Need help - Fixed gantry design

[triumph406]

Yep, I quite agree.........rolls pins are the answer to an engineers prayer when it comes to simple location and multiple removal.

.

Engineers use pull pins for location into reamed/bored/honed holes that need removal.

[NIC 77]

Hi triumph406, good to see you're still active here, I for one, value your input.

As to the question of boltzzz design, I personally don't see the need for pins, as I mentioned, not trying to open that can of worms again, but I did think to suggest one thing.

Square aluminum tube (not T-slot!) perhaps 3/8" or 1/2" thick, could be filled with epoxy granite or high density polyurethane, could even press in plywood ribs, and a bolt together design without welding.

Aluminum generally comes flat enough for our purposes. I suggest taking a long straight edge to the metal merchant to check out the flatness before buying.

This is different from the original design as it doesn't involve trying to bolt together all different shapes to make a gantry, but actually using aluminum tube instead of steel because it is easier to work with and generally comes flat.

Steel is stiffer than aluminum, but aluminum weighs less, so you go with a thicker wall and consider filling it, if the pieces come flat enough, perhaps being easier to work with might justify the added cost in this instance.

That's really all I have. Good luck, and keep us posted boltzzz.

[ger21]

A thick walled aluminum tube appears to be rather expensive, from the couple online sources I looked at.
Epoxy doesn't stick well to aluminum, and if the tube is subject to large temperature changes, I would expect whatever you fill it with to eventually separate.

[RCaffin]

Aluminum generally comes flat enough for our purposes.
Aluminium comes with a 'straightness spec', which tells you straight away that it is not really straight. (Sorry about the puns.)
Ground tooling plate is available - for a price.
I guess it all depends on the customer's requirements or expectations

By the way, Gerry, I think some epoxies do stick to clean oxidised aluminium rather well.

Cheers
Roger

[handlewanker]

Hi, my 2 cents worth.....if you use ally at all the expansion rate from the ambient variations is almost as bad as having welded steel tube construction except welded steel tube doesn't move once you weld it unless you heavily machine into it.....also the cost for welding steel is minimal.
Ian.

[RCaffin]

Worth remembering: high precision machines are usually water-cooled (with embedded pipes) and are run in tightly air-conditioned rooms.

Cheers
Roger

[handlewanker]

Hi, I once thought that it would be so simple if you cast in a solid block a complete machine body in one lump of aluminium and fitted a few linear rails and ball screws etc..........using it at zero degrees in Winter and 35 deg C + in Summer would be rather interesting when you're chasing a few thous of tolerance.
Ian.

[NIC 77]

A thick walled aluminum tube appears to be rather expensive, from the couple online sources I looked at.
Epoxy doesn't stick well to aluminum, and if the tube is subject to large temperature changes, I would expect whatever you fill it with to eventually separate.

If you buy it online you will pay much more. The best is to phone your local wholesaler. IMO, you are looking at double the cost from a local reseller and more from an online metal reseller.

Russel metals is an example of a wholesaler, you'd have to buy a full length and call for a quote. Probably the same place you might buy sheet from at work will also sell you the tube, no need to go to an online seller, make a couple calls and you might be surprised at the cost, if it's an item they regularly stock.

Worth picking up the phone to find out anyways.

Hi, I once thought that it would be so simple if you cast in a solid block a complete machine body in one lump of aluminium and fitted a few linear rails and ball screws etc..........using it at zero degrees in Winter and 35 deg C + in Summer would be rather interesting when you're chasing a few thous of tolerance.
Ian.

Who's chasing a couple of thousandths on this build? +/- 5 thou? Also, the free end of the ballscrew is able to shift in the bearing housing. The steel rails attached to the aluminum don't have this ability, and I don't honestly know how much this matters.

Who uses their CNC at 0 degrees?

I'm looking at 15C to 30C as my temp difference.

Have you guys seen Linux_fan's build? Steel frame with an aluminum gantry that is filled with epoxy granite, and it works awesome!

Plus I'm talking about making the whole thing out of aluminum except for the ballscrews and rails.

Worth remembering: high precision machines are usually water-cooled (with embedded pipes) and are run in tightly air-conditioned rooms.

Cheers
Roger

To my knowledge, we're not talking about a "high precision" VMC here. We're talking about an affordable DIY "acceptable precision" machine.

Aluminum generally comes flat enough for our purposes.
Aluminium comes with a 'straightness spec', which tells you straight away that it is not really straight. (Sorry about the puns.)
Ground tooling plate is available - for a price.
I guess it all depends on the customer's requirements or expectations

Puns add a bit of flavor, I like them. As you can tell perhaps from some of my other posts, sometimes I like to add a bit of humor, even edgy humor, to my posts.

I have some solid aluminum extrusion about 2" x 3", and the 2" faces are dead nuts flat within my ability to measure them. Whatever the spec is, IMO, from what I've seen, they are generally good enough, whether that means they come better than the spec or not I don't know. Also, if it's hit or miss, I don't know. That's why you should bring a straight edge with you to the metal merchant.

An example of this is all of the people building T-Slot machines. Flatness = Good E Nuff. But I wouldn't say the same is generally true about steel tube.

