Key Questions to Ask When Ordering V Track Rollers and Guide Rails

Full Discussion: Round shafts & ball slide blocks versus V-rails & V-wheels

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joepardy

I am new at this... What are the pros and cons of using Rails, Wheels, & Rack systems compared to Shafts, Bearing blocks, and Lead Screws? :confused:

Gerald D

Price and low maintenance (self-cleaning). It's a robust, simple technology ideal for woodwork. Sawdust gets in, but it self-cleans. No jams or binding due to lack of maintenance. Did I mention it's low cost?

Some argue linear bearings and ball-screws offer better "precision." Perhaps, but a few wipes with sandpaper suffice for woodwork. The "precision" difference doesn't justify the extra cost and lower reliability in a woodshop, in my opinion. (I haven't personally seen the supposedly better quality from more precise systems, but I've only seen three other CNC Routers in action).

smreish

Joe,
There are many design philosophies for linear "guidance" systems. My background is in machine automation design and fabrication, so each application has its pros and cons, usually balancing cost, reliability, and serviceability.

Ideally, the Mechmate offers a low-cost, easy-to-maintain method with common shop tools. It's a philosophy I was mentored in during graduate school: KISS... Keep it simple.

I'll let Gerald, if he chooses, explain the significant differences and challenges between lead screw, linear rail systems, and captured track, rack, and pinion drive systems for a CNC router table application. Other machines and movements require their own considerations.

Regards,
Sean

Gerald D

Go for it, Sean. I have drawings to complete and don’t want to get sucked into this thread :)

driller

I personally LOVE ACME screws and ballscrews for power and precision. However...

A 9-foot screw has to be thick to be spun fast enough to prevent second and third harmonics (that ump-rope bowing).

Simply put, it's a wrong application for such a long axis that has to spin fast.

Another viable technology is a fixed belt. It works similarly to the rack and pinion. The problem with using a belt is that YOU have to develop and test it. There are no cost savings, nor any real work savings.

Gerald has either stumbled upon the best technology for the application or learned through painful experience that some things don't work well. I think we all know he’s learned through experience.

No one will argue that you can get things to work. But in the end, you'll likely return to say this design is the easiest and just as effective.

As one who loves to re-design, I suggest pricing out both the current method and your choice to see the cost difference.

Assume Gerald's design works, and you can always swap it for yours.

My strong recommendation is to design the machine for his design and then make modifications if needed. That way, it's simple to revert if necessary.

Dave

Richards

An easy and inexpensive way to decide between rack/pinion or screw is to buy one of JessEm's Mast-R-Lifts (http://www.jessem.com/mast_r_lift.htm) for your router table. Though the Mast-R-Lift works well, you'll find the threads need daily cleaning (sometimes several times a day). Sawdust, grease, and lead screws are not a good mix.

smreish

Joe,
The short answer is they both have pros and cons.

Lead screws (acme, recirculating ball, pitched rod):
Pros:
- Great precision (acts like a worm gear in a gearbox).
- Great holding (position) if pitch is high enough.
- Ideal for vertical axis travel (like Z) due to holding properties.

Cons:
- Can be expensive and often requires machining (lathe work) to mount.
- Needs high axial speed for fast IPM cutting speed, which may require a servo motor.
- Collects dust and can quickly clog the nut. Recirculating ball bearings may jam, causing friction and premature failure.
- Harmonic and vibration issues if rotated too fast.
- Size matters. Larger screws require heavier motors and bearings, adding cost.

Rack and Pinion:
- Readily available off the shelf with no machining required.
- Extendable by joining racks for longer travel distances.
- Quick and easy installation.
- Self-cleaning if installed upside-down (like the MechMate).
- Easy to change speed/torque ratio with different pinions.
- Size generally doesn’t matter. One size fits all for a CNC router table of this size.

Guide Mechanicals:
- Nearly the same principles as Lead Screw relate to Linear bearings.
- Nearly the same principles as Rack and Pinion relate to guided track.

In summary, each application requires a clear, value-engineered decision. The MechMate is inexpensive, easy to maintain, and delivers repeatable performance.

Regards,
Sean

joepardy

Thanks for the insight.

bradyaero

Hi All,

Absolutely love the MechMate! Working on my own variation and wanted to ask, linear rails vs. knife edge for the X, Y, and Z axes. I expect to gain accuracy using linear rails compared to the knife edge and bearings. Is it worth the extra effort (I'm aiming for 12" on the Z axis for 3D foam cutting)?

Thanks, Greg

Gerald D

Hi Dex.

What is a vbx rail?

Sounds like something that is round? Why do you think it will improve accuracy?

bradyaero

Hi Gerald,

I believe it will improve accuracy because the rails are machined to high tolerance and the bearings are quite tight. Knife edges are 'coarser' and I think they wouldn't be as accurate as the rails/bearings. I'm not sure which way to go; my MechMate is for 3D foam cutting for molds.

Thanks again, Greg

Gerald D

Sorry Greg, I can't hold a discussion based on such vague concepts.

bradyaero

I'm sorry for being vague:

I'm referring to using 20 or 25mm round rails with slide blocks containing linear bearings. The rails are supported by pylons bolted to your X-axis rails. Here's an example:

http://www.vxb.com/page/bearings/CTGY/25mmLinearMotionSystems

If this is off-topic, I apologize. I'm just trying to figure this out.

Thanks, Greg

Doug_Ford

Hey Greg,

A few months ago, we debated various rails and rollers before concluding Gerald's design was perfect for our needs. You might find some posts here informative:

http://www.mechmate.com/forums/forumdisplay.php?f=6

The short version is that wood swells and contracts, so more accurate rails and rollers aren't necessary and add to the cost.

