Shopbot is double cutting and overlapping design.

[williams_architectural]

I just baught a 96" PRT Standard about 6 months ago and I just began using it about 1 month ago. I was rough cutting a 36" diameter carving and it was supposed to cut a total of six passes @ 1/8" each with a 1/4" endmill. It made three passes and was looking great so I left it alone to do it's job while I ran some errands. When I got back it had moved over about a foot and started the whole process over making three more passes before it was stopped by the limit switch. The piece was ruined and it also began doing the same thing on other pieces. The jog speed was set at 10 ips and the cut speed at 5 ips. I checked the gear mesh on the motors and made sure they were tight. I have a ground wire from one of the chasis bolts to an outlet plug ground but do not have the dust collector system grounded to the chasis. Please help me figure this out.

[Brady Watson]

William,
The problem is most likely in your design & toolpathing software. You should run the file (or post it here as an attachment) in Preview mode of the control software befroe running it on a live tool. If you run the file in Preview and it shows the double cutting, then you know you did something in CAD/CAM that is completely outside of the ShopBot machine controller. Have a look.

-B

[cnc_works]

William, I have a PRT standard like yours that I upgraded to Gecko drivers. My memory is that, before I upgraded, a 10 ips jog speed or a 5 ips cut speed could very well have caused me to lose steps. Your symptoms match what could happen if you lost steps. Even now I don't try to use my SB quite that aggressively, I'm more interested in accuracy than speed. Before my upgrade, I believe I was jogging maybe 3 or 3.5 ips and cutting at 1.5 or 2 ips.

Donn

[williams_architectural]

Thanks Brady & Donn, sorry I'm just getting back. As soon as I posted the message I left on a trip. Brady, I ran the preview mode and it looked fine and showed to do what it was supposed to. I also looked close at the simulated toolpath and it looked ok. The design and toolpathing software I'm using is ArtCam Pro. Still it puzzles me as to why it would just start over instead of just moving over and then cutting deeper. Donn, when you say that 5ips is possibly to agressive does that mean that the shopbot may have jumped out of position during turns or mainly just bogged down during cutting. I was cutting red cedar and it didn't seem to have any problems cutting the first three layers. Even though I think I'll still try cutting at 1.5 or 2 ips on some scrap wood and if it still moves over and does the same then I'll know it was the toolpath and if it doesn't then I'll know it was my speed settings.

I appreciate your input guys and I'll let you know what happens.

William

[Brady Watson]

William,
Donn is probably right with the machine being pushed too hard. You're really maxed out to about 2.7 to 3.0 IPS on a standard PRT before the motors loose torque and position. In reality, the PRT cannot run faster than about 4.5 or so IPS since it runs out of voltage. Your best bet is to pull back on the speeds or upgrade to a 4G (which I always recommend). I run an Alpha spec machine now...and don't miss my PRT ever...

-B

[gwilson]

William,
I bought mine about a year ago and remeber that problem. With mine if I cut any kind of curves or circles I can not run faster than 3 ips and that is after I change the slow corner speed. Brady better explain that one. If I am cutting straight down x axis, just back and forth I can run upto 6 ips.

[burchbot]

And never, never, never leave your bot running unattended.
Dan

[williams_architectural]

Brady,
I'm new at this so please excuse my ignorance but what is a 4G?, how do I upgrade to a 4G?, and how much faster would I be able to cut?

[williams_architectural]

Gerald,
Which bot do you have?

[Brady Watson]

A 4G is the 4th generation PRT tool. It uses better electronics to get 2.5X the resolution of the previous generation PRTs and about 2X the speed as well. You can read all about it here: http://www.shopbottools.com/PRT_4g_Upgrade.htm

-B

[gwilson]

prt96

[gwilson]

I bought it used. It is a 2003 model.

[williams_architectural]

Thanks Brady,
I'll check it out.

[williams_architectural]

Mine is 2007 model and I guess it needs an upgrade already.

[Brady Watson]

William,
If it is an '07, then it is a 4G version tool. It sounds to me like you need to reduce your speeds and/or stepdown as you may be pushing the tool harder than the Standard tool was designed to cut. Alpha tools have feedback that self correct loss in position. PRT and PRS Standard tools with the 4G drive system do not.

-B

[williams_architectural]

Brady,
If I have a 4G version then what speed and stepdown do you advise? I did slow the bot down over the past couple of days to 2.5 and had zero problems other than it taking a while to cut. How am I supposed to make any money at that speed? I carved a 36"x 40" sign today and it took 7 hours. I was hoping to not go over 4.

[Brady Watson]

William,
It depends on the material, the bit, router or spindle and how you are holding down the material. It would be impossible to give you 'training' on the net on how to run your tool and know what to look for. You can cut most designs efficiently using the Standard version of the tool up to about 6 IPS. Just watch how much cutting force you apply to the cutter. Use the chipload calculator in SB3 under tools to get you in the ballpark. If you find the tool losing steps, then reduce the chipload.

Without knowing what material you are cutting on a regular basis and what type of cutting you are doing, it's kinda hard to make suggestions. As a *general rule* keep your stepdown depths at .25" for most woods and .5" for foams. This will help minimize the load on the motors and reduce bit deflection giving you a clean cut. By observing and listening to what the tool is doing (how does it sound? Labored? Screaming?) you can make adjustments on the fly to get the right speed and RPM for any given job.

-B