Building a new table, any tips?...

[davek0974]

I was dreading the Alu job, used to use fine-cuts on the 30XP, but was pleasantly surprised at 40A

Moving on...

Torque and RPM...

Is it best to use a small drive pinion on the rack with higher motor rpm or larger pinion with lower motor rpm??

A 20t pinion will move 62.83mm/rev and at 3:1 reduction I will need 429.73 motor rpm for 9000mm/min travel.

A 32t pinion will move 100.53mm/rev and at 3:1 reduction I will need 301.58 motor rpm for the same speed.

I am fairly certain a larger pinion meshes better than a smaller one, but what size would be best considering my 3.1Nm motors.

My existing table is belt drive but works out at 100mm/rev travel and 300 motor rpm for 9000mm/min speed. When i did my high speed test it was running at 450 motor rpm so I know they can reach at least that.

I have a heavier gantry on the bigger machine, but also twice the motor power as i'm using two slaved motors.

Any views guys?

[davek0974]

Looking at direct drive now,

with a 17t pinion it seems i can keep the rpm way down in the high-torque area of the motors, lose the added losses in the reduction boxes, and get a simpler build as well.

I have also just secured a good package of larger 4Nm motors, DSP drivers and higher voltage PSU so I can keep my existing table going right up to the last minute - just swap the Z-axis and breakout board over.

Adding reduction boxes later would be easy.



Gantry material, Whats best here?

Needs to be about 75mm/3" tall to take the wide HiWin rail on the face, steel tube of 75x50x5 is easy to get and would weigh around 12kg for my 1500mm gantry. Or do i need/is it worth looking at aluminium box ??

Alu seems to only come in thinner wall section of 3mm from what i have seen so far.

Will a few kg make any difference here, especially as i have now possibly lost the weight of two reduction boxes by going direct drive????

[SeanP]

I'm sure 3mm wall is fine if using steel especially, I did add a strip just last week on the inside of 5mm x 12 to thread into for mounting rail/rack,,,solid!
Infact I might be tempted to use 2mm wall using that method if I was doing it again.

[davek0974]

Ah, so you have inserted a strengthener inside the box section ?

That's a pretty neat and easy idea, may copy that, it would make more sense to go thinner on box.

I think 50x50x3 for the frame and 50x75x3 for the rail supports and gantry will work ok.

[davek0974]

Bracing??

First frame design idea...

Frame1.jpg

Should i be thinking of gussets or bracing anywhere???

[SeanP]

Ah, so you have inserted a strengthener inside the box section ?

That's a pretty neat and easy idea, may copy that, it would make more sense to go thinner on box.

I think 50x50x3 for the frame and 50x75x3 for the rail supports and gantry will work ok.

It's not so much of a strengthener more to give a decent bite to the threads, being 3mm wall when I tapped that it pulled out of shape a little, but adding this strip with the threaded hole to match the rail and clearance holes in the 3mm box section could really nip it up tight.

Luckily I had a decent size hole in for the cable which let me slide it in, you should be able to tack it in position before welding ends into the gantry cross member, use at least 6mm there.

[davek0974]

Reality check time...

How do you decide what table size is the right one?

I know the 2'x2' I have now is too small, it works ok for most of my jobs but i can only cut with the plate square on the bed so no quick twisting it to get another cut out etc.

I can physically fit the 4'x4' (1250x1250) into my shop, thats ok.

I cannot lift a sheet of steel that size, have little possibility of lifting gear etc. as its only a double garage. A sheet of 3mm which is my most popular is about 36kg so only just about ok but the other one i use a fair bit of is 4mm and would be 50kg, no way i can lift that.

Would it be better to go 3'x3' instead?

Cost difference in build is minimal, but what makes sense - having the real estate and not using it or saving floor space etc??

[acourtjester]

Size is something us small shop guys have to deal with I have built a few table and ended up with a 4X4 that I kept for my use.
At 72 I too have a problem moving sheets around I cut mostly 1/4" so I cut the sheet into 2X4 size for handling reasons. Here is something that SeanP built that would be a help and don't break the bank either.
https://www.youtube.com/watch?v=ubyRYxWP4dU

[davek0974]

Thats some nice ingenuity there

I'm still leaning towards the 1250x1250 at present, of course, i don't have the space to use a handling system like Sean's

[davek0974]

View from the front of the latest gantry idea..

Gantry 1ai.jpg

This is for the more common open-bed style table with the longer gantry arms. The linear guides can be easily covered from the top.

The gantry beam will pass through the side support plates and be welded up and gusseted inside with some nicely cut parts, the outer face of the vertical supports can be strengthened with a couple of ribs to resist side-sway.

