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  #1  
Old Tue 16 October 2007, 06:05
joepardy
Just call me: Joe
 
Lebanon, OH
United States of America
Round shafts & ball slide blocks versus V-rails & V-wheels

I am new at this but ... what is the advantage/disadvantage to using Rails, Wheels, & Rack system instead of Shafts, Bearing blocks and a Lead Screw?
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  #2  
Old Tue 16 October 2007, 06:39
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Price, low maintenance (self-cleaning). Good, simple, stupid, robust technology completely suitable for woodwork. The sawdust gets in, but it works out by itself again. No jams or binding due to lack of maintenance. Did I mention low-cost?

Some will argue that the "precision" with linear bearings and ball-screws is better. Maybe it is, but a couple of wipes with sandpaper is the great leveller for woodwork. The difference in "precision" is not worth the extra cost and lower reliability for a woodshop, IMHO. (I personally havn't seen the supposedly better quality from the more precise systems, but then I have only seen 3 other CNC Router's work).
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  #3  
Old Tue 16 October 2007, 06:47
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
Joe,
There are many "school's of design" when it comes to linear "guidance" systems. My background is in machine automation design and fabrication, thus each use has it's pro's and con's -
Usually, weighing the difference between cost, reliability and serviceability.

Ideally, the Mechmate has found a way of using a inexpensive, easy to maintain and low cost method of achieving all three - with common "layman's " shop tools. It's a philosophy that I had the privilege of being mentored in during graduate school. KISS....keep is simple.

I will let Gerald, if he chooses, to explain the big differences and subsequent challenges with lead screw, linear rail systems vs. captured track and rack and pinion drive systems for a cnc router table application. Other machine and movements require their own considerations.

Regards,
Sean
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  #4  
Old Tue 16 October 2007, 07:13
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Go for it Sean, I have drawings to complete and don't really want to get sucked into this thread
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  #5  
Old Tue 16 October 2007, 07:54
driller
Just call me:
 
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 is a wrong application on such a long axis that has to spin fast enough.


The other very workable technology is a fixed belt. It would work similar to the rack and pinion. The problem with using a belt is that YOU have to develop and test it. There is no cost savings, nor any real work savings.

Gerald has either stumbled blindly on the best technology for the application, or has painstakingly learned from experiance that somethings will not work WELL. I think we all know the correct answer to that, and 'pain' may need to be capitolized.

I think no one will argue that you could get things to work, but in the end, you will come back and say the design is the easiest and every bit as effecive.

As one who loves to re-design, may I sugguest you price out the currrent method and the method of your choice and see the price diference.

Figure you know his will work, AND, that you can always swap it out for yours.

My real hard recomendation is to design the machine for his design, then add what you need to make yours work. That way it is a simple matter to come back to his design if needed.


Dave
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  #6  
Old Tue 16 October 2007, 08:45
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
An easy and fairly inexpensive way to decide whether you want to use rack/pinnion or some type of screw is to buy one of JessEm's Mast-R-Lifts for your router table. Although the Mast-R-Lift works very well, you'll find that the threads have to be cleaned daily (and sometimes several times a day). I don't think that God ever intended that sawdust, grease and lead screws be used together.
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  #7  
Old Tue 16 October 2007, 10:29
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
...running off at the keyboard.
Joe,
The simple answer is "they both have advantages and disadvantages".

Pro's and Cons, this is a gross overview. FOR a CNC Router

Positioning mechanicals.

Lead screws (acme, recirculating ball and pitched rod)
Pro.
- great precision (because it acts like a worm gear in a gear box.
- great holding (position) if pitch is high enough
- great for a vertical axis travel (like a z) due to holding properties.

Con
- can be expensive and usually requires machining (lathe work) to mount properly.
- needs great axial speed to have fast IPM cutting speed at tool bit. (This speed may exceed stepper motor limits and require servo motor to drive the screw.
- collects dust and will quickly clog the "treaded" nut. If recirculating ball, then bearing are subject to jambing and potentially inducing great friction into drive system. This will make your drives work (heat up) quicker and cause premature failure.
- harmonic and vibration issues if rotated to fast
-size matters
-1/2" dia screw would be fine for a 12-18" travel....but a 2" dia screw may be required for a 8' travel...more weight, bigger motor, bigger bearings...bigger cost.

Rack and Pinion
- "of the shelf" availability and no machining required.
- can add lengths of rack to get travel distance. (join 3 - 1M racks to get 3M of travel)
- quick and easy installation
- if installed upside-down (like the MechMate) it's almost self cleaning/clearing
- easy to change speed/torque ratio with different pitch dia pinions. Easy to experiment with.
- does not require different diameters of screw with relation to length of travel...
- size really doesn't matter. Generally folks. You do have to pay attention to shear angle, pitch and torque on the right size rack teeth. But generally, one size does fit all for a CNC router table of this size.


