New CNC Mill on the horizon

[Jonathan]

Rough dimensions: 120 x 800 table size

Typo surely, 1200mm? Rest of my post is based on the assumption that it is 1200*800. What Z-travel?

Since you want to machine 'aluminium at a reasonable DOC' then an adjustable height bed, or very small Z-travel is critical. Clearly a small Z-travel is quite limiting for machining non-metals, so the adjustable height bed is preferable.

You should be able to get the profile rails on eBay for a good price - certainly a lot less than new. 15mm rails will be good compared to any size supported round rail since it is the pre-load that makes the profile rails more rigid. If you find bigger (20-25mm) rails then that's good, but it's not worth spending a lot more on them. Since you seem to have waited 5 years to post I guess waiting a bit longer for the right price isn't a problem!

You could try 4 rails on X as you suggested, I considered it for my machine. The problem with the supported round rails is that the bearing blocks are not preloaded, so even though load ratings are plenty high enough adding more bearings doesn't magically pre-load them so it will still be worse.

[fvfdrums]

Jonathan,

Thanks for the response, yes it was a typo 1200 is correct. I am not sure on the z travel at the moment I am looking to keep it reasonably short this was the reason for the adjustable bed. What would you consider a reasonable z travel based on machining aluminium?

So I could go as small as 15mm ok that helps I was expecting to need 20mm dont ask me why just thought that would be more the size.

I see you use those bearings on your machine, by saying they will still be worse than the profile rails what exactly am I likely to notice between using the two? (thats not me posing that questions as if im not expecting the profile to be better because I am just wanted to know what key differences in the machined end product)

[JAZZCNC]

If you have been lurking long enough you will have probably seen my dislike of round rail/bearings and complete louthing of unsupported rail.!! . . IMO really is no comparison between round rail and profile rail.! Profile rail makes the difference between DIY machine and Pro machine that will out last the machine and out perform round rail by magnitude of 10X.

To me if your serious enough to invest couple of grand and several months of hard graft then the extra cost is just not worth skimping on and like Jonathan says if you've waited 5yrs then don't spoil the broth for a pinch of salt and wait untill you can afford them.

Regards size then 15mm will more than handle the load but I find 20mm is the better option due to bearing size being slightly larger offering better support and easier mounting. 25mm is just over kill unless really large/heavy machine and waste of money plus has well has being inefficeint and stressing the motors more.

[fvfdrums]

Jazz,

To be fair I expected that comment and yes I do agree that profile is the way to go just got to find a good supplier!

Any suggestions?

[Jonathan]

just got to find a good supplier!
Any suggestions?

eBay is by far the cheapest place. Unsurprisingly shorter rails are more common so you may have to wait a while to find rails for the X-axis, but it's clearly worth it.

[routercnc]

Hi fvfdrums,

When I made my machine a few years ago I bought profile rails from this place on ebay. For example:
http://www.ebay.co.uk/itm/THK-SSR20-...item4165411fab

They are second hand parts, so you take a chance, but I ordered a pair at 700mm (Y) and a pair at 300mm (Z). 700mm was about £140 for pair of rails and 4 blocks, and 300mm was around £80 for pair of rails and 4 blocks. Vaguely remember paying import tax on one of these orders. They had a very smooth action and work very well on my machine although I only really cut plywood, liteply, and balsa.

[Jonathan]

Forgot to mention, those are all rails/blocks with 4 rows of balls which is what you want since their load rating is equal in all directions and the moment load is greater. Two rows is still fine if they're quite big and clearly still far superior to supported round rails.

Probably Stolen goods. . Lol

I better keep them locked up else they will be!

[JAZZCNC]

Forgot to mention, those are all rails/blocks with 4 rows of balls which is what you want since their load rating is equal in all directions and the moment load is greater. Two rows is still fine if they're quite big and clearly still far superior to supported round rails.

I very rarely see 2 row bearings for sale.! Where do you see them.?

I better keep them locked up else they will be!

Better still get them working and earning there keep.!!

[Jonathan]

Agree with jazz about the X-screw - RM2005 wont help. The problem is the pitch is half what you get with RM1610, so you have to spin the screw twice as fast to get a given feedrate. Going from 16mm diameter to 20mm doesn't make the critical speed twice as much (only 25% more) so you actually end up worse.

RM1610, critical speed - 1460rpm => 1460*10=14.6m/min
RM2005, critical speed - 1825rpm => 1825*5=9.1m/min

The feedrate with either screw is plenty, but since 20mm is more expensive and in addition to the above a 20mm screw requires significantly more torque than 16mm, so you only gain stiffness/lifetime which on a router like is completely negligible since the 16mm screw is strong enough and will last for many years.