What size ballscrew and linear slides?

[kerino]

Hi,

I am just designing my first CNC router table for woodwork, and possibly light metal work. The router is 1/2" and the work area will be in the order of 36" x 36". The speed of cutting is not critical.

My question is how do I decide what size linear bearings and ball screws do I use. The size is calculated acording to the loading, but I have no idea how much force is exerted when routing out a piece of wood?

Can any of you experienced guys out there give me a helping hand please?

I am looking at rectangular section linear guides, but should I go for 15mm, 20mm or larger?

With ballscrews, will 12mm be sufficient, or do I need 20mm or larger?

Many thanks
Kerin

[Oldmanandhistoy]

Hi Kerino

Nice to see another UK member.

Unfortunately there are no simple answers to your questions without a lot more information.

I would suggest you have a good look through the DIY-CNC Router Table Machines and CNC Wood router project log for your answers.

Look for machine being built similar to what you require.

Look for threads that involve the part you plan to use.

Ask the builder a few questions about their choices.

Unfortunately unless you work from free or purchased plans, designing and building a cnc machine is a long process.

I hope I have helped and as a person who has built a cnc router already would like to say stick with it as it is well worth the learning curve to see you machine cut it’s first part.

Best regards, John

[kerino]

Thanks John,

I will persevere (just ordered the metalwork for the frame, and I already have the stepper mototrs and controllers).
I think I will go down the traditional route of linear slides and ballscrews, as it seems to be the favourite. The software I wrote some time ago for another project, and just need a bit of modification.

Thanks again
Kerin

[phantomcow2]

Well its hard to say with certainty without more detail of your setup.
But 15mm rails will probably suffice, the load ratings of even these somewhat narrow things are very impressive. I am using them for my router in progress. Look for C1 in the model#, which is light preload. This really makes it rock solid!

As for ballscrew, I think you could get away with 12mm, but I would be concerned about whip at high speeds. 5/8"-.2 would do it okay, 16mm would do it fine also.

[ViperTX]

Well speed is never important....until you notice yourself growing a beard waiting for the router to finish its work.....

Don't use low profile rails...unless you already have them.....use at least 15mm rails....2 trucks per rail....also remember when you size up the rails....well the trucks also increase in size.....so, you'll have to use longer rails to get the cut envelope you desire.

Ballscrew for the x and y axis.....minimum of 5/8 inch with 5 revolutions per inch....for the z-axis....use an ACME screw from MSC-direct.com....something line 5/8 inch X 8 (turns per inch).....they are made by Nook....

[kerino]

Thanks Guys, thats very helpful.
I think I'll go with 20mm slides. Incidently, whats the problem with low profile ones?
For the ballscrews, I'll go with 16mm.
I have a question for ViperTX - why do you favour ACME screw for the Z axis, rather than a ballscrew? Is it to do with friction or something?
Thanks

[ger21]

Originally Posted by ViperTX Well speed is never important....until you notice yourself growing a beard waiting for the router to finish its work.....

I disagree. Not being able to cut fast enough can cause burning of the workpiece, and can dramatically shorten tool life. Heat is the greatest cause of tool wear. Slowing down the rpm's can help, but high feedrates are the best answer for this.

Unfortunately, increasing the feed rates may require a stronger, stiffer machine, possibly bigger steppers and matching drives, as well as a more powerful spindle. All of which will add to the cost, some significantly. So most people don't worry to much about the speed, because they're usually on a budget. But speed is important. How important probably depends on your wallet.