The Luthier's Pantograph

[Prostheta]

An idea I was throwing around yesterday; a reduction-based pantograph specifically for the luthier to do inlaying on fingerboards and headstocks. For those unsure of what a pantograph is, it's a simple mechanical system capable of duplicating a pattern of movements with opportunity of ratio alteration, eg. doubling the size of the original.

https://en.wikipedia.org/wiki/Pantograph

The project bounds are simple:

• benchtop-sized unit
• works with a Dremel or end-pen type milling tool
• approximately 5:1 - 4:1 reduction
• simple templating system
• simple centring and alignment system

I'll draw some CAD sketches over the coming weeks and develop a prototype. The end objective is to have a full design supported with a complete how-to published within a month or two.

[Prostheta]

Doing a bit of CAD sketching today to understand the constraints of the design when it comes to practicality. I'm also typing sort of stream-of-consciousness here. Bear with me and feed in by all means.

The current rough size of the bed is a metre by a half metre (about 40" x 20"). Any larger than this defeats the "desktop" purpose. Smaller would be nicer, however that of itself causes certain other issues.

...namely the stability of the pantograph. It's all well and done reducing the movement of the pilot at the cutter, however the moving structure should ideally be supported at all points. Perhaps the strut through which the cutter is mounted needs to bear flat onto a fixed surface to be more or less vibration free. Perhaps having it running face flat on the top of a small "box window" over the top of the cutting material will give it the support it needs. This does beg the question about the amount of adjustment the cutter needs in the Z axis. It's likely that an end user might want to cut a board already on a neck just as much as a blank. That's a consideration. The more mechanisms around the place increase potential for vibrations and slop.

This box sitting over the face of the cut does increase the possibility of chip removal....

Going for a ratio which is of a high reduction in the XY axes also has the effect of doing so to the Z axis. The upshot of this is that the pilot needs to be lifted/plunged by a large distance in order to lift/plunge the cutter itself. The fixed point of the mechanism would probably benefit from being mounted using some sort of universal ball joint. That's an umm and ahh right there. Solution with no answer as it were.

The pilot might benefit from a sprung plunger of some sort. Maybe a set of pilots sized up in the same ratio as the pantograph on the end of a rod fitting into a tube with a spring? Hmm.

More to come.

Edited September 25, 2015 by Prostheta

[Prostheta]

A casual Google search came up with a bunch of other people's pantographs for more or less the same idea. It seems that in many ways I am overthinking the sturdiness. It shouldn't be too much of an issue. A lot of these pantographs are fairly overbuilt which I think is an option I'd rather not incorporate into this design. A simpler "everymans's" project which can be scaled up from a quick day-build through to a week's work for a more permanent version.

[SwedishLuthier]

I have been thinking along the same lines. Altho the idea that started me was when I saw a non-reduction pantograph, were a inlay was first cut and glued upside down on a flat piece of something. Then friendly plastic were pressed down over the inlay. After it had set the FP was flipped 180 degrees and a pantograph with a dremel at the business end was used to make a very accurate cut for the inlay using a stylus the exact same size as the dremel bit as a follower in the FP. Its going to be interesting to follow this...

[Prostheta]

Good call on the moulding plastic. That never occurred to me!

The reduction ratio should be at the very least 2:1. I think perhaps 3:1 might be more sensible than 4:1 in terms of compactness and rigidity. My personal project bounds in this include that it can be made from simple bits and bolts that you can pick up at the hardware store. Nothing exotic or specialist. That is why the fixed point is throwing me a loop. I think I'll have a look to see how other people have solved that one. No shame in doing that. It's not like it is an original idea.

[curtisa]

Neat idea.  I hadn't considered a pantograph reduction mechanism as something that could be applied to guitar making.  I'd be curious to see how well it goes with making inlay cavities for fretboards and the like - will the gearing reduction translate into ultra-fine detail?

