HELP! 5-Axis 4'x4'x30" CNC Router - Page 3

# Thread: HELP! 5-Axis 4'x4'x30" CNC Router

1. ## Re: HELP! 5-Axis 4'x4'x30" CNC Router

Originally Posted by routalot
Thanks for the clarification.Its good to know that the sensors are there,but I return to my point about calibrating them so that the installed spindle is known to be perfectly aligned when the machine is homed.It would also require that the mounting for the B/C head is offset with perfect accuracy so that the spindle is concentric with the C axis and parallel to the table.I don't think the task is impossible,but given the required accuracy it won't be easy.By way of illustration;a few years ago I had a boss who quoted for a job that had a 0.002" tolerance because he saw that the axis readout on the router went to three decimal places.I told him that I knew a fellow that owned a Yugo which had numbers all the way to 130 on the speedometer-didn't guarantee that it would actually do it though.
Even if a Yugo could go 130 (possibly terminal velocity driving off an airplane) I wouldn't want to be in it! I've seen quite a few designs where the spindle is not concentric to the C axis. And it doesn't really matter as the CAM handles that. As for alignment, it's one reason I suggested the use of ground tooling plate as a table. It's never easy, but for a team of smart and motivated interns, I think they'll figure it out.

2. ## Re: HELP! 5-Axis 4'x4'x30" CNC Router

I must admit to never seeing a design that didn't have the spindle concentric with the C axis and while I suppose it could work,it still gives you the problem of ensuring that the distance between them is actually the dimension that you have given the CAM system to work with.I can do concentricity alignments,but would find working to a specific distance a lot more difficult.I share your belief that there is probably sufficient mental capacity to work out a practical solution as they seem to be asking intelligent questions and endeavouring to find solutions.

I would like to comment on the proposed reduction in Z axis travel as it might seriously reduce the usability of the machine.You need to keep in mind that with the spindle horizontal you are cutting at a height of half the spindle/gearbox (whichever is greater) above the table and with a cutter of any length you are reducing the height of the highest cut accordingly.Too short a cutter will run the risk of the corner of the spindle/gearbox hitting the job on a concave surface.Additionally,on the topic of tool length,how do you propose to determine the distance from tool tip to A or B axis pivot?

We haven't seen any illustration of the revised design yet.Mention of greater stiffness is encouraging but will almost certainly increase the weight of the Z axis column.Unlike the other axes,the motor alone must carry the weight and will need to be strong enough unless you add a counterbalancing system such as gas struts or just a weight and a motorcycle chain.It would also reduce the risk of the axis running down because of it's own weight with the power off.Do you have a weight estimate for the Z axis assembly?

For alignment of the machine some things need to be absolutely correct and one of the most difficult is ensuring that the Z axis is perpendicular to the table.A huge engineers square or angle bracket will be valuable for this,most other alignment can be done with a couple of magnetic stands and a pair of dial gauges.

3. ## Re: HELP! 5-Axis 4'x4'x30" CNC Router

Originally Posted by routalot
I must admit to never seeing a design that didn't have the spindle concentric with the C axis and while I suppose it could work,it still gives you the problem of ensuring that the distance between them is actually the dimension that you have given the CAM system to work with.I can do concentricity alignments,but would find working to a specific distance a lot more difficult.I share your belief that there is probably sufficient mental capacity to work out a practical solution as they seem to be asking intelligent questions and endeavouring to find solutions.

I would like to comment on the proposed reduction in Z axis travel as it might seriously reduce the usability of the machine.You need to keep in mind that with the spindle horizontal you are cutting at a height of half the spindle/gearbox (whichever is greater) above the table and with a cutter of any length you are reducing the height of the highest cut accordingly.Too short a cutter will run the risk of the corner of the spindle/gearbox hitting the job on a concave surface.Additionally,on the topic of tool length,how do you propose to determine the distance from tool tip to A or B axis pivot?

We haven't seen any illustration of the revised design yet.Mention of greater stiffness is encouraging but will almost certainly increase the weight of the Z axis column.Unlike the other axes,the motor alone must carry the weight and will need to be strong enough unless you add a counterbalancing system such as gas struts or just a weight and a motorcycle chain.It would also reduce the risk of the axis running down because of it's own weight with the power off.Do you have a weight estimate for the Z axis assembly?

For alignment of the machine some things need to be absolutely correct and one of the most difficult is ensuring that the Z axis is perpendicular to the table.A huge engineers square or angle bracket will be valuable for this,most other alignment can be done with a couple of magnetic stands and a pair of dial gauges.

As for the Z, we've yet again changed our minds. We haven't quite totally given that up. The results we've obtained so far suggest that we could make it happen. If in reality, when we assemble this thing and start testing it out, we can't obtain our tolerances we can always raise the bed up or drop the height of the frame down. We've changed our target tolerance to 0.005", and if its a little more than that, we're happy to keep the height to work with foam and other composite materials.

Concept 2:

FEA Results

Again, note that the weights are distributed along assumed rigid bearing blocks, and the deflections are under worst-case scenario 400 lbf loads in both the X and Y axes on the spindle/head ass'y mount. With 150 lbf loads, we are within our target tolerances for deflections. I look forward to hearing comments on these analyses and any suggestions that y'all might have. Let's use all of our wisdom and experience to make something great!

