splintery plywood

[FabLab Wageningen]

O guys, what are you doing to me?

I had my dinner and wine and now you force me to do all those imperial-to-metric conversions... Do I really get it right that Don cuts 3/4 plywood (= 19 mm.) in two passes and a little ? Thus a good 8 to 9 mm.a pass with a 1/4" (6,3 mm) bit ? Then you don't even need a mortise compression bit because the most 'ordinary' compression bits would have an upcut part of 6 to 7 mm. ?!?

an extremely confused

Marcel.

[JeffZ]

I used a 1/4" mortise compression bit at first. specifically, amana 46350. My guess, as to why i got better results with the down cut bit instead, is that my work holding is inadequate. I just clamp in a few spots around the edges, so i think the downcut bit helps push the sheets down and hold it still.
I'm still getting different stuff brought in and don't think this particular ply is going to be good for machining no matter what i do. Thanks everyone. Will post pics with my results.

[dmthomson]

O guys, what are you doing to me?

I had my dinner and wine and now you force me to do all those imperial-to-metric conversions... Do I really get it right that Don cuts 3/4 plywood (= 19 mm.) in two passes and a little ? Thus a good 8 to 9 mm.a pass with a 1/4" (6,3 mm) bit ? Then you don't even need a mortise compression bit because the most 'ordinary' compression bits would have an upcut part of 6 to 7 mm. ?!?

an extremely confused

Marcel.

Yes, I do cut 19mm (3/4"), 12mm (1/2")and 16mm (5/8") plywood using 2 passes cutting in a climb direction and final pass in a conventional direction.

For 19mm, the first pass is about 10mm. Second pass is about 7mm. The upcut portion of the bit is buried in the plywood. For 12mm, I do one climb cut at 10mm. For 16mm, first pass is about 7mm, second pass 7mm.

On the final conventional direction pass, the upcut portion of the bit is about 2mm below the bottom surface of the plywood. This makes for a VERY clean cut of the bottom veneer.

I use a vacuum hold down system so the plywood is very flat on the table. I also use a 3mm sacrificial piece of MDF on top of my spoil board so when I cut thru, I'm cutting into this sacrificial board instead of my spoil board. I surface my spoil board before each job to make sure it is flat and true. Been doing it this way since 2009 with excellent results.

[FabLab Wageningen]

Don, thanks for the reply.

You cut way deeper than I learned, but I'm sure gonna try your way. The idea of using just a little extra scrapboard between work piece and spoilboard to make a clean cut on the downside is pure genius.

I have both my Cadence 'Jenny' and the Amana mortice compression bit now in for comparison - let's rock and roll !

Marcel.

[martin54]

Don, thanks for the reply.

You cut way deeper than I learned, but I'm sure gonna try your way. The idea of using just a little extra scrapboard between work piece and spoilboard to make a clean cut on the downside is pure genius.

I have both my Cadence 'Jenny' and the Amana mortice compression bit now in for comparison - let's rock and roll !

Marcel.

While testing & experimenting are always a good think to do Marcel you may find that you can't match what Don is doing simply because your CNC is not capable. My guess from Don's posts is that he is running an industrial CNC which will be far more ridged & have a much higher powered spindle than the machine you have. He could probably cut 19mm ply in a single pass with a 1/4" bit but has found that 2 passes in opposite directions gives a much better edge finish
The vacuum hold down will also ensure the sheet stays completely flat & in contact with the sacrificial sheet which will help a lot with preventing any tearout on the bottom of the sheet.

[dmthomson]

I've got a 2009 Shopbot PRSAlpha with ATC. 5.5HP variable speed spindle. However, prior to this I had a manual tool change 2.2HP spindle and cut the same way. The results have been the same between the low powered spindle and the high powered spindle.

My limitation on cutting feed speeds is my vacuum hold down system. I'm at an elevation of 762 meters (2500 feet). Because of this elevation, I can't pull the vacuum levels that people at lower altitudes can pull. Just the physics of altitude. The 3mm sacrificial board saves my spoilboard. Prior to running a load of plywood, I flatten my spoilboard. Then I run a surface routine on the sacrificial board taking off about .39mm on each side. This cleans the shiny surface of the MDF and makes the air flow better thru the material. It also perfectly flattens the sacrificial board, to the spoilbaord, which is VERY important when using the CNC to make joinery, like mortise and tenon. Using this method, I only have to replace my spoilboard once a year. To align plywood with the CNC X axis, I use a hightech system. See pictures.

This has been the way I have done my plywood cutting since 2009.

For solid wood (carvings, furniture parts, etc.) I cut a piece of 25mm thick MDO. I cut rabbits into the plywood to accept T-slot channel. I use this to hold parts. The MDO is held to the table with the vacuum system. See pictures.

[FabLab Wageningen]

Martin & Don, rest assured

I'm not going to blow up a modest Ooznest WorkBee by cutting 18 mil plywood in one go... Although: the machine is not that bad: Vectric has one they use in the demo's so it can take some hitting. So far, we very conservatively set pass depths of 35 - 50% of tool diameter (*). It seems we can go a (careful) bit above that in some cases.

But the extra board between the 'common' spoilboard and the material is something we have thought about before, and this we will certainly experiment with. 3 or 4 mm MDF is readily available and within finacial reach.

Marcel.
(*) for example: we cut acrylate in 2 mm passes on a 6 mm bit