Comments sought on new build: A CNC Router for RC Gliders and Planes

**njhussey** · 12-01-2015

Originally Posted by Jonathan

Quite likely yes - but how much force will bending the bearing to 'flat' impose upon it? The answer is a lot, since the stiffness of these bearings is still very high in the rolling direction. It also depends on how stiff the gantry is and how wide it is - something has to bend. A fairly high force wont make much difference to how smoothly the bearings roll, since as I mentioned in the last post their co-efficient of friction is very low. It will still shorten the life of the bearings though.

It's different with round rail bearings - clearly they have zero stiffness in the rolling direction, so angular misalignment doesn't affect the force of the bearings (within reason).

If you look in the datasheet for the rails and bearings you'll find specifications for the maximum linear and angular misalignment. These are based on getting the rated lifetime out of the bearings, so if you exceed it by a little, the bearings just wont last as long. Though on a home machine it might still outlive you!

That's a good indication that they're close, but since you've got the straight edge you might as well measure it carefully. The issue with what you just did is it's hard to measure very small angles and you don't get a reading for what the angle actually is. Instead you need to do something like in my build log - stick the dial indicator on one of the bearings and mount the straight edge parallel to the rail, but some distance (say 200mm) horizontally away from it. Make the indicator touch the straight edge, with it's movement perpendicular to the surface. That will convert the angular reading into a linear reading and amplify it. You don't need to worry too much about getting the straight edge level, as if you plot the readings on a graph that will just make a linear error which you can subtract.

Just out of interest, how much did the straight edge cost?

I'm not too worried about the life of the bearings, as you say I'm sure it will outlive me and for what it's being used for. I will measure it though out of interest as I have the straight edge ...

The straight edge was from Axminster http://www.axminster.co.uk/veritas-a...-straight-edge it's got a claimed accuracy of 0.075mm over the 1270mm length which although not super accurate is more than good enough I think for my purposes.

**Neale** · 12-01-2015

Picking up Jonathan's comment re stiffness in roll and manufacturer's tolerance...

HiWin 20mm rails have a maximum permitted height difference of 0.130mm with 500mm rail spacing. That corresponds to a difference of 0.013mm across that 50mm or so bearing block. Don't know about you, but that's about a quarter of my thinnest feeler - and that's the maximum allowable tolerance. As for stiffness - and I don't recommend you do this at home as I only did it by accident - my gantry structure probably weighs around 30kg, and I managed to leave it hanging while only attached to the carriage at one end. It probably dropped by no more than 25mm or so. I hate to think of the abuse of the pair of bearing blocks taking the weight, but what's done is done. I shall be reporting on my experiments with epoxy (i.e. mistakes and how I've tried to correct them) elsewhere when I'm actually happy with the results, but in simple terms, it's a lot easier to get the rails level than to remove twist. The main reason to get the rails level (given that you will take out minor variations by machining the spoil board anyway) is that with, say, a 1000mm rail spacing, the max rail height difference spec only allow a height difference of about 0.26mm (10 thou). That's not too difficult with epoxy, and I would expect rather better with just a bit of care. But twist? As long as it is constant, then the gantry to bearing block joint can be shimmed/adjusted to suit but you only need a tiny amount of varying twist to go way out of spec. I'm still trying to get mine somewhere near acceptable. I'm using a sensitive level (0.02mm/m readings) to check.

It would be easy to say that the problem is that I'm chasing irrelevant numbers with tools that are working at their limit, except that a trial bolt-down of rails with gantry mounted gave clear signs of graunching bearings even when I was happy with levels. Suck-it-and-see tells me that there's something wrong.

**njhussey** · 12-01-2015

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These are the NSK specs on running parallelism....seems in theory I have about 16 microns for my 800mm spaced rails to play with....to be honest I think you can keep chasing numbers till you're red in the face and for a router that's going to be used for wood and plastics mostly with some aluminium then I'm not going to get my knickers in a twist about it.

I'm going to do a quick test as Jonathan did (I'm intreagued now) and then once the gantry is made bolt it to the carriages and see what they sound and feel like.

**EddyCurrent** · 12-01-2015

I bet you wonder how you ever managed without a straight edge now

**njhussey** · 16-01-2015

Picked up the cut tooling plate just now....gantry build can start next week though first got to design and build a pantograph arm for putting in M6 UNC bolts and grease nipples into 1000 units we're currently assembling for a contract....oh, that's after I've finished machining a custom unit that's just been ordered. Work keeps getting in the way of playing!