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Building a new table, any tips?...

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Building a new table, any tips?...

Postby davek0974 » Tue Jun 16, 2015 1:54 pm

Following on from a recent thread of mine which highlighted the poor build of my current table, I need to build a new one.

I have a Hypertherm 45 and need to cut thinner material so I need high speeds and high acceleration and a firm Z-axis.

Re-using some of the stuff I have, gives me 3.1Nm motors for X&Y, matching microstepping drives, CandCNC MP3000-DTHC2 THC, pre-built Z-axis with floating head, Mach3 and Sheetcam.

Can anyone recommend a design style or plans that will give a suitable table to get the most out my new PM45??

Already proposed has been ballscrew drive with direct coupled motors and HiWin linear rails.

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Re: Building a new table, any tips?...

Postby davek0974 » Tue Jun 16, 2015 3:57 pm

Looking around, I see torchmate and precision plasma stuff uses 3"x3/8" flat steel for the rails, is HiWin linear stuff really needed in this application??

One uses ballscrews and the other uses rack and pinion drive, is high-helix screw needed here as I thought steppers were pretty limited on speed??

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Re: Building a new table, any tips?...

Postby SeanP » Tue Jun 16, 2015 5:02 pm

I used this rack from Zapp, I would use it again no problem, I worried it might not have been big enough once I ordered it, but is perfect no need for anything bigger.

http://www.zappautomation.co.uk/mod-10-en8-rack.html
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Re: Building a new table, any tips?...

Postby BTA Plasma » Tue Jun 16, 2015 5:33 pm

You will see a difference in the cut with precision rails. You don't need a ballscrew but it has its advantages.

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Re: Building a new table, any tips?...

Postby davek0974 » Wed Jun 17, 2015 5:33 am

SeanP wrote:I used this rack from Zapp, I would use it again no problem, I worried it might not have been big enough once I ordered it, but is perfect no need for anything bigger.

http://www.zappautomation.co.uk/mod-10-en8-rack.html


Thanks, interesting site, bookmarked ;)

Rack does seem the way to go, ballscrews need a very aggressive lead in order to get speed out of a stepper, looking at some charts on the web, it seems I can get 400RPM from my motors, seems a little slow?? That would need a 25mm lead on the screws which is pretty steep, does not seem a good recipe for accuracy to me, I may be wrong of course. The CandCNC site shows their ballscrew/stepper combo at 2000RPM no idea how they managed that though?

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Re: Building a new table, any tips?...

Postby davek0974 » Wed Jun 17, 2015 5:39 am

BTA Plasma wrote:You will see a difference in the cut with precision rails. You don't need a ballscrew but it has its advantages.


Is that with HiWin style or the round fully supported style of rail??

The most popular seems to be a flat section rail with bearing skates running on it with the racks underneath the outside of each rail, the motor pinion is used to hold the skate down on the rail, I'm guessing with some preload to eliminate lift/backlash?

This is similar to my current build which uses rail and skates but i unwittingly mounted the rails vertically as well as in the dirt zone - both bad ideas :oops:

Are TorchMate and Precision Plasma builds ok or is this a bad style to follow??

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Re: Building a new table, any tips?...

Postby BTA Plasma » Wed Jun 17, 2015 8:47 am

Hiwin, IKO, Thopmson, Bosch, NSK, SKF, Lintech, TBI style are the smoothest, most rigid and most repeatable. The round rail is nice but it will forever and always have a slop to it that will get worse. Its the nature of that style of bearing. Even if you go with a split bearing you are challenged to tighten it enough so it wont bind or gouge the rail. The ultimate rail has zero preload and zero slop. Reciprocating ball style with a profile like Hiwin EG or even HG are light years beyond cold rolled steel.

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Re: Building a new table, any tips?...

Postby davek0974 » Wed Jun 17, 2015 9:25 am

BTA Plasma wrote:Hiwin, IKO, Thopmson, Bosch, NSK, SKF, Lintech, TBI style are the smoothest, most rigid and most repeatable. The round rail is nice but it will forever and always have a slop to it that will get worse. Its the nature of that style of bearing. Even if you go with a split bearing you are challenged to tighten it enough so it wont bind or gouge the rail. The ultimate rail has zero preload and zero slop. Reciprocating ball style with a profile like Hiwin EG or even HG are light years beyond cold rolled steel.


