Chroma - Lasercut Acrylic 4x5 Field Camera

[Nomad Z]

If you're tapping into the sides of the slot, then that will be a potential weak point. M5 into a 4mm slot will give you something that has about 32.4% of the overlapped area (between screw and threaded shape in the rail) compared to a proper round tapped hole. Depending on what the rail is made of, and how thick it is, there's a risk that the threads will strip. More so with M5 - it isn't a particularly coarse thread, meaning there isn't a lot of material in the ridges and valleys to resist tightening forces.

To be honest, I wouldn't do it like that, especially if it was intended to be used by others (almost guaranteed to get stripped threads). If it was for my own use (and I have good mechanical feel and sympathy), I'd use M6 (or a 1/4" 20tpi tripod thread). The coarser thread means deeper valleys (so more material to try and rip out), and the bigger diameter means a bigger overlap for the given slot width (48%).

That's a bigger overlap of something that's 1.5x stronger than M5, meaning M6 tappings in a 4mm slot would be about 1.7x stronger than what you'd get with M5 - but note that a proper M5 hole is 3x stronger than the M5 tappings in the slot.

Like I said, it depends on what it's made of and how thick it is. If it was 3-4mm thick stainless steel, it likely wouldn't matter so much (next to impossible for ordinary mortals to strip a proper M5 hole in 3-4mm stainless using a thumb screw and human fingers, so would still need a fair old effort to do it in the slot), but if it was the same thickness in aluminium, brass, die-cast zinc or the like, I wouldn't trust M5 (and would still have misgivings about M6). If it was something for others to use, I'd do it differently.

 

[Nomad Z]

A picture to illustrate the above...



To put it another way, the easier it is to tap, the easier it is to strip. That said, both of those are easy to tap, although adding the tapping hole for M6 to a slot is unlikely to be fun (can get away with no tapping hole for M5, especially if the slot is a smidge more than 4mm in reality - but the wider it is, the weaker it will be).

 

[stevelmx5]

A picture to illustrate the above...

View attachment 102434

To put it another way, the easier it is to tap, the easier it is to strip. That said, both of those are easy to tap, although adding the tapping hole for M6 to a slot is unlikely to be fun (can get away with no tapping hole for M5, especially if the slot is a smidge more than 4mm in reality - but the wider it is, the weaker it will be).

Thanks again for your advice. The thread will be cut into a 4mm thick steel plate (with a 4mm slot down the middle of it). I'm not concerned about the thread stripping as I'll be using plastic threaded knobs so if anything's going to strip, it will be them. They will only ever be tightened by hand so I'm hoping that they will be solid.

I originally intended to use 6mm threads but I've now changed to using 5mm layers of acrylic to build up the front standard instead of 6mm so I will only have a 5mm space to work with.

 

[gunnar]

@stevelmx5, how did you embed the magnets into acrylic?

 

[stevelmx5]

@stevelmx5, how did you embed the magnets into acrylic?

I laser cut 5mm holes through the acrylic then pushed the magnet through, followed with a few drops of super glue to hold them in place.

 

[gunnar]

thanks, nice idea. I will probably use it in next iteration of my digitising rig project

 

[Nomad Z]

Thanks again for your advice. The thread will be cut into a 4mm thick steel plate (with a 4mm slot down the middle of it).

Does a magnet stick to it?

I'm not concerned about the thread stripping as I'll be using plastic threaded knobs so if anything's going to strip, it will be them. They will only ever be tightened by hand so I'm hoping that they will be solid.

The idea is to make it strong enough to not fail in normal use - defining the failure component and hoping it will be okay should tell you something.

The problem with something like this - a thing that's tightened by a human - is that the applied torque is variable. Yes, it will clamp the two bits together. It may well clamp them well enough to stop them sliding against each other. But you can't be sure that a human will only ever apply sufficient torque and no more. For that reason, it needs to be over-engineered to some degree. Ask yourself this: When is the best time to get the design of this bit right?

What sort of plastic is the screw made of? A purchased ready-made component or 3D printed?

Am I right in thinking that the thumbwheel part of the screw is about 30mm in diameter? (Looking at the disc under the hinge.) Unless it's some special material, it's almost guaranteed to fail. The plastic is unlikely to be tough enough given the poor overlap of the threads due to the slot, and the large diameter thumbwheel will make it easier to apply too much torque. I seriously urge you to reconsider how this part is approached.

