100 Watt CNC Laser Installation

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Hey gang I'm building a cnc table for a laser engraver and I have the following question. I am installing newer more accurate servo motors. The old motors were nema 34, the new servos are not. This changeover will require that I use a belt to connect between the ballscrew and the servo motor. I can supply a picture to show why I have to go to a belt system but basically the larger servos are so large they dont fit in order to line up directly with the ballscrews.

So I'm making a mount that will place the servo over to the side of the ballscrew. The configuration I'm considering will use a 1/2 inch wide timing belt between two pulleys. The pulleys will be 1.5 inches in diameter, the distance between the the pulleys will be 2.5 inches from center to center.

Would anyone care to comment on the type of backlash I'll get with such a system? The overall accuracy of my table will be 10,000 steps per inch and if I'm adding a lot of slop in the drivetrain I'll rethink my plan.

Alternatives for more accurate belts would also be more welcome.

thanks,

owen

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What you're planning to do is the same thing done on some very high quality lathes with C-axes. Some of them position to 1/10 degree or less, with great repeatability.

I assume you have to calibrate the backlash compensation but, once it's done, they seem to work fine.

Ed Huntress



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If you keep your belts snug, the backlash via the belts will be in the small tenths.

The Very early OmniTurns had a belt running between the ballscrew and servo in Z, and would hold 2 tenths minimum all day. Id be surprised of one of the OmniTurn owners here didnt have one of the early ones, with the Z axis servo underneith the slide, rather than to the far end of.

Just be sure to match your belts with the pulleys.

Gunner

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Just as a point..if the encoder is separate and mounted to the screw, the backlash between the motor and the screw becomes insignificant. If you using a combined system then BF Goodrich (I think) makes a "0" backlash type of timing belt where the
tooth is trapezoidal and fits the pulley differently. Another point to consider is belt reforcement..there are several differ material available with different strengths. But as most others have said, almost any timing belt and pulley, properly tightened will work.

R. wink

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Hello Owen,
you are right to worry about the reflected inertia of the table but usually it is low compared to screw and motor...I dont know details in your case.

It is indeed a good idea to have encoder on the screw. This may still leave you with transient (dynamic tracking errors) if the timing belt is too compliant..the arrangement will of course look after steady state errors and get rid of backlash....depends on screw pitch and inertia of table. How the actual errors look will also depend on the way system..the amount of friction or damping involved.I looked at a robot drive many years ago that had this sort of arrangement.. a very large inertia rotary axis driven by belt and harmonic drive...the dynamics due to the belt were quite visible..and easily calculated,

regards

Ian

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I believe that you will get better results using the metric round profile tooth belts rather that the trapezoidal style belts. The round tooth belts have a smaller pitch, with more teeth engaged, for a given belt size. This results in less backlash within the teeth and higher torque capacity. For absolute accuracy I think that belt stretch is a larger factor than backlash between belt and sprocket. For that reason I wouldn't use a belt at the top end of it's load rating.

I have seen serious examples of backlash with trapezoidal profile belts. I worked for a robot company for a short while. They had some timing belt sprockets made in Russia. The Russians copied the tooth profile from a large diameter sprocket to a small one on an encoder. When you wrap a timing belt around a small pulley the tooth profile is substantially different! That encoder sprocket only engaged the belt through about 30 degrees so it introduced al kinds of backlash error.

Gary H. Lucas

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Owen,
A large part of our machines (in excess of 200) use a timing belt between the ballscrew and the servomotor on at lease one axis (usually x on lathes) We have no issues holding 20 micron tolerances with this set-up. As long as you have proper belt tension and the proper pulleys for the belt you are using, you should be fine.

--
Anthony

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Toothed belts, if good grade and run on good quality sprockets will have no backlash. You need to keep the belts under a LOT of tension to prevent it from climbing the sprocket teeth anyway. If you put the encoder on the end of the ballscrew, then there will be no effect on accuracy. The only error the belt might introduce would be cyclical error due to variations in belt pitch, and runout errors on the sprockets, if the encoder were also mounted on the motor shaft.

