4' x 4' x 8" Router Build

[JonnyFive]

Thanks for the replies.

Graham:

If you are still thinking of the pinion drives then please be aware that the spur gear they use is a US imperial pitch size

I knew it was an imperial rack but had assumed it was of the standard DP type that isn't too hard to find? Do you know what pitch it is? What is the solution you have come up with?

routercnc:

1) An extra 1" travel will be fine to allow the machine to go slightly over the home switch/prox and reverse back to settle at the home position. Gives a small amount of room before any physical hard stop.

OK, I'll go with 50" travel, I'll round up to 1300mm as I'm a metric kind of guy.

2) When the Z is at maximum travel the collet needs to be close enough so that the cutting bit can level the board. I notice on your design that there is still plenty of Z travel left when nearly fully down, which is a bit of a waste. It also means when the Z axis is fully up/home it is still fairly close to the board which will limit the maximum height of the part you can cut. If you want to do vice work this gap looks too small. Most people work to 150 mm from a typical tool tip to the spoil board when fully raised. Yours looks considerably closer.

This is why I was asking so that I can adjust the offset between the spoilboard and the gantry, it's at a random arbitrary dimension at the moment which I agree looks too small. I guess my question should have been how much does the shortest cutter stick out from the spindle, so long as that just touches the spoilboard then I can adjust my gantry height from that point.

Just check this is what you want the machine to do. You will see other machines on here use vertical risers on the bed, or vertical extensions of the side members for the base frame to get the main beam high enough that the gantry then runs on.

Not entirely sure waht you mean by this, can you explain please?

4) I've seen racks shimmed in using a DTI mounted to the carriage of the rail, ensuring it runs parallel. Either use proper shim stock or aluminium foil if the gaps are not that big.

I can see the advantage of a ballscrew as there are only two points that need shimming.

One thing to consider is that the stiffness of that axis depends on the stiffness of the rack mounting so I wouldn't favour angle brackets and the like, rather a very solid shimmed mount back to the main gantry.

I hadn't considered this, it's a really good point thanks!

Best bit is when it is all built and moving you will be able to say JonnyFive is alive

This day is a long way off but I can't wait!

Cheers!

[routercnc]

Ok the vertical risers comment- I meant at the moment the side members that the long axis rails sit on would need to move up to push the gantry up and that it would be stiffer if the vertical parts of the lower frame were longer rather than finding a rectangular section which was very tall and slim. In fact I would probably not go with side members quite as rectangular as you have drawn them but make them a bit more squarish. Then get the height you need by making the base frame vertical parts longer. Very tall large open rectangular sections will start to lozenge when they get pushed sideways by the cutting forces. Away from pc so I can’t sketch something.

[JonnyFive]

Understood. They were originally 50x50x3 like the rest of the frame but I changed them to increase the second moment of area. I can change them back easy enough.

[GrahamWhite]

I bought my rack while the parts where on their way from USA, noobie error on my part! lol
I only matched up the 20 degree angle not the DP
End up buying new 60tooth gears with a hub on and new spur gears for mod 1.0 rack and drilled out and reassembled the drives, at least now if I ever need to replace the spur gear they're only £5 :)

[Neale]

Good luck - it's a fun process!

I built something roughly similar a year or two back - here. Probably more useful if you look at the "what I would do different next time" comments!

Joe's CNC guys go rack and pinion because it is a heavily US-based community. R&P can be bought from US manufacturers who don't seem to make ballscrews which generally come from China. Not a popular choice for US builders, unlike UK/Europe. Seems to be almost a fashion thing rather than technical choice. Both can work but unless you really need R&P, ballscrews are much easier.

[JonnyFive]

Thanks for your input Neale, just read your thread - great work on documenting everything so thoroughly!! It'd be nice to see some photos of the entire machine, looks like I'm heading along a similar path to you.

Interesting what you say about the R&P. My understanding of the ballscrew / R&P decision was that although ballscrews are more accurate & efficient anything over 1m should use R&P because the ballscrew whipping limits performance - is that correct? There doesn't seem to be much of a cost difference between the two?

[Neale]

Interesting what you say about the R&P. My understanding of the ballscrew / R&P decision was that although ballscrews are more accurate & efficient anything over 1m should use R&P because the ballscrew whipping limits performance - is that correct? There doesn't seem to be much of a cost difference between the two?

This is where we get into real design decisions! I'm using 2005 ballscrews. There are a few online critical speed calculators available; the general consensus for 1750mm, fixed/floating bearing, screws is around 900-1000 rpm. That gives me 5000mm/min rapids, which is slower than ideal but which works for me using a hobby machine. In retrospect, I could have used 2010 and run faster. However, I am also using 2x3Nm motors to drive these. Pulley drive, but 1-1 gearing. This is because the motors are just about reaching their corner speed (where torque starts dropping rapidly) at about the "whip" speed of the ballscrews. I'm not entirely sure (because I haven't studied other people's designs) if the motors would handle the load of my heavy gantry using 2010 as there is also the acceleration/speed trade-off. I doa fair bit of small fiddly machining where acceleration is more important in overall cutting time than speed.

All I'm really saying is that you have to balance up all the conflicting requirements for an engineering compromise that is right for you and your intended use. My design/build might not be optimal, even for me, but at least it works! There are plenty of posts one this forum with other designs successfully using ballscrews of this kind of length and I would happily do the same again.

[JonnyFive]

I bought my rack while the parts where on their way from USA, noobie error on my part! lol
I only matched up the 20 degree angle not the DP
End up buying new 60tooth gears with a hub on and new spur gears for mod 1.0 rack and drilled out and reassembled the drives, at least now if I ever need to replace the spur gear they're only £5 :)

Sounds like it might have been a costly way of doing things? I had considered designing my own version as there's nothing clever about it really but thought the costs would be more than buying from CNCRP.

Out of interest where did you get your aluminium extrusion from please?