How to get belt drive PRTAlpha smoother yet?

[Jasono]

Hello,

I have a PRTAlpha that I just built belt drives ( 2.77:1 ratio ) for. But I'm still not getting the smoothness I want, compared to our other PRTAlpha with the 7.2:1 gearhead upgrade. I remembered that along with the 7.2:1 motors I also upgraded the drivers in the control box. I'm wondering if the new drivers had something to do with getting a smoother cut, or if the problem on the beltdrive machine is strictly the smaller gear ratio?

I've been able to improve the cut some by tweaking settings, and I could switch from 24 tooth pinions to 20 tooth, but I'm not sure that that'll do enough.

-Jason

[Jasono]

I think I have my answer, Ryan @ ShopBot said the new drivers don't make a ny difference to the smoothness.

[gerryv]

Hi Jason,

Can the big gear on the belt drives be increased in size assuming you can get the right length belts? if yes, do you see a downside to going with the higher ratio such as too much torque from those big 1:1 steppers?

Otherwise, are you happy with the performance and style of your belt drives?

I'm considering doing the same thing to my 2006 PRT Alpha with 1:1 steppers.

- Gerry V

[richards]

I used a 3:1 belt drive on my PRT-Alpha (20 tooth on motors and 60 tooth on the axle). My pinions were the stock 20-tooth (1-inch pitch diameter). Ted Hall suggested that I change the pulse per rotation from 1,000 to 500, which I did. That means that the effective change in resolution was 1.5:1 instead of 3:1; however the torque was increased 3X.

I cut a test pattern with circles, rounded corners, and 45-degree straight lines, i.e., the type of cuts that had previously had chatter marks. With the 3:1 belt-drive, there was only minimal chatter. (I posted some photos several years ago to show the improvement.)

Later on, I bought the upgrade motors and drivers to replace the AS911 motors on the X and Y axes. I ran the same test and got identical results. Visually, I couldn't see any difference. However, the upgrade included 30-tooth pinions instead of the 20-tooth that I had been using, but PPR was set to 1,000 instead of 500, so resolution with the new 7.2:1 motors was (1.5 diameter X 3.1416 / 7.2 ratio/ 1,000 ppr) = 0.0006546 inches per pulse compared to (1 diameter X 3.1416 / 3 ratio / 500 ppr) = 0.0020944 inches per pulse with the belt drive and (1 diameter X 3.1416 / 1,000 ppr) = 0.0031416 inches per step with the original non-belted motors. The numbers show that the 7.2:1 geared motors should have had a 3.2X smoother cut than the 3:1 belted motors. That was not the case. I'm assuming that the machine's flex accounted for the lack of improvement. (The test was run at 5-ips, which is the speed that I normally cut most MDF and I wanted a 'real-life' test.)

Because the numbers didn't compare well with the visual and tactile examination of the parts, I decided that the increased torque on the 'off' axis (the axis at rest when the 'on' axis was moving) held that 'off' axis in place so that the 'on' axis could not push the 'off' axis around. If that was actually the reason that the cuts improved with the 3:1 belt drives, it would help to explain why the 7.2:1 geared motors didn't significantly improve the cut. If you remove the 'push around' effect from the 'on' axis motor and the 'off' axis motor, you still have the machine's flex. That normal flexing seems to account for the fact that the geared/belted cuts were visually identical.

Later on, Jeff Bishop and I ran some 3-D cuts. He has a PRS-Alpha 48X96 and I had a PRT-Alpha 60 X 120 upgraded with the 7.2:1 motors. The cuts made on the different machines were almost identical, with my 'heavy iron' PRT giving slightly smoother cuts on some of the patterns than his 'lightweight aluminum' PRS. Actually, the results had both of us scratching our heads, wondering why my old machine had very slightly less chatter on some cuts. We decided that the spring-loaded motors on my PRT might have had very slightly less back-lash than the non-spring-loaded motors on his PRS. (Actually, the cuts were visually identical, at least to my old eyes, but you could feel a very slight difference between the cuts with a finger. The difference could have been just the difference in the two pieces of pine that we had cut.)

[ken_rychlik]

I had my prs std with 3.6 geared motors. I upgraded to the alpha system on it with 7.2 motors and the cuts look the same to me. More power and smoothness of the machine, but no difference on cuts.

If your Machine is tight and tuned up, you should be able to do pretty well with it. Follow Mike's advice about the switches. He has been through what you are doing.

With the machine locked in, can you get any slack out of the machine by trying to push it back and forth?

I have a friend with an ez router with belt drives and he has very bad chatter. I grabbed his gantry and can move it back and forth at least 1/8 inch when the motors are locked in.

Having two machines, you can lock both in and do a push pull comparison to see how it stacks up to the better cutting one.