Purfling marker or cutter?

[JonGeo]

Good day all

ive arrived at the perfling stage on my build. I’ve read and watched and watched and read on how to do this but there’s no substitute for the wisdom that come with experience.. 

I recently bought a purfling tool as pictured and was wondering what is the proper/best way to set it up. I’ve read reviews on this and it was fairly well received for it price but a lot of the reviews would say something to the effect of “it works fine but I had to touch up the blade first.” My question is.. what does that mean? It comes with 4 blades. 2 for the tool and 2 spares.. the 2blades together in the holder are 3mm wide and taper down to a point which is around 2.5 mm. The purfling channel is supposed to be 1.2-1.3mm depending on the purfling. So I assume I’ll need to do something to narrow that. Secondly I’m sure I want the walls of the channel to be perpendicular to the surface so I’d need the out side edges of the blades to be flat.. making an upside down M. Aside from the grinding that would take to get each blade down from 1.5mm to .65mm at the thickest thinning the blades would make them flimsy…. I would think. So I’m wondering.. am I thinking of this tool all wrong.. perhaps I should think of this as a marking tool? And I need to grind the sharpened tips down to a width of 1.2-1.3mm? Or use it single blade and mark the purfling channel which would take longer and possibly more tedious.. 

and lastly.. what kind of bevel for the blade… or just sharp tips?

[Davide Sora]

 

You can find many other endless discussions on the topic here on Maestronet.

[nathan slobodkin]

The purfling cutter in your photo looks as if it is made to use on an already assembled instrument. Most people who purfle the plates before assembly prefer to have a handle above the cutters rather than below them. As far as the width of the cutters if you are only marking a line to be cut later with a knife then a double bevel should be fine and you should grind so the blades pressed together have the points at less than the minimum width of the purfling. You can then use paper shims between them to adjust width. If you are going to try cutting to depth with the guided tool then a single bevel and a single cutter will work best.

[JonGeo]

1 hour ago, nathan slobodkin said:

The purfling cutter in your photo looks as if it is made to use on an already assembled instrument

The blades can be flipped around. Thanks for the pointers it helps.

@Davide Sora thanks for the link I’ll be checking it out.. your videos have been very helpful as well.. thank you

[MikeC]

I got that one.  The needle tips had to go.  I reshaped it to a rounded knife edge.

I figured the needle tips are just going to scratch the wood and tear fibers.   Didn't even try it that way, I reshaped them as soon as I got it.  

  I find that with both bevels facing the same direction the distance between the cutting edges is just right for my purfling.   Well, it's a hair loose maybe but that makes putting the purfling in easier and there's no visible gaps except where I mess up.  Messing up is not the cutters fault though.  heh 

edit:  Back plate is finished,  part way through I decided to try just one blade cutting at a time and that worked better than using both at the same time.  

 

 

 

 

 

[JonGeo]

@MikeC I was wondering if shaping them like that was helpful.. did you use a grinder or did you do it on a stone.. I can’t get a good enough grip to hold it for my combination belt disc sander so I’ve been grinding away on my diamond plate.. did you bring the thickness down on the individual blades? I think I said they were around 1.5 stock.. did you have any issues with the bevel facing out compressing the wood? If so did the hot glue swell it back? Did you notice any difference between the maple and spruce… I’ve been thinking about going single blade. Do you cut to full depth single blade or just score then cut later with a different knife?

 

sorry so many questions 

[MikeC]

It has been a while but as I recall I shaped them on a diamond stone.  I may have touched them to a grinder just to remove the sharp needle tip,  but I don't remember.   I didn't reduce the thickness of them.

   On my last build I used them as a marker cut not very deep so no problem compressing the wood.  And then cut the channel with an xacto knife. 

  I planned to do that on this build so I marked all the way around and then started using the xacto and then thought about using the cutter with one blade and turn the other blade around so the flat end serves as a depth stop.  In this case I had the bevel facing the channel.  

Here's a short video I made when I was using it.  I had already dug out some of the channel when I had the idea of using it single bladed so that's how I finished the channel.  It took a couple of passes around.   Digging the scrap wood out between passes.  

 

[MikeC]

By the way, I only used the cutter up to the curve of the corners,  from there I used a knife just in the corners.  Oh and I had to use a knife across the button.    Also in the C bouts because of the arch being steep and close the the channel I had to raise the depth stop so the blade would cut a little deeper.  

[David Burgess]

Single blade with shim as a depth stop, used to cut to uniform full depth. Bevel on blade goes toward inside of groove, leaving outside wall straight and uncompressed. Stabbing the blade in a bar of soap periodically provides lubrication, helping it cut much more easily.

 

[JonGeo]

@MikeCi had seen your video in my research over the last week or 2.. it all makes sense now. Thanks for that.

@David Burgess that’s interesting.. I like the soap idea.. it doesn’t effect the varnish later.

[Claudio Rampini]

1 hour ago, JonGeo said:

that’s interesting.. I like the soap idea.. it doesn’t effect the varnish later.

It doesn't affect the glue too, it's a very good idea to save our tired fingers.

[nathan slobodkin]

10 hours ago, David Burgess said:

Single blade with shim as a depth stop, used to cut to uniform full depth. Bevel on blade goes toward inside of groove, leaving outside wall straight and uncompressed. Stabbing the blade in a bar of soap periodically provides lubrication, helping it cut much more easily.

 

David,

Do you run this all the way around the instrument in the same direction?