[Prostheta]

This is what I'm working on rattling out. I'm going to use common skateboard bearings to eliminate binding flexure from the outset. Originally the design in my head was based on single leaf arms which could introduce twist, whether that be distortion of the mechanism through flex or a means of allowing vibration.

To be fair, I've found that this is not a unique idea. In some ways that is good because they expose or solve problems, however bad also since I prefer fixing problems through directly invented solutions. Convergent evolution is going to be likely however it develops of course. 

This is what convinced me to go for stacked arms: https://woodgears.ca/pantograph/inlay_followup.html

 

[Prostheta]

This is a rough idea of the basic reduction mechanism I threw together.

...it's in no way representative of the more compact design I'm aiming for though. The relationship between the pilot (lower right corner) and cutting point (mid-arm, towards the top) is 4:1, which turns out to be a very large reduction ratio. Probably an impractical amount of potential detail since we likely wouldn't use bits getting towards the 0.2mm mark (1/128"?). It appears that a lot of existing examples use 2:1 with no problems. I do think that the weight of the arms is about right for structural stability though, albeit with at least two stacks. They'll happily work in conjunction with 608ZZ bearings and 8mm steel rod.

Any thoughts on whether I should go into this with Dremel tools in mind (~125W) or something a little beefier like a palm router motor (600W)? I don't think Dremel tools are really up to snuff for this kind of thing. Foredom pens, sure.

[ScottR]

Dremels on a router base are plenty for fretboard inlays which rarely need to go deeper than 2 mm. I guess for something John's Hand of God or a full dragon it might start to feel a little small.....

SR

[Prostheta]

The beauty of this kind of setup is that you can set up enlargements of standard things like logos and repeat them time after time with accuracy.

Thoughts on this one? I love the guitar knob adjusters!

[Prostheta]

Okay, so a bit of fiddle farting with the design left a very usable 3:1 ratio pantograph. There are a few designs out there with one that is adjustable between 2:1 and 3:1 however I think that constant reconfiguration will wear the mechanism out, introducing play and inaccuracy. Building securely in the first instance guarantees a unit that will last a long long time and produce stellar results.

The long rear arms are curved to allow the front cutter mounting arms to achieve better range. I haven't worked out the theoretical maximums of this design yet, however they're clearly far larger than most people will use regularly. More than enough for an inlay spanning several frets or an entire headstock!

[Prostheta]

Looking at brushless DC motors on eBay; anything around the range of 200W-400W seems a good choice. Dremels are around 125W and basic Foredoms 150W. Speed control would be handy for doing plastics or shell.

I was wondering to myself whether it would be feasible - or even safe - to mount the fan assembly from a small 15mm-30mm DC fan to the cutter as a way of evacuating material....

[komodo]

Spectacular. The only thing I don't like is wasted material, since it costs so much. I can be pretty careful in placing pieces on a sheet of pearl to get maximum yield. That said, the accuracy and speed might be worth it. I'd also be pretty worried about using a fan for evacuation, since you don't want that fine shell dust in the air. It would be super cool to have a vacuum somehow. 

After watching that vid, it's clear that accurate depth adjustment is critical. Also, a really well made template from acrylic. I wonder how much time you would end up saving in the end? If it's a repeatable, logo or something then the benefit would be awesome. 

Edited October 30, 2015 by komodo

[curtisa]

Based on personal experience with the baby CNC machine, a 200W spindle motor will comfortably have enough oomph to do this kind of work. Whatever solution you settle on will need some way of securing and changing bits to allow for different reduction ratios and cutting detail.

Fitting a fan is an option, but you either need to have it blowing into the workpiece and spreading the dust everywhere, or drawing air away and having some way of collecting the dust being drawn through the fan. For such small work it may be easier to just let the cutter do its thing and have a vacuum cleaner handy to clean up as you go.

[Prostheta]

At this stage, the prototype is being left at one single reduction ratio. Once built it can be kept as-is without constant changing and reconfiguring. Rebuilding any jigs causes more slop over time, and then you end up with a crap tool!