My thoughts, off the top of my head...

Use the original 1/4" screw as the fixing. Find a way to lock it such that it doesn't rotate in the slot. If it has a metal loop on the underside (or any irregular shape) it should be possible to make a sqaure/rectangular 3D printed part that will fit around the shape as well as fit into the gap under the rail. Might need to fix the anti-rotation block to the screw to hold it in place when the rail extends beyond the adjustment block (or make a shape that is held in by the screw itself in some way).

Use a metal disc with a 1/4" tripod thread in the middle in place of the thumbwheel on the screw. 3-4mm thick aluminium would work okay, but basically as thick as the design will allow for, up to the protruding length the of the screw. Brass will work and feel better, but can be expensive. It's feasible to use something smaller as the metal part and 3D print the thumbwheel, but again needs an interference fit for robustness (easier to make this at home with limited tools). The disc (or insert) would need to be tapped with the matching tripod thread, so drill and tap needed (taper tap okay for this thickness).

This would be far more robust and has the added advantages of allowing the standard to placed anywhere on the slot, and no need to unscrew it to move to a new position - just slacken, slide, and tighten. The thumbwheel could have long lobes rather than be a disc, which would be easier to use (it's a potentially fiddly bit to get into with a finger and thumb, and would be hard to apply decent torque with only one digit on one side - lobes make that much easier, and would only need the thumb at the front if access was tricky at the back).

 

[stevelmx5]

Does a magnet stick to it?

The idea is to make it strong enough to not fail in normal use - defining the failure component and hoping it will be okay should tell you something.

The problem with something like this - a thing that's tightened by a human - is that the applied torque is variable. Yes, it will clamp the two bits together. It may well clamp them well enough to stop them sliding against each other. But you can't be sure that a human will only ever apply sufficient torque and no more. For that reason, it needs to be over-engineered to some degree. Ask yourself this: When is the best time to get the design of this bit right?

What sort of plastic is the screw made of? A purchased ready-made component or 3D printed?

Am I right in thinking that the thumbwheel part of the screw is about 30mm in diameter? (Looking at the disc under the hinge.) Unless it's some special material, it's almost guaranteed to fail. The plastic is unlikely to be tough enough given the poor overlap of the threads due to the slot, and the large diameter thumbwheel will make it easier to apply too much torque. I seriously urge you to reconsider how this part is approached.

My thoughts, off the top of my head...

Use the original 1/4" screw as the fixing. Find a way to lock it such that it doesn't rotate in the slot. If it has a metal loop on the underside (or any irregular shape) it should be possible to make a sqaure/rectangular 3D printed part that will fit around the shape as well as fit into the gap under the rail. Might need to fix the anti-rotation block to the screw to hold it in place when the rail extends beyond the adjustment block (or make a shape that is held in by the screw itself in some way).

Use a metal disc with a 1/4" tripod thread in the middle in place of the thumbwheel on the screw. 3-4mm thick aluminium would work okay, but basically as thick as the design will allow for, up to the protruding length the of the screw. Brass will work and feel better, but can be expensive. It's feasible to use something smaller as the metal part and 3D print the thumbwheel, but again needs an interference fit for robustness (easier to make this at home with limited tools). The disc (or insert) would need to be tapped with the matching tripod thread, so drill and tap needed (taper tap okay for this thickness).

This would be far more robust and has the added advantages of allowing the standard to placed anywhere on the slot, and no need to unscrew it to move to a new position - just slacken, slide, and tighten. The thumbwheel could have long lobes rather than be a disc, which would be easier to use (it's a potentially fiddly bit to get into with a finger and thumb, and would be hard to apply decent torque with only one digit on one side - lobes make that much easier, and would only need the thumb at the front if access was tricky at the back).

I haven't tested with a magnet. On hindsight it's more likely to be aluminium but I'll test later.

That's a good call on using the captive bolt although I'd need to replace it with a longer one as it's currently designed to go straight into a tripod socket on the bottom of a camera so is around 25mm too short. The only issue I can see with that method is that if it's loosened so that I can swing the front standard, there's a fair chance that the bolt will also move forwards/backwards in the slot at the same time which will throw out the focus.

