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

[njhussey]

Sliding carriages along the rail gives some indication of the straightness, but very little indication of the twist. Fortunately for us these rails have very low co-efficients of friction, but when it comes to aligning them this makes small errors in straightness harder to detect as they don't produce much force.

Using an indicator mounted on one rail carriage to measure and fix the distance between the rails is good once you've got one rail straight. If you do if before then, you'll just copy the error - so if one rail is banna shaped the other one will be the same banna shape. To be fair that's still better than having a banna and some other fruit...

From a practical point of view, if you've got the rails aligned well enough that the bearings run smoothly then the bearings should last a long time, so it's arguably OK to use but you will need to have realistic expectations of the accuracy.

Surely the twist will be sorted when the rail is bolted tightly down, that's assuming that the epoxy is perfectly flat of course?

I'm going to use the sliding carriage method to drill and tap the holes, there is a clearance of 0.5mm either side of the bolt in the rail hole so plenty of room for adjustment. When I get the tooling/eco plates I'll be machining a reference edge to mount the carriages to the bottom piece of Ali, then the gantry will have a reference edge to mount the carriage plates to and thus I hope the gantry will be straight/perpendicular/parallel etc. etc. so that when I bolt the carriages to it the second rail will be referenced from the first which will be as straight as I can get it without having a machined reference edge in the epoxy to go on.....if that makes sense?

I think I'll get a straight edge, that will help with twist and straightness....plus the above and it should all be as acurate as it's possible for a bodger like me to get

[JAZZCNC]

Surely the twist will be sorted when the rail is bolted tightly down, that's assuming that the epoxy is perfectly flat of course?

Gezzzz People have too much trust and expectations of epoxy.? You can't beat checking so buy or make a straight edge to confirm if you really think you need that level of accurecy in a router.!!!

When I get the tooling/eco plates I'll be machining a reference edge to mount the carriages to the bottom piece of Ali

Why waste money on Eco-cast if your going to mill the ground surface away.? . . . The whole point is the accurecy of the ground plate and no matter how careful you won't mill to same level as grinding gives.!

What I do with ground plate when I need a ref edge is mill pockets on the ref line and insert aluminium dowels 3mm proud of surface. Then I machine half the dowels away to the plate surface leaving me machined ref edge without disturbing the ground plate.
Like this.

[mekanik]

Have to agree with Dean, can't see the point in going to all trouble of designing your machine, getting feedback from the experienced members and then making a horses arse of aligning the rails at the last hurdle.
No offense intended.
Regards
Mike

[njhussey]

None taken Mike, im all ears with regards to how its done. That's why I keep saying how I think I'm going to do things then listening to the advice given by the more experienced members.

The last thing I want to do is make a horses arse of aligning the rails having got this far :)

[mekanik]

Glad about that Neil
I assume you have tried the reference edge of your rails when placed face to face? can you get a feeler between them, not that that is a definative test as they could both be bent the same. I know the squares and straight edges are a substantial investment that doesn't seem justified on possibly building a couple of machines,are you near any engineering or tech colleges that have this sort of gear a suitable deposit might get you the loan of some suitable kit.
Good luck with the build
Regards
Mike

[njhussey]

Mike, I'm ordering a straight edge and a couple of engineers squares tomorrow along with the tooling plate.

I'll try the feeler gauge tomorrow and see how that is but I do remember when I first got them that I had them together every which way and couldn't see any light between the rails.

[mekanik]

That sounds promising if you cant get a feeler gauge between the reference edges you could use the rail as a straight edge. have a look @ Silyavski's post before you buy any kit he did a lot of researching looking for straight edges and squares, you want the stuff that's made to a decent spec can't remember the BS number of hand but it's there in his thread.
Regards
Mike

[JAZZCNC]

so it's arguably OK to use but you will need to have realistic expectations of the accuracy.

Bingo he's Hit the nail on the head.! . . . . . . . People are getting silly on this forum regards the level accuracy they are trying achieve or THINK they are achieving with what is essentially Cave man tools in engineering terms.

Be realistic and remember it's a router based machine that will probably spend most of it's time cutting materials that have high variations of in-stabilty that far excede the accuracy machine can cut too.!

