Confusing Timing infiormation

[NickB]

J have just finished designing a set of parts for making models of Civil War tents
The parts consist of a couple of pyramid shaped parts and a cross bar. I nested 12 sets of the parts on a 24" square 1/4" plywood part. The tool paths look good and the confusion starts when I calculate the time to cut.

The VCarve says 12 minutes to cut the parts. but when I start the shopbot control software in preview mode and run the program it comes up with 82 minutes. I checked the speeds and they look to be the same. Any ideas why the big discrepancy in timing? The scale in the VCarve timing is set to 2.0

[adrianm]

Do you have the same feed rates set in the tool databases for both programs?

[NickB]

As far as I can tell Yes. I set them when I set up the toolpaths and checked the codes in the SHB file. I'll take another look tonight

[adrianm]

It might be that SB3 takes account of ramping etc whereas VCarve doesn't (as it doesn't know the settings for the machine it's outputting to).

If there are lots of points the tool needs to slow down for corners that would account for it.

I ran some test of simple and complex shapes and that certainly seems to look like it might be the case.

[rcnewcomb]

You should adjust the scale factor in VCarve to a larger number to more closely match what the SB software is showing.

For some applications I've had a scale factor of 4.5. Try different values until it more closely matches what SB estimates.

From the VCarve manual:
Scale Factor The nature of different styles of toolpaths means that they may be simple 2D cuts or require simultaneous 3-axis moves, the more complex the toolpath then the more chance the CNC machine may not actually achieve the programmed feed rates. This can be compensated for by multiplying the times by the Scale Factor.
The inaccuracies generally come from the acceleration / deceleration of the machine. If the program tells a machine to move at 200 inches / min along a 200 inch straight line the machine will probably take pretty close to a minute.
If instead of a straight line we ask the machine to cut say a gear wheel with a total circumference of 200 inches, the machine is likely to take considerably longer. This is because the machine will have to slow down to go around the tight curves of gear tooth profiles and will probably never reach its programmed speed before having to slow down again for the next change in direction.
The scale factor in the program lets you approximate this slowdown for your machine, but it will vary depending on the type of work you are doing. Many people will use one scale factor for simple 2d work and another for 3d or VCarving. The best way to calculate it is just to take a note of estimated and actual machining times of a period of time.
If a job actually takes 6 minutes to cut and the estimate was 3 minutes, the scale factor would be set to 2 (actual time / estimated time = 6/3).

[NickB]

Thanks Randall, I guess I'll just have to time things as I cut to get the scale right.
I've only cut one sign for practice and am just beginning to do real stuff.