CarBEN Model -- 1/24th scale model and 3D computer model for EV

[CFECO]

Very nice progress!

[Sven7]

Wow, i haven't been keeping up on this thread. Lookin' great. Forgive me, but are you planning on molding a fiberglass body from the sections or what?

[NeilBlanchard]

Yes, the sections will be glued together in a stack, smoothed and then the inside and outside will be fiberglassed; forming a composite sandwich.

The main hurdle at this moment is to devise some side rails with wheels for the PhlatPrinter 3 to hold the foam sheet straight. If the sheet can stay straight, then the second pass of the cutting bit will be in the groove of the first pass and then the heat won't build up and the cotton candy glob won't form.

Any ideas on wheels I can use on the side rails?

[JethroBodine]

Surplus Center always has some cool stuff kicking around. I saw these rollers and thought of your Phlatprinter. Might be used mounted sideways fore and aft of the cutting head.

[NeilBlanchard]

Thanks Jethro -- I wish I had seen this a few hours ago! I bought some fixed caster wheels at Lowes that should work bolted to and metal angle. Those strips might be better if I can't rig up something with what I have.

Edit: I'll return the casters these look much better, and they'll cost less even with the shipping.

I put together the 3/4's of the F2 section tonight, and I do need to figure out how to cut most of the way through. My brother called me about the longer bits, and I need to check them out.

[Rokeby]

Neil,

The conditions you're encountering must be making you crazy; so close,
yet so far!

What type of cutting bit are you using now?
Are its cutting edges parallel to the bit's axis, or do they spiral around the
axis?

I didn't anticipate the overheating and "cotton candy" problem that you are
encountering. Still, I'm willing to take a stab at a work around.

Of course, the low melting temp of the foam material is a major factor. You
don't typically have this problem with wood -- the feed rate is way too fast
and/or the bit is dull -- or thin foam sheets.

I'll guess that one problem is that as the bit advances into the foam
material, it is cutting around the half-perimeter on the leading side of the
bit, and this is causing the bit to heat up.

The bit may be dragging a little on the sides of the cut as well. This may
be made even worse by the foam swelling a little when it is locally heated. I
suspect that when the foam heats up the bit then has to cut around more
than its perimeter.

I'm thinking that the solution may be to cut a channel in the foam that is
wider than the bit. This would require making side-by-side passes, with
the second pass along the actual cut line. I'm thinking an offset of 1/2 to 3/4
of the bit diameter. Cutting out would be making a series of two-pass slots
separated by small tabs.

Rather than try to suction foam chips out of the cut, it may be more effective
to blow the chips out with a stream of compressed air, which may help cool
the bit as well. But you'd also need a larger, but highly effective vacuum
hood to contain the flying chips.

He's my reasoning. On the initial cutting pass, the bit has to cut around its
leading half perimeter just to make forward progress. Here's where the heat
buildup originates. And if the foam is swelling -- due to expansion of the air
in the closed cells? -- bit heating is accelerated. FIddling with bit translational
speed and rotational speed may reduce the rate of heat buildup.

At any rate, when making the second cutting pass along the intended cut line,
the bit would be cutting around much less than half its perimeter. Also there
would be an air space on the non-cutting side of the bit. I would think this
would lead to less heating of the bit, more opportunity for cooling, faster
clearing of chips, and the probability of a much cleaner cut line.

I hope this makes sense!

[NeilBlanchard]

It makes total sense! I am looking to improve the 3 or 4 critical things:

I ordered 4 of the rails with nylon wheels that Jethro linked to -- this is the key point both for the bit overheating, and for the cutting accuracy. The upper part of the shank is where the cotton candy is forming when the second cut has wandered from the first cut.

My brother Nathan told me about a company nearby that has some longer bits that should let me (safely) cut through the sheet, and hopefully they have flutes that are 2"+ so they can cut and clear the foam if it does wander (a bit). My ideal bit would be 2.5-2.75" (63-70mm) long, ~4-5mm (5/32"-3/16") in diameter, have a cutting edge that is 2 1/8" (~55mm) long with an up spiral flute. I am waiting to hear back from them.

