C-Beam CNC Part 3 – Making PTFE Anti-Backlash Nut Blocks and DIY ACME TR8x8x2 Tap

In the previous video I finalised and assembled my Y and Gantry Plates. I am now going to make the PTFE anti-backlash nut blocks which will attach to them.

As I am building a lead screw machine, I will need a block with the same thread as the lead screw to attach to all the moving plates. There will be two for the Y axis, one for x and one for the z.

The individual pieces would have cost about £40 altogether to buy. This piece cost £15 and I should be able to make 8 if I am careful.

This is the nut block that I will be re-creating, so I am not designing one from scratch. It is an open source design, and I found the OpenBuilds diagram which I then used to recreate the lines as vectors in ArtCam.

There is version available to buy which is made from Delrin, like the wheels but I will be using PTFE or what is sometimes referred to as Tteflon.

This material is manufactured by the same company who produce the material for the wheels, and it has the properties of very low friction, and resistance to high temperatures and chemicals, which makes it perfect for the task.

I then exporting the vectors as an SVG file to easel. I am going to cut the nut traps and holes out first, before screwing the block down to the waste-board and cutting the outline.

This way I can avoid using tabs, reduce scrap and cut up to 8 lead screw nut blocks from this piece of 100mm x 150mm of 12mm PTFE.

Ok I am going to see if the locking nuts fit, and they do.

This is probably the nicest material you can ever machine just because of how little friction it has on the bit. Seeing the success of the first nut block, I decided to cut the other three.

Ok let me check how these came out, 32.5, 32.46… I now need to secure them to the plates, and drill and tap the holes in the correct position. The original designed had a recess on the underside for some aluminium spacers, but I have not milled these out not bought those spacers.
I think I can alternatively use several wide washers to roughly position the nut block so it is in line with the centre of the bearing hole.To do this, I am going to place the y plate onto a piece of c-beam and attach the end caps with its bearing in place. Just by looking through the bearing hole I can tell I need more spacing washers.

These are two MM washers, and the ones I have already added on here are 1mm and that will create a 3mm spacing.

I have these transfer punches, which I can square through the bearing using two small engineer’s squares, and tapping the transfer punch mark the PTFE.
I had to do this a couple times before find a position that seemed correct.

I just flipped this the other was around to get a clean surface, the is just a bit too close to where the punch went and pulling it into the original mark. But I have also worked out that to the edge of the c beam my mark should be 32.40 mm in. So I will be able to take some measurements and work out where it should be using the vernier caliper.

While drilling the hole, I make sure to clamp some stop blocks to the table, to allow me to repeat the positioning with the other nut bocks.

I now need to make my own tap for the lead screw, which is a TR8x8x2 – That is an 8mm trapezoidal lead screw with a lead of 8mm and a pitch of 2mm. The lead being how far the nutblock will travel with one full rotation of the leadscrew. I bought a tap which had a lead of 2mm that I mistook for the pitch. Thankfully it was quite cheap, but I couldn’t find the correct tap and decided to make my own.

I did this by cutting a small piece of lead screw around 70mm, and turning it on the lathe while grinding a slight taper on one end. You could do this on a pillar drill with a file providing you don’t over apply force and possibly mess drills bearings. Remember if you use excessive force something will undoubtedly go wrong.

I set the lathe fence to guide a small multi tool with a metal cutting disk, so I could make two narrow lines on either side of the threaded rod. I then ground lines those with a larger disk, trying to carefully keep what will be the cutting teeth of the tap sharp. So with the taper facing towards me, that meant slightly tilted the grinding disk to the right. I then used a file to clean up some of the burr and also ground a square end to fit the tap on a handle.

The tap worked pretty well, although the teeth did tear the PTFE a little. Considering I’ve just made this by hand, and more importantly the movement on the lead screw is really smooth, I’m happy to drill the tap the remaining blocks.

I also drilled and tapped a hole for the tensioning grub screw, and then assembled a tester C-beam, to check if everything lined up. And lo and behold, the bearing holes and anti backlash nut block all line up.

So in the next video I will make some locking collars, and assemble the X and Y axis of the machine.
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