[woodhouse]

Hi All, and thanks for any advice anyone is able to provide, its greatly appreciated.

We've currently got a project where we are building a massive 2.5M x 4M bed machine with a multi function bridge/gantry.
Its primary use is for a nailgun, for nailing in a pattern but while we are building it it might as well be able to machine as well.

Anyone who isnt sure what we mean just google 'MBA Mobi one' or 'weinmann wms 100' but we intend ours to be much more compact as space is at a premium.

We all ready have the massive overkill NSK 55 profile rails and the bed/frame side of things is not a problem. What we cant decide on is the design for the gantry itself...

so far we have come up with the following...

1) use a large hollow section with the rails tapped into it but would a piece of flat bar be needed under to increase the thickness to get a good tap?

2) use a large c section with the rails bolted through?

3) angle welded flush to face of a hollow section to allow bolting through?

4) smaller hollow welded to produce the gantry?

any ideas and experience of any of these??

I'm assuming that one rail on the front face and one on the top face of the gantry would provide a better design or does it not really matter?

We also still haven't decided on the drive side for it either, whether rack is the way to go or rotating ballscrew?

thanks in advance

Tony

[SatanKlawz]

A machine that is 4m long I believe is going to be seriously pushing the range for any screw type setup. I believe the majority of people that have machines approaching 5' and longer are using rack.

[ger21]

Just use a hollow tube with enough thickness for the threads.

I'd put both rails on the face.

And industrial machines with screws that long, will usually use a rotating nut.

[woodhouse]

Yeah, I was thinking that was the only viable way to go for the long axis too.
The 2.5m axis across the gantry I'm still wondering if I should use a 50mm ballscrew I can get access to from a friend or if it's just going to over complicate and cost more in the long run for all the bearing blocks etc.



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[pippin88]

Two rails on one surface are much easier to align. Much harder to get right angle surfaces true. Those rails will have equal load rating in all directions.

A closed section (e.g. tube) is much stronger than a C section / open section. A tube is made much stiffer by addition of bulkheads. They could be welded in prior to machining the rail face, or even epoxied in.

If you can, get everything stress relieved.

[woodhouse]

Thanks for all the replies.
Is there any significant advantage why some commercial manufacturers have the rails on right angle surfaces?
My only reasoning for doing so was due to the space on the front face from the bearing carriages on the 55mm rail, they are 140mm wide with the flanges


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[woodhouse]

Attachment 285752

This is what I was originally thinking from a RHS point of view (ignore the gantry supports)
They are 250x150mm at 6mm thick would be 100kg on it's own


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[woodhouse]

Attachment 285754

Just amended to show what you guys are recommending.
Where would the best place be for the rack then? As far to front edge on top to limit twist? Or...


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[ger21]

You can get away with a 32mm ballscrew on the gantry, running between the rails. Just be sure to use a high pitch, like 25mm. This will give you plenty of speed, and keep the screw from whipping.

Is there any significant advantage why some commercial manufacturers have the rails on right angle surfaces?

On the $100,000+ routers I've used and am familiar with, I've never seen it done that way.

[woodhouse]

Thanks ger21, I think it might be that on the two multifunction bridges I was looking at both were at right angles, but used the extruded guides so possibly needed to be?


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[woodhouse]

Thickness of the RH section, will 6mm be enough to tap?
The cap screws are m16 so fine threads are 1.5
4 threads hold??


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[ger21]

You do realize that on a big, similarly sized industrial router, that they'd usually use no larger than 25mm rails.

You could easily hang a car (or two) off of your gantry with those massive rails.

Those massive rails are going to require you to make everything much larger than it needs to be in order to accommodate them.

There are formulas for screw thread engagement. I have no idea what they are.

[pippin88]

Use 250x250 tube. A lot of the force will actually want to twist the gantry rather than just deflect it vertically.

[woodhouse]

Oh I know, each carriage is rated at 120,000N.
The only real problem I can see from using them though is the increased mass of the gantry, there is no real difference in friction.
I had considered getting smaller rails for the axis across the gantry but not sure if it will save more in the long run buying smaller?


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[wizard]

Oh I know, each carriage is rated at 120,000N.