My current bit is ~2 1/4" long and the up spiral flutes are about 1 3/8" long and are 4mm in diameter. (Sorry for the mixed units!) I am only cutting down about 1 3/4" which makes removing the cut pieces from the sheet quite messy and problematic.

The third thing that I'll probably need to do is rig up an air nozzle that blows directly at the top of the bit. I have also cleaned out the filter on the central vacuum system -- it is very powerful, but obviously it works better when it is not got a ton of fine gunk in the filter.

The fourth thing that I can do is shorten the lengths of the cuts, and set it to cut through the foam, and insert the tabs. If the bit lifts out of the foam frequently, and if the cut lengths are kept short, and if the dust can drop out of the bottom of the groove, then I can get quality smooth edges (and even then increase the cutting speed), I will be on my way!

It will be GREAT to get started on building the chassis of CarBEN EV5! I'll need to learn about fiberglassing, and I need to design and build hinges for the doors, the hood and the front wheel skirts. I'll need to buy a windshield (and the wipers) of a Smart ForTwo and fit that into the chassis. Then I need to find a front wheel drive front suspension with manual steering. And the rear suspension will probably have to be custom fabrication -- probably trailing arms with a lateral link and coil springs.

Then a driver's seat (in the center), steering wheel, dashboard, etc. -- and then a dual motor direct drive drivetrain with a 50kWh+ battery pack in the floor!

That's all... :-)

[NeilBlanchard]

A small update on CarBEN EV5: one step forward and two steps back on using the PhlatPrinter 3, unfortunately. It was working but I was having trouble with the sheet of foam skewing i.e. drifting so the second cut pass was not lining up with the first. I think I have the solution for that (side rails with rollers to hold the sheet straight) and I redid the layout of the parts on the sheet to reduce the length of the cuts, and to add attachment tabs so the bit lifted up out of the foam regularly to both cool it and to help clear the dust.

But, when I tried to run the revised sheet, the X-axis direction was "compressed" to about half of what it was supposed to be.

I had bought a stronger stepper motor on the X-axis (that I understood would still work with the Planet CNC driver board), and the stock motor was quite warm to the touch after this brief aborted run. The X-axis is doing a lot more work than the other two, since it has to move/accelerate the entire sheet of foam.

I decided to then install the stronger stepper motor but the motor pulsed about 3 times, and blew the fuse. So, I reinstalled the original stepper motor, replaced the fuse (2.5A) and it blew the fuse after only a second of being powered up.

Also, I have figured out that the deepest I can cut is ~1 7/8", due to the limits of the Z-axis rails on the gantry. I can live with this, but I have asked Mark and Trish the good people who designed and built the kit for the PhlatPrinter 3 whether a revised gantry could be made with ~2 1/2" of Z range could be made. This would allow there to be 2 1/4" of cutting length on the bit, so ~1/16" would stick through the sheet, ~1/8" would be above the sheet so the dust / cuttings could be cleared out, about 1/16" of straight shank out of the chuck, and there would be about 1/8" clearance on the topside when the bit is traversing.

[JethroBodine]

Current problems not withstanding, would flipping the sheet over and "mirroring" your cut page be accurate enough to cut through from the back side? Setting your cut depth to 2/3 the thickness of your sheet. Or even 3/5.

[NeilBlanchard]

Hi Ken,

That is something I had considered in relation to doing 3D shaping on the edges of the sheet. If the side rails were fixed and the Home/Zero point was accurate enough, this might work.

I think though that if I can get a bit that has a cutting edge the is ~2.125-2.25" long and if the gantry can be modified to get the Z-axis motion up to ~2.375-2.5" then that would be easier and more accurate.

At this point, I think my controller board has a fatal flaw in the X-axis channel, and I need to buy the 10A version of the board:


(click on image for link)

The plate underneath is the heatsink.

The stock board is rated at 2.5A, and the cooling for the output stage is much less robust:


(click on image for link)

My board has heatsinks with less surface area, though they did not get very warm either.