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cropwell
22-01-2014, 01:34 AM
I have been considering a rebuild of my MD machine and I have been musing on how to counter various problems. One of them is the crap falling on the leadscrew on the 'X' axis. It is under the bed of the machine. Would it not be better to mount the leadscrew above, so gravity helps keep the crap off it ?
If there were a sturdy gantry, you could hang everything off it like a travelling crane.

Is this a reasonable idea or am I too stoopid to see the flaws ?

I am also considering a design which pulls the Z axis as near as possible to the Y axis, with the stepper motor for the Z axis acting as a counterbalance to the spindle, to minimise twisting forces. This would mean a toothed belt in the drive chain of the Z leadscrew. What problems might there be here ?

I have already got a quote from Chai for the bits I need for rails and screws, and will probably start building in Late February, or when the bits get delivered. At the moment I am completely undecided how much of the original machine to use. It will probably be little more than the steppers and the spindle mount, though there might be some profile I can use.

I welcome your comments.

Cheers,

Rob

JAZZCNC
22-01-2014, 03:00 AM
I have been considering a rebuild of my MD machine and I have been musing on how to counter various problems.

Rob don't bother mate just sell it complete and start again.!! . . . . I've been down this road with Micheal marino who was doing the same and he'll agree with me you'll just be wasting your money.
For very little more money you'll build a far far better machine than an upgraded MD could ever hope to be.!!. . . . . You can't turn a Sow's ear into Silk purse no matter how hard you try.!!

Bit of advise is don't order anything until you have a finalised design and direction to head in.!

GEOFFREY
22-01-2014, 09:32 AM
Listen to Jazz's advice or you may be taking wrong steps in the wrong direction!!! Good luck with whatever you decide. G.

cropwell
22-01-2014, 11:35 AM
Hi Chaps,

There are (at least) two problems with selling my current machine. a. Overcoming the shame in admitting I bought it and b. I wouldn't wish it on anyone else ! The wee beastie is really only a collection of bits of ali profile and some trapezoidal screws. When I mean rebuild, I only intend to use the steppers, spindle mount and T slot plate. Some of the remainder of the machine might furnish profile and motor mounts, but the rest would go into my non-ferrous collection.


Chai has quoted me on supported rails and bearings, leadscrews, ballnuts and end bearings. I have requested they are only machined on the driven end, so I can turn them to the final fit. For the small cost of the rails and leadscrews, I have been generous in length allowance. The machine can only be 600 x 500 x 200 MAX. I have a MIG and 4 saws capable of slicing steel, as well as 4 angle grinders (there is a short but very boring story as to why 4 of each !).

On a small machine, the usual practice seems to be to drive the long axis with one leadscrew centrally under the machine. How problematic would it be to put it to one side ? Could any skewing be sorted with a parallel motion mechanism of wires and pulleys like draughtsmens' tables ?

My main thought at this early stage is 'why put a leadscrew where it gets shit thrown at it ?' Are there any major mechanical disadvantages to starting the design as a gantry with the long axis screw at the top ? I think that I probably ought to get some drawings done to clarify my thoughts.

Cheers,

Rob

routercnc
22-01-2014, 12:02 PM
Rob,

There's no shame in buying an MD machine, or any starter machine. At that stage you need to be sure this hobby is for you and that is one of the ways to do it. You will also learn a lot about what is important and what factors are required for a really good machine.

Many, myself included, have considered pulleys and wires to slave the other side but in the end I've not seen many builds like that. In the end with wire tensioning, stretch and setting up concerns etc. most opt for just buying another screw and belt slaving or double motors etc. I would suggest you do the same. At 500-600mm wide it be well worth it to go to double screw.

Going back to your thoughts - if you put a single leadscrew at the top in the centre, it will have to be very high to clear the Z axis and stepper. The drop to the gantry will be so long that you will loose considerable stiffness with the mount.
If you put a pair to each side, but still high up then you will have long supports up from the bed to mount the steppers to (assuming raised gantry design), again loosing stiffness.
If you put a pair to each side, just above the bed then you have a fairly optimal position in that the driving force is similar in height to the cutter. The stepper mounts are short and simple. I think if you use side shields down each side of the bed (e.g. perspex), or dust extractor for wood you will be OK.
You can also run the screws even lower down, outside the gantry sides and run the screw level / lower than the bed, and this also works. I think it is not quite as good as the previous for cutter force offset, but this might easily be outweighed if the mounting is more convenient and can be made stiffer.

