Gavztheouch's Mk2 Laser

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Gavztheouch's Mk2 Laser

Postby gavztheouch » Wed May 23, 2012 10:36 pm

I thought I had it cracked, turns out I need to go back to the drawing board to fix backlash issue in the y-axis. So this will be my build thread as I attempt to integrate ballscrews into my design and get the backlash down to a usable level.

I am building a 40w 1.2m*0.9m laser cutter with servo motors and a Lightobject dsp.

Here are a few pics of the laser setup with the old y-axis HTD 3mm belts. The run of belting was chopped down to about 300mm as a test to see if it would reduce the backlash, which it did suggesting it is possibly belt stretch

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P1020827.jpg
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Re: Gavztheouch's Mk2 Laser

Postby iGull » Thu May 24, 2012 8:26 pm

gavztheouch wrote:Here are a few pics of the laser setup with the old y-axis HTD 3mm belts. The run of belting was chopped down to about 300mm as a test to see if it would reduce the backlash, which it did suggesting it is possibly belt stretch


Hi Gav

Worrying, that's a similar type of belt I'm using (from motionco - HTD 9mm wide 3mm pitch , aluminium pulleys). After replacing my slotted shaft couplers for the same ones you are using (marchantdice), my backlash virtually disappeared. I say disappeared in the loosest sense, there is obviously still some, but it's barely visible (it's measurable of course- approx 0.1/0.2mm 'ish). To be honest, visible is more important to me than measurable these days as I no longer build parts for the hubble :D

I'll follow this new thread with interest, are you considering trying HTD in a different type of material? If so, then I'd be interested in any improved results (so I can change mine if it works, or let you spend the money if it doesn't :lol: )

Ballscrews should prove interesting, high helix screws are certainly not cheap, and in the size you need, they'll need to be rigid - you won't get away with little teflon coated jobs at 6mm diameter and 20mm pitch :D I'm guessing min 15mm in X?

Good luck with the mk2, it's looking good. I do fancy another build, slightly smaller perhaps (700-900mm X) and with decent quality slides like yours. I have a huge THK linear slide sitting in the garage, but it's just too hefty for a laser - more suited to that heavy duty cnc router.

Cheers

Neil
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Re: Gavztheouch's Mk2 Laser

Postby gavztheouch » Thu May 24, 2012 9:39 pm

Motionco HTD 9mm wide 3mm pitch that's the very same, I got my pulleys from there as well.

Im thinking of giving it one more go with the belts, this time I will use some aluminium for the pulley supports and bolt them onto the side of the Misumi extrusion instead of my rather woolly 9mm MDF top. This should stiffen the assembly up and take some of my worries about the chassis flexing under load away.

Currently, to tension the belts I am only using my hands and a screwdriver as a lever to pull back the idler pulley and then lock into place once I have moved it far back enough. Do you think it is possible to get enough tension in the system using the method, to give you an idea of how tight the belts are if I pluck one it rings like a guitar string, but with the motor locked I can still move the carriage back and forward as the slack is taken up. I like the way Maralb (a poster in this forum) uses a simple screw system to tighten his belts.

Immagine 017.jpg


I have found some ballscrews from Zapp automation that have a lead of 20mm and a diameter of 15mm these look perfect for the job and cost around £80 plus £65 for the ballnut each. Then you need support bearings, about £100 for enough for both screws. So around £400 with delivery, a lot really, and more than I can currently afford right now.

Another issue with the ballscrews is if one of my motors takes a flaky and takes off on its own, it will cause the gantry to twist and no doubt break something, if not bend the critical x-axis extrusion. To that end I will prob need to look at linking the two motors together with a chain or belt.

Just to add, I recently read about a belt driven router that had a backlash figure of something like 0.003mm. I know it is possible, its just a case of time, money, and know how all of which I need to acquire.
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Re: Gavztheouch's Mk2 Laser

Postby iGull » Fri May 25, 2012 8:00 am

Had a quick look at the belt specs - not that much info really - mine aren't overprinted with a manufacturer.
As far as I can see on their site, tension should be 100N. Without getting involved, let's call it 10Kg or 22lb if you are metrically challenged. That should be easy to pull by hand without resorting to a screwdriver lever :-)
Similar specs at gates show +\- 0.03mm at 1200mm belt length variation (I'm assuming manufacturing variation).
As for stretch, I don't know.
Perhaps someone with higher mechanical knowledge can suggest what a ballpark value might be for HTD 3M style (not the company) 9mm wide, 3mm pitch. I believe the core is glass with neoprene casing - maybe kevlar is better (although I can't think why kevlar would stretch less than glass).
As an aside, gates have a wee tension gadget where you pluck the belt and it listens for th tone - you put in some values and it tells you what tension is. I have an app (free I think) on my iphone that I use to tune my guitars, maybe that would work - given a similar overall length ?
Need to ask the guys at motionco methinks.

