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roncruiser
09-14-2013, 01:49 AM
Hello All,

CRP will be able to provide a custom kit for me: 5' x 3'.
I'm trying to get a feel for how much more deflection I'll get on my 5' machine. Has anyone measured deflection at the tool on their 4' machine?

With deflection data from a 4' machine, I can then extrapolate the deflection on a 5' machine.

Maybe I'm a bit too worried, but I don't enough to know a difference. It would still be nice to see deflection data from these machines at some set Z height with a load of 15lbs. Measurements taken in both the X and Y.

I would greatly appreciate any input from the forum.
Thank you!

pippin88
09-14-2013, 06:46 PM
So you are planning for a 5 foot gantry? With the other axis 3 foot?
Unless you have to do it that way, you'd be better off with a 3 foot gantry.

roncruiser
09-15-2013, 02:36 AM
So you are planning for a 5 foot gantry? With the other axis 3 foot?
Unless you have to do it that way, you'd be better off with a 3 foot gantry.

Space limited. I have to do it that way.

CarveOne
09-15-2013, 06:21 AM
This is sometimes referred to as a "wide router" design. Most regular 4' x 8' router tables have a nearly 5' wide gantry and it's no problem unless you build a flimsy gantry. Use a larger size extrusion (or use the same parts as is normally used on the larger router tables and it will be ok. (Rocket science mathematics is really not required.)

Whatever mid span flexing remains can be handled by finding out the feed rates and plunge rates your machine can handle best for the different materials you will be cutting and use those numbers on future projects. If you just don't like the results, get a bigger extrusion and change it out. Ease of upgrading is the big advantage to extrusion designs. Unless you have to dismantle the nearly the whole machine to get to one part. :)

roncruiser
09-15-2013, 08:05 PM
This is sometimes referred to as a "wide router" design. Most regular 4' x 8' router tables have a nearly 5' wide gantry and it's no problem unless you build a flimsy gantry. Use a larger size extrusion (or use the same parts as is normally used on the larger router tables and it will be ok. (Rocket science mathematics is really not required.)

Whatever mid span flexing remains can be handled by finding out the feed rates and plunge rates your machine can handle best for the different materials you will be cutting and use those numbers on future projects. If you just don't like the results, get a bigger extrusion and change it out. Ease of upgrading is the big advantage to extrusion designs. Unless you have to dismantle the nearly the whole machine to get to one part. :)

Thanks for the reply.
I tried looking for a larger extrusion than the 8016 the CRP PRO KIT calls for. I suppose something else can be done. What I was after most was the amount of deflection coming from elsewhere contributing to overall deflection at the tool. Not just from gantry beam itself. This post here was very telling to me:


You mentioned you were measuring the deflection at the "tool", which I will take to mean you mounted something like a dowel pin in the collet of the router? If you did, hats off to you!

Most people just think in terms of how much the gantry beam is going to deflect. But there is a lot more to it than that.

First, you have the bearings in the router itself. The Z axis has bearings and also can have a bit of deflection. The Z is riding on the beam with bearings, more clearance. Of course the beam has deflection. Then the gantry is riding on bearings. It may or may not be on risers which can deflect.

You also have some slop in the drive mechanisms, whether it be R&P or screw driven. If you ever notice, they are termed "anti" backlash as a rule, not "Zero" backlash. (Zero backlash is available but is usually only available in expensive industrial grade equipment).

Then we have the tool itself. Tools deflect. Even on industrial milling machines they deflect when cutting. It is one of the reasons why machinists and toolmakers take roughing passes and finish cuts.

You have to have all of these clearances or else the machine would not move at all. If you "tighten" up the machine, then you will encounter greater wear and it will require more power to move the machine. It is a balancing act.

So, the idea is to rough out your material, then if you are doing fussy work, take a finishing pass so that you are not putting a lot of pressure on the tool, which gets translated to your machine, to get a quality cut.

An interesting side observation: A couple of machines that I know of that do not transfer tool pressure to the machine are lasers and water jets. However, they do have "tool pressure" in the sense that the beam or jet will "bend" as the feed rate is increased, if increased too much, they will "break" and stop cutting.

Until I had read that post, I was under the impression that deflection was only from the beam. That being said, just having a deflection measurement at the tool on a CRP PRO KIT machine would be helpful understanding the machine and ultimately my future machine. I suppose I can build an Ansys model but that would require more time and effort than I'd like to invest. I'd rather leave that to the experts.

Thanks,
Ron

CarveOne
09-16-2013, 05:44 AM
He's right. There are a lot of things at play that can contribute to flexing under load. The cutter itself is one. Long narrow cutters can flex. The upright supports for the gantry need to be very rigid because it can sway in the direction opposite to the cutter travel. Even with stiff gantry supports the bearing carriages being just loose enough to roll easily can let it sway a small amount. (action = reaction) Anything that can move or bend - will. How much it reacts is a function of how hard you are driving it. Anything excessive (for your particular machine) will show up in the cut results.

The CNCRP designs have a lot of considerations for stiffness versus cost versus ease of assembly in them that you won't notice at first.

ahren
09-18-2013, 03:50 PM
Hi all,

It's difficult to predict actual deflection due to all of the variable mentioned above, but anecdotally, we have a customer with a custom sized machine (using 3060 extrusion) with a 7' axis (6' of travel). They use this to sculpt MDF and plywood layups for fiberglass vehicle parts, and are quite pleased with the stiffness of the machine. They even have a slightly longer z axis, and have raised up the gantry risers a bit. Ron, I'd say you will be fine with the configuration quoted, as your gantry will be shorter, it will use the taller 8016 metric extrusion, and it will be mounted to the PRO risers.

Since you're primarily cutting sheet goods, to really increase stiffness the best thing you can do is lower the gantry so your z is not hanging out as far during cutting, and this can easily be done by swapping the riser extrusion for a smaller series.

Ahren
CNCRouterParts (http://www.cncrouterparts.com)