CB CNC Part 20 – Old CNC Machine Cutting New One From Acetal

In this video I will cut the redesigned plates for my CNC machine using acetal, and begin the process of replacing the original plywood plates.

I built my CNC machine with plywood plates at first to keep cost down but also as a test if the design was sturdy and accurate despite the plate material.

I always intended to replace the machine plates with ones made from more durable material, and I decided to go with acetal, mainly because it would be a lot quicker to cut than aluminium.

My first step was to cut the larger 500mm by 500mm sheet of 12mm thick acetal into smaller section so they could fit on my current machines waste-board.

I don’t have a nesting module on the software that I am using, so I had to do this manually by moving pieces around within the software until I was happy with the amount of waste.

Ok I’ve secured it down with three mending brackets. I’m going to drill out the holes first for each plate that’s going to fit on this piece of acetal. And then I’ll screw it down using these cup washers and screws, so each individual piece is held. Then I’ll remove these and cut the remaining cuts out.

This method allows me to cut pieces a lot closer together and without having to worry about larger clamps getting in the way of the toolpaths.

I can’t tell how  well you can see this but there’s a lot of different size holes. Now what I’m going to do is start securing stuff down.

One, two, three, four. I should be able to take these off now.

Because I set my origin position at the beginning I could return to that point, load new gcode files and make follow on cuts – cutting the plates out in stages.

I’ve secured each piece, so there’ll be four pieces I’ll be cutting out from this one block of acetal, with two screws and cup washers each.

Ok it’s cut out. Dust has built up. The extraction isn’t as strong as I’d like.

Before unscrewing the pieces I check that all the holes had cut out correctly, and also fitted a few bearings within their allotted recesses.

I also physically check the plates were identical by placing them over one another.

Well it feels like it lines up.

I am cutting the second piece of acetal now. I used the same method of screws and cup washers to secure the material. It’s worth mentioning between cuts I gently sanded any raised bumps on the MDF wasteboard caused by the screws. This will allow the acetal to lie flat against the wasteboard.

Once this had completed I decided to check the temperature of the CNC machine by using this laser temperature measurer. I’ve been cutting on and off for a couple hours now and well I’m just curious.

It’s over thirty.

The results are good on the machine and not too bad in the enclosure considering I haven’t put any extra cooling fans inside yet.

I then cut the two sides of the gantry. I didn’t redesign this part to use smaller v-wheels.

I’m going to have to clear up the back a little bit but it’s looking really good. Really nice material to work with. If you’ve never tried halloumi board I really recommend it.

So now I’m going to do this plate here. I am now cutting the z axis plate. This will be a little more complicated because I am going to change over to a larger 8mm bit to area clear a trench for the spindle mounting parts. The spindle mounts will be cut out from a 250 by 250mm sheet of 16mm thick acetal.

I measured this material with a veneer calliper and changed the dimensions of the trenches to compensate. I added half a mm to this, so the fit won’t be super tight. As long as the trenches are parallel this will help me fit the spindle mounting parts, so the spindle is square to the plate.

Ok here goes, the last cut.

I added half a mm to this, so the fit won’t be super tight. As long as the trenches are parallel this will help me fit the spindle mounting parts, so the spindle is square to the plate.

So that’s 16.1mm. That’s the same. The depth is pretty good, it’s 2.2mm. Wow, ok that’s what I put in. I’m bleeding all over the place.

I should check this piece fits in there. It does that way. But this side here doesn’t. So I can either leave it like this, and make sure I position the cuts so that the pieces that will fit onto the grooves are on the thinner side, or I can quickly run another toolpath while I still got the origin set. Let’s see what time it is? Do I have the time? I don’t think I’ve got the time to do that, so what I’m going to do is get some tape.

That’s ok, that’s ok, that’s not ok.

Thin, and thick.

Here’s the final piece. Now I need to decide what I’m going to do about the spindle mounting plates, and in particular the hole for the extraction tube. The redesigned files on fusion 360 have this a little further towards the spindle, just to make it look a little more squat. Obviously if I cut that out I’ll have to make the bloody dustshoe. And I’m not sure I can really be bothered to do that.

These are the spindle mounting parts. I decided to change the location of the extraction port so the dust shoe will also have to change.

Ok I took the first section out coz the tabs didn’t hold that well and I was worried they were going to move around. Lets just test this fits.

I made the openings for the spindle exactly 80mm – but when I originally cut the plywood plates I had shrunk this by ½ a mm or so. Annoyingly, the spindle is a little loose now. I don’t want to have to re-order new material to cut this out again so what I could do is either wrap some Kapton tape around the area of the spindle which I’ll clamp against, or glue some neoprene tape onto the smaller clamping section of the mount, although it’s notoriously difficult to glue anything to acetal. I could even drill and tape some holes through the side of the mounting pieces for flat faced grub screws, which I can use to tighten against the spindle. It’s not ideal but I have subsequently changed the fusion 360 and DXF files again in case anyone decides to cut these plates out. I’ll make the mistakes so you don’t have too.

