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Prostheta

The Luthier's Pantograph

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Prostheta    1,254

Having put some thought into the usable range in terms of fingerboards, etc. I think that the 45° corner origin is limiting uses. Parallel to the rear plane is likely better for working on larger wider pieces.

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Prostheta    1,254

First test cut today using templates made via Guitars and Woods' laser-cutting service (more on that in this week's article!). Far from perfect results, since I have found that the Dremel mount is a little imperfect. The tool deflects under load and this needs eliminating to increase absolute accuracy. It actually looks worse than it is; I didn't use a spoil board, so I stopped shy of cutting fully through so there's a bit of a lip of waste on the bottom edge. Cutting geometric shapes like circles and ellipses definitely shows any problems with the pantograph. It's a tough test.

You don't learn from your successes.

IMG_8169.JPG

 

....yes, I used the poor trick of "two pieces of masking tape and superglue" since I haven't got any double-sided tape on hand. It's no excuse, I know. I could feel the tape shifting slightly even under slight pressure....eeek....

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Prostheta    1,254

The issues have been identified, if not completely fixed for the long term. The second test cut turned out perfectly, even if the stylus (10mm) is a little too large for the cutter (3mm). In reality, this is probably the preferred combination of stylus to cutter sizing, since we don't want the coarse 3mm size stylus to cut right up to the finished edges.

If that were the case, a 3mm cutter would require a 9mm stylus. By using 10mm, the cutter leaves an internal/external margin of:

(stylus diameter / reduction ratio) - cutter diameter
(10 / 3) - 3 = 0,333mm

A third of a mm is fine for the smaller cutters to chip away at, so by taking all my cutters (1,0mm - 3,0mm in steps of 0,5mm) and working this backwards so that each cutter leaves a tinier margin, then the final finest cutter can simply do a finish pass.

IMG_8170.JPG
click for supersize

The stylus is currently a 10mm/M10 bolt through an insert in the arm. The slop in the thread is taken up by locking a second bolt from the top with a spanner, which centres and secures the stylus. That source of flexure within the mechanism was eliminated by snugging the upper bolt with the spanner rather than finger tight. Huge difference. Secondly, the lower through mount for the Dremel was epoxied into the crossmember. That part is the threaded bit, and since it's a large weird thread there was no good way of making a screw mount. I hate Dremels anyway, so it being more or less permanently-stuck is no big deal.

I may need to fabricate a new stylus arm with a different stylus holding mechanism. This is probably as simple as they can get (short of a small M10 thumbwheel perhaps) so I'll do more test cuts and see how confident I feel about it, or more accurately, how much confidence it gives me....

More advanced pantographs could include mechanisms for raising/lowering the cutter motor itself in addition to the stylus. This would allow for better control over the Z axis alignment of fulcrum/cutter tip/pantograph tip but isn't hugely necessary at this level. I simply want to produce as simple a working design as possible that anybody can throw together.

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Guitaraxz    30

It looks awsome. Thank you for sharing your design with us! I'm thinking of making one but in order to hold the Dremel in the Pantograph using this http://www.stewmac.com/Luthier_Tools/Tools_by_Job/Tools_for_Inlay_and_Pearl_Cutting/Precision_Router_Base/Precision_Router_Base.html 

I see the 3:1 ratio clearly, but how are you setting the depth of cut?

 

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Prostheta    1,254

You're welcome!

This is how the Dremel is mounted; fitted into a U-shape the same diameter as the body with a retaining clamp to add a little pressure and stability. The lower mount was drilled through and fitted with epoxy. It'll probably twist out if I really need it to. This means that the Dremel tool is fixed in the Z axis, so there is no adjustment there other than how far the cutter is fixed in the collet.

The adjustment comes from the stylus more than anything. Given the thin nature of inlays and pockets, any slight discrepancy from misaligned depth of cut is virtually impossible to distinguish. The focus is in the accuracy of the X and Y axes, which is manages extremely well.

The arm that holds the stylus will be being modified for a different method of holding the stylii. I don't like the current method as threads have so much slop in them, even when locked down.

----

The cutters I use are 0,5mm through 3,0mm in 0,5mm increments. I'd decided that it doesn't make much sense to go through every cutter, one after another however I'm going to try and make up (or have made up) a set of stylii which incorporate a slight offset so that only the final sizes (0,5mm or smaller if I get any) are taking finish passes. That would require the following sizes:

 

3:1 Reduction Pantograph Stylus Chart

ROUGHING
Cutter Size > Stylus Size > Offset From Finished Size
3,0mm > 12mm > 0,5mm
2,5mm > 10mm > 0,42mm
2,0mm > 8,0mm > 0,33mm
1,5mm > 6,0mm > 0,25mm
1,0mm > 4,0mm > 0,16mm

FINISHING
Cutter Size > Stylus Size
0,5mm > 1,5mm
0,2mm > 0,6mm

 

I don't have ready access to a metal lathe, and I doubt I could take off all that material using a cordless hand drill and a file! :lol:

IMG_8177.JPG

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Prostheta    1,254

@Mike.Mara made a few very useful suggestions re: stylii. Having given them some thought (and that this is something that I'd like anybody to be able to build) I decided to use 10mm aluminium tube. Instead of turning down solid rod, epoxy in cheap drill bits! Nice.

