Small Gantry Router

[mike os]

Well I did say I would be asking questions..... :pride:, this is my first machine build, reading the various build threads has given a lot of inspiration, hopefully enough to get the design right with a little (lot!!!) of help. This is a DIY "at home" project, and am looking to minimise any external machining etc. so lathe and hand tools mainly

Base for router, mainly wood, if I can get design right then a small amount of non ferrous as well (would be nice for small parts)

work table is 900 x 800 and about 220Z, so am expecting about 900x 700mm max machinable width, overhangs on rails sufficient to maximise work area and allow edge drilling at one end, clean pass through to allow longer if required

Structure is 50x50 and 100x 50 steel box, welded. Support rails for linear track is bolted to allow alignment with epoxy putty or shims between plates and if required tig brazed/tacked to secure. Ends are bolted on to improve rail stability and allow adjustment, bolted in place and the drilled & reamed for pins.


http://www.mycncuk.com/attachment.php?attachmentid=14073&stc=1

To me this feels over engineered..

any thought, suggestions or observations gratefully received

[EddyCurrent]

It looks a lot like my frame, but, there was quite a bit of discussion regarding where to place everything and I'll try to list them all;

1. don't position diagonal tubes right in the corners.
2. don't bring two diagonal tubes together in a point at the top
3. don't attach parts directly in line with other parts
4. diagonals across the bottom will remove the need for small corner braces in the bed

At the moment you have no support around the edges of the bed but if you rotate the 100x50 you will.
Don't use that supported rail, it's nowhere near as good as Hiwin type rails.
You might want to put adjustabe feet on the legs.


You can see what I mean by looking at one of these photos; http://www.mycncuk.com/threads/6565-Ready-Steady-Eddy?p=54294#post54294

[mike os]

thanks Eddy...more questions..

1, diagonals into corners why not?, ditto connecting 2 at the middle (sorry, I am one of those people that like to understand the why:miserable:)
3 I get as it eliminates resonance... easier to draw symmetrically for this stage...:stung:
Bed support is not a problem... just no need to show for the frame development... it will be there when made
feet will be on a level pad and shimmed and bolted down

Re rails... I have been looking for some real comparative data.... (similar to the info on round vs supported rails on the forum) from what I can find... only looking as I wont be buying for a while... profile rails & bearings seem to be about 2x the cost of supported rails ( 20mm supported compared to 15mm profiled but I am guessing I will want 20mm? which puts it nearer 3x) ie I need info to justify the additional cost (in all likelihood I would end up that way anyway, these were the first 3d model that I found & thought "hell that will do for development, regardless of what I eventually use)

[EddyCurrent]

thanks Eddy...more questions..

1, diagonals into corners why not?, ditto connecting 2 at the middle (sorry, I am one of those people that like to understand the why:miserable:)



Jazzcnc
Be careful when welding the diagonals where they meet in the centre, don't weld both at same time has you'll be putting a lot of heat in one spot.

I assumed same might apply to diagonals in the corners, also it was suggested by silyavski to put the diagonals so they were about 1/3 way up the legs and also supported the upper rail at more places than just the centre point so I put them at 45 degrees which spread them across the upper rail but not at the same place as the short upright stubs supporting the linear rails beam.

[mike os]

Assuming that the base is basically OK.... will update drawings in final draft for asymmetric layout

What is a reasonable weight for a gantry given the OP? planning on steppers for drive, I guess 2 rather than 1 and belts.
ballscrews or rack for Z.... or does it make little difference at this size

been playing in steel, bolted/glued rather than welded as I don't want to have to take it to be machined & weight seems very excessive...

I am Aiming for about 240mm spacing on y rails and at least 150-200mm on z, does this seem about right?

[mike os]

playing with gantrys... please ignore drawing errors....just reused parts to get some ideas together

1, steel, with welded beam (100x50 box), 4/6mm brace. Ends & bearing carriage are 10mm bolted and tacked or TIG brazed if necessary and bolted to gantry beam. gantry approximately 930 wide, weight approx. Mass approx. 52kg

[mike os]

Gantry 2
Still all steel, this could be fully bolted, which eliminates any heat distortion issues, again can be TIG tacked or TIG brazed if necessary
A little heavier, but easier to ensure no distortion and should be a lot stiffer? however this weighs in at 66kg!!!
Cant help feel that is getting a bit much, but I really don't know

[mike os]

Gantry 3

Steel ends, aluminium box and plate gantry beam, all bolted
This checks in at a more reasonable 40kg

[mike os]

Am I worrying too much over the weight?
Which design is heading in the right direction? or am I going the wrong way?

on all the above the COG is low and just forward of the middle of the gantry beam

[EddyCurrent]

Only the first version has a space for the Y ball screw I think. One good thing about aluminium box section is that it's quite accurate dimension wise, I put my rails top and bottom and using a vernier the distance between them was the same right across the gantry.

