Questions about cutters and speeds

[richards]

I've just about decided to buy a PRT 120-60 with the Colombo spindle, but the transition in thinking of cutting with a handheld router and cutting with a CNC router leaves some questions.

Last week I posted a question about cutting Baltic Birch and received a lot of good advice, especially from Eric in Florida. After looking at the Onsrud web site and the Colombo web site, as well as reading posts in this forum, I started to realize that there is going to be a substantial learning curve.

Anyway, here are a few more questions.

1. When cutting through 3/4" Baltic Birch plywood, which 3/8" Compression Onsrud cutter should I use to get the best top and bottom surface for cutting parts out of the material: the 60-121 (CEL=7/8") or the 60-120 (CEL=1-1/8")? Or would another cutter be better? (I've never seen a compression spiral cutter, so I don't know how far the two spirals extend.)

2. The Onsrud catalog has a feed chart for wood using a cutter with a CED=1/2". If I use the 3/8" cutter should I factor in the difference in the cutter's circumference? For example each revolution of the 1/2" cutter takes 1.57", while each revolution of the 3/8" cutter takes only 1.177". I know that there are a lot of other variables that have as much effect, if not more, on the feed speed than the difference in circumferences, but it is a question nevertheless.

3. When cutting multiples with a 3/8" cutter, can I nest the parts 3/8" apart (so that two pieces share the same tool path for a portion of the cut) or do I need to space the parts 2 x 3/8" plus another 1/4" or so, so that the cutter has a constant resistance? My concern here is that having a cutter alternate between cutting air and cutting wood every few seconds might create too much banging on the spindle's bearings.

4. There is a suggestion to ramp the cutter down when entering a cut. Does Shopbot's software have a routine for doing that?

5. Is there a program to optimize cutter paths when importing an AutoCadLT dfx file. (I usually lay out parts with a base line and then do offsets and connections to make a series of rectangles, then finish the piece with the necessary radiuses, etc. In short, the Shopbot would be jumping all over the place if it cuts lines in the order that I create parts - just watching my old HP Draft-Pro plotter verifies that problem. I have downloaded PartsWizard, but the demo doesn't allow saves, so I can't look at the code to see what happens to an imported file.)

[elcruisr]

Hi Michael,
Eric again! I have dozens of 60-121s around, my favorite cutter for 3/4 ply. I use the 7/8 CEL But either one is fine. When I nest I use the Artcam Insignia nesting program and nest the parts about .5 apart and hold together with positionable tabs. In parts wizard you can't exactly position the tabs so you'd need to leave the parts maybe an inch apart.

You can select cutting order in Parts Wizard manually. IIRC Parts Wizard will not do ramped entries automaticly but you can add a few lines of code manually to do the same thing.

The feeds and speeds on the charts are only rough guides to use as a starting point. A little experimentation will get you the best settings. Remember you want to see chips not dust!

[Scott Smith (Unregistered Guest)]

From AutoCAD to a SBP file, going through PartWizard is your best bet. You are correct that the DXF converter is a pain when it comes to cut order. When you save a part file from PartWizard select the output format “Shopbot[arcs][inch][*.sbp]. This will give you 1 or 2 CG commands for arcs rather than hundreds of segments per arc.
I’ve herd good things about Vector but it isn’t included with the PRT any more.

I have nested parts both keeping a small strip between the parts and using the same tool path (I usually step over a 1/8” not to run along the edge of the previously cut part). Both have worked out fine for me, I hold the parts with a vacuum table.

[richards]

Thanks Eric and Scott. I'll start with the 60-121 cutter from Onsrud for Baltic Birch and use Parts Wizard as a starting point for parts placement.

I downloaded the ArtCAM demo program and played with it for a few minutes. It looks great - probably something that I'll add sometime in the near future. The goal right now is to find the right combination of tool(s) and software to cut parts from baltic birch sheets efficiently. If I understand correctly, Parts Wizard will enable me to choose the tool path (for example, select a point near 0,0, start the parimeter cut and end up back at the starting point).

The project that led me to need/desire a CNC solution has 3 sets of parts nested eight times on a sheet of baltic birch (24 parts total per sheet). Total cut length is a little less than 1200 inches. Using Eric's numbers, 100 inches per minute, from a former posting, as a guide, gives me a minimum time of 12 minutes per sheet. Doubling that time and then rounding up to 30 minutes per sheet (better to underestimate until training and ability prove otherwise) gives me 8 sheets in four hours, which would give me as much product that I could ever hope to sell, given my existing marketing program. Having a few extra hours available per day to cut other 'stuff' would allow me to learn to use the machine by doing a lot of little jobs - if I can use AutoCadLT to lay out the parts, manage the tool path in PartWizard, and use a standard cutter to route the parts. The idea is to start simple and stay simple until experience makes more complex jobs seem simple.

[dave_west]

If you downloaded the artcam demo, you can't save but you can cut and paste into part wizard, Nest in the demo and paste into you part wizard. I'm not sure if the program is worth the money they are asking. I've been playing aroung with it some
but am using my bot mostly for cuting aluminum sign backs, plexi, and a small amount of wood working 2d mostly.

[richards]

I apologize for not doing my homework. During the last few hours I studied other parts of the forum where the capabilities of PartWizard have been discussed.

The more I read, the more amazed I've become about the capabilities of the Shopbot and of the software available for it; and, the more I read, the more I realize how many of you have given freely of your time.