Straightness of steel/alumunium box section

[Jonathan]

... if your 2 rails and blocks can still handle the load at 25% capacity then its not worth worrying about and save your y travel

They should do, they are rated for 862N/1372N which is well above the cutting forces. I'm not sure about the torque ratings of the bearings though which is probably more of an issue.

It dose look a bit narrow to me tho. dont forget that making it wider will reduce the torque arm caused by the long z axis. (fun this design stuff eh....)

It's 160mm wide and the centre distance of the rails is 100mm. I don't really understand which torque you're referring to that could make a difference to the Z rails?

From www.clickmetal.co.uk the 20mm aluminium plate (1220x160x20 total) will cost £88 inc. VAT and postage. I could get a 2 meter length of 6"x0.75" flat from www.aluminiumwarehouse.co.uk for £93 posted....I can't find anywhere nearby to get it :(

[Ross77]

They should do, they are rated for 862N/1372N which is well above the cutting forces. I'm not sure about the torque ratings of the bearings though which is probably more of an issue.

Ok i will try to explain, but im not the best. Maybe some else can re-phrase my ramblings......

Basicly you have a force at the cutter that is then transmited over a distance to the bearings (force x distance = N/mm) if nothing is able to resis this force then it will rotate about the centre point. In this case it will be in the middle of the 4 bearings, so yuo will have 50mm from the centre line to the bearings.

So say you have 10N cutting force over the 350mm (for aguments sake) thats 3500 N/mm, now as all forces have to equal and opposite (one of Newtons laws.....) the bearings will have to resist the 3500N/mm but only over 50mm so the bearing wil see 3500/50=70N

So you see that the actual force on the bearings isnt just the cutting force and in this case is 7x the cutting force, although divide that by 4 and each bearing will onlys see 17.5 N from the cuting force

It's 160mm wide and the centre distance of the rails is 100mm. I don't really understand which torque you're referring to that could make a difference to the Z rails?

Its the centre line of the rails that counts, as far as the lever arm is concerned. theres also the fact that a long thin beam will more suseptable to vibration........( but that a whole over can of worms):naughty:

The danger is that this calc only deals with cuting force and on the face of it makes the bearings look overkill. In reality there all sorts of other forces that need to be allowed for. but rather than spend the rest of you life trying to calculate them just make the machine as ridgid as can and then cross your fingers :whistling:

[Jonathan]

Ahh thanks for the description. I was thinking in the wrong plane...now it makes sense!

I'll put the rails right on the edge of the plate since that gains about 10mm.

rather than spend the rest of you life trying to calculate them just make the machine as ridgid as can and then cross your fingers

Yes, I'll make it as it is now. I'm currently waiting on my Mum/Dad to give me permission to buy the bearings. Then I hope to find somewhere better to buy the aluminium plate!

I'm intending to have this router finished before I start University, so 21-09-10.

[routercnc]

Can I be picky and say torque is measured in Nmm, not N/mm. That's stiffness.:tup:

[Jonathan]

Can I be picky and say torque is measured in Nmm, not N/mm. That's stiffness.:tup:

Haha, I thought that just wasn't going to mention it. Strictly speaking isn't it N·m! But not a Joule...

[Ross77]

Can I be picky and say torque is measured in Nmm, not N/mm. That's stiffness.:tup:

Duh me bad only fair since its normally me who's picky still if thats all I got wrong I'm happy, thought it was going to be ripped apart.......

Haha, I thought that just wasn't going to mention it. Strictly speaking isn't it N·m! But not a Joule...

you should have, I don't mind, everyone makes mistakes and you never know everything, I'm by no means perfect and isn't that the point of the forum that mistakes get corrected?:naughty:

I don't think it has to be Nm tho (dynamics maybe), as manufactures rearrange units to fit tables or make them look better with big numbers, I used mm to keep the units the same as you quoted, as long as you keep then the same through out the calc it doesn't matter as pointed out its force x dist (Nm) and then divide by dist to get a force (N).

What are you studying at Uni?

[Jonathan]

What are you studying at Uni?

Electronic Engineering at Nottingham - purely because I've always been interested in electronics and that sort of thing. I'm hoping they'll let me use the workshops there since it'll be a bit disappointing not having access to big milling machine/lathes after having had access at school for several years. I was in the workshop every lunchtime/free period in the 6th form.

[irving2008]

Good course... I did Electrical Engineering with Electronics at Imperial many many moons ago now ... you have to get to know the labtechs in the mech eng dept :lol:

[routercnc]

On your original question of straightness of steel box section, this guy went with a steel gantry and got the THK rail mounting surfaces ground. Cost him $200.
http://www.woodsolutions.com/diy-cnc.htm

Also of interest is the spindle setup (one for Ross and Steve), and the unusual X axis bearing arrangment.

[ecat]

A quick question if I may.
http://www.valuframe.co.uk/Technical-details.html

The Valuframe profiles have a 0.1mm camber either side of the profile slot. To my mind, mounting a profiled rail to this slot slightly to the right or left of centre may result in the rail ending up rotated with respect to the plane or possibly twisting slightly along its length.

A) Does this in fact happen?
B) Is it a significant effect?