By using the next size of drill bit than the stylus requires in larger cutter sizes, the cuts they make will be undersized. The 1,0mm size is for the final cuts.

1mm cutter - 3,0mm stylus - final cut
1,5mm cutter - 5,0mm stylus - 0,167mm spacing
2,0mm cutter - 6,5mm stylus - 0.167mm spacing
2.5mm cutter - 8,0mm stylus - 0.167mm spacing
3,0mm cutter - 10,0mm stylus - 0.33mm spacing

That leaves the 3,0mm cutter as the roughing bit, and a choice between the middle three sizes for moving towards the final size and into details. I have a lot more aluminium tube left and a full set of cheap drills, so I could feasibly make a few more intermediate sizes.

Fitting them into the tube was fun, and needed some thought. The 8,0mm went straight in of course. The 6,5mm bit needed a few wraps of painter's tape. The 5,0mm had a couple of layers of heatshrink and a bit of tape. A length of a pencil fitted neatly into the tube for the 3,0mm stylus.

Neither masking tape nor heatshrink are going to keep the stylus tightly centred, so I chucked up each rod in a hand drill and ran it to check for concentricity. I'll show methods of guaranteeing this when the project goes to video....

The pencil will need centre drilling. Again, that's a fun thing for video!

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Prostheta    1,254

Negative template for cutting pockets. Brain farted and mixed up my stylii/cutter sizes. This is a 4,5mm stylus (designed for a 1,5mm cutter)....

IMG_9087.JPG

 

....running with a 1,0mm cutter. This means the pocket will be out by a bit....

IMG_9083.JPG

 

....as will the positive template when cooking up the inlay....OOOOPS

IMG_9088.JPG

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Prostheta    1,254

Test piece was cut from a sample chip of pearlescent plastic from Galaxy Plastics. It's a little more transparent that I would like. The test has some pinholes in the edges of the cyanoacrylate used to fit the inlay. Otherwise, it's more or less perfect. The 0,5mm discrepancy could make this even better.....

IMG_9089.JPG

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Prostheta    1,254

If I hadn't have sanded the inlay flat whilst the superglue wasn't fully cured (this test was for fit and nothing more) then acrylic debris wouldn't have been embedded into the edges. If this were a build, it would be a big negative point.

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Mike.Mara    48

That's some great work there! That panto-router worked out a treat :D.

When the pearl plastic is inlaid I actually quite like it, gives a misty ghost like effect... Very cool!

Speaking of which... That would look great on that ghost tele. ;)

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Prostheta    1,254
21 minutes ago, Mike.Mara said:

That's some great work there! That panto-router worked out a treat :D.

When the pearl plastic is inlaid I actually quite like it, gives a misty ghost like effect... Very cool!

Speaking of which... That would look great on that ghost tele. ;)

 

It's a good point. I'm not one for inlays, which is weird given how this project is totally bent in that direction! The Ghost Tele will have something that is virtually invisible if anything at all....and then that might be dots....! :blink:

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Mike.Mara    48

Was worth mentioning, I'm sure it'll turn out great anyways!

If nothing else it would make a great makers name inlay. Or set it up to engrave lettering too. Either way, great job!

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Prostheta    1,254

Second test was to do a two-stage cut, firstly with a 2,0mm cutter and 6,0mm stylus then with the 1,0mm cutter and 3,0mm stylus. The very edges when finishing up details and tight recesses with the fine cutter were kissed slightly after roughing with the larger one. Perfect.

IMG_9093.JPG

 

The inlay went in with a few taps from a small hammer, protecting the inlay with a piece of flat plastic. No real force was required for it to go in, however it was almost gapless, hence finger pressure alone wasn't enough. The real enemy in this test was the translucency of the inlay material, shadowing at the edges. Other than that, 100% perfect result. The Wengé and inlay were sanded to 1000 grit and given a light wipe with linseed oil.

IMG_9106.JPG

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Prostheta    1,254

Re-sanded back through from 150 grit to 1000. I definitely think that a second fill with water-thin CA will help tidy the outline. Otherwise, this is super tight.

IMG_9117.JPG

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