[mike os]

Hi Eddy

I thought if I used either of the others the ballscrew could go at the back? (behind the gantry beam...or should it be as close to y axis rails as possible?)

It is the dimensional accuracy that is tempting me with the aluminium section... one less thing to think/worry about, and a 15kg saving on weight cant hurt... or will the extra mass help with damping?

would a bolted aluminium gantry be stiff enough for light aluminium/brass work?

[EddyCurrent]

The ball screw on my machine is behind an L shaped gantry and it works fine. I fitted a plate to the front, it's all bolted and glued and I think it's stiff enough to cut aluminium okay. There are other designs on the forum more suited to metal, silyavski for example has a nice design; http://www.mycncuk.com/threads/6457-Sturdy-and-Fast-all-Steel-CNC-my-first-build?p=59577#post59577

I don't know if you saw this, if you are doing calculations, it's a pity more people didn't contribute;
http://www.mycncuk.com/threads/7155-stiffness-measurements-cnc-mk3?p=56572#post56572

[mike os]

yes I saw both of those, and like both designs, I spent several weeks reading various design/build threads before I started, but for a first build I am finding is a bit much to remember everything

will have another read of silyavski's post... my concern there is welding introducing unwanted distortion but think I might be able to come up with a suitable compromise :nevreness:

[mike os]

gantry based on 200x100 RHS, pretty much all bolted.... this has the benefit of simplicity, which I always like, no need to weld any of the joints, just weld in bolting plates to the ends, only drawback is closeness of y axis rails hence the layout

can I use hiwin rails in this configuration?

[EddyCurrent]

Yes you can use Hiwin rails vertical or horizontal, just make sure you can get them lined up exactly in both planes.

[JAZZCNC]

Don't stress over the gantry weight you'll be ok upto 80Kg with 2 motors using the correct pitch, motors,drives,voltage etc.

Regards the gantry then again don't stress. Any of the designs you have showed will be more than strong enough but the last one will be by far the easiest to make.
Most of the Steel based Gantry's your are seeing on the forum at the minute are hugely over engineered and quite unnecesserily so in my experience considering the relatively small 2.2Kw spindles they are using on them. Whether that be wood or Aluminium they are cutting.!

I mostly build small/medium routers using much smaller aluminium profile arranged in an L shape which I'm sure you have seen on the forum and they easily handle cutting woods soft or hard and light duty aluminium work. So I can assure you at this size your design using 200x100 RHS is still OTT for just cutting woods.
My larger 4x4 and 8x4 router design uses 120x80 in the L shape arrangment and it also easily handles any cutting forces that a 2.2kw spindle can throw it's way. At your width this would be ideal and massively strong for your needs and very easy to build.

So please don't get caught up in all the Hysteria and paranoia regards strength that seems to be sweeping the forum at the minute.! IT'S NOT NEEDED FOLKS.!!

Your design so far is fine and I'd only make a few changes to the design and Eddy as mentioned them both.

Don't let the diagonals meet in the centre if welding as there is too much heat generated in one spot.
Thou I don't agree about diagonals not going into corners.? Your better having the corner to corner rather than in the middle of a tube as there will be less pulling forces trying to bend the tube. But in both cases it's a not a big deal and much better than none at all.

Forget round type rail it's junk and profiled rail from China as come down so much in price it's not worth not using them.

Don't get too wrapped up in the design your nearly there, you have a good proven design that will easily fullfill your needs just iron out the small details and get building.

Good luck.!