Mounting the support rails is harder on the open bed style though, I was going to bolt them on but this style really wants a box frame at bed level.

[SeanP]

You look to be getting high up there, you see so many builds a high gantry and a tall z when there isn't a need really.
What thickness are you using on the gantry sides?

[davek0974]

Its not to scale, the gap from bed top to beam bottom is about 175mm, I can maybe alter that by redesigning the z axis mount.

For the sides i was looking at 3mm steel but with gusset plates and stiffening ribs on the outside where needed. Pretty easy to tweak later if needed.

[davek0974]

Ok, stuff is on order.

The final result is 50x50x2 for the gantry beam and frame legs, 50x80x2 for the motion support/bed frame.

The final capacity is going to be the half sheet 1250x1250 / 4'x4' size.

Direct drive rack and pinion with a view to change to belt reduction drive or even ballscrews later on.

[davek0974]

Little bit messing with the CAD software, getting it into some sort of scale now, gantry side plates...

Gantry Side.jpg

The top box is the gantry beam, 50x50, the motor sits down the bottom on a plate that swivels on the point shown. It will be spring loaded but restricted so it cannot jump out of the rack and chew it up.

A couple of 75mm gusset plates under the beam on the inside and a rib or two on the outside should stiffen things up a fair bit.

[BTA Plasma]

Make sure you know where the front of your torch is to the end truck. If you want I can have one of my guys do a FEA analysis on it and send you a .dxf.

[davek0974]

Torch centre is 77mm back from the front edge, whats an fea analysis ?

[BTA Plasma]

Its a software analysis for torque..ect on the gantry sideplates.

[davek0974]

Its a software analysis for torque..ect on the gantry sideplates.

sounds good, look forward to any info:)

[davek0974]

Steel came Friday, nice and straight and after a few stress tests (me standing in the middle of a 7m long beam) I realised just how rigid this stuff is

Construction question:

I was thinking of welding on some 100x50x10mm pads under each corner of the motion frame and similar to the top of each leg, then bolting the two parts (welded motion frame/bed and welded leg assembly) together, just to give the opportunity to shim out any variation in leg length etc. Good idea, bad or not worth it???

Next, the bed/motion frame - should I attempt to weld it up as one piece, or weld closing plates flush into the ends of the cross-members, drill/tap and then bolt together?? If yes then how to weld the closer plates in - only way i can think is to heavily bevel the plates then insert and fill bevel with weld tying into the tube sides??? Plates would be 46x76x10mm.

Joint1.jpg

[davek0974]

Project drift again...

It seems that as I am now building a faster machine, my MP3000-DTHC2 THC from CandCNC will likely not be able to keep up with the machine.

I am now looking at getting the MP3500 EtherCut system, this will allow me to move away from XP and parallel ports etc as well.

Anyone here using the MP3500/EtherCut, is it good??

Supposedly a lot faster as it controls the system itself and not Mach3

Going to add a lot to the budget but I figure it might be worth it in the long run.

[SeanP]

I really don't find there is much wrong with the thc you have Dave, it's same as mine and it keeps up with anything I cut down to .8mm. It would need a serious breakdown before I would be replacing mine.

[davek0974]

Thats good news, thanks.

When you run the .8 what sort of speed would you be at ?? Book??


I could do without spending the cash of course but want this one to be the best i can get it.

I have just heard from Tom at CandCNC and there are issues with ethercut that cannot be resolved on a windows machine, it appears they are going towards LinuxCNC instead. Thats probably a very good thing as XP is dead now and Linux can usually be run from a USB stick so no hard-drive Early days yet though it seems.



BTW, how parallel are your sides Never having used linear guides before, I have no idea what sort of accuracy to shoot for for the main frame section.

[SeanP]

I'm away at the moment so can't check speeds for the thin stuff Dave.
As for the sides, have a look on the build photos I didn't weld the side rails they were adjustable so I could get them spot on, I would do the same again I reckon as well, would still weld the rest of the frame though.

[SeanP]

Sorry didn't quite read that properly.
You will likely have a little flex in the gantry sides so that might even it out, I think I got mine within .1mm at least.

[davek0974]

I see your construction method and it works well, however I only have four legs and four mounting points for the motion frame - no opportunity to pull any bow out, I can only shim up or adjust parallel.

The top of my slats are going to be flush with the top of the side frames - open plan style for easy sheet loading etc. I was going to make the motion frame a complete box - my table is smaller so i'm not sure if the frame-and-rails method you have is called for here??? Most small tables seem to have a welded or bolted frame at the top.