Question about Linear Rails Sizing

[bml6116]

I am (trying to) building a CNC router loosely based on the MadVac design. Cutting area is 4x8 with 8" (travel) z-axis.
The gantry (y and z-axis with spindle) weighs in at about 240lbs with an overall height of 26".
So here is the question: Will 20mm linear rail (Hiwin AG20) with 4 trucks work? The trucks are mounted 22" on center, drive is rack and pinion on both rails. I.e. Will te gantry fly off if the E-stop is hit during jog? I am not concerenc about the weight, but the moment force.
I have a hard time calculating the forces involved.
Any help is greatly appreciated.
Cheers,
Bernhard

[ger21]

Should be plenty strong.

[bml6116]

Gerry,
Thank you! I was hoping for that answer. Just was a bit leary, because Hiwin calls those rails " Medium Duty".

Cheers,
Bernhard

[svenakela]

Check the specification. 650 kg per wagon is a lot of load.
http://www.hiwin.com/lg/images/agh-sa_agh-ca.gif

[bml6116]

Originally Posted by svenakela Check the specification. 650 kg per wagon is a lot of load. http://www.hiwin.com/lg/images/agh-sa_agh-ca.gif

I do have the specs; my problem is that I do not know how to calculate the dynamic force and moment for my homebrew gantry.

Cheers,
Bernhard

[svenakela]

I think you don't have to worry. Just make sure your parts are tight/preloaded and backlash free.

[bml6116]

Originally Posted by svenakela I think you don't have to worry. Just make sure your parts are tight/preloaded and backlash free.

Thank you. Now I have 2 positive answers, soooo I am going to get the rails.

Cheers,
Bernhard

[jimashworth]

Originally Posted by bml6116 I am (trying to) building a CNC router loosely based on the MadVac design. Cutting area is 4x8 with 8" (travel) z-axis. The gantry (y and z-axis with spindle) weighs in at about 240lbs with an overall height of 26". So here is the question: Will 20mm linear rail (Hiwin AG20) with 4 trucks work? The trucks are mounted 22" on center, drive is rack and pinion on both rails. I.e. Will te gantry fly off if the E-stop is hit during jog? I am not concerenc about the weight, but the moment force. I have a hard time calculating the forces involved. Any help is greatly appreciated. Cheers, Bernhard

Bernhard,
When your looking at linear guides, they are often purchased by how strong the unit looks. In your case, the bearing in our size 20 has a load capacity of over 3,000#, which would more than meet you requirements. You could get by with something much smaller, but the ways would look small and undersized. Starting with a size 20 is a good place to start. If you think the unit will have a high cycle life, than move up to 25. To acheive longer life you move up in size. In a low cycle application, SBG20FL guide would be fine. Just one more word. When using 4 ball circuit linear guide blocks, you can often get away with using only one guide and bearing block due to the ability to take high moments. A great cost savings
Always willing to help.
Jim

[lgalla]

Bernhard
This is the machine I am using.The rails seem to be slightly under 25mm.I am sure you won't be building heavier than this.
Happy New Year
Larry

[neilw20]

Put energy absorbing buffers at the stops. Absorb inertia over some distance.
Either rubber, or pnuematic with blocked off outlet.

[bml6116]

Originally Posted by jimashworth Bernhard, When your looking at linear guides, they are often purchased by how strong the unit looks. In your case, the bearing in our size 20 has a load capacity of over 3,000#, which would more than meet you requirements. You could get by with something much smaller, but the ways would look small and undersized. Starting with a size 20 is a good place to start. If you think the unit will have a high cycle life, than move up to 25. To acheive longer life you move up in size. In a low cycle application, SBG20FL guide would be fine. Just one more word. When using 4 ball circuit linear guide blocks, you can often get away with using only one guide and bearing block due to the ability to take high moments. A great cost savings Always willing to help. Jim

Jim,

Thank you for the info. I looked your website (lee linear), but couldn't get through the registration screens to get to the info. Also, -on a more general note- I wish more companies would post prices and have on-line ordering; saves both parties time and effort.

Cheers,
Bernhard

[bml6116]

Originally Posted by neilw20 Put energy absorbing buffers at the stops. Absorb inertia over some distance. Either rubber, or pnuematic with blocked off outlet.

Thanks for the tip! A mechanical stop with shock absorber is one more thing I hadn't considered, will come in handy in case of limit switch/estop/software failure.

Cheers,
Bernhard