[mike os]

Thanks Jazz :peaceful:

I did/do wonder about the requisite stiffness, but as I have no practical experience in this field I have to read, listen and assess as best I can... I am on a few photography forums and the nit picking arguments about lens quality etc is the same sort of thing I think... sometimes best just getting on with it and having fun rather than worrying about the tiniest improvements.

will make the changes to the frame in final ( hahaha who am I kidding... it will change up to the day building starts ) draft ... lots of stuff to learn/ work out how it goes together yet...but some of the builds here have really given me ideas... and tripled the original guestimate LOL

[mike os]

still plodding on... a question about the best stepper position for x & Y axis... is it better to connect directly or use timing belts? Obviously Z seems better belt driven but it does not have to deal with so much mass/inertia and helps to move the COG back towards the centre of the gantry ( at least that's how I am thinking?)

[EddyCurrent]

Use belts, they dampen resonance, and if need be, you can change the gear ratio later on by making one pulley larger than the other.

[mike os]

15 or 25 mm belts? What is the "rule" for minimum radius on pulleys for this kind of application

[mike os]

can anyone give me a link for Chai/Linearmotionbearings2008 cant find on ebay... put user name in the sellers field in advanced search and just get everyone

[cropwell]

[email protected]
[email protected]
It's one or both of these

[mike os]

thanks cropwell

[mike os]

more questions..... have I got this right?

ballscrew sizes... fixed & floating bearing for mounts (BK & BF?)
x axis approx. 1000mm between bearings, 20mm screw, 5mm pitch, critical speed about 2000rpm = 10m/min?
Y axis approx. 780mm between bearings, 16mm screw, 5mm pitch, critical speed in excess of 3000rpm=15+m/min
z axis approx. 400mm between bearings, 12mm screw.. critical speed<6000rpm

does this look ok?

[Clive S]

more questions..... have I got this right?

ballscrew sizes... fixed & floating bearing for mounts (BK & BF?)
x axis approx. 1000mm between bearings, 20mm screw, 5mm pitch, critical speed about 2000rpm = 10m/min?
Y axis approx. 780mm between bearings, 16mm screw, 5mm pitch, critical speed in excess of 3000rpm=15+m/min
z axis approx. 400mm between bearings, 12mm screw.. critical speed<6000rpm

does this look ok? It probably would be Ok with 16mm screws all round but in general for wood 10mm pitch would be used for x and y and 5mm pitch for Z ..Clive

[EddyCurrent]

If you order ball screws then refer to the 'standard machining' diagram and request dimension F = 30 mm, this will give you enough length to fit a pulley otherwise it's a bit short.

[mike os]

no where near ordering yet,,,, lol, I wish

I put 5mm pitch as it seems to be used a lot, any thoughts on 5 or 10mm pitch? not really worried about rapids as only a small machine

[EddyCurrent]

I was recommended 10 mm pitch for X and Y and 5 mm pitch for Z, I use a 1:1 ratio pulley/belt drive and it works very nicely.

[JAZZCNC]

more questions..... have I got this right?

ballscrew sizes... fixed & floating bearing for mounts (BK & BF?)
x axis approx. 1000mm between bearings, 20mm screw, 5mm pitch, critical speed about 2000rpm = 10m/min?
Y axis approx. 780mm between bearings, 16mm screw, 5mm pitch, critical speed in excess of 3000rpm=15+m/min
z axis approx. 400mm between bearings, 12mm screw.. critical speed<6000rpm

does this look ok?

Nah at this size don't bother using differant size screws just go with 16mm all round and use 10mm pitch for X & Y 5mm on Z axis.
Critical speed isn't an issue if using steppers as you'll never get stepper to spin above 3000rpm and have any torque left.

One reason for 16mm is the intertia is less so you can have slightly higher acceleration. Connect the motors to screws with timing belts and pulleys to help with resonance but the other reason is that if you find you need higher resolution or more torque then you can apply a ratio to increase.

10mm pitch gives a good balance of speed and resolution. 5mm is ok if you need high resolution but not so much speed. This is why 5mm on Z axis because resolution is more important in this axis. Also the larger 10mm pitch means the motor/screw is spinning half the speed of 5mm pitch for the same feedrate. IE: 5mtr/min on 5mm pitch=1000rpm compared to 500rpm for same feed rate with 10mm pitch.
This puts the motor in better place on the motors speed curve giving more torque while cutting and still gives half decent feedrates. It also means the screw is spinning half the speed so less chance of whip for same feedrate as 5mm pitch.!
For cutting woods and plastics correctly you'll definately want 10mm pitch on X & Y for the faster feed rates.