Cleanest possible cuts in thin plywood?

[metabot]

I am new desktop shopbot owner. I will be cutting thousands of small parts in 1/8" to 1/4" plywood and I need the pieces cut as cleanly as possible.. Some of the assemblies I am working on have up to 700 individual pieces so sanding each one by hand for even a minute or two begins to add up fast. Prohibitively so.

Anyway.. I am using a pressure foot with an 1/8" downcut spiral bit at 18,000rpm with a feed rate of 3 inches per second in two passes. The plywood flat and firmly attached to the spoilboard with no lateral shifting. The top of the cut looks pretty good (8/9 out of 10) but the bottom is suffering from tear out (which I suppose is to be expected due to the downcut bit.. but a spiral upcut will only make the top worse, no?).

How can I get both top and bottom edges CLEAN.. requiring only very minimal post-machining sanding/clean up?

Options that are on the table include... tweaking feed rate, bit diameter, spindle speed, etc. Not sure where to begin with those adjustments though..

I can also use two operations - First a downcut spiral to cut most of the wood. Then an upcut to clean the final onion skin. But I would rather not have to retool for every panel I cut.

I also read about compression bits (dual up/down spiral cutting geometry).. Would they do the trick?

Any other advice for getting this dialed in? Getting cleanly cut parts is going to make or break me on this one...... :confused:

Thanks for reading..
Tim

[knight_toolworks]

I seldom get tearout on the bottom with a downcut bit. is your spoilboard smooth? You want to cut a little bit into the spoilboard and that will help. Myself if you want good hold down one pass then you don't need the pressure foot unless you don't have any space around the parts. usually I plan on solid material between parts then the sawdust holds the parts in place. 8k is usually plenty on rpms 1 or 2ips one pass 8k .01 deeper then material and you should be good. I do this all the time and only some really cheap plywood tearout and that's only if I don't cut deep enough. I am using vacuum though and that may make a difference.

[pkirby]

I've used compression bits with great success, but only on wood thicker than 3/8". I've never tried using it on thinner material. The mortise compression bits I order from Centurion Tools have around .200" of upcut at the bottom and the rest of the bit is downcut. So this would most likely work for your 1/4" material but not 1/8". For the 1/8" you will need to find a bit with an upcut stroke shorter than .100".

Another option would be to use a downcut and an upcut as you suggested. As a way to cut down on tool changeout time, I setup a "fence"/"stop" along 2 sides of my table so that I could accurately position my panel at 0,0 each time. This way if I had 10 panels to run with 2 different bits, I would just run the downcut bits on all 10 panels first, then switch bits and run them again. Here's a sketch of the fence:
15629

[gene]

One thing i would try is to re surface the spoilboard between each sheet you cut. What i am thinking is that the tear out on the bottom is because of there being no support in that area as the cutter passes. in cabinet work when you pass a cabinet door rail thru a cutter without a back up piece there is a lot of tear out , with the backer there is no tear out . try it and i hope this helps . are you using a vac hold down ?

[metabot]

Well... the spoilboard started off smooth. :D

So correct me if I'm wrong... If I want the cleanest results, I need a perfectly smooth spoilboard for each sheet?

Would it be effective to have a separate spoilboard for each cutsheet pattern? The bit would then be traveling in the same .1" grooves for each operation.. In my case each board would only be 24x18" 1/4" mdf.. I would be using several different boards for each project. If I indexed them accurately on the baseboard, they should be as good, or very close to the quality achieved with a smooth spoilboard, no?

If not, I suppose I will be resurfacing between each sheet (thanks Gene). Seems time consuming though.. What's the fastest rate I can do that using .5" bit?

I'm also going to try a slower feed rate in a single pass (thanks Steve).. and I'm going to skip the compression bit for now (great point about thickness Paul).

[gene]

You are on the right track. Give it a try and see if it improves , it should. I use a 1.5 bit ( Magnate.net ) and i surface my table prt alpha 48x96 @ 12 ips

[gene]

also a way to register your spoilboards would be to machine pins or dowels in the corners of your table . then re run the files for that spoilboard

[knight_toolworks]

I forgot the sawdust holding can be good or bad depending on the plywood. the harder the plywood the better. the russian ply's or baltic birch are about the best. but some work well some don't
Anymore I never worry about a clean backside as long as I cut deep enough. I only have a problem when I don't cut through the bottom layer well enough.

[metabot]

OK great! I am going to try this out tomorrow. I'm going to grab a larger surfacing bit too. Thanks!

Steve, I'm using latvian birch. It's pretty good stuff (and reasonably priced).

[Mike Kelly]

If you don't want to cut into your spoil board you could use a secondary spoil board, 1/8 or 1/4 mdf or what ever else is cheap in your area.

[metabot]

OK gentlemen.. re: cut quality.. your feedback worked like a CHARM. After pre-sanding all my sheets, the pieces were perfect off the machine. I will be sharing more details and pics of my setup once I get more grounded with the rest of my workflow.

THANK YOU!

[gene]

Great Glad to hear it worked:)