compression bit with no problems. The beauty of doing this with a 1/4" bit is you get enough bit and machine deflection that you don't need to use any offsets in the final conventional cut.

I is important to ramp in but beware with a compression bit you first have to plunge about 1/4" then ramp in. Otherwise you will get a lot of tearout as the upcut part of the bit goes through the veneer.

I can take parts off the CNC right the edgebander with no sanding required. I've got a PRSAlpha with the 2.2hp spindle. Feed is 6 ips with spindle speed set at 10K rpm. Sweet spot for the bits I use. Your results may vary depending on the bit manufacturer.

Been doing this method for about 3 years with excellent results.

I cut prefinished a lot on my bot.

I have a standard.

I cut 3 passes with compression. .37,.58, and last pass .15 (your 3/4" is actually .7 if its Chinese)
2.9 inches per second gives the best top & bottom. Faster I get veneer problems.