4. Originally Posted by ScottMcCormick

As for the Z, we've yet again changed our minds. We haven't quite totally given that up. The results we've obtained so far suggest that we could make it happen. If in reality, when we assemble this thing and start testing it out, we can't obtain our tolerances we can always raise the bed up or drop the height of the frame down. We've changed our target tolerance to 0.005", and if its a little more than that, we're happy to keep the height to work with foam and other composite materials.

Concept 2:

FEA Results

Again, note that the weights are distributed along assumed rigid bearing blocks, and the deflections are under worst-case scenario 400 lbf loads in both the X and Y axes on the spindle/head ass'y mount. With 150 lbf loads, we are within our target tolerances for deflections. I look forward to hearing comments on these analyses and any suggestions that y'all might have. Let's use all of our wisdom and experience to make something great!

For Z axis of large travel I would suggest mounting the rails onto the Z carriage and the block to the Y/Z saddle...

5. ## Re: HELP! 5-Axis 4'x4'x30" CNC Router

Originally Posted by louieatienza
For Z axis of large travel I would suggest mounting the rails onto the Z carriage and the block to the Y/Z saddle...
I agree.What I am a bit uncertain about is the effect that having an upright at each corner of the frame will have on the cutting envelope.Could we see a model with the spindle tilted at say 45 degrees at either side of the table?It might make sense to offset the frame a bit from the cutting area.The other advantage of modelling the spindle is that it will be easier to visualise the cutting area and how much of the table can be used in X and Y.

6. ## Re: HELP! 5-Axis 4'x4'x30" CNC Router

Originally Posted by routalot
I agree.What I am a bit uncertain about is the effect that having an upright at each corner of the frame will have on the cutting envelope.Could we see a model with the spindle tilted at say 45 degrees at either side of the table?It might make sense to offset the frame a bit from the cutting area.The other advantage of modelling the spindle is that it will be easier to visualise the cutting area and how much of the table can be used in X and Y.
Sorry, haven't chosen a spindle and importing the head into the ass'y causes FEA to fail because of imported geometry. For that reason, we don't yet have that ass'y modeled. Though there shouldn't be too much worry, the total dimensions of the frame are 6'x6', so we have 12" on either side of the usable workspace for leeway.

7. ## Re: HELP! 5-Axis 4'x4'x30" CNC Router

Don't forget that your spindle will be way in front of your Z...you should model it in just to make sure you can reach where you need to reach.

8. ## Re: HELP! 5-Axis 4'x4'x30" CNC Router

Originally Posted by ScottMcCormick
Louie, Wizard, Jim, Andrew, and ger21, routalot:

Thank you for your responses and insight. I appreciate you all backing me on the disparaging comment made off the bat by one of the users. I stated right away that this was a preliminary design, and was in no way final or even feasible. We are interns, tasked with a seemingly impossible task, and are young and inexperienced with respect to this industry. Your comments and wisdom drawn from experience have proven to be useful, and we are rethinking our approach. First off, I think that we are scrapping the idea of cutting aluminum for sure, and potentially scrapping the thought of cutting wood also. We are looking away from Mach 3, as suggested. We are rethinking the feasibility of 30" Z-travel, and looking more towards 12-15". The gantry has since been redesigned, as well as the z-axis assembly. More supports have been added to the frames.
If the work to be done requires a 30" Z then it needs to be considered in the design. In other words the machine needs to be designed to meet the requirements of the jobs that are expected be done on the machine. Since we don't know exactly what will be done with the machine we can only assume that you need the 30" Z.

The problem with the long Z is that every things adds up to the point that getting the minimal deflections you seem to need with be rather tough in my mind. You won't be able to assume perfectly rigid bearing blocks for example, your modeling must consider deflection in the bearings and any clearances that might exist.

As for wood and aluminum I'm pretty certain you will be able to machine them to some extent. I'm actually sitting here asking myself if tolerances of 3 thousands of an inch in wood and foam, on a machine this big, even makes sense. The wood worker in me thinks that even if you held dimensions as you pulled the part off the machine, the following day could very well see the part out of spec. Between thermal effects, moisture effects and stress relief, I'm not sure the specs make sense in the context of wood.

More design iterations to be had, and more updates to come. Again, any further insight is welcome and appreciated.
Component dimensions, on the drawings, might help. I believe I got a bit confused previously about the sectional sizes. In a later posting you have a drawing that is interesting but it was hard to gage the size of components. I also had a hard time imagining how you would get the saddle the Z axis runs on machined and built. Make sure you have a plan for actually manufacturing the assemblies you design.

9. ## Re: HELP! 5-Axis 4'x4'x30" CNC Router

Judging from the lack of updates,might we assume that the project has come to a screeching halt?

10. ## Re: HELP! 5-Axis 4'x4'x30" CNC Router

The project has not halted. Mind you this is a side project, and we have other clients and contracts to fulfill. This is also not as trivial as bolting together aluminum extrusions and throwing some plywood down. I plan on giving a proper update sometime this week. A lot has happened since the last update, so it may be a lengthy one.

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