What am I doing wrong?

[Simops]

Hi,

I have a problem that I'm hoping someone can solve for me. I don't think it is the Buddy(brand new), but if it is I'll call Tech support.
Anyhow been cutting some sample practice pieces OK. Tried to cut my first more elaborate project and hit a snag. As per the picture attached the second circle is offset and not centered around the first circle cut. Yet in the drawing it is symetrical and the toolpaths appear correct. See attached sbp file. The Partworks file is too big to attach...won't let me.

BTW the MDF is positioned and screwed down correct and the Buddy homed to XY zero and Z zeroed.

Can anyone point the ways of my error here?

Thanks in advance...cheers

Mike

[adrianm]

File looks ok to me. 2 ips is a lot slower than I cut MDF at but I wouldn't have thought that would cause an issue like this.

It does look like you've lost steps somewhere. Trying doing it with a 1mm depth of cut only to take all the pressure off and see if you get the same results.

If you do then I'd say something is binding up somewhere on the machine.

[Simops]

Adrian, I slowed it down because I broke my 1/4" end mill at the default 6 ips....even with 4 steps down through 18mm MDF!

Cheers

[dana_swift]

It helps to know what kind of shopbot you have, whether it is standard, or alpha. If it is a alpha machine, this is a puzzle. If it is a standard, cutting 6 ips is very fast. Try 3 ips or less if it is a standard.

If you are breaking bits at any speed something is VERY wrong. What rpm? Spindle or router? Which model? Was the spindle/router running? Did it labor, or cut easily?


D

[dana_swift]

One other note.. your file is in metric. You are quoting move speeds in inches per second. Mixing units is a great way to get confused and use very inappropriate values.

Partworks allows mm/sec speed settings which is more appropriate for metric work. 75mm/sec is a good cut speed for a shopbot standard, while an alpha should have no difficulty with 125mm/sec. With either machine, start with a pass depth of the bit radius. If you are using a 10mm bit, make the pass depth 5mm. When you get more experience feel free to be more aggressive, but get it working first.

Eventually you will learn to cut some materials at 2 or 3 times the bit diameter in a single pass. Cutting a hard wood with a 3mm bit at a pass depth of 30mm at a speed of 100mm/sec is guaranteed to break the bit. It may make a few heroic cuts before snapping in half, but it will not live long.

Stepping up to a thicker stiffer bit can make the same cut with no problem and the bits will last a long time.

When you are learning how to make the machine work well in any given material, keep the feed rate high and the pass depth low. The feed rate is determined by the chip size you need to make and the RPM and number of flutes. The depth of cut is determined by the horsepower available to push the cutting edge of the bit through the material, the stiffness of the bit, and the density of the material.

Hope that makes sense.

You will figure this all out fairly quickly.. welcome aboard-

D

[adrianm]

For what it's worth I work in metric as far as measurements go but I always use IPS for the feed rates.

As the CNC industry seems to be mostly US driven nearly all information on feed rates is in imperial so I find it much easier to work that way rather than having to convert to metric.

There's no issue with mixing units as the Partworks Post Processor converts everything seamlessly to metric. Quite often I'll use someone else's imperial unit file and output it as metric so I don't have to switch units in SB3.

As Dana says something is definitely up if you're breaking 1/4" endmills on MDF.

[Brady Watson]

I know you said the circles are not concentric...do they appear/measure oblate more in the X than the Y?

If so, check the motor to rack engagement and make sure it is solid.

-B

[Simops]

Thanks guys for the info so far...

Dana, I have a Buddy 48 standard with 2.2hp spindle. The reason I used imperial feed rates is because all my bits I have are imperial and in fact the bit I used was from the Shopbot starter kit. The file was created in metric and saved as a MM shopbot file. In the control program I selected mm and then loaded the file. I think the bit broke, correct me if I'm wrong, because I used a downcut spiral 2 flute 1/4" end mill and as it plunged down about 5mm on first cut and moved at the default 6ips got around about 3/4 in the circle and must have fouled with chips (being downcut) and labored till it broke?? When I changed the speed to 2ips and this time using an upcut bit it went around easily.

Brady the oblong cut is indeed in the X axis. I checked the power stick and it is engaged correctly and when I use the keyboard control moves smoothly from one end to the other. The interesting side to this is that the first inner circles cut perfectly round and in the correct place. But when it came to cutting the outer circle is when it went oblong. You would assume that if there was an issue with the machine it would have done it for the inner circle cuts as well?? Is this a correct assumption?

For your info before I loaded the file to cut I zeroed the X & Y position using the proximity switches and fixed the 18mm MDF material in place so that it was orientated correctly to the part works drawing.

This is indeed perplexing, as far as I can tell, as I seem, on the surface, to have followed things correctly but obviously something is not right and just can't put my finger on it just yet??? As always it will be something simple and stupid....just can't work it out yet!

Cheers
Mike

[robtown]

I may have missed something in the thread, but I'm wondering if the your 0,0 is the same after the cut. If you're losing steps, it won't return to the same 0,0 spot.

[dana_swift]

Michael, what is your pass depth set to on your current bit definition? If you are using a 1/4" up spiral.. I will suggest you start using 0.125 for a pass depth, 3ips for a cut speed and 12000 rpm.

Your explanation of the bit break makes sense, you probably figured that one out correctly. When you are new to the machine breaking bits is more common than later when you have used it a lot. Keep spares around..

If it isn't just losing steps, I dont see where the error is. Standard shopbots can lose their position if they are asked to cut at too many ips.

Also dont jog faster than 5 ips, the machine can lose the steps at jog time as well as cut time. Becuase the circles dont appear to be correct as individual circles, the jog speed being too high is a good suspect.

Hope that helps-

D