I'll figure something out about the extraction. It's certainly possible since there is very little depth of cut and range of movement. A simple "walled off downdraft" is likely to have good results. I'll get the main mechanism finished first, then once the working prototype is bagged we can brainstorm making a better mousetrap.

[Prostheta]

ZOMBIE THREAD TIME!

 

[Prostheta]

This week has truly been one of catchup. Not having access to the big machinery at the workshop over summer means I have to prepare a lot of stuff for home projects. I figured that working on the pantograph would be a good one.

I made new templates for the various parts and produced them on the pin router. The large rear arms are bent so that they do not interfere with the Dremel mount. I didn't install bearings on this one as I think it will be fine otherwise. Perhaps oiling the parts and waxing them should be enough. It doesn't need to be beautiful as long as it is perfectly functional.

When I have finalised a few things, the base will be marked out with corresponding squares to indicate placement of templates and workpieces....

[ScottR]

The tools and aids you build are works of art.

SR

[Prostheta]

Thanks Scott. I appreciate that. When I get good time to concentrate on doing work well, they turn out excellently. Unfortunately, since my time with the big toys is very limited these days projects like this drag out over months rather than days. In some respects, it does mean that I can plan out these ideas rather than "making it up as I go" in the workshop. Things like the arm components are all relatively simple to lay out in plywood, then copy across to "real wood" with the pin router.

Off to go check out hinges for the main articulation point. I am suspecting that they will have a bit too much play for my liking, so I might need to alter the main mount into something using bearings and a shaft. Some nylon spacers between the arms will help things smoothen up a bit too. I'm hoping there's as little play in the mechanism as possible. I think it should be good as it stands.

[Prostheta]

I just realised that the mounting arm is the wrong way around. It just needs flipping. The cutting tool needs to be in a direct line between the stylus and the common barycentre.

[Prostheta]

Today I made a pair of bearing blocks to hold the main articulation point. A hinge is clearly not going to cut it here. Any play in the mechanism is bad enough, but that far up? No way.

First test cut:

 

It went flawlessly. The stylus is 8mm in diameter, which is incorrect for the 2mm bit. 6mm would be right (3:1 reduction ratio, stylus diameter included). Still, I'm impressed how smoothly the entire mechanism works. Little to no racking, and certainly at these small tool pressures even that won't cause issues. The arms were given a chamfer to lighten it up a bit and also to reduce part-to-part friction. When my nylon washers arrive, it'll be even smoother. Hoorah!

[Prostheta]

The blocks contain a pair of standard 608 bearings (22mm OD, 8mm ID x 7mm) which were sunk into the inner face of each block. The rest was drilled out to 12mm so that the shaft doesn't contact the block, only the bearing. Again, less friction for more better.

[Prostheta]

I marked out the baseboard with gridding for the templates and workpieces. The next job will be to relieve the bearing blocks a little and cut a small relief in the lower side of the rear lower arm since they both clash at the extremes of movement. Not hugely so, but enough to become bothersome. Freedom of movement and the ability to "park" the pantograph away from the workpieces would be useful.

The finish the whole thing off, I need to order a bunch of carbide endmills and make up corresponding styli out of M8 threaded nylon bolts. That might not happen until August since this would be easier to do on a lathe....probably not best done in a hand drill....

Oh, and to fit more permanent hardware.

[Prostheta]

As a last "note to self", I'm considering adding small reinforcing blocks between the upper and lower arms front and rear to increase internal rigidity....

[Prostheta]

Off the back of this pantograph, I'll be writing an article on how to produce usable vector drawings for companies offering laser cutting. For obvious reasons, being able to produce a highly-accurate positive and negative template set for logos or inlay of any sort means you can cut both the inlay and the pocket itself. Many companies charge by the hour plus materials, so reducing setup time by producing files that can more or less go straight to the laser drastically helps your back pocket contracting external services like this.

The layout of this pantograph isn't 100% conducive to say, inlaying a fingerboard however moving some bits and pieces around isn't too difficult for this purpose.