Definately a scrapyard challenge

[motoxy]

Hi guys. This is my first attempt at building anything like this myself. I have a history in motorcycles for 45 years and used to be a mech eng designer. Its a bit diferant without a full blown workshop doing your bidding. This project has a limitation on funds but I will not skimp if possible.

As it is the first I have decided to make a torsion box base. If this all works then I can upgade to a metal frame later. I also thought that linear rail bearings would be better than bearings on angles.

I realise that regidity is essential so the y gantry has been constructed from 10mm Ally. Although it came cheap the angle was not exactly 90deg . Bit of patience and it appears to be fairly accurate. When the rails arrive for it the will be measured off the x rails to locate. If it ends up being too heavy then I can cut sections out to lighten the load. It rolls well but inertia maybe a problem.

I have ordered ballscrew, 2 for the x and 1 for the y. My next decision is stepper size (3nm maybe) and whether to build or buy the z.

Comments are always welcome. Already had great advice on spindle choice.

Bruce

[Jonathan]

I too started with my X-axis rails mounted to a torsion box made from MDF/pine with a gap left either side to hide the 2 leadscrews. I'm still using the torsion box as the machine bed. Making the steel frame for mine made a huge difference. What size are your X-axis rails, they look like only SBR16 or 20mm?

Plate is weak for the gantry since when the machine is cutting parallel to X the cutting force will try and bend the gantry, and that's in the direction that's worst for a rectangular cross section. It's also the worst cross section to choose for twisting. Adding extrusion, or more plate perpendicular to it to the back would help, but you could try it and see and make sure you leave space to add it at a later date if required. What is the width of the plate, so I can calculate the deflection (roughly)?

I'd be inclined to add some triangular pieces to the aluminium angle to stop that flexing.

3nm is a good choice for the stepper motors. Lots of people, including me, use them. If you can afford 70 volt drivers (like MDS752) then you will get a higher feedrate than 50V (like MDS542), but it's more difficult/expensive to find/make a suitable power supply. The mass/inertia of the gantry is not a problem with these motors. Chip's and mine (and many others) using the same motors weigh a lot more.

Have you bought the ballscrews yet? If not then get 10mm pitch (RM1610) for X/Y to get a higher feedrate and use HTD timing pulleys (although 1:1 ratio is about right for those motors and 10mm pitch screws adding the pulleys still helps as it reduces resonance). 5mm pitch screws (RM1605) will work but it wont go anywhere near as fast, or more importantly the acceleration will be less and possibly vibrate more as the screw is spinning twice as fast.

[motoxy]

Thanks for the heads up guys. All the material is 10 mm thick and 150mm wide. I had wondered about the the plate for the gantry but thought I could add box/angle to the back if required. the rails are 16mm

Unfortunately I have already ordered the lead screws and they are rm1605. I really should ask first DOH!. I am going to look into ordering the motors controllers this week sometime.

The angles on the rails were left hanging for, just as you say, the support for the nut on each side. I also thought that this allowed for the Y screw supports to be further apart and thus allowing me to use more of the bed width.

I really appreciate all your help guys
Bruce

[motoxy]

x and y screws plus the y rails arrived today. Had a thought about the gantry design and have to agre especially with the angle on the rails. Looks like a bit of a rebuild required. I will also have to think about the z gantry.

[Swarfing]

Jonathan

Have you bought the ballscrews yet? If not then get 10mm pitch (RM1610) for X/Y to get a higher feedrate and use HTD timing pulleys (although 1:1 ratio is about right for those motors and 10mm pitch screws adding the pulleys still helps as it reduces resonance). 5mm pitch screws (RM1605) will work but it wont go anywhere near as fast, or more importantly the acceleration will be less and possibly vibrate more as the screw is spinning twice as fast.

He is using an MDF box and i think the screws he has ordered are much more suitable, you need to stop pressing everyone with the need for speed? shake rattle and role will not bode well here.

Bruce your screws will be just fine for the application you have there.

[Jonathan]

He is using an MDF box

So was I, and I am now though it's no longer there for strength. To start with that will be a limiting factor, but there's always the option to add a steel frame ... or even a simple rectangle on the bed made from steel box would help a lot.

you need to stop pressing everyone with the need for speed? shake rattle and role will not bode well here.

That's why I mentioned you'll get better *acceleration* with 10mm pitch screws, and less vibration from the screw as it's not spinning so fast. I agree high rapid speeds are pretty much useless, but good acceleration is important. When I cut the mayan calender using lower aceleration values (as I was limited by the computer) than Jazz did added over an hour to the time it took. That's quite an extreme example admittedly, but the general trend is true.

Yes they will work fine, but not as well as could have been.

[Swarfing]

The problem is every time you mention 1610 screws it makes it sound like everybody has to rush out and buy them?????? just remember to when you first started mister and how difficult it was. I use 1605 screws and belts on mine and it is more than enough for it's job, i could have my X axis twice as fast as yours with the belt but it would be pointless.

Thats it my rant is over and i'm stepping back from the computer for a cup of tea.

[motoxy]

I can appreciate both sides of the argument. My mistake was getting confused on some of the other sites and deciding to go it alone.....and then finding you guys. This is not a quick project and will take some time. I have many commitments between my work as a landscaper and my wood turning as well as other Things. Need to decide on the z so I can order parts for it. Probably just use a plate with 2 rails I still have some of the 10mm plate left. I will have to order some rails and a short ball screw.

[Jonathan]

The problem is every time you mention 1610 screws it makes it sound like everybody has to rush out and buy them??????

In my opinion 1605 has very few advantages over 1610 for a router, which is why I recommend it so often.

just remember to when you first started mister and how difficult it was.

When I was designing my router, the rotating ballnut design was far more difficult that picking the ballscrew, but that's just my opinion.

I use 1605 screws and belts on mine and it is more than enough for it's job, i could have my X axis twice as fast as yours with the belt but it would be pointless.

Even if you manage to get 60m/min, it will only be for a short time before failure. Realistically it's impossible. In terms of speed and acceleration 1610 screws outperform 1605 by a large margin. Even hypothetically running a 1605 at 2:1 is still inferior in performance to a 1610 at 1:1 due to higher K.E.

I started with a 2mm pitch screw on Y and 2.5mm on X since that was the biggest I could afford at the time (not ballscrews), and I made a rotating nut for X to compensate and that funny wonky bearing drive, that I wish there was a simple name for, on Y to lower the friction.

[Swarfing]

Bruce i apologies for tramping your thread but things like this puts doubt in new comers minds and they then abandon their projects because it gets too expensive. Advice on your Z though is definitely not go for 1610 screw enless you are using high powered steppers. The greater the thread angle the easier it is for the stepper to not be able take the weight. Please do not ignore Jonathan's advice as he has a lot of experience learned first hand. Just stick to your budget and if it is not enough for what you really need then save rather than waste it on buying the first thing that comes along.

A great effort on your build so far