Thanks, I'm looking for pictures of tables built with HiWin etc so i can get an idea of the relationship between the drive pinion/rack and the rail, proving a bit difficult to find ;)

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Re: Building a new table, any tips?...

Postby tcaudle » Wed Jun 17, 2015 8:51 pm

Stepper motor RPM is a function of the power supply voltage, motor inductance and the drivers. Generally the higher the voltage the more RPM you can get but BEWARE! The faster a stepper motor spins the less torque (acceleration) it can provide. At 48 volts you can get about 600 RPM out of a good hybrid stepper with a moderate load (more at no load but what use is that?). At that RPM you have very little torque. You need to operate the motors at about 50-% of the top RPM to have enough torque left for acceleration. So if you design your table to be able to cut up to about 300 IPM to handle thinner metal down to about 18ga then looking at the numbers you need to run the motor at 300 RPM so you need a linear ratio that gives you 1" of motion per revolution. (300 RPM = 300 IPM) Ballscrew would need 1 TPI Underpowered tables with heavy gantries have poor acceleration and that results in rounded corners and loss of detail in smaller cuts, A lot of designs will run out of "pookey" at about 100 IPM or less. You need at least 25 ips/sec accel to cut at 180 IPM and about 35IPS/sec at 225 IPM. Compare that to some designs that offer acceleration numbers below 10 IPS/sec and some below 5! You can of course do things like corner slowdown but its a bandaid and brings other challenges to the cut process (wider kerf, torch dive from THC and increased slag to name a few)

the 1" spec is easy to hit with a rack and pinion by using belt reduction

Ballscrews are nice but you will be paying for accuracy you can't use and you have to find a way to keep all of the nasty plasma dust off them

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Re: Building a new table, any tips?...

Postby davek0974 » Thu Jun 18, 2015 3:26 am

Thanks Tom,

So I'm going towards rack and pinion with 3:1 belt reduction.

A 20t Mod1 pinion will give me 62.83mm movement per rev (20mm pcd x pi),

9000mm/min / 62.83 is 143.24rpm at the pinion,

3:1 reduction at the motor will mean 429.72 shaft rpm - I think that is pretty well matched to the 410rpm that a motor calculator showed me based on 36v/4.2A/3.2mH motors, will also mean around 9Nm of torque on each side of the gantry (sounds a lot but I think more torque is good?)

Are my figures correct here?

Also, is it worth going linear rail like HiWin etc or using the flat plate and skate style?
Linear seems really nice but is it overkill, an expensive part so I need to get that right.

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Re: Building a new table, any tips?...

Postby davek0974 » Thu Jun 18, 2015 3:12 pm

Some encouraging test results...

In a moment of discovery, I thought I should try and find the limits of my transport system as it is now as a basis for the new machine.

It currently has one 3.1Nm Nema23 motor on each axis, each with 3:1 reduction on them, running at 36v 4.2A.

In Mach3 I pushed and pushed and gave up at 15,000mm/min and acceleration of 3000mm/s/s (0.3G) :shock:

I set up a home position indicator and deliberately tried to fudge it by bashing the rapid jog on the keyboard as violently as possible and it still returned home on the spot, amazing.

Naturally, the machine was far less than happy, it was shaking violently and the torch could be seen bouncing and wobbling all over the place, but the transport is amazing, so with two motors on the Y axis, even with a heavier gantry, things should be good still.

So, it seems my original transport design was 100%, let down by a lightweight chassis. I helped it a lot by fixing it to the garage walls but even these were shaking.

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Re: Building a new table, any tips?...

Postby davek0974 » Fri Jun 19, 2015 4:36 am

Ok, a specific question, looking at a cross-section of the gantry beam, what style will give me the best stability A or B??

beam.JPG
beam.JPG (44.07 KiB) Viewed 2478 times


Both have two linear rails, I was guessing to put two carriages on the lower rail and one on the top rail for type A or two on the front and one on the back for type B

I am looking at high speeds and want zero wobble induced.