EddyCurrent
22-01-2014, 02:14 PM
For the parts you intend to slavage, weigh up what they would cost new. Now you know what it's worth to you in bits, would you get more for it complete on ebay, so put it on there for a week with a break even starting price and see if you make a profit, if it fails to sell then strip it down.

cropwell
22-01-2014, 10:35 PM
I hear what you say Eddy, and I might go that way, just take off the motors and spindle mount and sell the frame. I don't actually use the T slot bed, there's always a sacrificial MDF board.
I want to keep the controller box I made.

I am probably going to have a twin screw long axis and would need to buy a stepper and driver. I have got 4 Astrosyn MY103H702 steppers sitting in a box (bought for a project which never happened - cos I bought the MD machine). These are smaller than the 57BGYH7630's on the MD machine, but two of them should be OK on the one axis, but it might be as easy to get another 57BGYH7630.

The driver is a Tonman PD2064M and I will need one to drive the slave motor - anybody know a supplier that is not in Exeter ?

Jonathan
22-01-2014, 10:44 PM
I hear what you say Eddy, and I might go that way, just take off the motors and spindle mount and sell the frame.

If you're going to eBay it, sell it as a complete working system and you'll get a lot more for it. CNC Routers seem to go for over the odds on eBay at the moment, regardless of the build quality...

irving2008
23-01-2014, 12:55 AM
Don't bother with the Astrosyns, they are too low torque and too high inductance to be useful in any reasonable machine. I too have a box of 4 that never got close to a machine...

JAZZCNC
23-01-2014, 03:19 AM
Ebay the lot and start again trust me on this you will only regret not doing it.!!

Irving's correct those motors are no use for anything much more than cutting air.!

Forget offset screws wires and all that rubbish again it's just doomed to failure and those that have done it have only done it to try get around the mess they made of first attempt.!! . . . . And that mostly that doesn't work so ends up getting scraped anyway. . . . .They just tend not to mention this bit.!!

Also depending what you want to cut and how machines built will determine if you need twin screws or not. If just cutting wood and softer materials then 500-600mm is no problem for a single centre driven screw machine. Harder materials or wider than 700mm then go with with twin screws.

m.marino
23-01-2014, 10:22 AM
Again building on what Jazz has said above, depending on what your cutting and width/length of the machine you could also look at rack and pinion set up. There are some pretty good sources these days for both and I have seen a few sources for helical which has some serious benefit in resolution over standard R&P set up. I have seen one set up in the US that has an inverted rack under the table, but the designer pointed out that crude still can get on the pinion without proper shielding.

As far as the MD machine, sell it on ebay as a whole unit. I sold mine to someone who wanted to do some poking around and testing and for what he wanted with the care to keep it completely flat it does what he wants. Anyone wanting to get very into serious CNC activity I would strongly NOT suggest any of MD's smaller line as I have yet to see any product that will not end up twisting itself or otherwise creating misalignment just from running. That is my experience and opinion. -Michael

cropwell
29-01-2014, 12:18 PM
Thanks for the comments Jazz, but I don't agree with the bit about the motors, they are certainly strong enough to rip the threads out of delrin and jam the whole axis. I am considering belt drives with a 2:1 reduction, so this would double the torque and halve the speed, but the leadscrews will have a bigger pitch anyway.

Flying in the face of advice, I have got the rails and screws I need sorted out from Chai. I have ordered longer than I will need (based on space considerations in my studio) , machined only on the driven end, as I will turn the floating ends of the ballscrews to the final dimensions. I am going with supported rails and probably a similar design to the MD machine (i.e. a gantry), but beefed up a lot. I can't get my head around Sketchup, so my design may be modelled in foamboard (cos I've got 15 A3 sheets of it) after the initial fag-packet drawings. Who knows, it may be rigid enough to run with it :distant:....

