Back to the drawing board

[darryl]

A long time ago I had an articulated arm that held a plastic square- a drafting machine. This was pretty basic- for angles you'd use other plastic drafting tools with it. Now I'm finding that I can make use of this again- for my 24 wide x 16 deep pull-out shelf.

The concept is pretty basic- each 'arm' has two control rods with pivots at each end, and there's a two pin mount fixed to the table, a four pin 'inter-tie', then a two pin mount on the plastic square. You can move the square around the table and it maintains its orientation.

I'd like to make one, and because I'm prone to over-engineering things, I'd like some input as to how I should make the pivot points and keep this very simple. It does have to be very low profile, because when the shelf is closed there's less than an inch from the top of the shelf to the underside of the desktop that it slides into. My old one used rivets, and it wasn't too bad, but there was play there. I want zero play, but of course nice action. Has anybody made one of these?

[Paul Alciatore]

I have never made a shelf support system like that and frankly, I do not have a lot of confidence in being able to. IIRC, those drafting machines were made to be supported by the table and when they were lifted off the table they flapped in the breeze. They were 2D devices, not 3D. So, just how much weight will this shelf need to hold?

The first thing I would suggest is to keep the weight of the shelf itself as low as possible. No particle board or steel here, perforated aluminum comes to mind.

I take it that the shelf will store under the table, out of the way when it is not in use. But then, how far out will it need to extend? Normal drawer slides would normally be used for a slide out shelf and then it only slides out for it's own depth. And even those are known to flex when used. A computer keyboard is hard enough to support and much beyond that is going to be a real challenge.

I am not saying this is not possible, but it is going to be a challenge. I am making some slide out shelves for some of the cabinets and a shelf unit in my shop. The cabinet slides are small but even there the standard, hardware store slides, are marginal. I purchased two pairs of heavy duty slides for the shelving unit: they cost me around $90 each. They will each support a 2' x 4' shelf and I am wondering just how much weight I can put on them. Some bottles of oil and solvents? Probably. My 10" RT and it's tail stock? Probably not.

A thought: instead of the drafting machine type mechanism, how about two pairs of drawer slides, mounted at right angles to each other. That way you can position it in both directions but the support will be a lot firmer.

Another thought just hit me. Are you thinking of using this mechanism in a vertical orientation instead of horizontal? That would seem to make more sense.

[darryl]

Ok- here's what I have. My desk top is 3/4 mdf, laminated both sides, laying across two cabinets. The space between them is about 25 inches. In that gap there's two pullouts- one is for the keyboard and it's on regular 3/4 extension slides. The other is above that, and angled downwards to some degree. When it's pushed in all the way, there's about 3/4 inch gap at the back edge, and a bit more than 1-1/4 inch at the front. This pullout is on full extension ball bearing slides. Both pullouts are 1/2 inch mdf, laminated both sides, and the upper one has aluminum channel put over the left and right edges so the screws for the slides don't split the material. Both are 16 inches deep- the upper one pulls out all the way. Obviously I can't operate the keyboard when the upper one is pulled out.

It's the upper one that I want to put the drafting arm on. I'm left handed, so it will mount near to the right side, and at the back of course.

It will be pretty much horizontal, nothing can change there. When I pull the shelf all the way out, there is clearance off the right side for the arm to use.

Right now I have a lip put on the front edge for plastic squares to align to. I can use that with an adjustable stop to maintain a back and forth position if I want to- in other words the 'head' can move left and right while maintaining the same line- but it might be more difficult to do this with the other motion. Doesn't matter anyway. I have already played with the x and y axis thing- it has its benefits for sure, but it isn't going to fit here.

The length of the arms I use for the drafting machine might be somewhat critical- I'll probably mock it up using cardboard arms and pins just to make sure before I commit to cutting materials.

[dian]

not sure if i get it, but if you want a mechanism to hold the shelf attached at one point, look at the arms for tv screens and computers.

[darryl]

The shelf is good- it slides in and out on ball bearing slides. It's mounted under my desk top. I pull it out when I want to work on a layout, then push it back when I'm done or want to use the keyboard. The drafting arm which I want to replicate will mount to the shelf, and slides in and out with it.

I'm mostly concerned with how to make low profile, zero play and low friction pivot points. It appears to me now that the back pivots will have to hold the first pair of arms up in the air in order to keep the whole mechanism clear of the shelf. The only part that needs to touch down would be the head where the 'calibrated drafting square' mounts. I don't want to over engineer this, but I do want to be happy with how it works. As I said previously, my original drafting machine just used rivets to hold the ends of the arms. I want to keep it simple, but do something better than that.

[darryl]

If this works, it's a picture showing the basic construction of the machine. I would of course make it sized to fit my working area. Looks like the pivot points are little ball bearings- could be just bushings.

[dian]

a possibility for the pivot points are screws/studs with a spherical end into cupped receivers. at least thats how this works:




also keep in mind, in case you need friction joints, that multiple surfaces work better. you know, instead of bolting two plates together you "fork" the ends and get three surfaces instead of one.