The linear rails are no better that the material supporting them. In this case you have an interesting problem in that what is needed for a nail gun is most likely dramatically different that what is needed for a large spindle. For a nail gun I can't really imagine deflection being a huge problem nor a bit of vibration. For a router that could be a huge problem depending upon what is expected from the machine.

In your case I really think you are under sizing the gantry beam by a massive amount if the goal is high precision quality routing. The unfortunate thing here is that you can't rely upon the experience or guess work of others, because of the limited amount of people actually building such a massive machine. In a nut shell your gantry is longer that most people's X axis. As such you probably should run the design through the Engineering calculations to be sure it will meet your needs.

The only real problem I can see from using them though is the increased mass of the gantry, there is no real difference in friction.

The problems are massive size result in massive support requirements needed or not. For example your sketch shows that there is a minimal distance that the rails can be separated by. My thinking right now is that your gantry beam is too small anyways

In another post you said (maybe I read it wrong) that the mounting bolts for the rails are M16. If so that implies a rail that is far larger than im imagining. In fact that is so large I'm pretty much convinced you are wasting money. In any event I bring this up to answer another question, which is how thick does the tapped material have to be to support the screws. In steel the common practice around here is to have one bolt diameter worth of threads, which is considered more than enough. That is a 6mm bolt would need a gantry beam that has a web thickness of at least 6 mm. However at is an optimistic value because you need to take into account machining the beam for rail mounting and ideally material to counter bore the threads. Sometimes it makes sense to buy your rails tapped and through bolt.

Another way to look at this would be to grab a nut for the bolt in question and measure its thickness. This should give you an absolute minimal size that you would want to use for Web thickness after machining. The gotcha here is that nuts are replaceable, replacing threads in a stripped out machine frame is a lot more work. Also in most cases the materials are matched in nut and bolt usage this might nit benthencase in something tapped

On a side note about threads, a guy on one of the gun smithing sites did the calculations required to determine hor many threads are of use in a threaded connection. His math demonstrated that most of the strength comes from the first 5 threads and that by seven you gain little.

I had considered getting smaller rails for the axis across the gantry but not sure if it will save more in the long run buying smaller?

For a nail gun you might get away with round rails, not that I'd recommend it in this case. The only reason I could see to go to a larger than normal profile rail is that the rail can handle the shock loads better. I'm not even sure that is the case and I don't know how much energy a nail gun would inflict upon the bearings.

In the end you are wasting money buying components that are too big in an exceptional way.

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[woodhouse]

Thanks wizard for such a thorough reply.

Sorry, but I can't work how to quote...

You're absolutely right, there should be very little force exerted from the nailgun apart from a little vertical shock but that can be dampened out on a tool like that.

I already have the 55mm rails and carriages that I got from a machine that was being replaced for a very very low price. Its not like I'm crazy and off to spend £12k on new ones!!

What I mean by save money is will buying a smaller profile save me money in other parts of the build.
Apart from the extra mass of the rails which is about 50kg a piece will mean the gantry needs to support them without deflection, but Is this not negligible compared to the force a spindle can put on it?

I think I'm going to have to have a play on inventor tonight to see what the stress calculations look like for the different size RHS

With regards to bolting through, with a little contortionist there should be enough room to get nuts on the back using a long socket and a couple of round access holes.

And thinking hypothetically, not necessarily to do with this build, With regards to twist to a gantry from a spindle, am I right in thinking that a rail front and back at the same height and connected round with suitable size material would cause no twist as the levering action from the spindle would be translated into an upwards force one side and a downwards the other??!!






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[wizard]

I must have missed a few things in my Communications here.

Thanks wizard for such a thorough reply.

Sorry, but I can't work how to quote...

You're absolutely right, there should be very little force exerted from the nailgun apart from a little vertical shock but that can be dampened out on a tool like that.

I already have the 55mm rails and carriages that I got from a machine that was being replaced for a very very low price. Its not like I'm crazy and off to spend £12k on new ones!!

If you already have the rails that does change things a bit. If that is the case then simply consider a beam of a much larger size that gives you the support and the Z travel you will need.

Speaking of which, if the machine is to nail things together, I'm imagining you will want significant clearance under the gantry. This would imply Y axis linear bearings separated by a good distance anyways so to better support the router when it is in operation.

What I mean by save money is will buying a smaller profile save me money in other parts of the build.