Jon

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Jon,
This is not true, you do NOT need lots of tension on timing belts, and all the manufacturers warn about that. In fact many timing belt drives are designed with fixed distance sprockets where you can't adjust the tension. If you are have a problem with timing belts jumping teeth using the specified tension for the belt, then you have a design flaw. Common design flaws for timing belts are things like too small of belt for startup loads which may be far higher than running loads. Insufficient number of teeth engaged is also a problem. This can come from using sprocket diameters too small for a particular belt size, or from having insufficient wrap, say a small sprocket that drives a very large sprocket and they are close together. These problems with timing belts happen a lot because timing belts in general are so good that they tend to be abused without the user realizing that is the case.

Gary H. Lucas

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Okay guys, for archival purposes I'm following up on my original post.
The responses seemed positive when I asked about the accuracy of a
timing belt and pulley system for the servos on a cnc table. The table
is a 18inch by 18inch ballscrew and rail system shown here:

http://nilno.com/laser_dir/cnc_table1.jpg

I spent a lot of time poking around Stock Drive Products:

http://www.sdp-si.com/

They have piles of cad drawings, pdf files specifying pulleys and
belts, and a straight-forward ordering system. Once I picked out the
pulleys and got the diameters from the sdp site I had to make an
educated guess about the length of the belts. I put the geometry of
the motors, pulleys and ballscrews in autocad and figured out the
number of teeth required for the correct length belt and ordered
everything up.

I could also then design up how much the slots of my motor mounts
should move around to adjust a nice tight fit, and made drawings for
the motor mount plates:

http://nilno.com/laser_dir/motor_mount.jpg

I faxed the drawing to a shop, got a quote, and the shop made the
parts in two hours. I was quite happy with the work. So my first
experience with you machinists is that you're alright. :-)

After I got the pulleys, belts, and motor mount plates I cut up some
sheet metal for straps and welded those up. Then I put the whole
kit-n-kboodle together this evening. Here's the assembled work:

http://nilno.com/laser_dir/motor_mount2.jpg

If you're interested in my project, it is development of CO2 laser
system that will cut sheet metal. I put together a blog and you can
follow my progress at:

http://nilno.com/laser_dir/blogger.html

I anticipate that it will be approximately one year before I'm
completely operational so be patient. I'm almost done with the
machining portion, I am getting some adaptors for optics made. In this
case I'm working with a shop that is much more experienced with lasers
and optics. The remainder of this month will probably be dedicated to
to construction of (my third) motor driver circuit. Hoo boy.

Thanks to alt.machines.cnc for the opinions and considerate answers
like I said I currently have a very positive attytood about
machinists. :-)

Owen

Had insomnia so I generated a list of categories to list blog entries by:
operation
cooling
optics
cnc table
enclosure
house wiring
ventilation
electronics
software
gas lines
product design
other

then I edited all my previous posts to include the categories in the title. Then I went back to bed.

I got this message from Gilbert:
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Dear Owen,

We received the components you sent to us between the holidays. Upon our evaluation, you will need the following components to complete the beam delivery system as per your requirements:

(2) Custom Adapters for Haas to LM delivery systems $ 110.00/ea.

Delivery, at this time, is 3-4 weeks. Terms are COD. Visa and MasterCard accepted.

Please let me know if you need any additional information.

Sincerely,

Gilbert J. Haas
President
Haas Laser Technologies, Inc.
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and called in the credit card order. Gilbert was very friendly. I asked him if the optics seemed like they would be packing the gear to cut sheet metal and he thought they should work fine, or possibly we'd put a new nozzle on the cutting head. Gilbert also said he had spoken with Kevin Radloff a couple times. One big happy family.

Built a work table:


Worked on the motor mounts:


And got cats: