[High Seas]

I was checking constant force springs on mcmaster -- maybe a prob for overseas shipping -- but a pair at 25 bucks each or so they seemed like a good options. I imagine if the spring tension was less than the gantry weight - you'd still get the same benefit of nearly no backlash and a lower cost spring. Good info at mcmaster even if you don't go that route.
Good project - looking forward to the build and pics - in my to do list too.

cheers - jim
ps a great way to get my parking space back in the garage.

[joecnc2006]

I have seen some with just a simple pulley and weight system, this way you can add or delete weight as needed.

[martinw]

Counterweights add inertia to the motor and all the rest of the trouble. Constant force springs may not.

BTW, High Seas, safe passage....

Best wishes.

Martin

[High Seas]

thanks - Martin

BTW The details I thought was at Mcmaster was actually at MIT -- go figure same alphabet placement

http://pergatory.mit.edu/2.007/kit/s...nstspring.html

also here
http://www.smallparts.com/products/descriptions/cfs.cfm

heaps of other neat info if you sleuth thru their pages there.
cheers - Jim

[jimluu]

Thanks for the input guys. I'm thinking hard on the counterweight system. Constant tension spring seems interesting. I've never use them before...which is a good excuse to look into them now. Seems that the spring tension in the links above seems inadequate to hold up a 75-100lb gantry, even at a 25 degree angle. A pulley system may be easier. To counter the issue of inertia, I was thinking about using a long spring on a pulley, or perhaps a hydraulic cylinder rather than free weight. I've have to think it over some more.

As for the frame, do you suggest bolting my linear slides to an 8020 section, then bolt the 8020 to the steel square frame? I'm thinking of an A-frame ~30 degree or so, one side to be used for the cnc router, the back side to be a panel saw. I'll be able to clear out my table saw and save even more space

[joecnc2006]

Ok, i remember having a picture of a simple one, don't remember where i got it, but it will show the counter weight and maybe spark some ideas.

[Oakley1975]

I was thinking that the y axis would be up and down... the gantry would actually be x and it would move across the sheet ( the 8' way) .... that way it would only need to pull the weight of the router and z axis assy up and down ( 4') rather than the weight of the whole gantry. Pushing the gantry sideways instead of up and down would be alot easier I would think and may only require one motor to do so. Know what I mean?

[joecnc2006]

Yes thats how I would do it, that way the weight of the Z-Axis is alot less than the whole gantry.

Joe

[BMG]

If you use a counterweight system, you might consider leaving it a bit on the light side so that you get the benefit of no backlash and reduced inertia (compared to a fully couterbalanced system). I would suggest that it is a full wrap system meaning the support cable wraps round the lower side of the table and closes the loop and provides the ability to mount the drive stepper behind the table.

With the increased wear of the leadscrews/ballscrews, since it will be mainly on one side, the screw can be flipped after wear becomes an issue.

For braking, you can use a solenoid to trap the counterweight cable or pulley. If you lose power, the solenoid closes, braking and or holding the axis.

Interesting Project.

[martinw]

If you use a counterweight system, you might consider leaving it a bit on the light side so that you get the benefit of no backlash and reduced inertia (compared to a fully couterbalanced system). I would suggest that it is a full wrap system meaning the support cable wraps round the lower side of the table and closes the loop and provides the ability to mount the drive stepper behind the table.

With the increased wear of the leadscrews/ballscrews, since it will be mainly on one side, the screw can be flipped after wear becomes an issue.

For braking, you can use a solenoid to trap the counterweight cable or pulley. If you lose power, the solenoid closes, braking and or holding the axis.

Interesting Project.

Dear BMG,

Good points.

IMVVHO, I still think that anybody should take backlash seriously rather than relying on the one-directional weight of an axis. Cutter forces may catch you out. I could well be wrong.

Thanks,

Best wishes,

Martin

[joecnc2006]

If you look at the photo carefully you can see a spring on the far side to counteract the pulling of the counter weight in one direction, which i believe relieves the pulling or wearing on the lead screw on one side of the threads, I'm sure he has a spring on both sides just can see the one closer to the camera.

Or i could be mistaken, but a good idea anyway.

Joe

[High Seas]

Sure looks like it has great potential to sag in the center - something that would be less likely with the gantry aligned vertically. Nice looking rig though. Another plus - mounting on an angle - a gain from gravity for holding in place - vice being dead vertical.
Takes up the footprint of a workbench --- so I really could get my parking place back.
cheers - Jim

[harryn]

Hi, I am "in the middle" of such a project now. There is some info in my build log. Might still be too early to gain a lot of real info from my project. My general plan
- Using the Y axis vertical
- Z is horizontal
- X is a moving table

- Counter the mass of the gantry with constant force springs from McMaster
- Linear motion managed with sliding surfaces (budget constraint)
- Drive with steppers using timing gears on a 50mm wide belt (sort of like a belt version of rack and pinion)

There are some very real challenges / limitations with my plan, so don't go down that path blindly. A number of challenges remain - like - will it work when I finish it?

[jimluu]

My bearing blocks just arrived from musumi today. I'm not sure how it fits. On the PDF file of my ballscrew, the fixed end of my ballscrew is 25mm and the support end is 1". Misumi doesn't make 1" bearings so I bought a set of 25mm, and hope to sand down the support endshaft. I'm surprised how tight the bearing fit on the fixed end. How tight are the bearing supposed to fit? I thought it just slides on smoothly, but even trying to wiggle it on, I can't get anywhere. Should I grind down the shaft a bit?

btw....I was going to use a wire brush to grind down the shaft. Any suggestion?

[bemfarmer]

I would suggest a bearing heater to slightly expand the bearing, but since I have never done this, perhaps there is someone out there who could explain how to go about this?

[beone]

Be aware of the cycle life on the cf springs. It may look like a lot but a cnc can cycle a lot.
Dave