El Beast - Initial design phase, comments and critique welcomed!

**Chaz** · 25-09-2017

Awesome work.

Some questions.

What 3D printer / material are you using?

In terms of your bed, you always seem to have screws for workholding in the correct places. Are you drilling the holes (or milling them) and then drilling / tapping into whatever material your bed is made from on your mill? I struggle with workholding at time yet you make it look easy.

Thanks

**Zeeflyboy** · 25-09-2017

I'm using my trusty old makergear m2... I've had it for about 5 years now and it's still churning out lovely prints. I actually designed myself a new super duper printer which I'll make one day once this CNC machine is done, but that's really just because I want a bigger beast with quad extrusion and dual independent heads. Oddly enough makergear just announced their new version that does have dual independent print heads, but I would still want the larger build platform to make best use of them (I love the idea that you can print two sets of the same items simultaneously, effectively doubling output of parts if you need more than one of anything).

Material is some stuff called Cheetah by ninjatek. It's a flexible rubbery material a bit like hard skateboard wheels, 95A on the durometer scale... fairly easy to print with if you have a direct extrusion head. I believe it even works with some bowden printers.

Workholding wise for these big pieces I have been using a 12mm sacrificial sheet of SRBP, skim it level and then where the parts have suitable holes I run a "jig drill" drill cycle through into the bed that then allows the hole to be tapped and a screw inserted without moving the part. For the exterior clamps I either try to find a previous hole that works or just drill a new hole and tap. I keep going until either I skim down too far or it becomes like swiss cheese and then I replace with another sheet.

It's not the cheapest option I'm sure (a sheet that size costs around 30 quid) but it's a wonderful material for stability, machinability, coolant resistance and accepting threads/dowels etc.

**Chaz** · 25-09-2017

Thanks for that. Can you explain a bit more about how you might do the drilling / tapping into the SRBP in one step without moving the part?

Thanks

**Zeeflyboy** · 25-09-2017

Nothing particularly clever I'm afraid... I took some more photos today of the second carriage mount so perhaps an explanation with the pictures is best.

So on this part for example, I have some 5.4mm holes in the part for M5 screws.... After I've finished drilling/milling those holes while the part is held down using external clamps I would add an extra drill cycle using a mm drill bit on the holes I want to use as a fixture, setting the hole top as the stock bottom and the hole bottom as stock bottom minus eg 10mm.

External clamps, run the milling/drilling of internal holes to be used as hold down points, then run jig drilling cycle.

Once that's done I used compressed air to blow the SRBP dust clear, using a hand drill I whizzed a spiral m5 tap down through to the bottom of the hole and again give another blast of compressed air to clear the hole.

While the part is still clamped down with the external clamps, I then find some appropriate length screws (important that they aren't too long, thus preventing proper tightening down of the part, but you want to make sure they at least get a good 5-6 turns of engagement so that you can snug them down tight. It pays to have a nice assortment of various length screws in typical sizes - m4, m5, m6, m8 is what I tend to use.

Once those are tightened up you can remove the external clamps which lets you run the remaining cycles... just be sure to check your tool paths to make sure it isn't going to try to mill out those holes again as it'll find a bolt there of course (adaptive can be a little bugger for that, just make sure you have ticked "rest maching" and selected "from prior operations" so that it doesn't try to remove the same material twice).

If you've thought about it a little before hand and the part is suitable, you can use the same holes to hold it down on any bottom side operation, I like to use 6mm dowel pins in the bed to butt the part up against to ensure it's aligned to the axis.

With it held down, I find a suitable area to probe to define zero again

If possible I do a quick gross error check to make sure zero looks good:

And then run the rear operations:

**Chaz** · 25-09-2017

Thanks, excellent explanation.

I suspect you can also just set your retract height higher than the bolt height. It might waste a bit of time but better than cutting into the metal etc.

Certainly easier if you do rest machining or similar as you have said but you never know what path it will take when it crosses over. Always good to watch the CAM 'preview' before actual cutting IMHO.

**Zeeflyboy** · 25-09-2017

Yes absolutely, make sure if the bolts are sitting proud of the surface you set an appropriate retract height. I actually always just set it at 25mm as a matter of habit, it doesn’t waste much time at all as all moves back down to the top height are done at rapid’s speeds.

**Edward** · 26-09-2017

I just got a piece of SRBP to try. It's incredible that it's made out of resin and paper, it looks really tough and quite heavy. I only got a 400x300 piece, in 15mm, as it's quite expensive, I think you use larger sheets?. I am guessing that Tufnol is a brand name, but the same thing?

I just wanted to ask roughly how thin does it get before you discard it, with regards to having enough thread left in the holes with sufficient strength to clamp the piece, 8mm or so? If you use M5 bolts, I suppose you need a good 5mm of thread? Secondly, I assume that you don't skim it every time you cut a new piece, provided you have enough of a flat, clean area left for the next piece to sit flat, is that right? Also, what's your favourite tool for skimming the SRBP? Does it machine similarly to Delrin? (the only plastic I have machined).
Keep on posting about your progress, the more the merrier:)

Edward

**Chaz** · 26-09-2017

Originally Posted by Edward

I just got a piece of SRBP to try. It's incredible that it's made out of resin and paper, it looks really tough and quite heavy. I only got a 400x300 piece, in 15mm, as it's quite expensive, I think you use larger sheets?. I am guessing that Tufnol is a brand name, but the same thing?

I just wanted to ask roughly how thin does it get before you discard it, with regards to having enough thread left in the holes with sufficient strength to clamp the piece, 8mm or so? If you use M5 bolts, I suppose you need a good 5mm of thread? Secondly, I assume that you don't skim it every time you cut a new piece, provided you have enough of a flat, clean area left for the next piece to sit flat, is that right? Also, what's your favourite tool for skimming the SRBP? Does it machine similarly to Delrin? (the only plastic I have machined).
Keep on posting about your progress, the more the merrier:)

Edward

Its pricy alright, £60 for my size machine. That said, if its makes machining a lot easier, its worth it. Also keen to understand if there are variations of this material that might be worth looking at.

**Edward** · 26-09-2017

I paid £44.32 including the £15 for UPS, for the 400x300 piece. I guess if you ordered a few pieces it would just be the one-off transport fee. At least they provided an invoice, so I can discount it off my tax as a legit expense.

They also call it Phenolic Sheet.

Edward

**Desertboy** · 26-09-2017

At prices of Tufnol I'd glue it to a 12mm sheet of mdf then you can mill through the whole lot screw wasting any.

If you have issues with losing 12mm of travel on the Z then you really have issues lol