Guide Mechanicals.

Almost all the same principles of Lead Screw relate to Linear bearings
Almost all the same principles of Rack and Pinion relate to guided track.

I hope this description is general enough for the novice and not "wrong" enough for the advanced designers.

---
In summary....each application requires a clear, concise and value engineered decision. The MechMate is very well designed. Inexpensive, yet easy to maintain, fabricate and delivers repeatable (predictable) performance.

Now, without sounding like a marketing campaign, I will get back to drafting. As like Gerald, I too have a design deadline that needs my attention.

Regards,
Sean
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  #8  
Old Wed 17 October 2007, 04:24
joepardy
Just call me: Joe
 
Lebanon, OH
United States of America
Thanks for the insight
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  #9  
Old Tue 04 December 2007, 21:44
bradyaero
Just call me: Greg #19
 
Smiths Falls, Ontario
Canada
vbx rails

This post originally started as separate thread, now merged in to this topic

Hi All,

Absolutely love the mechmate! Working on my own variation and wanted to ask,
linear rails vs the knife edge for x,y, and z axis, I expect to gain accuracy by using linear rails of the round type vs the knife edge and bearings. Is it worth the extra effort (I'm trying to get 12" on the z axis for 3D foam cutting)?

Thanks, Greg
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  #10  
Old Tue 04 December 2007, 22:08
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Hi Dex.

What is a vbx rail?

Sounds like something that is round? Why do you believe it will gain you accuracy?
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  #11  
Old Wed 05 December 2007, 06:07
bradyaero
Just call me: Greg #19
 
Smiths Falls, Ontario
Canada
Hi Gerald,

I believe I will gain accuracy because the rails are machined to a high tolerance and the bearings are quite tight. As the knife edges are a 'coarser' product 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
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  #12  
Old Wed 05 December 2007, 07:36
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Sorry Greg, I can't hold a discussion based on such vague concepts.
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  #13  
Old Wed 05 December 2007, 13:51
bradyaero
Just call me: Greg #19
 
Smiths Falls, Ontario
Canada
I'm sorry for the vagueness:

What I am referring to is using 20 or 25mm round rails with slide blocks that contain linear bearings. The rails are supported on pylons bolted to your x-axis rails. Here's a link to one supplier as an example:

http://www.vxb.com/page/bearings/CTG...rMotionSystems

If this is too far out of scope I apologize, I'm just trying to figure this one out.

Thanks, Greg
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  #14  
Old Wed 05 December 2007, 17:11
Doug_Ford
Just call me: Doug #3
 
Conway (Arkansas)
United States of America
Hey Greg,

A few months ago, we discussed all different sorts of rails and rollers before deciding that what Gerald designed was perfect for our needs. You might be interested in reading some of the posts here

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

The short version is that wood swells and contracts so rails and rollers that are more accurate than what we have are really unnecessary and simply add to the cost of building this router.

Gerald is a mechanical engineer with lots of experience and his design is incredibly well thought out. Like most engineers, he doesn't do anything on a whim. The design of every single part of this machine was carefully considered.

No one is trying to be rude but I guess we're pretty much worn out and tired of discussing this particular topic since it won't make our machines any more accurate, faster, prettier, or cheaper.
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  #15  
Old Wed 05 December 2007, 17:15
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
...and I will sit quietly sipping coffee while applauding Doug on a polite, yet accurate response to the linear rail item. As a footnote, I actually have moved to the MechMate machine "because" of the rail system and to get away from a VXB, Thompson or other round linear rail. ...back to keeping it simple in florida.
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  #16  
Old Wed 05 December 2007, 18:54
Marc Shlaes
Just call me: Marc
 
Cleveland, OH
United States of America
Send a message via Skype™ to Marc Shlaes
Group,

To be fair, everyone who first discovers this site is overwhelmed by the volumes of information. These volumes grow every week. It is completely impossible for a "new guy" to take it all in and make an assessment if this is the right machine.

Additionally, if you look at the Big Iron machines, they almost all use the the type of linear bearings that Dex is describing. I am sure that Dex saw this and is probably wondering why / why not.

I also assume Dex knows his needs better than anyone.

Dex, Gerald has designed a large format machine primarily for cutting sheet goods such as MDF, melamine and possibly plywood. Doug is absolutely correct that these materials are seldom used to build extremely high-tolerance items. This might not be the machine for NASA. Although, I believe it was Fabrica who submitted some pretty cool photos of carvings he did on the MM.