Or this design which also keeps the z-axis mount very close to the beam face???
beam2.JPG
beam2.JPG (23.89 KiB) Viewed 2477 times

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Re: Building a new table, any tips?...

Postby davek0974 » Fri Jun 19, 2015 7:47 am

Looking at the main side rails now, likely 100x50x3mm box section, HiWin 15 only has M4 mounting holes so not good in side load support.

My idea is outlined in the picture - fit a strip of 3mm x something to the box, this could be drilled and plug welded or something, then bolt the Hiwin above using the plate as a support as recommended in tech specs, plus i can then fit the rack against the lower surface of the plate giving me a (hopefully) parallel relationship of linear rail and drive rack 8-) 8-)

beam3.JPG
beam3.JPG (41.05 KiB) Viewed 2472 times


I have no idea how parallel the sides of a strip of 3mm x maybe 30,40 or 50mm cold-rolled is, any ideas if it's good enough here??

Looks like I can fit a bigger build in my shop, hoping for a 4'x4' or 1200x1200mm cut area so looking at the racks and rails being maybe 1500mm long??

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Re: Building a new table, any tips?...

Postby davek0974 » Mon Jun 22, 2015 11:54 am

Ok, so a well known engineering forum is favouring a 20mm lead ballscrew, linear guides, direct drive, my steppers and drives etc, I'm still workington this though, not 100% convinced yet.

Also looking good is a single wide linear guide on the X axis with one carriage on in it, far easier than aligning two rails and less friction etc, rotational forces look good so should make a decent X axis, price is very favourable too, cheaper than two smaller rails.

Suggested to use 50x50x5mm all over as well, welded frame and bolting the motion frame so it can be shimmed etc.

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Re: Building a new table, any tips?...

Postby davek0974 » Wed Jun 24, 2015 2:14 pm

End view of gantry would look like this i think...

Two HiWin carriages, rail on top of support, beam welded to plate with gusseted corners...
gantry 3.jpg


The ballscrew would be on the outside face of the support.

End view...
gantry 4.jpg

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Re: Building a new table, any tips?...

Postby davek0974 » Sat Jun 27, 2015 6:36 am

Just to muddy the waters a bit more, someone mentioned the SwiftCut table, If i had the cash i would buy their 1250 table tomorrow! It is exactly where i want to be, has the full cut speed range I want, pity the pictures are not clearer;)

I see that they have R&P drive but use V-Rails all round, can't seem to find that stuff? I have read that they are self-cleaning?

I like the below-table idea too for the transport, places more twist stress on the gantry support but obviously works.

The motors are not much above what i have - they use 4Nm steppers and I have 3.1Nm, both hybrid so thats a good sign.

Now, can i build that table with Linear rail or does anyone know where I can source the V-rails????

Really need to get this project moving now, too many options.

Common sense is nagging me to just follow the commercial builders and use R&P but....

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Re: Building a new table, any tips?...

Postby acourtjester » Sat Jun 27, 2015 7:20 am

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Re: Building a new table, any tips?...

Postby davek0974 » Sat Jun 27, 2015 7:34 am

Thanks,

I have just been given a link to a UK supplier, way more expensive which sucks, almost equivalent to a linear rail when you take into account the cost of the rails, rollers, mounts, even worse if you add in the time to drill and tap six million mounting bolt holes!

Maybe stick with linear is better!

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Re: Building a new table, any tips?...

Postby SeanP » Sat Jun 27, 2015 4:37 pm

Is it not more expensive going with ballsrews Dave? the rack I got seemed good value I thought
What way do you reduce the gearing if you use ballscrew?

I looked at those swiftcut machines as close as I could when building mine, like you say they're not cheap, but you would want that if you were building them.
I'm happy enough with what I got for 1/3 of there price, I reckon it's stronger, far better water table and cuts just as good.

I had a good look around for v rail and couldn't find anything then, it's a job to know which way to go I bet.
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Re: Building a new table, any tips?...