Probably not! The reason being the gantry needs to be fairly large due to the distance spanned. Your primary concerns likely will be twist and deflection when the router is in use.

Something that just occurred to me, it might make sense to have two gantries on such s machine, one for the nailing apparatus and one for the router. They have the potential of running at the same time and one could be serviced while the other runs.

Apart from the extra mass of the rails which is about 50kg a piece will mean the gantry needs to support them without deflection, but Is this not negligible compared to the force a spindle can put on it?

It depends upon what the spindle is doing.

I think I'm going to have to have a play on inventor tonight to see what the stress calculations look like for the different size RHS

With regards to bolting through, with a little contortionist there should be enough room to get nuts on the back using a long socket and a couple of round access holes.

I was actually thinking here that your rails would be tapped for bolting through. I'm not a fan of nuts for something like this. However either way you still have the issue of how you will drive the nut or cap screw. That may require a lot more holes to be drilled in the beam.

As for drilling and tapping you haven't indicated if the M16 value is right for the rail bolts. If it is, I honestly wouldn't bother with these rails on the gantry as that would imply gantry wall thicknesses of around 16 mm, in a beam 12-14" square that is a huge amount of mass.

And thinking hypothetically, not necessarily to do with this build, With regards to twist to a gantry from a spindle, am I right in thinking that a rail front and back at the same height and connected round with suitable size material would cause no twist as the levering action from the spindle would be translated into an upwards force one side and a downwards the other??!!

I'm not sure if I follow, sounds like you want a saddle that wraps the gantry beam. Not good in my mind but that depends upon how much time you put into this. There are reasons why the overwhelming number of machines out there have their liners rails on one surface. One of the primary reasons is that it is easy to get the machining and thus rail alignment right. It would be incredibly difficult to do otherwise.

As for what the spindle will do it full depends upon what the spindle is doing at the time. The spindle can even pull the gantry down if it happens to cork screw a drill bit into something and jam. In Normal operation the forces depend upon the cutter used and the aggressiveness of the cut.

So with respect to the nail gun, the spindle would have a greater impact on the beam. It is up to you to determine how much of that impact is too much. Most guys in this forum are trying to achieve fairly good precision with their machines. In this case it sounds like you want to prefab walls for houses, as such the routing quality might not be important at all. I can't read minds here so I'm defaulting a machine design where higher quality is expected.






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[woodhouse]

thanks guys, some amazing stuff coming up here...

I must have missed a few things in my Communications here.

Speaking of which, if the machine is to nail things together, I'm imagining you will want significant clearance under the gantry. This would imply Y axis linear bearings separated by a good distance anyways so to better support the router when it is in operation.

Yes thats right, it does need a fair bit of clearance... Weinmann manufacture a multifunction bridge although on a larger scale for house sized wall cassettes. the stuff we make is much smaller.




looking at the picture, the the gantry beam doesn't appear to be overly large and if you assume his head with cap on is 300mm ish... obviously there is no way to know the thickness, and this is on a much wider machine.
the distance between the gantry is much higher than i was considering too, this hight worries me on the twisting!

Your primary concerns likely will be twist and deflection when the router is in use.

does anyone know what kind of magnitude of force we could be looking at at the spindle? i presume it depends on the cutter but a worst case senario? can work out the deflection from the levering action.

Something that just occurred to me, it might make sense to have two gantries on such s machine, one for the nailing apparatus and one for the router. They have the potential of running at the same time and one could be serviced while the other runs.

yeah it is something i had considered.

As for drilling and tapping you haven't indicated if the M16 value is right for the rail bolts. If it is, I honestly wouldn't bother with these rails on the gantry as that would imply gantry wall thicknesses of around 16 mm, in a beam 12-14" square that is a huge amount of mass

yes they are 16mm holes, but with fine pitch 1.5 threads would give 4 threads in 6mm and 5 in 8mm.
for a standard m10 1.5 pitch nut the thickness is 8mm

[woodhouse]

For anyone that's interested I found this...
http://www.steelconstruction.info/in...nternal_link=1
It's excellent info from tata steel on torsion forces with torsion tables for all the sections at the end!!


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[woodhouse]

There is also this paper on gantry design...
http://www.mech.utah.edu/~bamberg/re...e%20Design.pdf


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