BTW, locking your two Y leadscrews together when both are being driven is not a great idea. However, driving one and belt to the other is ok - but you're then back at square one :-) Its really only a problem when one fails (note the word 'when' :-).
I know of lots of systems that use two motors 'though - shopbot springs to mind.

Cheers

Neil
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Re: Gavztheouch's Mk2 Laser

Postby gavztheouch » Mon May 28, 2012 12:45 pm

As far as I can see on their site, tension should be 100N. Without getting involved, let's call it 10Kg or 22lb if you are metrically challenged. That should be easy to pull by hand without resorting to a screwdriver lever.


Hmm, If that is the case, tension is probably not the main cause of the errors and its more down to the wide of the belt. I found this picture of a similer sized epilog laser in there service area of the website. As you can see their belts are huge.

epiplog_wide_belt.png
epiplog_wide_belt.png (35.11 KiB) Viewed 3124 times
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Re: Gavztheouch's Mk2 Laser

Postby iGull » Wed Jun 06, 2012 8:01 am

Those belts look about 30mm?

Like most things in life, I didn't give belt width much thought for mine - just picking something that was available rather than researching what would be best (cost is a huge factor of course :D )
I suppose (like resistors) a large x sectional area will give less 'resistance' - in our case stretch.

That's what you should do then - buy wider belts and pulleys, fit, test and report back to us here :lol:

I can't imagine epilog fitting wider belts for no good reason (unless they buy stock on a one size fits all scenario where the price cut is high :-))) I'm assuming that they have found this a problem too.

Cheers

Neil
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Re: Gavztheouch's Mk2 Laser

Postby gavztheouch » Thu Jun 07, 2012 8:11 am

Sadly I have lost faith in the belt system, I added some inline tensioners to each side of the gantry and it made very little difference with the belts running at super high tension. So Im going to go with ball screws, if nothing else it will be an education installing them.
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Re: Gavztheouch's Mk2 Laser

Postby Liberty4Ever » Mon Jun 11, 2012 2:39 am

Sorry to hear you've lost faith in timing belts.

I use a friend's VersaLaser. With a 12" X 24" working area, it's somewhat smaller than your 1200 mm X 900 mm laser, but the VersaLaser does not seem to have any problems with backlash due to belt stretch, and it uses a tiny belt. I could take another look, but it looks like a much longer version of the .25" MXL timing belt used in the ORD Bot. It's a very light duty belt, but the carriage of the laser has very little mass and it glides easily with little friction. Maybe VersaLaser is doing some sort of backlash compensation? It cuts very round circles, though.

When I've finished my current projects, I'd like to build a 60W laser with a 12" X 24" or slightly larger working area, so I'm reading the build logs trying to get an idea of what works and what doesn't. Thanks for documenting your build.
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Re: Gavztheouch's Mk2 Laser

Postby gavztheouch » Tue Jun 12, 2012 12:06 pm

Ballscrews were ordered today at a cost of £400 plus vat..yikes, fingers crossed this works. There is still plenty of design and fabrication to get them onto my system, End machining of the ball screws looks to be the biggest challenge. I found this nice tutorial online that has given me the confidence to try.

http://www.5bears.com/cnc16.htm

I went with 15mm dia ball screws with a pitch of 20mm, hopefully this will mean I won't lose too much off my top speed.
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Re: Gavztheouch's Mk2 Laser

Postby Liberty4Ever » Tue Jun 12, 2012 1:10 pm

Thanks for that hobby CNC ball screw turning tutorial. Bookmarked! That was pretty much what I had envisioned, but the tips and tricks were excellent. I've been procrastinating the ball screw update on my CNC lathe project.

Too many projects, not enough time! Too much time on BuildLog.net? :)
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