My current design uses two layers of 40mm disks, one with a 16mm hole in the centre and one complete, which the ready-made plastic bolt is bonded to. I've tested the plastic bolts in an existing 6mm threaded hole in the focus rail and there seems to be a strong joint with good torsional strength but I agree that there's always a chance that someone could over-tighten. If I use a metal bolt instead of the plastic one there's a higher risk of stripping the thread in the plate but the bolt will be stronger. I reckon I need to do some crash-testing :0)

 

[Nomad Z]

I haven't tested with a magnet. On hindsight it's more likely to be aluminium but I'll test later.

That's a good call on using the captive bolt although I'd need to replace it with a longer one as it's currently designed to go straight into a tripod socket on the bottom of a camera so is around 25mm too short.

It's not clear to me why it would need to be 25mm longer, unless something has changed since post 242. In that, there's the top of the rail, then a fairly thin U-shaped piece holding the hinge. Under the hinge, there's a disc (or sandwich of two). I don\'t have the dims, but I'd guess 3mm thick for both the bottom part of the U-shaped piece, and for the disc, which would make 6mm in total. The tripod screw has to extend to the top surface of the disc, and doesn't need to go any further. In the pictures in post 242, if the screw was 25mm longer, it would pass through the hinge. Am I missing something?

The only issue I can see with that method is that if it's loosened so that I can swing the front standard, there's a fair chance that the bolt will also move forwards/backwards in the slot at the same time which will throw out the focus.

That's a good point.

My current design uses two layers of 40mm disks, one with a 16mm hole in the centre and one complete, which the ready-made plastic bolt is bonded to. I've tested the plastic bolts in an existing 6mm threaded hole in the focus rail and there seems to be a strong joint with good torsional strength but I agree that there's always a chance that someone could over-tighten. If I use a metal bolt instead of the plastic one there's a higher risk of stripping the thread in the plate but the bolt will be stronger. I reckon I need to do some crash-testing :0)

I thought the screw was M5? Bear in mind what I said about tapping into the sides of the slot, though - the less complete the hole is, the less overlap in the threads. An M5 or M6 nylon screw (for example) can be quite strong in a complete hole, and still feel pretty decent in a partial hole (like a threaded bit right at one end of the slot to get the tripod screw in before sliding it along the slot). But in a hole with so little overlap as M5 in a 4mm slot (see earlier diagram), the torsion is concentrated into a much smaller area of the threads on the screw.

 

[stevelmx5]

It's not clear to me why it would need to be 25mm longer, unless something has changed since post 242. In that, there's the top of the rail, then a fairly thin U-shaped piece holding the hinge. Under the hinge, there's a disc (or sandwich of two). I don\'t have the dims, but I'd guess 3mm thick for both the bottom part of the U-shaped piece, and for the disc, which would make 6mm in total. The tripod screw has to extend to the top surface of the disc, and doesn't need to go any further. In the pictures in post 242, if the screw was 25mm longer, it would pass through the hinge. Am I missing something?




That's a good point.




I thought the screw was M5? Bear in mind what I said about tapping into the sides of the slot, though - the less complete the hole is, the less overlap in the threads. An M5 or M6 nylon screw (for example) can be quite strong in a complete hole, and still feel pretty decent in a partial hole (like a threaded bit right at one end of the slot to get the tripod screw in before sliding it along the slot). But in a hole with so little overlap as M5 in a 4mm slot (see earlier diagram), the torsion is concentrated into a much smaller area of the threads on the screw.

Sorry for the confusion. The diagram in post 242 is another version of the front standard I've been working on which has individual movements for swing/tilt and rise/fall. The 'basic' standard has a single threaded knob that passes through the bottom section of the standard (16mm thick) then into the thread in the focus rail. When that's loosened, the standard will swing but if I have a fixed thread the standard won't move forwards or backwards.

I think I may have found an alternative solution which will be considerably stronger. If I drill three 6mm holes along the slot, I can embed three of these M5 tee nuts from the top;

http://www.ebay.co.uk/itm/122340439...49&var=422823896421&ssPageName=STRK:MEBIDX:IT

These then offer a complete 5mm thread which will give a stronger mount.

