Thread: Wobblycogs CNC Mk1
Wow, thanks for the really generous offer, I think I'll probably be taking you up on it one I figure out all the parts I need.
I had noticed that metals4u were expensive but I hadn't started comparing prices yet. I checked out metal superstores as someone recommended them, the total order cost was less than a third. The downside is that metal superstore essentially don't deliver, it is technically possible to get stuff delivered but their maximum weight is so low I'd need to place about a dozen orders!
No problems mate...once you've set on a design PM me with the lengths etc and I'll go to our suppliers and get a price for the length(s) you need. I'll cut them up in my lunch hour(s) and let you know when I've cut them and you can pop over and pick them up.Neil...
Ok, last update to the frame unless anyone can spot something really wrong.
I've extended the X-axis rails to 1800mm which will mean a serious re-arrangement in the workshop, an extra support has also been included to up the stiffness. Bracing, in the form of 50x5mm plate, has been added to help prevent the frame from distorting. I think the bracing could be better but it should help once it's bolted down tight. The only other change is to move the long stretcher (at least that's what it would be called in woodwork) to the inside of the leg as I think it looks better that way. The weight comes in a at around 100kg which I can live with - I just won't invite anyone into the workshop :-)
That looks much better than the initial design. Your steel sheets will help a lot to do the job of the diagonal bracing pieces, but they could cause problems with resonance since it's a large unsupported thin area, but that should be easy to solve by adding some thin strips to them should it be needed.
Are you not having the height of the bed adjustable? It's a pretty big advantage in terms of rigidity, although I doubt it will be moved much...
1800mm for the X-axis is a tricky size for the ballscrews. The cheapest option is just to use RM1610, but due to the critical speed you'll be limited to about 6m/min using the standard bearing blocks and assuming the screw arrives without any bend. To go any faster, without rotating the nut, is going to require a larger diameter screw (e.g RM2510) or larger pitch (e.g RM2020), which gets expensive as you would most likely need Nema 34 motors.
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Before I rush off and redesign the frame again I'd best ask a few questions :-).
What is the maximum length length I could reasonably use RM1610 up to and still get a good feed rate? I see people talking about 7 and 8m/min as the top speed they run their machine at so 6m/min doesn't seem so bad. I suspect though that 6m/min on this length is like getting 100mph out of a Nissan Micra - possible, but not advisable.
Lets say for a moment that I find a few more pennies down the back of the sofa and I can afford RM2510 screws and Nema 34 steppers. Would I then have to beef up all the electronics as well? The 34's seem to be about four times the price of 23's (on Zapp) if that's also the case with the electronics I think it puts it out of reach. Could I use 23's on Y and Z or would they have to be 34's as well?
Am I right in thinking that the problem with the screw having a bend is a reduction in the critical speed? If I understand correctly the critical speed is the point at which the screw starts whipping around, yes?
Yes, the bed will be fixed for rigidity. On another thread someone suggested that for machining aluminium I'd be best off raising the piece up on a stand to reduce the Z-axis extension which seems like a simpler solution than trying to make the bed move up. I want to work aluminium but I don't see myself machining large pieces so a stand is a pragmatic solution.
The side sheets were there mainly to prevent chips flying everywhere but I'll add some additional brackets to the design so they act as bracing.
Just a quick Q Wobbly.... Am I right in thinking that you've extended your overall length 1500 -> 1800 in order to get a working length of 1220... It is possible to get working 1220 within an overall length of 1500?
I think it's possible to get a 1220 working length out of 1500, I'm not sure my design will be good enough to do it though. My reasoning went something like this: 1500 - 1220 = 280. That means that I can have a maximum of 280mm from the back of the Y-axis bearing to the centre of the spindle assuming there is nothing else blocking the movement and the front Y bearing is not ahead of the spindle centre line. That's certainly achievable but... with this frame design the legs will, I think, limit my maximum X travel to 100mm less than the rail length which would mean the gantry would have to be no more 180mm and that feels too narrow to me. To be fair I could probably manage it with 1600mm on X but I figured if I'm going to make it larger I might as well make it comfortably large :-)
Ah, the legs.... I missed those... And with the extra length you could space the bearings further apart for improved rigidity... So size does matter! ;-)
05-11-2012 #19So size does matter! ;-)
I'm having a love hate relationship with the legs where they are.
The two upsides that I can see are that I can have a more rigid gantry without losing X travel that wouldn't have been lost anyway and it gives me a 50mm recess each side that I can move the Z-axis into. I'm hoping that will allow me to achieve 800mm of workable length on the Y axis but I'll be happy with 700. The downside of course is that the machine has to be that bit longer in X. The longer X is though the small the loss is as a percentage of the total length - yet another reason bigger is better :-)
You should be able to fit the gantry into 280mm, maybe 300mm, quite easily, especially if you let the Z-axis stick out at the front. On my machine the X-axis bearings are 300mm from end to end, which is adequate. It's certainly good to increase that if you can, but not absolutely necessary. The same is true for getting the centre of mass to lie precisely between the bearings on X.
You can make a sufficiently strong Z-axis 160mm, maybe 150mm, wide so the frame doesn't need to be wider than the Y-travel plus 160mm. Again, if you want to make the Z-axis wider and space the bearings out more that's a good idea to get better rigidity.
Nema 23(/24) motors will be fine for the Y and Z axes. For X, try using irving's spreadsheet:
BUILD LOG: Wobblycogs CNC Mk2By Wobblycogs in forum DIY Router Build LogsReplies: 113Last Post: 10-02-2015, 12:37 PM