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

I don't actually know whether anyone has actually performed repeatabillity tolerance testing for the Mechmate but I would guess that it would be within a few thousanths. If I remember right, three thousanths is about a sheet of paper.

I am also guessing that will meet your needs and you can do that with this very elegant design that won't be 10% of the Big Iron price.

So, welcome to the best forum on the web for production quality, yet DIY CNCing.

BTW: Foam for molds to create what?
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  #17  
Old Wed 05 December 2007, 21:56
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Quote:
Originally Posted by bradyaero View Post
What I am referring to is using 20 or 25mm round rails with slide blocks that contain linear bearings. The rails are supported on pylons bolted to your x-axis rails. Here's a link to one supplier as an example:

http://www.vxb.com/page/bearings/CTG...rMotionSystems
Which one are you using, the 20 or the 25mm - there is a big difference.

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

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

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

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

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?

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

Where will you attach the rack?
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  #18  
Old Thu 06 December 2007, 08:09
bradyaero
Just call me: Greg #19
 
Smiths Falls, Ontario
Canada
The existence of pre-machined V rails makes all the difference in the world to me. The idea of griding my own rails to a decent tolerance was the reason I was looking for alternatives. As my application calls for shaping medium to high density foam my tolerances are approximately a couple thousandths.

I have a master machinist as a good friend who has volunteered to help me out, between him and you guys I hopefully can be kept on the straight and narrow!

Graham, my X will be v-rails for sure. I'm still considering using the round linear rails for y and/or z; it's important for me to try and increase the z-axis travel. (bigger blocks of foam).

If I use the round rails for y or z I would machine an extrustion that would run the entire length of the rod, using a ball end on top to make the rail fit exactly; bolt spacing 8-10 inches.

This is getting ahead of myself but to make these round rails work for the Y axis I would consider changing the orientation of the Y Gantry to vertical and place the rack on the top of the gantry.

I know talk is cheap but I want to get this right before I end up down a blind alley so thanks for the responses.

Marc, I want to build a mechmate to help out with other projects that I am working on. I belong to another DIY group, http://www.fieldlines.com/ we make homebuilt wind genny's I want to use my mechmate to make better blades than by hand. Laminated foam core blades will be lighter, stronger and more efficient than wood blades. I used to own a big iron machine that I made custom motorcycle wheels with but thats in the past now. Here's a few pics:
Attached Images
File Type: jpg blade.jpg (37.6 KB, 1111 views)
File Type: jpg plane.jpg (32.0 KB, 1115 views)
File Type: jpg wheel.jpg (47.4 KB, 1108 views)
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  #19  
Old Thu 06 December 2007, 10:50
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Dex, your post was deleted until you answer my questions.
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  #20  
Old Thu 06 December 2007, 16:46
bradyaero
Just call me: Greg #19
 
Smiths Falls, Ontario
Canada
lol well i dunno why my post has to be deleted... as you know some of that information is not posted on the vxb site....

Last edited by bradyaero; Thu 06 December 2007 at 16:58..
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  #21  
Old Thu 06 December 2007, 16:54
Leko
Just call me: Leko
 
Kaukapakapa
New Zealand
hmmmm....I'd go with the 20mm for that.....
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  #22  
Old Thu 06 December 2007, 17:03
bradyaero
Just call me: Greg #19
 
Smiths Falls, Ontario
Canada
>>Which one are you using, the 20 or the 25mm - there is a big difference.

None at the moment, but it would be minimum 25mm maybe even 30mm. My project now includes using the 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 the split open slider?

There is no pylon spacing, as reported it 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 length of the rail with a ball end groove cut into the top of it for the rail to sit in.

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

If dirt is going to be an issue for these bearings then maybe a felt/teflon bushing at each end would do the trick.

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

No idea, I have to get one to find out, it would certainly depend on the quality of the manufacturer. Vxb is just one example of a round rail/bearing block importer. There are other high quality companies like Thompson bearings that manufacture fantastic linear bearing and rail products. I used vxb as an example, not as my de-facto 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 thing I can see doing in this case 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.