I agree, it's about the customer's expectations.

The machine I'm working on is mostly steel tube, but there is also considerable aluminum. Not sure yet if this is going to cause a problem for me re: thermal expansion. Guess I'll find out.

All I can do is give my opinion, and sometimes it's wrong. This was just a suggestion as a route to explore based on the lack of welding and metal working experience or equipment from the OP.

[RCaffin]

Who uses their CNC at 0 degrees?
Yeah, well, my workshop has been at 0 C and very close to 40 C.
And yes, I DID notice the effect of thermal expansion! Right puzzled me at first though.

By the way: T-slot aluminium tubing: it is NOT flat across each surface. There is deliberate inwards bowing in the die, so that when you do the bolts up the metal is sprung slightly. To be sure, it makes for more secure joints, but precision can go out the window.

Cheers
Roger

[handlewanker]

Hi, back around '82 or 3 when I first moved to OZ we had temperatures in Melbourne that put ice on the windscreens of cars overnight.

I worked outside at night in a 10 foot square tin shed and used a home made wood burner made from a 25 litre tin can and some down pipe for the chimney.....temperature got up to 10 deg C.....not cosy but bearable........a 3 hour stint was the most I could take.

If it came to a CNC work load.....no way would I work in those temps.

Now I have a brick garage with a fan heater and an insulated roof.....that makes evening work in Winter OK.
Ian.

[RCaffin]

You're lucky. I have an old galvo barn built around 1900, with a LOT of drafts. For real.
With resident skinks and barn swallows.

Cheers
Roger

[boltzzz]

LeeWay

With regards to your post (#51), is this what you mean? Uprights down to the base tube?

[LeeWay]

Not quite, but could work well if filled and capped. Think more like the columns being a separate entity and bolts to a flat surface that the base tubes will be secured to. IE the floor or a base plate. However what you have modeled would work fine on a smaller machine I think.

[RCaffin]

If you are building a router, that is probably fine.
If you are building a MILL, then at least double the wall thickness on every tube - and cap and fill them.
Some extra reinforcing would not go amiss.

Cheers
Roger

[handlewanker]

I just did some "conceptual imagining".....regarding the design in post #92.....depending on the working envelope.........imagine if those tubes were actually solid hot rolled steel bars 100mm X 50mm welded together.......no chance of distortion etc......I don't think there would be enough heat build up to warp a steel bar of 100mm X 50mm.

Welding would be so ,simple.......just short welds all round that even a 'tomic bomb couldn't move.

The weight would be quite large but then so would a mill if it was made from cast sections and it would be ever so rigid......weight in a machine is a desirable factor.....lightness is not.

I would weld the 4 base pieces first then machine the Y axis rail seats on top.....after that the two side support columns can be welded on and finally the pre machined cross beam.....no bolting as this means machined interfaces.

if you had to make a mill or router from castings all the design,pattern and casting work would cost hundreds and then a large amount of machining.........and that would take a big mill and very detailed drawings.

What you would have would be a moving table router type machine that had a larger work envelope and was capable of machining steel like a mill.......call it a portal mill if you want.......just thinking.
Ian.

[RCaffin]

All solid - back of envelope suggests about 1 tonne.

Cheers
Roger

[rocketflier]

All solid - back of envelope suggests about 1 tonne.

Cheers
Roger

2x2x0.25" is 5lbs/ft. 2x3 is 7, 2x4 is 9, 2x6 is 12. If the base is made up of 4-2x4 24" pieces, 72 lbs. 1 1/2 x 4 solid is 20lbs/ft, 160lbs.

[RCaffin]

Yeah, hollow tubing is a LOT lighter, isn't it?
Is there much difference in rigidity between solid bar and 1/4" tube? Some, but not a lot. The hollow tube will however ring a lot more I think.

Cheers
Roger

[handlewanker]

Well it all depends on the work envelope to get the frame size.......I think a 6040 size mightn't come to that figure...........but it would be quite easy to build and the weight, once on a base, would not move the Earth beneath your feet.

A Bridgeport weighs about a ton last time I moved one.........I doubt a 6040 to the design mentioned would come to that figure......and as it would be a moving table type, the frame would be the easiest part to build.

This is just a concept idea, but when you consider the amount of work....and alignment factors...... that a composite in aluminium or steel takes, it is an interesting concept......the design is so simple, without being a compromise, and it could be up and running in a month......why spend 6 months on something only good enough to cut wood.

I think this could be construed as getting out of the pail and thinking on the backs of Giants.......LOL.

BTW, a lick of paint and you'd never know the makeup of the build.

Just as a matter of interest, the concept of a solid build occurred when I was stacking some off cut pieces of 90 X 45 timber from a bathroom revival campaign........seeing the sections as they laid across one another generated the idea.
Ian.

[RCaffin]

What I have not seen yet mentioned is how you align one of these builds.

Figure on spending quite a bit of money on a few precision instruments for this - and I do NOT mean a couple of $1 eBay string levels!
A lot of eBay devices have a carpenter's resolution (or worse). A precision device can give you 0.0005 "/ft. It's the only way to get rid of twist in the guide-rails.

Cheers
Roger