If you take your time to align the rails with sensible means IE: Clive S suggestion of taught wire along with feeling of bearings etc you'll be very close if not spot on for the use of the machine.
Yes rail alignment error will affect performance and live span of the rails but with care and patience the level of error will be so small to make little difference at DIY levels. Even in small/medium scale production enviroment it will take good few years to have any affect on wear if care is taken on setting up and alignment.
Just throw them on and you'll pay the price but with careful setup and sensible attention to detail you won't go wrong.!! . . . . Stop over thinking it and take sensible precations with checking straightness/twist along with feeling your way and you'll be fine.!! . . CRACK ON

[mitchejc]

Neil your build is looking great. I bet you can smell the wood chips already :-)

Please take what I’m about to say with a huge pinch of salt as I just fitted my first rails ever on my first build, so no real experience here but I’m putting it out there hopefully to get corrected or affermed. Look, I don’t know, maybe my expectations of final accuracy is just low or I got very lucky with the batch of 15mm Hiwin rails I received but those reference edges are as straight as Charles Bronson. Keep in mind my build is relatively small so I guess on longer rails it might be a totally different story. Few of us has the right equipment but I believe Jazz’s way with machining the dowels is the best approach assuming the mill you cut it on is accurate over that distance. I’ve seen builds where straight edges are clamped to the rail before drilling but I don’t think a straight edge is still straight after clamping it to something that’s potentially not straight?

I did not use a clamped straight edge for my reference rail. To ensure I didn’t bend it out of shape I clamped it to the epoxy base, drilled, tapped and fastened the ends and then repeated the process by moving the clamps for the rest of the holes always making sure I only drilled next to a clamp if that makes sense. On my rails the holes are 4.5mm and the bolts are 4mm which leaves a huge amount of adjustment as far as these things go assuming one does not stuff up drilling and tapping the holes.

For drilling I used a method proposed on here, I think Silyavski : Using the rail holes as reference, I took a 4.5 mm drill and drilled though the epoxy to the point where it made a good tapper dimple on the steel below and then drilled the rest of the hole with a 3.3mm drill for the tapping. That 3.3mm drill has no choice but to run to the centre of the dimple. Touch wood, I’ve done 40 something holes so far and not one out of centre so it appears this method is working really well. The important thing is that there is no binding whatsoever when you put the bolt in otherwise it will push the rail out of straight or best case you won’t any room for adjustment later. The steel on the inside of these rails where the holes are, are softer than I imagined so I guess the outsides where the bearing run are only case hardened. Not ideal but I’m totally prepared to enlarge the hole in the rail a little to ensure the bolt is not touching sides due to bad tapping or drilling the tap hole at a slight angle. I then used my straight edge and a feeler gauge (ok, it’s not really a straight edge but it’s a good quality stainless steel ruler and I know the one edge is relatively good) to check that the rail. I believe I have enough adjustment in the rail holes to fine tune it somewhere in the future if I get a real straight edge but I’m pretty confident it’s already better than my expectations of accuracy.
I then used my gantry after aligning it perpendicular to my first rail etc to align the second rail and then repeated the clamping drilling, tapping and fastening exercise on that side. As pointed out by Jonathan, I realise that these rails are quite slippery and it would require a huge force to actually get them to bind, but I did the best I could to move it around while aligning the second rail to feel for any sort of friction and I’m very happy with how smooth it’s running.
Even with this potentially dodge method I followed it still took me more a full day to get my x rails on so no quick wins here

Best of luck with fitting your rails.

[njhussey]

Well I've stopped buggering about and started drilling and tapping the first (reference rail) rail. I put the two reference sides of the rails together , as suggested, and I couldn't get a 1 thou feeler gauge to even start to go between them.

I've drilled and tapped about 7 holes so far with my set up as below and all the holes are nicely central with approx 1/2mm each side of the hole so plenty of room for any adjustment if needed.







Tooling plate is on order but will be 5-6 days and now going to order a straight edge from the same place as Silyavski in Germany to make sure that there isn't any (or negligible) twist in the rails.

Thanks for all the comments and suggestions guys!