Last edited by bradyaero; Thu 06 December 2007 at 17:21..
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  #23  
Old Thu 06 December 2007, 17:17
bradyaero
Just call me: Greg #19
 
Smiths Falls, Ontario
Canada
Here's a pic of what such a rail looks like:
Attached Images
File Type: jpg linear47.jpg (3.9 KB, 1116 views)
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  #24  
Old Thu 06 December 2007, 17:23
garyc
Just call me: Garyc
 
Charlotte, North Carolina
United States of America
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, When I bought mine vxb did not offer the supports for the 20mm that they do now, I had to make my own. The seals do not keep out the dust unless the particles are big enough that they get pushed out of the way(which is rarely the case with wood, especially mdf!!), So basically there is none or very little sealing against contamination mine are full of it! I have to remove them every so often to clean them out because the balls will actually stop recirculating and just slide. I can tell when this happens because my ball screw is in the middle of the X axis and it will start racking badly, the edge of the cut will look like saw teeth(conventional milling) when it comes back in the opposite direction(climb milling) it will smooth up the edge. There is no way to preload the bearings that I have(open) so you get what you get which also adds to the racking problem, This is most noticable in my z axis, The only I can explain is to say the z axis jitters back and forth. It was also noticable when I surfaced my table. I used a .5" end mill and what happens is the end mill get pushed opposite the direction of travel which results in the cuts not being flat because the end mill begins to ride up on the rounded part of the tool rather than on the flat bottom. I now am able to afford thk linear rails to solve the z axis problem and I am working on that as time permits. As far as nicks in the rod go the metal is very hard 60 - 65 rockwell if you dropped something on it hard enough to nick it, their most likely would not be any use in worring about the nick because the tonnage it would take to nick it would surely destroy the machine. The only way to lubricate my bearings is to dump oil on the rods directly, needless to say I have ruined a few pieces. So what do I think about using vxb bearings? it is this if you want a cnc router as I did and it is all the finances will allow then go for it, I did! As time goes on and you can afford it change them out just like I am doing. In the long run it will cost more money to do it this way but it seems like a lot of people end up building more than one machine anyways.
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  #25  
Old Thu 06 December 2007, 22:57
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
My impression is that "vxb" rails purport themselves to be "linear bearings" which conjures up images of "accuracy". All lovely vague concepts and people chasing brand names.......

Dex, the picture of the rod on the support is pretty, but it doesn't show you that the rod has holes tapped in the bottom and the there are screws at finite intervals. The screw spacing is the pylon spacing since there is nothing else holding the rail down between the screws. Cheap rails have a thin electroplated layer of chrome on the outside (if lucky) - sometimes it flakes off.

The purpose of this forum is to help guys who stick to the plans, or to fill in the gaps on the plans. If you want to start off by saying that you are not sticking to the plans, then rather go to another forum.
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  #26  
Old Fri 07 December 2007, 08:35
bradyaero
Just call me: Greg #19
 
Smiths Falls, Ontario
Canada
Roger that Gerald,

Since my needs are different from the typical needs of the people who build mechmates I'll only comment and ask questions on aspects of the mechmate that don't deviate from your plans. I appreciate what you've done here; hats off to you.
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  #27  
Old Fri 07 December 2007, 09:10
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Dex, I am not convinced that your needs are non-typical. You made the assumption:

"The existence of pre-machined V rails makes all the difference in the world to me. The idea of griding my own rails to a decent tolerance was the reason I was looking for alternatives. As my application calls for shaping medium to high density foam my tolerances are approximately a couple thousandths."

Pre-machined V-rails are available from BishopWiseCarver and Superior and now fit straight into the MechMate design:


See the bottom of this thread.

Neither vbx, nor BishopWisecarver, publish sufficient specs to decide that one system is more "accurate" than the other. (Nor do they publish sufficient specs to prove that a hand ground rail will be inferior).

However, it is known that the V-rail runs virtually free of clearance, is self-cleaning, lubrication is not essential and is available as true surface hardened (no chrome).

The straightness of both types of rails is determined by the person screwing them down.
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  #28  
Old Fri 07 December 2007, 12:28
bradyaero
Just call me: Greg #19
 
Smiths Falls, Ontario
Canada
That sounds great to me. I'm a new convert to v-rails If I raise the sides of the Y-Gantry to give myself more z clearance and extend the z-axis for more travel do you think that I can keep the existing design ? (I need 10-12 inches of z to do what I want to do).

Thanks, Greg
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  #29  
Old Fri 07 December 2007, 12:41
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
Dex,
I can't find the thread link at the moment...but the 9 nov 2007 download of plans accommodates a longer stroke for just your foam cutting requirement. Additionally, changing the side rails from 7/8" channel to a taller 12 or 14" channel will give you the gantry clearance needed. Or, you can do as I have and made the side rails at 8" (to maintain the rack and pinion distance that matches the current laser cut parts) and make the center part of the table removable so I have a net "24 INCH DEEP WELL IN THE MIDDLE" that allows me to put billets of foam on the floor and carve the top of them and maintain the "standard" gantry clearance needed when cutting sheet goods. My table is set up to cut a net 60" x 120" and the longer z-stroke slide for those foam mold making days. Good luck. Sean
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