 

[Nomad Z]

Sorry for the confusion. The diagram in post 242 is another version of the front standard I've been working on which has individual movements for swing/tilt and rise/fall. The 'basic' standard has a single threaded knob that passes through the bottom section of the standard (16mm thick) then into the thread in the focus rail. When that's loosened, the standard will swing but if I have a fixed thread the standard won't move forwards or backwards.

Ah, it makes sense now.

I think I may have found an alternative solution which will be considerably stronger. If I drill three 6mm holes along the slot, I can embed three of these M5 tee nuts from the top;

Diameter d1 for the M5 part is 6.4mm, so without measuring the real bit, you'd need a 6.5mm drill (easier and cheaper to find, guaranteed clearance with minimal slop). I got a set of these from Screwfix recently, and very happy with them...

http://www.screwfix.com/p/hss-metal-boxed-drill-bit-set-metric-25-piece-set/14117

I needed some new drills of known sizes (rather than an assortment of crap in random boxes) for clearance holes above 6mm, and took a punt at 12 quid.

http://www.ebay.co.uk/itm/122340439673?_trksid=p2060353.m1438.l2649&var=422823896421&ssPageName=STRK:MEBIDX:IT

These then offer a complete 5mm thread which will give a stronger mount.

Yes, they look like a much better way to make the threaded holes - no strength issues there. The top of the threaded part will stick up above the plate, so you might need to add some clearance to the underside of the standard (6.5mm, deep enough to make room). Do you have a drill press / bench drill of some sort?

 

[stevelmx5]

Yes, they look like a much better way to make the threaded holes - no strength issues there. The top of the threaded part will stick up above the plate, so you might need to add some clearance to the underside of the standard (6.5mm, deep enough to make room). Do you have a drill press / bench drill of some sort?

Thanks, I think they'd be a much stronger option too. There's a 2mm thick rubber strip on the top of the focus rail to provide grip so I'm planning on cutting a slot in that that same size as the plate on the threaded insert. That way, the front standard still sits flush on the rubber. Forgot to say that yes, I've got a drill press here :0)

 

[Nomad Z]

You want the flanges on the fixings on the underside, not on the top. That way, the force is a clamping one, all made of metal. If you put the fixings in with the flanges at the top, only the bond is resisting the forces. If the flange is a little sub-flush of the surface of the rubber, the pull force applied by the screw will be whatever it takes to compress the rubber under the standard down to the thickness of the flange. If it's as firm the typical rubber that's found on tripod heads and the like, there's a pretty high risk that the bond will give way. If the flanges are proud of the rubber, the pull force is relieved, but the whole standard is being clamped onto a small area, which will have scope to rock a bit. If that motion were to occur with a shock loading, the bond could again break (a bump near the top of the standard would create a big lever).

Much better to put the fixings in with the flanges on the underside, glued in with something like 24-hour Araldite (stronger than the Rapid). When they're in place, use an M5 screw from above to pull it up hard to the underside of the rail and let it set. If the tops of the fixings are proud of the rail (to act as a locating pin, but viability depends on the scope to make a good hole in the underside of the standard), stack up some M8 washers with an M5 at the top so that the screw bears onto those and down onto the surface of the rail. Use a greater clamping force than would be expected with the thumbscrew when used by a gorilla. That way, the glue well set with minimal thickness between the fixing and the underside of the rail, and will be very unlikely to experience any compression forces that would worry it.

If you want the tops of the fixings to still be sub-flush of the rubber, the height can be reduced by filing a bit off with the part held in a vice, then spin a countersink in the top of the hole to remake the lead-in for the screw (if you have an M5 tap, you can run that through as well to clean up the thread at the top). The listing says they're carbon steel, but doesn't say what grade - a hacksaw might be tricky (hard to get it to bite cleanly at the start, so cuts could be a bit wayward), while a file will be easier to control. A good b*****d cut file should get through that pretty quickly. Even if the fixings were cut to half of their supplied height, they'd still be plenty strong enough (an M5 nut is about 4.5mm high, and no ordinary mortal will strip one with a thumbscrew).

