Speed Confusion...

[jeffreymcgrew]

OK, so, I've got a PRT96 with the standard PC router.

On another thread about breaking bits, I got some advice to run the RPMs of the router slower and the speed of the shopbot faster. So I was running my PRT96 at 4 ips, taking a 1/4" deep cut into plywood, and it seemed to work just fine. Normally, I was running 1.5 to 1.75 ips before, and being able to double the speed of the 'bot is really, really great.

However, from reading other threads, I see that 3.4-4 ips is about the 'top end' for a PRT96 in terms of speed. Yet I see other folks posting that they are running 8, even 10 ips, and jogging at 30 (!). Is this because they are running an Alpha, or have a Spindle, or both? Or could I run my PRT96 at those speeds?

How do you know when you're pushing it too fast?

[harryball]

"How do you know when you're pushing it too fast?"

Some symptoms...
Bits break,
The bot looses its position,
Bearings in your router wear out fast,
Poor quality of cuts (tearing and/or chatter)
router bogging down,
lots of gantry flex,
to name a few


Alphas run faster, as I understand it the controller can keep up and stronger motors are on the Alpha. A spindle also helps, 4HP with lots of torque blows away any Porter Cable.

I recently started running at 12 ips but my jogs are still at 20. Remember, the faster you go the faster things can go wrong.

Robert

[hines]

Dirk has a way to upgrade you PRT to Alpha type speeds. http://www.botrods.com/index.html

[jeffreymcgrew]

Robert, you're cutting @ 12 ips and jogging @ 20 with a normal PRT96 & a Router?

So it does sound like I could run a lot faster than I have been... hm...

[zeykr]

Jeff,
He's not cutting at that speed with a stock prt! He has an alpha.

There are several upgrades to a prt that have just become available that can allow your prt to perform faster AND smoother.

Shopbot has a new 4G upgrade board: www.shopbottools.com (http://www.shopbottools.com)

Botrods (Dirk) makes an upgrade system: www.botrods.com (http://www.botrods.com)

and custom cnc has the Ascension system: www.custom-cnc.com (http://www.custom-cnc.com)


There has been a fair amount of discussion recently on these upgrades on this forum.

[jeffreymcgrew]

Ken, thanks for the links. Someday in the future I'd like to update the table to run faster, so these will come in handy.

Today however, I'm still confused about what speeds are sane with my standard PRT96 and PC Router.

I'd like to cut hardwood plywood at 3.5 to 4 ips. I've run a few jobs this way now, and haven't had any problems (so far). Now, in the past I was running everything at about 1.5 to 1.75 ips, but this was too slow for the compression bits (they were getting too hot and breaking). So I'd like to run at these faster speeds. I see a lot of different folks talking different speeds on the forums here, but they aren't saying in those threads if they are using spindles or Alphas, so I'm confused if the above numbers are too much for my lowly PRT96 to handle. So far so good, but we're in the middle of a big job and I'd really like to not find out the hard way (something important breaking) that we're pushing it too hard...

[richards]

Jeffery,

My machine is an Alpha with a 3hp spindle, so high speed and husky feed rates are possible. A stock PRT would probably have to be updated with one of the kits listed above to get feed rates in the 3 to 5-ips range (or higher, depending on a lot of mind-numbing details). A 3-hp Colombo spindle runs very well at full torque in the 12,000 to 18,000 range. It will run considerably slower, but at reduced torque. The standard bearings limit its top speed to 18,000 RPM.

As discussed before, slow feed rates and high RPMs don't work well together. But, you can change to 1-flute cutters, moderate RPMs and feed rates as fast as the PRT will cut to keep the cutters from frying.

[harryball]

I didn't mean to confuse, I'm running an Alpha with a 4HP spindle at 12 ips and 20 ips jog.

Robert

[Brady Watson]

Jeffrey,
Have a look at the PRT 4G Upgrade (http://www.shopbottools.com/PRT_4g_Upgrade.htm) on the ShopBot main site. It will allow you to increase your cutting speeds to around 6 IPS with your PC router & jog @ up to 12 or 14 IPS. It is a really nice upgrade that will totally transform your PRT into a smooth running higher resolution tool. Check it out!

In the meantime, running the WIndows control software (SB3), your PRT's torque curve falls off very quickly after about 2.5 IPS. You can tweak your ramping settings to cushion starts and stops to reduce the possibility of lost steps when moving at higher speeds. Taking smaller bites (stepdown) will also help to reduce lost steps on a standard PRT while running at higher speeds...but like I said, it sounds like the 4G upgrade is the solution you seek.

-Brady

[mikejohn]

Jeffrey
There has been lots on this, as has been said.
Look closely at all 3 options highlighted by Ken before you make your decision, and try and find Mikes posts about the 4G

All 3 solutions appear to have advantages and disadvantges, but you do have a choice of alternatives.
Or make your own


...........Mike

[richards]

Jeffery,
In response to Mike J's post, and to summerize what I've posted before about the G4 vs Dirk's AGek controller, the main difference that I've noticed is that Shopbot's G4 upgrade seems to be targeted directly towards someone who wants to use the stock motors, and still get smoother cutting and higher speeds. Dirk's AGek seems to have all of the features of Shopbot's G4 with the additional capability to be used with high current stepper motors. (If I had a PRT and wanted to upgrade it, I would first talk to Dirk at Hotbots.com. The main reason that I prefer Dirk's controller over the G4 is that his controller uses heat sinks on the Gecko drivers, thus allowing higher current motors to be used. Both controllers seem to be top quality and excellent value. Either could be used with stock motors.)

[jeffreymcgrew]

So it looks like we're going to go with the G4. The other options look great too, but we can't afford any downtime right now. So the G4 looks the best, for it sounds like I'll be able to swap it right in without trouble.

As for running better motors, I don't know if I really need to yet. We're not a high-production shop, and most of what we do is complex and/or custom. Getting a little faster without downtime or high cost is perfect for us. I mean, if I had 'real' money to put towards upgrading the table, I'd want a spindle first, and then look at swapping up to an Alpha. My PRT is one of the 'post-Alpha' PRTs, so it's the same as an Alpha other than the motors and brain, so swapping up to the full Alpha would probably be the best if I need to run faster yet...

[richards]

Jeffrey,
You'll be just fine with the G4 and the stock motors. I'm sure that that combination will do everything that the PRT was designed to do.