Japanese banjo tuning peg help


Hello everybody, I’m working on my first sketchup make project. I was cheeky enough to try and take on sketching a 3d model of a tuning peg for this shamisen instrument I want to build seen here on the far left of the image labeled suroku: https://shamisenjapan.com/wp-content/uploads/2017/03/Itomaki3.png

However I can’t seem to figure out how to sketch it. It is basically a hexagonal wooden block that slopes into a circular peg at the other end. The two methods I tried were 1. extruding a hexagon block and putting a cylinder on top of them and then tried to blend them together like this https://gyazo.com/b4cd827f68dc8bbba312cec38bc37671, didn’t work 2. Tried to trace from an image, couldn’t figure it out. If anyone has any ideas I’m sure this is really easy but I’m not too bright.


You are plenty bright and you are getting pretty close to success. Looks to me like your attempt is a very good start. If you were able to taper both the cylinder and the hexagon you would be there right? Well guess what… you can!

First let me say it’s easier for you to just post your whole model here in your reply, then we can see how your model is set up. Questions: are you using groups or components, and are your hexagon and your cylinder separate groups? Do you know how to hide or unhide things or turn on x-ray mode? Have you used the size tool yet? I could model this in a flash but better for you to “learn to fish”, then you can model it in a flash. answer these and post your first attempt skup file.


Thanks very much! I have not used any of those tools and I think they are separate components because I just used the polygon tool and the circular tool and just pushed and pulled them accordingly. I would like to know how to use the size tool so I will look that up. The taper is sort of odd, seen here: http://bachido.com/images/products/full-size/sakura_shamisen_18.jpg

Here is what I have so far.

itomaki.skp (146.0 KB)

I tried tapering with the move tool, the follow me tool, and the offset tool with no luck.


The Circle and Polygon have 4 Cardinal Points which you can find by hovering the Move tool over an unselected Circle/Polygon. See examples of playing with this feature.

When you don’t have cardinal points to play with, then select the geometry you want to tweak and use the Scale tool, Scale about center - Scale + Ctrl - is quite useful.

In your model, the hexagonal handle geometry doesn’t have any entities with cardinal points. That part will have to be redrawn if you want to play with cardinal points. But the cylindrical part still has Circle entity at the top and bottom of the cylinder portion. That still has cardinal points.


Yes, scale around center is your friend here. The trick is you only want to scale one particular face and let the rest of the faces get dragged along with it. Start by selecting the bottom face of your hexagon only, then scale that around center.

oops, that should have imbedded

Peg from Sean Riley on Vimeo.


Here is the file to play with. also check out view > component edit > hide rest of model for another helpful feature I used.

Peg.skp (104.8 KB)


Thanks a lot, I downloaded your skp but I’m going to try and follow your video first. Most importantly I have to figure out what the middle tapered parts measurements are. I know the circular has to be 12mm in diamater at the thickest part and the hexagon width is 25mm and the book says something about the tapering the slope of the peg to 2 degrees. it’s a little confusing. Thanks for all the help sean and cata. I will post the final thing when I can get it perfect.


itomaki2.skp (152.7 KB)

Would anyone like to inspect this I think I have finished? I’m going to have to drill a hole through it but I need to make sure it’s 3d printable.


The base group is solid so it should be printable. It’s got an unnecessary group level. Explode the outer one and you’ll be fine.

Where are you going to put the hole? How big is it? You might consider that drilling the hole in the completed peg will give you a nicer hole than 3D printing it.


my thoughts exactly on the hole, it needs to be 3/32 I think I will just use a drillbit


Good idea.


Not so fast on drilling the hole! While your model is a “solid” in terms of ability to 3D print, it is hollow inside!

So any hole you drill will cut material for only the thickness of the walls. If you need a hole THROUGH the part, then you’ll actually be drilling two holes - one as the drill enters the part and another as it leaves. In between will be no material.

If this hole will be taking a fastener that applies ANY pressure, tightening the fastener will distort the part!

There are a couple of solutions:

  1. Add a hole with material surrounding it. Diameter of the hole is desired diameter of finished product. If, due to shrinkage/expansion of material (which depends on the printing process and material you’re using), the hole ends up too small, then you can drill it out. Surface quality of the inside of the hole is something you’re not likely to care about - as it will be hidden by the fastener!
  2. Add an interior post with a large enough diameter such that, when you drill through it, there’s enough support material after you drill to resist the compressive force of the fastener.

If you’re printing yourself, you need to pay attention to the details your slicer program presents. In case #2, your slicer may want to make the interior post as a solid edge with a “filler” pattern within it’s bounds. You don’t want to drill through the filler pattern - it’s not solid! In case #1, your slicer program will apply the filler inside if the wall is too thick.

If, on the other hand, you’re having it printed by a printing service, you have no control over the slicer. Here, I’d recommend case #1 as the service’s product review will most likely recognize that the hole might have a compressive fastener applied to it and will make sure the wall thickness of the hole is OK.

Perhaps a picture will help:

Note that I use RED for my “inside” face color.
The cube (with parts hidden by section plane) on the left shows case #2, the cube on the right shows case #1.

Here’s the .skp file if you want a closer look: Two Holes.skp (59.1 KB)


The peg is not drawn with a hollow space inside like your box is.


True, but almost all slicers will give a thickness to the exterior that it will fill with material 100%, then “fill” in the rest with a less dense mesh - usually specified as a percentage.


Nice job. So I see that you blended the round portion of the peg with the hexagon handle using soften/smooth? This looks very nice but the result is that the round end you insert into the shamisen is actually only “roundish” for the first 10mm or so then it slowly starts to turn into a hexagon as you move toward the handle. My understanding of how this part interfaces with the instrument is that the peg fits into a tapered hole and keeps it’s position by friction around the surface of the taper. The examples you posted all had a smooth taper peg attached to a shaped handle.

The result may be that depending on how far you need to insert this peg into the hole it will not fit, or it will fit but will be bearing on a few contact ridges at the extremity of the hexagon shape, instead of a smooth contact between the pin taper and the wall taper.

it’s a square peg in a round hole :wink:


It’s an intense learning process! I don’t even know if the peg part is tapered to the right slope but I guess I will find out soon enough! I will let everyone know and obviously anyone can use this for their shamisen lol.


Are these pegs to be made of plastic? Or do you have someone 3D printing in ivory?

In your original post you say: [quote=“computerfr0d, post:1, topic:43997”]
It is basically a hexagonal wooden block that slopes into a circular peg at the other end.

If you are making them in plastic, will you be able to get the appropriate surface finish?


I was thinking about getting the wood strips for the 3d printer but for now I’ll just try plastic since it’s cheaper. they can also be made out of ivory I suppose, but I have no idea how to carve ivory!


Thanks @sjdorst , I don’t actually own the printer I was going to have my friend print them so I tell him this when I see him. I will take a look at your file to see what you mean first.


I was kidding about the ivory but I guess I’d be inclined to make them from something like holly. It wouldn’t be difficult to turn most of the peg and then add the details by hand. I can see in my head how to make a jig to use with a small trim router to add the details, too.

Are the tapered holes already made? If so, I would expect you could get the angle of the taper to transfer to the pegs