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Author Topic: Bridge Beam  (Read 3838 times)

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Bridge Beam
« on: December 11, 2008, 03:00:39 AM »
Hi All

Attached is a couple of images of a possible approach to a guide way.

I have used adjuster bars 20mm square Mild steel plus the pyramids to seat and precision locate the 25mm round shafting (Pre drilled at 120mm spacing as per the Thomson spec sheet)

These bars could be made on a small mill.

Ideally after rough cutting at the local plasma or laser cutter’s shop.
Only the ends, back and V’s would need to be machined. The rest could be left as is.
In fact there if the shop had one of the latest machines only a few thou would be left to finish square.

The spacing between the V’s could be set by milling one first then locking the table and using the first one as a reference for the other, for the final cut. If done carefully it should be possible to get it to a few tenths.

There is a fair amount of drilling and tapping to do, and a lot of alignment to set it up but the result should be “Spot on”. All the adjustments are lock nutted. The top and bottom adjustment jams the bar between the ends of the beam. The in and out adjustment is lock nutted through the web of the beam. Using the same threaded rod that is holding the Roundway.
Note the nut behind the bar to tension that. The holes in the web of the beam are drilled oversize to allow adjustement.
 
This beam 310 UB40.4 has a 6.1mm web. I guess that is the weakest link in this setup. It is beyond my ability to calculate the deflection under a load perpendicular to that face. But for wood or Aluminium cutting it should be fine? The illustration is about 1500mm long Although I plan to make it 3600mm long

It would be interesting to know the Deflection If any member would like to do it I would appreciate hearing from them. 

Just doing a bit of brainstorming at the moment, there may be a better way, in fact an alternative method is bugging me right now!

If it looks OK I will post it.

Cheers

John Mac
« Last Edit: December 11, 2008, 03:13:55 AM by John Mac »
Re: Bridge Beam
« Reply #1 on: December 11, 2008, 06:18:23 AM »
Hi All

Another way of reinventing the wheel.

This time I have used a length of 12 mm plate Laser cut with “H” pattern holes. Pairs of precision cut mounting blocks are inserted in the holes and backed up by a 5mm thick washer that covers the “H” holes. An M8 threaded rod and nut fitted between the pairs tightens the rail against the blocks.

The question for me is what sort of accuracy could be achieved by this system?
The plate would have to be mounted on a strong backing frame and carefully packed level. That covers one alignment plane.

The holes would have to be cut in one setting of the laser cutter as would the sides need to be trimmed to get parallel edges.

The blocks are a worry… They would need to be a decent fit in the holes.

One thought would be to put a hole in the washer and pump epoxy into the joints after manual fine tuning of the alignment is done.

The blocks themselves would reflect the error of the laser; Would the reflected error be consistent over a batch, it may pay to put a mark on each one with the laser (Say cut a corner off slightly) to enable them to all be oriented the same way when being installed.

I considered tapering the sides of the block at the insertion point, that possibly could make them lock in place but the tolerances would be hard to set up. Maybe putting a cut a few mm in from each side would allow a little give?

There is no drilling or tapping for this design. And it would be quick to set up., against that you have the cost of the Laser Jobbers time? And slightly higher material cost and weight.

Cheers

John Mac
« Last Edit: December 11, 2008, 06:22:57 AM by John Mac »

Offline Tweakie.CNC

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Re: Bridge Beam
« Reply #2 on: December 12, 2008, 03:41:01 AM »
Hi John,

Just out of curiosity, what sort of weight do you expect to carry on this slide ?.

Regards, Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: Bridge Beam
« Reply #3 on: December 12, 2008, 07:07:44 AM »
Hi Tweakie

Around 60kg for the work head in this case is a sewing machine running at 4500 RPM (Always slightly out of balance and with a needle striking the work intermittently) causing load pulses.
There is a matching beam underneath as per my previous post re Mach 3 sewing.

I have worked on other machines of this type the vibration can be really noticeable; the last one from an Italian company used a similar sized box section.

Ideally a deflection of less than a thousandths of an inch is needed for good stitch formation. You can tweak that a bit. Come to think of it that should be your job!

I like the “I” beam as a concept the flanges form a natural cover for the slide ways and drive components. Using a box beam these would have to be shrouded with extra metal
The back of the “I” beam will be a perfect place to locate the flexible plastic duct for the power and air supply. 

The 310 beam has a mass of 32kg per metre the 250 x 150 x5 RHS used on my previous post was only a little lighter at 29.9kg per metre

A kg = 2.20462262 Pounds

It may be possible to downsize the beam. At this time I don’t know. Empirically most similar machines use beams between 250 and 300mm. so I chose the larger.

Cheers

John Mac

Offline Tweakie.CNC

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Re: Bridge Beam
« Reply #4 on: December 13, 2008, 04:01:27 AM »
Wow !

I can't wait to see pictures of this machine when you build it. Please post pictures.

Good luck,


Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: Bridge Beam
« Reply #5 on: December 15, 2008, 12:00:18 AM »
A box section would be preferable.

As the loads (accel/decel, mass, cutting etc) can impart a twisting action to the beam, a closed section (like the box section) will resist this in ways an open section (like an I beam) never will.

You might consider closing in the back of the I beam with a plate but it will not be anywhere near as stiff as a box section of similar outside dimensions.