Building a Cnc Router

[Cold Fusion]

Here are the specs that I'm trying to get.

36"x24" table
5" Z travel
214 ounce steppers
Xylotex Stepper Board
Thomson linear round shaft for all the axis
10mm ballscrews for the X and Y
1.75hp Porter Cable Router
All machined aluminum frame

Here are some pictures of it in Cad.

The big problem that I forsee is shaft deflection. For the longest time I've been trying to figure out what diameter rod to use. Right now I've got it setup with a 1.5 on the X and a 1 on the Y. My gauntry will weigh roughly 60 pounds and will ride on single bearing Thomson pillow blocks.

Right now I'm fairly sure I can get it done for $1100. If you would like to see the price list I can type it up for you.

Ballscrew for X and Y

And now, let the critiques begin!

[Cold Fusion]

Another Picture.

[kong]

Ew, I hate criticising, so lets assume your model is un-finnished, ok?! I think you realy need to double up on the linear bearings, four per axis would be a lot more stable, and the increased width of the frame, both in x and y, would help your deflection problem. A cross piece between those gantry sides would also help stiffen it up nicely. Go and dig out Jeffs (HomeCNC) router thread, and take a look at his structure. If you're going to buy ballscrews, it's worth having a decent frame to mount them on, or you will not gain anything over using all-thread.
Sorry!

[Cold Fusion]

I thought about dual pillow blocks per side but the problem is travel room. I would need to add a good 10 inches to the X so I could retain my cutting area. This would mean more shaft delfection? Maybe I'm alittle obessesed with it, but it would be better to get everything right before I buy big items such as linear shafts.

[kong]

I asked the guys about shaft thickness a few months back in this thread. There is a link in there to a website which has a deflection calculator which may prove useful.

[InventIt]

Why not use supported linear shafts? No deflction with that. The extra cost could be reduced by using smaller shaft.

[kevincnc]

That's a great 3-D model Cold Fusion. Kong is correct about doubling up on the bearings. Round-shaft type linear bearings usually have some compliance to make up for less than perfect mounting. That's good under the right circumstances, but makes them nearly useless in your application. Even THK type bearings aren't great for this setup unless they are the extra-long type. Shaft deflection will be insignificant without a rigid linear bearing setup. You'll probably need longer axes to get the travel, no matter how much it hurts...

[buscht]

Cold Fusion, is you don't have the rails yet, don't discount using THK types rails and ways.

Especially if deflection is a concern, THK rails solve this problem.

Also, you don't need a very big rail, a 1/2" THK is stronger than a 1-1/2" round rail. (For load ratings)

That will keep your costs down. I've seen these on Ebay fairly regularly.

I also agree with the 4 bearing concept as being almost mandatory. Especially on a machine as big as yours. Check the Thomson website for shaft deflection. It's a different calculation for a two bearing load versus a one bearing load.

Good luck.

[Cold Fusion]

THK rails are alittle pricey for me. All together my X and Y will end up costing $200-$250. I don't think I could even get a pair of 36 inch thk's for that much.

[Cold Fusion]

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[Cold Fusion]

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[Cold Fusion]

1.25 shaft, dual pillow blocks, extrusion down the X for bracing.