Why is my 12' beam giving me trouble?

As many others are, i’m in search of the Holy Grail - cutting the entire 4’x8’ sheet of plywood with accuracy and precision. To do this I went to the 12’ beam, and for awhile I did get good results, but then my motors both had gear issues. Looking at the issue I think this was mainly caused by having a sled that weighs 42 lbs. It sounds like the Max. is 40 lbs? I got my machine back together with new motors and a 32 lbs sled, but now have troubles in the corners, somewhere around the 61 degree mark from horizontal (see photo). I’ve used @dlang spreadsheet quite a bit but since we don’t have enough data for what’s good and what’s bad, its hard to tell what would fix my issue and what wouldn’t.

I have data below but wondered if anybody could help with the following questions:
Should 12’ beam solve my cut the entire sheet issues, or at least in the corners?
Do I need to go to a 14’ beam? Anybody have success with this?
What is max weight for sled? <40 lbs? Ideally get 1000 hrs of cutting time before motor replacement or repair.
Anybody else have issues going past a 61’ degree chain angle in the corners like my photo below with a 12’ beam?
If I do go to a 14’ beam are their motor cable extenders I can buy? I found some parts to build some but not all parts.

Thought: It would be nice to validate the spreadsheet dlang has put together and understand the range of acceptable chain angles and tensions to achieve desired parameters, i.e. durability, reliability, accuracy, precision. I think we should have a running spreadsheet for users to fill out with their particular specs. - frame measurements, sled size and weight, and resulting cutting accuracy and precision. Actually as I type this I guess it would be better to just have a post calibration file uploaded to GroundControl HQ so that many data points can be gathered to help understand what is the true cut area.

My current machine info:
Motor Sep: 142 in
MotorHt: 18.77
Workwidth: 96
WorkHeight: 48
SledWeight: 32 lb
Frame Angle: 12 deg

Spreadsheet of frame
Photo of frame angles
Photo of setup with previous successful 42lb sled

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Tough scenario. Can you just move the two pieces to the top of the plywood, to avoid the bottom corners?

If you are able to increase the motor height? That might be worth a try before going to 14 ft beam.

The problem that you are having is not clear . Based on the photo, the cuts look good. Could you point out where the problem is happening in the photo?

a 40 pound sled is very heavy, the stock is about 20 pounds and we’ve seen 30 pound sleds causing problems (that’s why there is a 30 pound sled in the ‘bad example’ section on the spreadsheet)

60 pounds of tension means that your motors (rated at 66 pounds) are spending all their effort holding the sled in place, with almost nothing to actually move it from that point.

raising the beam higher will improve the angle in the top center, reducing the max tension, and should make the motors last longer.

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Joshua, the photo of the cut sawhorses was successful but this was with a 42 lbs sled just before i started having motor failure. So I know that with that amount of weight (i.e. normal force) I can achieve the proper chain tension to always overcome any cutting force but . . . i’ll have motor failure. So trying to find out if a 12’ beam really is a success for others to cut a full 4 x 8 sheet and if so with what weight sled.

I’ve been thinking about this a lot lately and with a 12’ beam and 30" rise a sled about 26-27 lbs is ideal IMHO.

Everything is relative a 40 lb sled would need about 16’ motor spread and 4’ rise. 18 lb’ft force in the corners is impressive considering the center “perfect area” is about 22 lb.ft force. However that would need a 9-10’ tall workshop and lots of chain sag?

IMHO the Holly Grail is not really that important because hardly anyone makes pieces that need a full 8’ length.

chain sag isn’t handled by the default calibration, you need to switch to holey
calibration to get any help on that.

David Lang

I’m pretty sure chain sag is handled by the default calibration

Edit: But @dlang is right, holey calibration will give you more control and is probably a good idea.