 

[stevelmx5]

Good idea. My reason for mounting from the top was that the whole plate/thread is around 8-9mm so will be proud of the top surface. You've got a good point about filing them flush though so I'll use that method instead :0)

 

[Nomad Z]

7.5mm total - if you like at the dimension h, it goes from the top of the barrel to the underside of the flange. If the rail is 4mm thick, and the rubber 2mm, then aiming for around 5.5mm height on the fixing above the flange should be fine (unless the rubber is very squashy). Even if it's a bit tall, it can't need much more than 1mm taken off. Best way to work it out is drill the clearance in the rail and stick one in - you might be lucky and not need to file at all.

 

[stevelmx5]

Yeah, I think it could be close so I've ordered some of the fixings and will try them out when they arrive. I'll remove the rubber sheet from the top of the rail before I fit the thread then file it flush and refit the rubber above it to keep it tidy.

 

[Ulfric M Douglas]

Just a quick note : Lidl sold a set of taps and matching drill bits recently, I find them really excellent. Only tapered taps, not flat-ended for each diameter, but the drill bits are precise (and the correct sizes) and very sharp.

 

[stevelmx5]

Just a quick note : Lidl sold a set of taps and matching drill bits recently, I find them really excellent. Only tapered taps, not flat-ended for each diameter, but the drill bits are precise (and the correct sizes) and very sharp.

Thanks for that. I might take a trip to my nearest branch to pick up a set. Always handy to have.

 

[stevelmx5]

I've been looking at the seals needed for the layers and have modified some of layers slightly to fit a couple of internal closed cell foam seals.

I've also added an extra 4 magnets in the rear standard so that the film back is held on with 8 magnets in both portrait and landscape orientation. I can't just have 4 magnets in a square shape in the centre unfortunately because they will cut into the rebate where the DDS clips in. Adding another 4 magnets won't add much to the material cost or weight so is well worth it for peace of mind.

Another question I've been thinking over is what type of threaded knobs to use on the front standard. I'd originally planned to use (and bought) standard round headed knurled knobs but am wondering about using either t-shaped heads or cross-head to give more grip?

 

[Nomad Z]

Another question I've been thinking over is what type of threaded knobs to use on the front standard. I'd originally planned to use (and bought) standard round headed knurled knobs but am wondering about using either t-shaped heads or cross-head to give more grip?

Thumbscrews are generally the most ergonomic and should enable sufficient torque to be applied without difficulty. Types with spokes tend to be more fiddly to use, especially smaller sizes. There are exceptions, like awkward access, where spokes sticking out like on a capstan can solve a problem (like the lobed idea for under the front tilt hinge that I mentioned in an earlier post). The type of fixing can also be chosen to limit the amount of torque that can be applied to prevent damage (small allen keys are shorter than large allen keys for this reason). In general, thumbscrews work well enough in terms of getting sufficient torque applied while tending to limit over-torqueing (they get painful to turn, so the user stops doing it).

 

[stevelmx5]

Thanks again for the voice of reason and experience!

 

[Nomad Z]

You're welcome, Steve. I think it's always worth considering it pragmatically - virtually all view cameras use thumbscrews to stop slidey things from sliding, except in places where access is awkward or space is tight. My Shen Hao has a largish central lever under the rear standard for shift and swing, and two pairs of little levers at the front (to lock the standard fore/aft, and for swing), and a total of 13 thumbscrews (6 pairs and a single one to lock the fine focus). They all seem to work, so unless the particular application under consideration has a movement or clamping requirement that's substantially different from the norm, the chances are that they'll all work there as well.

The devil is still in the details, though. Not all thumbscrews are the same - the thumbwheel part can vary in diameter by quite a lot for a given size of screw thread, and that directly affects the torque that can be applied. Although a given human will tend to apply a given amount of turning force, which can then be controlled by choosing a particular diameter, that can change. Sometimes, the conventional finger and thumb grip is compromised, like in the disc under the front standard in the revised version - that one is finger and thumb tips, meaning a given human will apply less torque compared to the conventional grip, so a larger diameter will help to equalise. If it was fiddly to get a digit to the back of the wheel, maybe larger diameter again such that only a digit tip at the front will apply the required force, or change to lobes for a different way of applying the force (no longer friction against a knurled wheel, but pushing a lever, at which point lever length becomes the way of managing the applied torque).

