New Machine Build- Revised design model

[jzurek]

Thanks to everyone for your help so far. This is my basic design. Overall footprint will be 63" x 48". Please take a look at what I've done so far and offer comments and potential problems.

1/2"-10 ACME lead screws
20mm Glacern linear bearings on X and Y
SRS25M THK linear bearings on Z

8020 sizes:
1515 and 1530

Z mounting plates 1/2" thick 6061 Aluminum

I look forward to seeing what you think.

[DonFrambach]

For my router, I thought about using Glacern SB-20 rails as you have drawn them on my gantry axis but decided against it because I was concerned that the fully supported rails placed as you have drawn them would force the carriages to be oriented supoptimally for left to right cuts. If you are cutting right to left, the bottom carriages would be pushed down against the rails (optimal). On the other hand, if you cut left to right, the bottom carriages would be pulled out of the rails (sub-optimal). I believe that the supported SB-20 rails are designed to take high side and down forces but not up forces. To get around this problem, I have seen designs where one rail was placed on the bottom of the gantry and the other rail was placed on the front. That construction seemed a little complicated for me. I have also seen designs where unsupported rails were used but those rails themselves can flex. That's why I decided to use a steel plate and carriages from CNCRouterParts.com for my gantry axis.

Just want to add an addendum. While it would theoretically be great if one rail could be placed on the bottom of your gantry and the other on the top, it would be extremely difficult to make them exactly parallel so binding would be a real problem.

[DonFrambach]

Sorry to be finding fault, but your Z axis will not give you the 12" of travel that you want. In order to do that, you should reverse the coupling so that the router is mounted where you have the Z axis rails and the Z axis trucks are mounted on a plate that is attached to your gantry carriages. I did that on my router in order to gain stiffness rather than more travel but it is required if you want 12" of travel.

Oops! Just realized that it is someone else who is building a router with 12" of Z axis travel. http://www.cnczone.com/forums/diy-cn...ce_please.html

But you may want to consider the suggestion anyway.

[DonFrambach]

Final thought and I'll shut up for the night. You'll want to use 5 start 1/2 x 10 ACME lead screws for your X and Y axes. Single start will probably be good for your Z axis.

[jzurek]

Don,

You are not pointing out faults... that is the purpose of me posting this now rather than during the actual build. You are simply increasing CAD time and saving me from scrap and aggravation later.

A.) I mounted the rails this way as to avoid hanging the bottom bearing block assembly with weight towards the open end. I had given no thought that the operation of the table would in fact create the same condition I was trying to avoid. Wouldn't having a rail on the bottom cause a problem? I do not want to use unsupported rail. I'll look into the rails you mentioned.

B.) I am looking for quite a bit of Z travel because the machine will have a 4th axis that I plan on being able to turn up to 6" spindles. I am a little confused by what you are saying. I'll have to read it again and think about it.

That said, thank you very much for the constructive criticism. It is very much appreciated.

Jeremy

One other comment... I would love to begin with 5-start ACME but considering I was originally planning on making the machine out of plywood my costs are already much higher than I originally planned. I plan on using the single start for now and once the machine is running and my credit card has cooled off swapping out to multiple starts later. I can deal with 40ipm travel for now.

[DonFrambach]

Here are some pictures from my router build, http://www.cnczone.com/forums/diy-cn...ry_router.html, that show what I mean by mounting the bearings back to back. The first picture shows the mount as I originally planned and is similiar to your current plan. The second two pictures show what I believe is a better mount that is much stiffer because it brings the bearings closer to the spindle and allows for much larger effective Z axis movement.

[skipper_52]

Back to the drawing board eh Jer? His point is well taken, something both of us overlooked. This forum has been great helping me build my machine and now yours.

Dad

[jzurek]

This is my first stab at the redesign. I see the problem you were talking about with the Z. I will probably tie the 8020 on the top and bottom together on the backside as well. New thoughts and problems?

Jeremy

[skipper_52]

Get to work

[jzurek]

I had today off. Where were you?

[jzurek]

So here is my next dilemma, is there a benefit to using these bearing assemblies on my x-axis rather than the open linear style bearings? I could build my x axis for cheaper. What is the accuracy of these bearings when they are compared to linear bearings?

Something like the attached pic. Has anyone had experience using these this way? Good, bad, problematic... feedback is appreciated.

[jzurek]

I've decided to go with the CNCROUTERPARTS.COM bearing assemblies. My question is, should I use two per x-rail or can I get away with one on a 42" gantry? Anyone?