 

Before I forget this is a tiny wafer of acetal. An offcut that was left between two of the parts. I think it was something like that. And I just didn’t realise how strong this material was until I picked this up and instinctively went to snap it, and I just literally cannot break it. I actually think maybe the thickness of the plates was a bit over kill for how tough this material is. If this was a bit of mdf or birch ply, just doing that would break it.

I decided that I needed to assemble as many of the plates sections as I could before I dismantled and upgraded the CNC Machine. If it turns out I made a mistake along the way, and need to recut something – well it would be very difficult with the machine in pieces.

I am waiting form some new wheels to come through the post but I was able to use parts from a cbeam linear actuator which I originally bought to upgrade the xcarve to test the machining and mock up a y plate.

I used my bench vice to push the eccentric spacers into their openings. And for each wheel I placed either an eccentric or 1/4inch spacer, a precision shim, a wheel and then a locking nut. I also used washers on the outside of the plate to make sure the machine screw didn’t protrude too far out and potentially scratch the aluminium C beam.

There are two types of wheels – which include Delrin and polycarbonate. Some of the Polycarbonate wheels actually fractured over time so I prefer to use Delrin.

I’m just looking down the aluminium extrusion, while I turn the eccentric spacer. Essentially what you want is, to be able to slide the plate along but to stop the wheel turning if you press your thumb or finger against it. You don’t want it any tighter than that coz then it will start top push the plate and the machine screws out.

What I found originally when I built the first machine, I used larger wheels at the top and even though I changed the spacing as I saw other people doing them, and as I was advised I was still not getting the wheels lining up as they should. It just makes more sense to stick with one type of wheel and to remove that problem from the build.

And the feet lift the y plate high enough so this doesn’t get in the way anywhere.

Ok, I though everything was going well and I suddenly noticed that there is an opening here that’s missing. When I tried to work out where this was going to go I ended up creating a new sketch, and somehow I forgot they were two separate section and I overlooked it. It’s quite faint but I’ve just cut the outline of this shape so I can pop it on top, and use that to then do the next cut.

That was lucky. It came out quite easily.  All the wheels, and parts and bearing as so on, have arrived. So I’m going to put them on to the different plates.

To speed up the process I use a vice to press four bearings at a time. I would do one side first, flip them over, place the precision shim between the two bearings before completing the wheel assembly.

I then made the z axis plate up next. Annoyingly two of the outer wheels on one side were just shy of the aluminium extrusion. I think while adjusting the eccentric spacers within the c-beam the plate shifted ever so slightly – and I didn’t anticipate this in the 3d model. I think AVE has a term fort type of tolerance and you’re welcome to guess what it is in the comments section bellow. Let’s just say you couldn’t cut a slice of cheese this thin, but you’re welcome to try and to send me pictures.

So I had to re-order some more 12mm acetal and I decided to also buy some 16mm as well to re-cut the spindle mounting parts.

So I’ve just cut a new plate out. The sight difference is these holes have been spaced in by about 1/2mm each, and that will just mean the wheels on the eccentric spacer will make contact with the c beam.

That’s going to be a problem. I need to counter sink these and use some other machine screws. Or I could buy some of these low profile machine screws.

I also assembled the gantry.

I thought I could make some PTFE washers to shift one side of wheels across so all sat properly on the aluminium extrusion, but in practice this resulted in a gap forming on the opposite side. I also tried replacing the combination of precision shims, spacer and two shims between the wheels with some 3/8 inch or 9.5mm spacer which I had from the x carve, but they weren’t any good either.

If I had an engineers lathe I could just make some the correct size but alas, I don’t.

I decided I should try assembling as much as I could before I dismantled the CNC machine. This included cutting out a negative of the smaller spindle mounting part which holds the extraction tube, which I could use to hold the piece while drilling its mounting holes.

It does feel quite close to the edge but the holes are only going to be 5mm.

I also drilled the tapped the opposite parts, and the mounting holes to the z axis plate.

I just put a bit of kapton tape there, and that’s a lot tighter now. If really really go for it, I can twist it but that’s pretty good. There is a little bit of a gap here, I feel like maybe the material opened up a bit after I cut it? So I took the tape off and it feels pretty good. It’s lining up ok. I mean really it’s all about this section here clamping downwards. I would have preferred it if it was a little bit tighter here. You can see a bit of light. The top is nice and flat. No light is coming through. I didn’t realise how translucent this stuff was.

This one I’m going to drill all the way through.

So previously the holes before tapping with a 4.2mm bit for the 5mm but this time I’v actually used a 4mm in the pillar drill so the thread is a little bit more pronounced.

One of the m5 clamping machine screws lost its thread so I had to replace those with M6 bolts. Tapping holes with a drill, even if you go slowly isn’t possibly the best method.

Ok that feels good. I was worried the spindle would be loose in the mount but the bolts applied a lot more pressure than I could by hand, and it seemed to hold quite well.

I’m just putting some 16mm flanged machine screw through this plate here, and it will hold the z axis.

In the next video I will being dismantling the CNC machine with the ply-wood plates and replacing those with the acetal ones which I’ve just cut.

As usual don’t forget to sacrifice a thumb to the algorithm gods, and let me know your thoughts in the comment section below. Thanks again.

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