Basically, I look at what is conventionally used in a given situation and take that as a starting point. At work, although I'm nearly always designing a new thing, the elemental functions of that thing are implemented by means that have been around for at least a century. In other words, virtually all mechanical motions and methods are a done deal - extremely well understood, and with countless empirical examples of how well, or not, they work in all sorts of applications. There are some constants that can be worked with, like strength of materials, friction between surfaces, conversion of forces (like torque to clamping), thermal effects, etc, and some less well defined semi-constants, like how much force a human can apply: Can a weakling make it clamp and unclamp? Can a gorilla break it? If they need a tool, which tool will work without risking damage, and will/can they use it correctly? There are also considerations of space, which I've already touched on. A great deal of this stuff can be done at the design stage, especially with some prior experience of the particular mechanical elements, even if the application is different.

Then there's the subtle stuff like ergonomics - it has to be usable, the controls accessible without being fiddly or becoming painful on the fingers, and the user has to know that it has done what it's supposed to do when a control is manipulated (like click-stops on aperture and shutter speed dials on a small camera used at eye-level - if you know here you start, you know where your adjustment will end up). Finally, there's the overall feel of the controls - there's a right amount of resistance in the clicks in an aperture ring, for example. It's much harder to design this stuff correctly from the start without specific experience in similar applications using similar mechanical elements. At that point, all you can really do is build a prototype, test it, and revise as needed.

Mechanical design is a fascinating subject which involves a whole load of stuff beyond the more obvious things like coming up with shapes and choosing materials. Especially so if something is to be placed in the hands of multiple end users.

 

[stevelmx5]

I think I may have just had an idea that changes the handling of the focus method entirely but simplifies it at the same time. I'm going to draw it up now.

 

[stevelmx5]

I've been thinking about the best way to create a focusing bed while still using layered sheets. I know that most LF cameras use a rack and pinion setup with a sliding bed so I've been going over some ideas in my head and come up with this option.

It removes the need to use the Pre-machined focus rail so should reduce weight as well as cost while at the same being more rigid. I've created a sliding bed in the existing base, using a combination of 5mm and 3mm parts, along with a 5mm thick pinion gear that protrudes over the edge of the camera base to allow for the focus to be controlled from the side, similar to the fine focus on my Kiev 4. The main benefit, apart from the reduced cost and weight, is that the camera is fully self-contained with Acrylic and the bed should be more robust. I've cut a slot into the bottom sheet of Acrylic with a small nub fitted to the sliding bed so there's a stop point to prevent the bed coming right out.

 

[stevelmx5]

As well as the stop point underneath, there will be a locking wheel on the thread fitted through the pinion gear so that the focus point can be locked easily. The multiple mounting holes for the front standard will have embedded brass threads that fit right through both layers (8mm total) for stability.

 

[stevelmx5]

The other benefit of the flush bed is that the standard is always fully supported rather than overhanging the sides of the metal focus rail as it is now. This should again help with stability.

 

[Steve Smith]

I like that focusing mechanism. I made mane with pinion gears from radio controlled car parts suppliers.

I would be inclined to have a smaller pinion attached to a larger wheel to increase the leverage and reduce the force needed to work it.

I would also add a cam or something similar to lock the focus position.



Steve.

 

[stevelmx5]

I like that focusing mechanism. I made mane with pinion gears from radio controlled car parts suppliers.

I would be inclined to have a smaller pinion attached to a larger wheel to increase the leverage and reduce the force needed to work it.

I would also add a cam or something similar to lock the focus position.



Steve.

Cheers Steve. I did think about adding a larger wheel underneath the bed, either as a permanent fixture or an add on for personal preference but that was at 2am so I needed to sleep :0) I'll have another look at it today with fresher eyes!

 

[ChrisR]

Looks good, Steve... is there a risk of the sharp teeth in that gear protrusion knocking against something (eg in your bag) and stripping the teeth, on either the gear or the bed? Also, could be quite sharp on the hand. A knurled wheel connected to a smaller gear, might that have smaller risk?

 

[stevelmx5]

Looks good, Steve... is there a risk of the sharp teeth in that gear protrusion knocking against something (eg in your bag) and stripping the teeth, on either the gear or the bed? Also, could be quite sharp on the hand. A knurled wheel connected to a smaller gear, might that have smaller risk?

