CB CNC Part 19 – Cutting Z Axis Top Motor Plate From Acetal 12mm

This is a piece of 12mm Acetal. It’s very similar to Delrin although this is the slightly softer version. Derlin is meant to be more duarable (but only slightly). I’m going do a practice cut of one of my parts – redesigned parts for the CNC machine. I want to this plate here with a mount so I can actually fit the proximity sensor onto it properly, instead of cable tying it the way I’ve done it there. This is just a test. What I am planning to do is to drill the holes out first, the mounting holes, and then I’ll screw the block down onto the MDF and then I will cut out the profile shape.

To do this in bCNC I positioned the bit above the area I want  to probe, and then set the x, y and z coronadites. I then set the probe along z direction to -10 and made sure the contact plate hadn’t moved away from the incoming bit. Once they made contact and had stopped I type the command G92 Z1.51. G92 sets the position of the bit end, and z1.51 is the thickness of the touch plate along the z axis.

I raised the bit, returned the dust shoe and ran the first stage of the cuts.This is the visualisation you’ll see if you use bCNC as well.

So far no problems.

Ok that’s seemed to have cut ok. I’m just going to press the home button now. So I move the spindle away from the work piece, and that’s just going to give me room to get the drill in and to screw the piece down. Its’ doesn’t look like a bad cut.

Ok, done.

So I could change the bit to a larger one, to do the remaining cuts. I’m not sure how confident I feel about doing that. That feels nice and soft – nothing feels like it’s melted. I think I’ll just stick with the bit as it is. I’m going to put these back. Just moving to the origin position.

I have to probe that again. It looks like it’s in the right position on the x and y but the z is a lot higher. So I shouldn’t press home next time I need to move the spindle out of my way. I re-probed the bit, and exported the next cuts from Artcam.

It’s come out really nice. This material feels like Teflon (PTFE). Before I unscrew it I’m just going to push the bearing in and that’s gone in perfectly. That is a really good sign. I haven’t missed any holes. I, 2, 3, 4 – 12.5 … 12.4 that’s pretty good.

I tapped the holes for the stepper motor brackets and began dismantling the original z motor plate.

I think when I build this again properly, and do the rest of the plates I don’t think I’ll be using these stainless steel machine screws. They are just a little bit too soft. The high tensile 10.9 just feel a lot more rugged.

So there’s always something you overlook when you are designing and it’s only when you start to put things together that you realise what that was. And in this case it’s this locking washer which is meant to go between the buts on the proximity sensor. And this is just a little bit too low (close) by about 1 or 2 mm so I will change that on the design.

I was planning on using 8mm acetal to keep costs down but I decided to go with 12mm, because it’s thicker and will be more rigid.

But it’s also the same thickness as the original plates, which will means I don’t have to change too many parts when upgrading – such as the spacers which I’ve already cut to size.

I’m really happy with this test piece. It’s cuts well and feel very rigid – it even looks like halloumi. As  for whether it’s a good material for CNC plates, only time will tell. If it’s good enough for the wheels I feel like it will be ok for the plates. So this is copolymer acetal. It works out cheaper than Delrin which is a homopolymer. In case you are interested I ran this job with a spindle speed of 15000 RPM, a federate of 620mm/min, the plunge rate was 200mm/min, step down was 1mm, and the step-over of 25% of the bit width.

I bought the test piece of acetal online form ebay, but will probably buy the remaining material from RStools or theplasticpeople. So I need to go make some more changes to the Fusion 360 file now.

Ok so I’ve just made some changed to the plates. I’ve changed the thickness to 12mm. I decided not to use 8mm Acetal, and to go with 12mm instead. But since I’ve done that I’ve had to change the positioning of the proximity sensor opening. I’ve adjusted this one here, I had a few areas where I didn’t have constraints on some of the openings, and they were shifting so I’ve changed that. I’ve also done the rear one as well. So it’s more or less in the centre. It’s not perfect but it will do. I’ve done the top as well. Ignore that hole, I have to get rid of that. But essentially I will be drilling the holes for machine screws to trigger the proximity sensor manually. As the design is now I will just have to do it in the centre. And the last thing I need to do is the Y plates. You can see that’s a little bit off. I right click, edit sketch profile. Rotate around, ah shit.

I want the proximity sensor mount to shift towards the slops and not push the mounting holes out of alignment. I would make a change and press control z and control y – trying to spot anything that had changed which would effect the overall design.

I’m going to pause it while I figure this out. Ok I’ve just fixed  a load of lines and I think that has solved the movement. I’m just going to change that back to 8mm. Enter.

Ok that’s moved, I really need a bigger screen. I press control z. So you can see, this is weird. You can’t see everything on my screen because I am cropping down to 720p. It’s really weird that the constraint is not on those sections there because I don’t think I’d have deleted it. Ok screw that, I’m going to pause you again while I work this out.

Lets see if this is going to work. I’ve deleted the lines and selected the ones I want to mirror, click on there, press ok. It looks better. Nothing looks like it’s moving that shouldn’t be moving. That’s ok. It’s moving along a little bit but it looks fine. So if leave that at 8mm, stop the sketch and check the component. It looks a little bit better but maybe I’ve moved too much.

I think when you’re doing something like this, don’t be afraid to take pictures of the screen or even write notes for yourself because it can get very confusing. You are relying on your short term memory, and you know, you’ve got lots of numbers and lines on the screen – that ok, almost there. You might think you’ve move something to the left and you check and it’s actually moved to the right. It’s how you add errors what you’re designing. And to overcome that the best thing to do is try and keep notes.

Again ignore that hole, that won’t be part of the design. It will be something I cut out myself. I think that’s fine.

Ok these are the final changes, I thought I’d just cut straight to the end. I’ve added some curvature of the spindle mounts just to help support the spindle. I felt like it was getting a bit thin on the sides. This actually looks like an interesting feature, I feel like it’s quite distinct for a CNC machine. Also with the mini v wheels on the side, and there will be some at the bottom here. I’ve extended the material around the mounting hole just so it isn’t so close to the edge and that additional material, as with the gantry bit here should help to support the wheels a little bit better. So my next stage is to start cutting out the pieces. I’ve bought a 500 x 500mm bit of 12mm acetal, and a 25cm by 25cm square of 16mm white acetal. I need to work out how I’m going to nest the pieces so that I can get an efficient cut section by section. Anyway I hope you check out some of the other videos of the build towards this CNC machine, and don’t forget to check out my patreon page where I provide advanced or additional material accompanying my videos. Anyway thanks again for watching and you’ll catch me in the next one.

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