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  1. #1
    Join Date
    Jan 2007
    Posts
    626

    How important are these?

    Was just wondering what others thought about the importance of these 2 design aspects.

    1. The spacing of the X axis linear rails. Obviously, the wider the better, to a point, I guess....ie Maximising the rail spacing on a single beam or moving to 2 maybe smaller beams to get your desired spacing.

    2. Trying to minimise the 'Spindle Overhang' by locating the X rails on the back of the Gantry and 'boxing' the Z axis.

    How do these features rate in importance and in relation to each other?

    We think about these sorts of things sometimes.....lol

    Steve

  2. #2
    Join Date
    Jan 2007
    Posts
    626

    Re: How important are these?

    I guess no-one has an opinion on this. No problem....I will try to incorporate both features in my new machine...one day hopefully.

  3. #3
    Join Date
    Jul 2018
    Posts
    6248

    Re: How important are these?

    Hi Steve - 1) The wider the better, but I think splitting a single gantry into two is a mistake unless they are very large which then counters the solution. I'd stay with one large gantry
    2) Then the z axis is still very cantilevered. That's why some machines put the rails on top and bottom of the gantry. But then this has to be machined to get it straight and parallel. No biggie if you have the correct machinery. You need a big CNC machine to make a small CNC machine. The cantilever of the z axis is not usually a problem if the bearings are correctly selected and wide apart.

    It the length of the Z axis thats the killer so on big machines the gantry goes up and down as well to keep the Z axis short.

    Large commercial mills and routers do not use both these strategies so I'd keep it simple and use conventional strategies. So what is your dream machine? steel, aluminium epoxy something? I'm now convinced to make a composite mill so I can make bigger metal and Tetrium parts...Peter

    https://www.youtube.com/watch?time_c...&v=f8nTNY0FO6o

  4. #4
    Join Date
    Jan 2007
    Posts
    626

    Re: How important are these?

    Quote Originally Posted by peteeng View Post
    Hi Steve - 1) The wider the better, but I think splitting a single gantry into two is a mistake unless they are very large which then counters the solution. I'd stay with one large gantry

    Thanks Peter. Ok, that makes sense. Its a comprimise on spacing, to enable easier fabrication, correct?

    Quote Originally Posted by peteeng View Post
    2) Then the z axis is still very cantilevered. That's why some machines put the rails on top and bottom of the gantry. But then this has to be machined to get it straight and parallel. No biggie if you have the correct machinery. You need a big CNC machine to make a small CNC machine. The cantilever of the z axis is not usually a problem if the bearings are correctly selected and wide apart.
    Putting the rails on top and bottom of the Gantry rail was going to be my stratagy, but I didn't realise that the T Slot would have to be machined flat. That may be the killer of the idea. Not sure if that service is avaialble in my area.

    Quote Originally Posted by peteeng View Post
    It the length of the Z axis thats the killer so on big machines the gantry goes up and down as well to keep the Z axis short.

    Large commercial mills and routers do not use both these strategies so I'd keep it simple and use conventional strategies. So what is your dream machine? steel, aluminium epoxy something? I'm now convinced to make a composite mill so I can make bigger metal and Tetrium parts...Peter

    https://www.youtube.com/watch?time_c...&v=f8nTNY0FO6o
    I am looking at T Slot and Aluminium plate machine.
    Something similar to this:

    Click image for larger version. 

Name:	Finished.jpg 
Views:	3 
Size:	172.4 KB 
ID:	430142

  5. #5
    Join Date
    Jul 2018
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    6248

    Re: How important are these?

    Hi Sterob - That's certainly a chunky machine. If your making a plate machine make the gantry from plate vs T slot. T slot is a very inefficient material see attached. Can you machine the plate parts yourself? I think people split the gantry into two because they think the extra spacing will make it better. But if you split the gantry and use two small members then its not as stiff as a big single. Especially if you use square rails as these take moments so one bearing will be dominant (usually the bottom one ) so it will take 90% of the moment and the top one will take 10%. If you have to split them its best to use round shafting and bearings so the loads are evenly split. But I wouldn't;lt use a twin gantry. cheers Peter

  6. #6
    Join Date
    Jan 2007
    Posts
    626

    Re: How important are these?

    Thanks Peter,
    Not sure if I can machine the Plates myself as I only have a 6040 at the moment. CNC Friends think I can.....

    What about using 2 large T slot beams butted up agains each other with an Ali plate across the back side? Overkill?
    Still not better than single plate?

    That would give good rail spacing and negate the need to mount rails top and bottom.

    Attachment 430146

  7. #7
    Join Date
    Jul 2018
    Posts
    6248

    Re: How important are these?