Postby davek0974 » Sun Jun 28, 2015 3:25 am

Hi

yes ballscrews are the most expensive option but also the absolute best for motion control. They do not need gearing in most cases, just choose the correct pitch of screw - finer pitch=less torque required, more force generated, more resolution, less speed

In my case i need a 20mm pitch screw which is quite coarse and therefore would need a fair amount of torque to drive it.

I was disappointed in the V-rail, its so cheap in the USA, but over here works out at the same sort of price as proper linear rail and blocks!


It really is a difficult decision but I think i have it now, almost.

Linear rails
Either rack and pinion or open belt drive(very similar to a rack)
Frame built from 50x50x5mm steel
Gantry rails high above bed with gap under
Some sort of downdraught bed or downdraught/water combination

Trying to build with the future option fitting servos and ballscrews later on

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Re: Building a new table, any tips?...

Postby jimcolt » Sun Jun 28, 2015 8:31 am

Ballscrews can provide accurate motion and positioning. They also can eliminate the need for gear or belt reduction if the pitch is fine enough. When you look at the best cnc plasma cutting machines (start at about $80k and go beyond a million dollars) you generally will not see ballscrews. This is because 1. they are expensive, 2. As they get longer in length the diameter is increased to minimize torque twist effects) 3. To overcome torque twist you can "spin the nut instead of the screw....complicated). 4. The weight of the screw adds to the rotating inertia, dramatically affecting acceleration. 5. Did I mention they are expensive?

Large precision industrial CNC plasma's use precision rack and pinion with a variety of mechanical backlash devices. Some use precision helical geared rack. The rack and pinion makes way more sense on machines with more than 4 feet of travel. Ballscrews can be an acceptable solution on short travel machines. Properly engineered a rack and pinion machine is every bit as tight and accurate as a ballscrew, and in most cases can have better acceleration characteristics (very important for plasma cutting) at a lower cost.

One of these days I'll shoot a short video of my 2 x 2 Plasmacam running at 1000 ipm with the acceleration turned up to 1G. It is impressive!

Jim Colt

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Re: Building a new table, any tips?...

Postby davek0974 » Sun Jun 28, 2015 9:05 am

Thanks Jim,

I think i have chosen a path now and that is to go with linear rails allied with rack and pinion drive with spring loaded pinion mount.

I am going to try and build with the option of future upgrades in mind, but being a steel frame means any alterations should be fairly easy.

That video would be very interesting to see.

On a good note, I cut one of my stock jobs from 3mm Aluminium today, first time on the PM45. Running at 4000mm/min and 40A the cut was amazing, practically no dross at all, I did suffer some blow-out on the ends of the cuts but I'm not messing with that until the new table is built.

It's something that plagues me on steel too - perfect lead-ins with no burn-out but the end of the cuts are bad, especially so when the drop falls out and you can't use a lead-out.

Sadly the cuts suffered from the speed-induced wobble i get on my Mk1 table, but they cleaned up with a file, this table is really only good down in the sub 2000mm/min speeds.

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Re: Building a new table, any tips?...

Postby acourtjester » Sun Jun 28, 2015 9:20 am

For blow outs maybe you should look at "path rules" to control your torch so it turn off before the very end of cutting.
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Re: Building a new table, any tips?...

Postby davek0974 » Sun Jun 28, 2015 9:30 am

Yes, I know all about them, well something about them ;)

I did start trying them on the 30XP but not had time on the PM45, I have one to slow down to 60% on small shapes, 60% on small circles but need to look into torch-off-before-end as well to try and tame the burn-outs and divots.

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Re: Building a new table, any tips?...

Postby SeanP » Sun Jun 28, 2015 9:48 am

Was just going to mention 'torch off' Dave it's very good in those cases, another way if you have room it to hold the drop out with a magnet to allow the lead out to cut if you have room.
Old hard drive magnet works well :)
https://www.youtube.com/watch?v=InPyY_CqVPk

Sounds like you are getting there now Dave, with some good advice along the way.
The ally cuts sound good, I haven't done any at that thickness.
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