The issue of a bare cog is in the back of my mind although the actual cog only protrudes around 12mm from the edge of the camera bed so it's not as large as it looks on the diagram. I'm thinking about adding a 1mm thick layer of rubber to the surface of the sliding bed (to add grip for the standard) along with a 1mm thick layer of acrylic over either side of the bed (to cover the cog to prevent fingers getting pinched or debris getting stuck in the teeth).

 

[Terrywoodenpic]

Have a look at the polaroid 120 focus mechanism it is unusual very stable, and could be adapted for layered plastic.

 

[stevelmx5]

Have a look at the polaroid 120 focus mechanism it is unusual very stable, and could be adapted for layered plastic.

You know what, that could be a perfect idea! I used to have a 110 myself and forgot all about the focus mechanism on that. I could fit the focusing knob on the underside right at the front of the base and use the same gearing. I'll have a play with that idea in a bit.

Cheers

 

[stevelmx5]

Yup, I definitely think that's a good idea @Terrywoodenpic

View attachment 102686



I've moved the rack to the inside edge of the bottom layer on the extending bed which means I can move the pinion gear underneath too. This means that the gear is out of the way of hands/debris and can be connected directly to a focusing knob fitted to the underside of the body. This should be far enough forward from the tripod mount too but I'll test that when I'm home and have mine in front of me.

 

[stevelmx5]

I'll probably add a trim section to the leading edge to prevent any open access to the gear too;

View attachment 102696

That will be bonded to the front face of the sliding bed.

 

[ChrisR]

Nice, Steve, solves my concern at a stroke. Looking at the model, though, will it be a pain that it wouldn't sit level on a table?

 

[Terrywoodenpic]

Yup, I definitely think that's a good idea @Terrywoodenpic

View attachment 102686

View attachment 102692

I've moved the rack to the inside edge of the bottom layer on the extending bed which means I can move the pinion gear underneath too. This means that the gear is out of the way of hands/debris and can be connected directly to a focusing knob fitted to the underside of the body. This should be far enough forward from the tripod mount too but I'll test that when I'm home and have mine in front of me.

Moving the gear mechanism nearer the centre like that should also make the focus smoother, as it is less likely to bind in the bed. With all the forces at the side You would tend get a cantilever lock from the twisting motion. I am not sure what the slot in the bottom of the bed is for as it weakens it.
If it is to stop the sliding par pulling right out by accident. It would be better to screw in a miniature captured ball catch to locate with a notch in the side of the the moving bed. This way the bed could be removed for cleaning or adding teflon powder to lubricate the parts from time to time.

 

[stevelmx5]

Yes, the slot along the base is for a guide pin on the sliding rail to stop it coming out of the front. It's only 3mm wide but there will also be a 5mm thick mounting plate (with a 1mm layer of rubber for grip) for the tripod thread bonded across the slot at the back which will help retain rigidity;



I might use a small grub screw rather than a fixed pin so that it can be removed if there is any requirement for maintenance of the bed etc. If I fit it with some threadlock it's not going to fall out accidentally but could be removed if need be.

 

[ianp5a]

The base plate, at the hinge end, does it need to be wider than the hinge? Does it need to taper in other words. Or is that just styling?

 

[stevelmx5]

The base plate, at the hinge end, does it need to be wider than the hinge? Does it need to taper in other words. Or is that just styling?

The rear is wider than the hinge for strength and to give more area to bond mainly. It's also a design choice rather than being a plain rectangle :0)

 

[Terrywoodenpic]

Yes, the slot along the base is for a guide pin on the sliding rail to stop it coming out of the front. It's only 3mm wide but there will also be a 5mm thick mounting plate (with a 1mm layer of rubber for grip) for the tripod thread bonded across the slot at the back which will help retain rigidity;

View attachment 102702

I might use a small grub screw rather than a fixed pin so that it can be removed if there is any requirement for maintenance of the bed etc. If I fit it with some threadlock it's not going to fall out accidentally but could be removed if need be.

This is all you need screwed in from the side of the bed and a notch in the side of the sliding part... and then no need to weaken the bed. the shear strength would be more than adequate.
http://www.wdsltd.co.uk/product/3052/spring-plungers-with-nylon-pin-metric-m3-to-m12-wds-606/