[JRouche]

If this works, it's a picture showing the basic construction of the machine. I would of course make it sized to fit my working area. Looks like the pivot points are little ball bearings- could be just bushings.

I get where you are going.

If you have ever used a nice drafting table you would know the glide and absolutely no jitters.

I like you idea. JR

[darryl]

I have one with a 6 ft long beam, ball bearings, smooth sliding mechanisms, nicely engraved markings, it has it all. I have nowhere to set it up. Sure is nice though.

[BCRider]

I recall seeing a sort of downscaled plastic version of what you're after. But that was then and this is now the age of CAD so I can't imagine where such a thing might be found any longer. I seem to recall that even it had a way to loosen up and set the square arms to some other angles. But it was all just plastic.

There was another option too. The ruler that ran on a set of cables so it was always parallel. Much like having a built in "T" square without the need to ensure it was up against the edge. What about something like that? Or perhaps a captured "T" square where it rides in a U track of extruded aluminium with two ball bearings that are fixed and a "slack eliminator" third bearing to push the other two against the side of the edge. This could be removable so you can lay drawings or paper down. It would require right angle squares for the vertical lines or you could make the "T" square with a U track in it with a similarly captured vertical mode "T" square. Give both axes of travel locks and you've have a pretty slick little drawing tool that might be easier overall to make.

[dian]

interesting, but no idea what your saying. he needs to stabilize the angle, right?

[old mart]

When I was at school, in one of the classrooms, all the desks had a horizontal plastic strip, about 2 1/2" deep by 3/16" thick. It ran up and down the desk and was the full width. Keeping it parallel to the top and bottom of the desk was a system of pulleys and Bowden cable on the back of the desk top. It made resting squares and protractors easy when making technical drawings.

[Dave C]

When I was at school, in one of the classrooms, all the desks had a horizontal plastic strip, about 2 1/2" deep by 3/16" thick. It ran up and down the desk and was the full width. Keeping it parallel to the top and bottom of the desk was a system of pulleys and Bowden cable on the back of the desk top. It made resting squares and protractors easy when making technical drawings.

My first job in the engineering department was designing transformer tanks. My 6 foot drawing board was equipped with a parallel bar like you mentioned. I envied a couple of other designers that had more seniority, when they got new beam type Bruning machines.

[BCRider]

Yep, that's the rig. The bar that slides up and down and stays locked horizontal by using the cable. It's very low profile. If you only have an inch of room to work with I'm not sure you could make the articulated arm style to fit that small a space. It would be a fair bit of work either way.

Years ago just before I self taught myself TurboCAD for 2D drawing I found an old used Vemco V Track machine which still needs a couple of small simple lathe turned parts to restore it to full functioning. But once I learned and got comfy with 2D Cad I just have not got around to fixing it. I should post it in the local CL as a freebie and let someone else take it or fix it up and sell it for a pittance so they can use it. I just don't have the room to dedicate to something of that sort or the need for it.

I did a lot of after hours drawing on the similar size Vemco at work for model airplane plans. And as a result I was the "Go To Guy" for any technical drawing that needed to be done. They truly are wondrous at what they do. But they sure as blazes won't fit in the little gap we've got here ! ! ! ! 😆

[darryl]

Well, I have a plan. I found some music wire pieces with a right angle bend on one end. I'm going to bore a pair of 1/2 inch bolts for a close fit of this wire. This is 1/4 inch wire, so it has pretty good sag resistance over the foot or so of length. I'll make a steel strap with two holes in it as a spacer, then mount the bored bolts to the back edge of the shelf. The wire pieces will insert into them and will then hover over the shelf. This will serve to carry the rest of the assembly just high enough so it clears what it has to. That central piece will have four holes in it to mount the arms, and I'll make six pivots from brass to fit those holes- four for the center plate and two for the last piece which holds the plastic square. I'll use the spacer I made as the square holder. With all holes spaced equally, the thing will work like it's supposed to. The six brass pivots will be cross drilled to fit the arms, which will be epoxied in place. I'll use a jig to ensure an exact placement so the arms have exactly the same length between points.

At this point I'll have the 'upper' arms mounted, and the center plate mounted. The lower arms will use the steel strap spacer I made previously as the holder for the plastic square. I have a square I salvaged from another drawing machine, but I'll have to cut one of the arms a little shorter so it suits the room available. Being left handed, the long arm will become shorter and the short arm will become the long arm, as this will be mounted on the right side of the drawing table instead of the left as is usual.

At any rate, I've pretty much answered my original question, which is how should I make the pivot points. I have some hex shaped brass lengths, which will make it easy to cross drill, and if I bore all the mounting holes first, I can then machine the brass to be the close fit I want as pivot pins. I'll hold them in place with button head cap screws. No rocket science involved here. I'll probably use TriFlow as a lube.

The square I'll be using is a Staedtler, and it has degree markings and positive lock at various angles. If I move it all the way towards me, the front edge of the shelf, it just barely touches the underside of the desktop when I push the shelf all the way in.