[Smiler]

Hi all.

Building a Solsylva 4'x2' router. I live fairly high up next to the sea and the air here can be very damp. For this reason I think MDF might not be the best choice to build the axes from. I have access to PVC sheet and bar at work which I could replace the MDF parts in easily. Not having had much to do with PVC as a construction material, it does seem to be a good substitute. It drills and mills well and takes a tapped hole no problem, glues and welds easily too. Weight wise it feels very similar to MDF, stiffness wise seems similar too, also moisture doesn't affect it in the same way. These are simply layman's impressions by the way.

The only downside I can see is possibly brittleness in cold weather. What do others think, is it a good alternative? I need something because my experience of MDF here is sheet one day, mush the next

Many thanks for any ideas.

Regards, Jeff.

[Geof]

Perhaps you should Google and see if you can find the properties of PVC.

Possibly you could find a copy of Modern Plastic Encyclopedia at work, it has all plastics.

PVC is really quite flexible, it is deceptive just trying to flex a small length because it appears stiff.

But I think a bigger drawback could be thermal expansion, this is much more than any expansion with a wood based structure.

I would suggest you look into something like the Baltic Birch plywood that is available at a (semi)reasonable cost.

[Smiler]

Hello Geof,

This is what I'm looking for. I'll look up the thermals as you suggest. I'd really like to use PVC if at all possible. Just done a little routing and boring with Forstner bits and it really does machine well.

I take your point regarding the flexibility totally. What I had in mind was to redesign the gantry, keeping the overall dimensions but as a hollow torque box type structure. Hopefully it would provide the stiffness I need.

Many thanks for the input.

Regards, Jeff.

[dansutula]

Smiler,
PVC is not a bad idea. But as stated earlier, you should consider the effects of differential thermal expansion with respect to the other long compents that may be attached to it, ie racks, ballscrews, linear rails, t-slot tables... etc.
You must also realize that thermoplastics such as PVC have are suceptable to creep. That is to say that they can relax, sag, and bend over time due to elevated temperature, and load.

If you can fab your components to relatively tight tolerances PVC can be bonded easily and effectively.
Good Luck!

[CanSir]

What about static build up on PVC? Could be bad for the electronics and make clean up a real nightmare.

[paulb]

I've built a 3' by 2' machine making the gantry body entirely out of thick chunks of delrin. Delrin is stiffer than PVC but still very easy to cut, drill and tap - a breeze compared to ally and steel. With limited tools/space I could put a machine together quite quickly.

The machine mills MDF and perspex but I'm unhappy with the amount of flexing and I'm now replacing/adding ally parts. But, the wonders of CNC, the drilling of the new parts can be done by the machine.

Thermal expansion is an issue -I believe a 1m length will expand by about 1mm over 10 deg C.

In retrospect I would have tried extruded ally instead.

[Smiler]

Hello everyone.

Well not much progress so far. I've decided that I'm not 100% confident in building a sufficiently stiff gantry from PVC/ABS. I'm either going with Pauls suggestion of aluminium extrusion or steel box section. I will still use PVC for the ends of the gantry and the Zcarriage though as I don't think they will be a problem.

I am about to order my motor/driver kit from Zapp Auotmation and the steppers are Nema23's of 3Nm holding torque. What I'd like to know is what cable I should order for wiring these up. Zapp do an 8 core and a 10 core shielded. Would any of the cables on the Zapp site be ok for driving these motors?

I've managed to pick up two precision round rails/mounts and slides plus a very nice ballscrew for my Z axis so the bits are finally coming together.

Regards to all, Jeff.

[capnjack]

I would also check out the MSDS on PVC. It might be wise to were protection when machining/heating it. Could have toxic vapours.

There is a pvc varient that is rated/ approved to cary hot water. That might have different thermal properties.

The last thing that came to mind; is the end product going to see UV light, it doesn't last long in the sun. If thats an issue then the gray electrical pvc might be a choice.

Capn

[CarbonKevin]

I'm going to second on Baltic Birch as a material. I would use a good sealer on the wood after the unit is assembled, then a nice shiny coat o' paint over that, and the humidity won't be as much of a problem.

If you want to be completely sure, though, go with aluminum extrusion.

As for plastic...unless it's fiber reinforced, like glastic, or carbon reinforced UHMW, I wouldn't bother.

Come to think of it, now, carbon fiber extrusions are hella-stiff and would dampen vibrations brilliantly...

[zeed]

What about using fiberglass?
You can make it thick as you wish,at 2cm thicknes it is solid as a rock...

[mactec54]

capnjack

Machining PVC does not give any Toxic Vapours that could do you any harm you have to burn it with a flame to a high temperature, when you see it turninig black from a flame that is when it gives off Toxic vapours/gas.

[capnjack]

Mactec54-

Thats good to know. PVC is so plentiful, cheap, easy to machine, weld, etc.. Thanks for pointing that out.

Capn

[wildwestpat]

Hi Jeff

Good to think about the shop climate at the outset. If the air is high humidity and salt laden you have a pretty tough environment to deal with. Yes plastics can be good but you need to get information on the actual materials. For example PVC is in reality only the gunge that holds the filler to gether and it is the filler that gives it stability. When you get onto laminates the gunge is reinforced with all sorts of materials from metal through galss to carbon fibre. However cost will rapidly become an issue.

You have a cheap source of some PVC type material that sounds as if it has a filler. Why not use this for the chassis with all the bearing slides made from stainless using stainless ball races in DIY trucks. A bit of careful design should ensure the accurate measurment bits are attached to the stainless steel tracks and slots in the PVC used to accomodate the differential expansion. This would give you a nice machine that should survive beach life and be very comfortable in a nice workshop.

Further more the metal tracks could also protect agains droop in the PVC over time.

Just more ideas to keep you awake at night!

Regards

Pat