I have been looking around the web for design ideas to evaluate and in my travels I found the V-slot system at www.robocutters.co.uk (http://www.robocutters.co.uk/) . Intriguing (!), but I would have misgivings about accuracy and rigidity. However, it seems a good idea for 3D printers, that need speed, don't have to handle cutting forces, or move a heavy spindle around, but for CNC milling, I can't have any faith that it would be strong enough or that the timing belt and pulley positioning system wouldn't stretch.

At some time in the future I may build a 3D printer using the Astrosyns and the Digiplan drivers I have, but I am trying to get projects completed, not take on any more new ones.

Anyway chaps, Grandson #2 is about to be born, so I will be away for a few days.

TTFN

Rob

As a bit of a PS, I will be using the old machine to build the new, and then it goes on eBay, but without the current PC and the controller my be repackaged, but still a plug and play system.

JAZZCNC
29-01-2014, 10:01 PM
Thanks for the comments Jazz, but I don't agree with the bit about the motors, they are certainly strong enough to rip the threads out of delrin and jam the whole axis. I am considering belt drives with a 2:1 reduction, so this would double the torque and halve the speed, but the leadscrews will have a bigger pitch anyway.

I was meaning the Astrosyn motors not the existing ones.! . . . . Your idea regards 2:1 reduction is a good one if you don't need the speed and have a decent amount of volts on the motors.
Don't worry about timing belts stretching because other than initial settling in they don't if correctly tensioned and good quality belts.

cropwell
30-01-2014, 09:09 AM
As far as the MD machine, sell it on ebay as a whole unit. I sold mine to someone who wanted to do some poking around and testing and for what he wanted with the care to keep it completely flat it does what he wants. Anyone wanting to get very into serious CNC activity I would strongly NOT suggest any of MD's smaller line as I have yet to see any product that will not end up twisting itself or otherwise creating misalignment just from running. That is my experience and opinion. -Michael

I have beefed up the frame and gantry on the MD machine. The frame now has 4 sides of 40x80 profile and the gantry is cross-braced with a 4x100 ali plate. my concern with the machine is twofold 1. the ACME and delrin leadscrews and the unsupported rails which adds up to a lack of rigidity at the spindle collet. I cut mainly wood, plastics and aliand if I take it slow I get adequate results for my purpose, which is prototyping a breath controlled synthesiser (MIDI harmonica). Though playing with CNC has rather diverted me from my intended task.

The fate of the MD machine was decided when the Z-axis linear bearings dropped out during a cut and I realised how basically cheap the design was to use profile end caps to retain the bearings. If they had been a press fit, then maybe OK. But I have got the bug and want better.....

cropwell
30-01-2014, 09:38 AM
The machine will be sold as a package without PC as I want to build a new controller with 4 drivers and I can't get a matching driver for the existing 3 Tonman PD2064M's. I will repackage the BoB and drivers as there are 'features' on the kit that would be best taken off. So I am going to have to find out more about motors and drivers.

cropwell
07-06-2014, 11:47 PM
I don't know what I have done, but I seem to have a buggered BoB. Can anyone suggest where I can get one of these Controller-Simple Controller (http://www.tonman.com/english/html/jykzb_93_27.html) (outside of Exeter:wink:).

JAZZCNC
08-06-2014, 12:05 PM
I don't know what I have done, but I seem to have a buggered BoB. Can anyone suggest where I can get one of these Controller-Simple Controller (http://www.tonman.com/english/html/jykzb_93_27.html) (outside of Exeter:wink:).

I recently worked on a Sbox controller that used Tonman BOB and it had a fuse have you checked this.?

BUT has it happened the person who I was fixing it for had bought a BOB thinking it was Dead which it wasn't it had just blown the fuse so he's got a spare I'll ask him if he want's to sell it.! . . . . . He's on this forum so he may well contact you anyway.!

cropwell
08-06-2014, 12:19 PM
Yes the fuse had blown, but after replacing it the device won't spin the motors.
Yes I am interested in buying !

Cheers, Rob

JAZZCNC
08-06-2014, 12:26 PM
sent him an email thou he may not want to sell if they are a little flaky.?

cropwell
08-06-2014, 04:07 PM
The BoB worked fine until I squirted 5v up it's Estop.