    Hi Sterob - The issue with a "composite" beam is the loadpath efficiency. Its difficult to predict the loadpath. One element of the beam maybe doing all the work, it won't load share evenly. Maximus' gantry back plate costs less then $500 so keep that in mind for your build as a benchmark. You can make a front plate to suit. The hole pattern suits a 15mm rail to minimise stack height of the Z axis. I can have a different hole pattern cut if needed or a different length. In this way the loads go into and out of the rail directly to the structure. Machining edge holes on large plates can be expensive depends on what machines they have available or if in home workshop patience and skill are required for good outcome. But if you place rails on plate edges the edges have to be machined on good machine in one hit to be parallel. Peter

    edit - if you epoxied the parts together then I think you have a chance at an efficient structure but see next post for suggestion. If you used Max's "gantry back plate" you could match a 12mm aluminium plate to the front and this would be an excellent gantry...

  8. #8
    Join Date
    Jul 2018
    Posts
    6248

    Re: How important are these?

    Hi Sterob - I suppose you are trying to make a machine that will cut aluminium? Does your current machine cut thin aluminium? say 6mm plate? If so you can laminate plates together to create thick plate. 4x6=24mm quite chunky. Use epoxy to join it, then finish profile it. Finish profiling is usually easy on the tool... This will be very stiff and damp due to the viscoelastic epoxy...Aircraft use a lot of laminated aluminium...I can also make slabs of tetrium for you but I haven't figured out its stiffness yet, designing a test rig to do that. Peter

    edit - for many many reasons I have been sort of forced to go down the moulded part route. But I don't have a large shed yet. Hopefully mid feb next year I have a large shed on my block so this sort of thing is easier. T slot extrusions are relatively expensive, convenient but structurally poor. Std Al extrusions are limited in availability and shapes for CNC parts. Steel sections are heavy but useful. Welding Al or St is problematic and needs stress relief. A "light" steel section of 150x150x10 (or 12) is a really good starting point. A section that is really stiff is 200x200x10 (or 12). So what do you want to cut and I'll put fwd some ideas? Width is important so how wide is the machine? I'm guessing this is your 1200x2400 sheet machine. So a 200x200x6mm steel SHS is required or a bigger aluminium section which is hard to get or use a 100mm front plate and the maximus SS rear plate. Can do it in mild steel just have to paint it. Where are you in Oz? I'm in Murwillumbah.

  9. #9
    Join Date
    Jan 2007
    Posts
    626

    Re: How important are these?

    Hi Sterob - The issue with a "composite" beam is the loadpath efficiency. Its difficult to predict the loadpath. One element of the beam maybe doing all the work, it won't load share evenly. Maximus' gantry back plate costs less then $500 so keep that in mind for your build as a benchmark. You can make a front plate to suit.

    Hi Peter,
    So you suggest something like this:
    Attachment 430216

    I suppose you are trying to make a machine that will cut aluminium?

    Yes. I want it to be able to cut Alumininium easily.

    Does your current machine cut thin aluminium? say 6mm plate?

    Err.....yes.....but its hard work, I think. Its a X61500GT. ( supported round rails for Y axis...unsupported round rails for the X and Z.Not the best....) Others say I would be able to cut new Ali side plates,but it would be very slow.

    If so you can laminate plates together to create thick plate. 4x6=24mm quite chunky. Use epoxy to join it, then finish profile it. Finish profiling is usually easy on the tool..

    Never thought of that. By 'Finish Profiling' do you mean run the tool around the perimeter to tidy the edges?
    I guess clamping the part in the correct position and getting the correct 0,0,0, point would be the hard part, for me....

    Width is important so how wide is the machine?
    Not sure yet....probably no more than 1000mm.

    Where are you in Oz? I'm in Murwillumbah.

    I'm in SW of WA...I guess the furtherest I could get from you....lol

  10. #10
    Join Date
    Jul 2018
    Posts
    6248

    Re: How important are these?

    Hi Sterob - Do I smell that bad?? Yes your diagram is correct. Now you have the bolts and rails transferring load directly into the structure and the structure can be sized correctly. Yes finish profiling is as you described but you can make the original plies with a 2mm allowance so zeroing is not such an issue. With a soft machine its the DOC that's usually the issue, so you will cut your plies slowly. Then glue the plies together then profile and drill. Another option is for me to send you some Tetrium to play with. I'll be making some slabs in the next week or so I can send a chunk.I'm looking for impressions and feedback on the stuff. I cut it easily on my router yet Scoot complains and mucks up when I cut aluminium. I've stopped trying to cut Al on Scoot that's one reason for me to develop Tetrium and make a mill. Peter

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