Gerald is a mechanical engineer with vast experience, and every part of this machine was carefully considered. No one is trying to be rude, but we're tired of this topic since it won't make our machines more accurate, faster, prettier, or cheaper. :D

smreish

...and I'll sit quietly sipping coffee while applauding Doug's polite, accurate response to the linear rail question. I've moved to the MechMate machine because of the rail system, moving away from VXB, Thomson, or other round linear rails. ...back to keeping it simple in Florida.:cool:

Marc Shlaes

Group,

To be fair, everyone discovering this site is overwhelmed by the information. As it keeps growing, it's impossible for a "new guy" to absorb everything and decide if this is the right machine.

Additionally, Big Iron machines almost all use the type of linear bearings that Dex describes. Dex probably saw this and is questioning why/why not.

Dex likely knows his needs best.

Dex, Gerald has designed a large format machine primarily for cutting sheet goods like MDF, melamine, and plywood. Doug is correct that these materials are seldom used for extremely high-tolerance items. This might not be a machine for NASA. Although, I believe Fabrica shared some impressive carvings done on the MM.

Are you aiming to cut foam to .0001 accuracy? I doubt it.

I don't know if anyone has performed repeatability tolerance tests for the MechMate, but I guess it’s within a few thousandths. Three thousandths is about a sheet of paper.

That should meet your needs, and you can achieve that with this elegant design that costs a fraction of Big Iron machines.

So, welcome to the best forum for production-quality, DIY CNCing.

BTW: Foam for molds to create what?

Gerald D

What I’m referring to is using 20 or 25mm round rails with slide blocks containing linear bearings. The rails are supported on pylons bolted to your X-axis rails. Here’s an example:

http://www.vxb.com/page/bearings/CTGY/25mmLinearMotionSystems

Which size will you use, 20 or 25mm? There’s a big difference.

What will be your pylon spacing? Only at the ends with a closed slider, or a split-open slider?

How will you get the pylons in a straight line?

How well does the slider block seal against dust? Does the split slider seal as effectively as the closed slider?

What is the clearance/pre-load between the shaft and the balls in the slider block?

What if you get a small nick in the shaft from dropping a clamp on it? Could you file/sand it out without interference with the sealing?

Will the grease lubrication stain your work or your clothing?

Where will you attach the rack?

bradyaero

The existence of pre-machined V rails makes a world of difference for me. The idea of grinding my rails to decent tolerance led me to seek alternatives. My application involves shaping medium to high-density foam requiring tolerances within a couple of thousandths.

I have a master machinist friend willing to help. Between him and you guys, I hope to stay on track!

Graham, my X-axis will definitely use V-rails. I'm still considering using round linear rails for Y and/or Z; increasing Z-axis travel is important to me (bigger blocks of foam).

If I use round rails for Y or Z, I'd machine an extrusion to run the entire rod length, using a ball end on top to fit the rail exactly; bolt spacing 8-10 inches.

To make the round rails work for the Y axis, I'd consider changing the Gantry orientation to vertical and place the rack at the top.

Talk is cheap, but I want to get this right before heading down the wrong path. Thanks for the responses.

Marc, I want to build a MechMate to assist with other projects. I'm part of another DIY group, http://www.fieldlines.com/. We build homebuilt wind generators and I want to use my MechMate to make better blades than those made by hand. Laminated foam core blades will be lighter, stronger, and more efficient than wood blades. I used to own a Big Iron machine for custom motorcycle wheels but that's in the past. Here's a few pics:

For more China Flat Iron Round Steel Customized information, please contact us. We will provide professional answers.

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Gerald D

Dex, your post was removed until you answer my questions.

bradyaero

lol, well, I don’t know why my post had to be deleted... As you know, some of that information is not available on the VXB site....

Leko

hmmmm....I'd go with the 20mm for that.....

bradyaero

>>Which one are you using, the 20 or the 25mm – there is a big difference.

None at the moment, but it would be a minimum of 25mm, maybe even 30mm. My project now includes using pre-machined v-capped rails for the x-axis. I am exploring the concept of the round rails for y and/or z axis.

>>What will be your pylon spacing? Only at the ends with a closed slider, or a split-open slider?

There is no pylon spacing. As reported, the pylon would run the entire length of the rail. The 300-degree C bearing with a split-block would be used to avoid the pylon.

>>How do you plan to get the pylons in a straight line?

As previously reported, the pylon would run the entire rail length with a ball end groove cut into the top for the rail to sit in.

>>How well does the slider block seal against dust? Does the split slider seal as effectively as the closed slider?

If dirt will be an issue for these bearings, then perhaps a felt/teflon bushing at each end would solve the problem.

>>What is the clearance/pre-load between the shaft and the balls in the slider block?

No idea. I'll need to get one to find out. It would depend on the manufacturer’s quality. VXB is just one example of a round rail/bearing block importer. There are other high-quality companies like Thomson bearings that manufacture excellent linear bearing and rail products. I used VXB as an example, not as my standard.

>>What happens if you get a small nick in the shaft from dropping a G-clamp on it? Could you file/sand it out without interfering with the sealing?

Good point. The only solution I see is to buy another one.

>>Will the grease lubrication stain your work or your shirt?

No grease lubrication is planned.

>>Where will you attach the rack?

The potential Y attachment point is subject to change based on its orientation. X-Axis will be v-rail for sure.

bradyaero

Here’s a pic of what such a rail looks like:

garyc

Gerald and others, I use VXB bearings on my router so I think I can answer some of the questions about them. Mine are 20mm shafts and bearings. VXB didn’t offer supports for the 20mm at the time, so I made my own. The seals don’t keep out the