[LCF]

18 hours ago, David Burgess said:

Single blade with shim as a depth stop, used to cut to uniform full depth. Bevel on blade goes toward inside of groove, leaving outside wall straight and uncompressed. Stabbing the blade in a bar of soap periodically provides lubrication, helping it cut much more easily.

 

 I haven't seen one of that type for a long time but I would like to make a copy of it. How big is the square hole that the blades fit into

[David Burgess]

11 hours ago, nathan slobodkin said:

David,

Do you run this all the way around the instrument in the same direction?

No. I go the direction in which the grain pulls the blade toward the center of the plate, and pulls the guide against the edge, at least at first. Once the cut is deep enough, the direction no longer seems to matter.

3 hours ago, LCF said:

 I haven't seen one of that type for a long time but I would like to make a copy of it.

I much prefer the type that MikeC shows in his video, and I use a similar technique, holding from underneath with one hand, and pushing and pulling with the thumb or finger of the opposite hand from above. One difference is that I clamp the plate firmly to the bench so it doesn't move around, and reposition the plate and clamp as needed so that the portion I'm working on is the part overhanging the edge of the bench.

On the tool I've shown, you can see how the tool would interfere with the C bout arching when the blade is retracted that far. With the blade a lot further out, it flexes to follow the grain lines more than I like. I think this tool is meant to only be used as a light marking gauge, followed by a knife. I just grabbed and photographed it to illustrate the concept of a single blade with a depth stop, during a previous thread on the topic, because it happened to be in a nearby drawer, and my other type was apart for some experimental modifications.

[Claudio Rampini]

On 8/20/2024 at 4:30 AM, MikeC said:

It has been a while but as I recall I shaped them on a diamond stone.  I may have touched them to a grinder just to remove the sharp needle tip,  but I don't remember.   I didn't reduce the thickness of them.

   On my last build I used them as a marker cut not very deep so no problem compressing the wood.  And then cut the channel with an xacto knife. 

  I planned to do that on this build so I marked all the way around and then started using the xacto and then thought about using the cutter with one blade and turn the other blade around so the flat end serves as a depth stop.  In this case I had the bevel facing the channel.  

Here's a short video I made when I was using it.  I had already dug out some of the channel when I had the idea of using it single bladed so that's how I finished the channel.  It took a couple of passes around.   Digging the scrap wood out between passes.  

 

Very nice tool, if not too long it could be used when purfling at closed body, because it can touch 2 edges, not just one. I'm using the tool in the attached photo, not pushing but pulling it along the edges, sometimes pushing and pulling it.

Out of curiosity, I'm showing a tool made by the italian luthier and bowmaker Renzo Bechini, it has a sort of spring loaded swivel head over a convex and concave body.

[David Burgess]

45 minutes ago, Claudio Rampini said:

 I'm using the tool in the attached photo, not pushing but pulling it along the edges, sometimes pushing and pulling it.

 

 

I much prefer that to the old German tool I pictured.

[JonGeo]

My first attempt at purfling.. 

on a piece of scrap maple…

I ground and shaped the blades down to .6mm.. they were fine.

i found I had to go over the wood with the tool carefully 2or 3 times.. then after that it would follow the grooves and the grain wasn’t as much of an issue..

I used a violin carving knife to cut the marks deeper.. I didn’t attempt to go the full 2mm on the first cut.. I cleaned out the channel then went over it again with the tool to remark the lines.. violin knife to cut to depth cleaned the channel.. it worked better then I was expecting… I found the exacto blade felt flimsy where the violin knife had the stability I needed… note.. untrained hands and eyes here…. that being said I also found the weight of the purfling tool was adequate and using the knife was more for defining the lines before cleaning out the channel.. no point really required a great deal of pressure.. just patience … 

oh and I found clamping the piece of wood down and holding the tool from below while manipulating the tool with my thumb and finger of the free hand gave me the best control. Again first time doing this.

I have a spare blade.. im going to flip it upside down and try using it as a depth stop on my next attempt.. cut and measure over and over got a little tedious.. again.. untrained hands and eyes.. experience and practice will go a long way to solving that.

 

Thank you everyone for your help I really appreciate it.

[MikeC]

looks good,  I like how you thinned the blades.  

[nathan slobodkin]

22 hours ago, JonGeo said:

My first attempt at purfling.. 

on a piece of scrap maple…

I ground and shaped the blades down to .6mm.. they were fine.

i found I had to go over the wood with the tool carefully 2or 3 times.. then after that it would follow the grooves and the grain wasn’t as much of an issue..

I used a violin carving knife to cut the marks deeper.. I didn’t attempt to go the full 2mm on the first cut.. I cleaned out the channel then went over it again with the tool to remark the lines.. violin knife to cut to depth cleaned the channel.. it worked better then I was expecting… I found the exacto blade felt flimsy where the violin knife had the stability I needed… note.. untrained hands and eyes here…. that being said I also found the weight of the purfling tool was adequate and using the knife was more for defining the lines before cleaning out the channel.. no point really required a great deal of pressure.. just patience … 

oh and I found clamping the piece of wood down and holding the tool from below while manipulating the tool with my thumb and finger of the free hand gave me the best control. Again first time doing this.

I have a spare blade.. im going to flip it upside down and try using it as a depth stop on my next attempt.. cut and measure over and over got a little tedious.. again.. untrained hands and eyes.. experience and practice will go a long way to solving that.

 

Thank you everyone for your help I really appreciate it.

Glad to hear you abandoned the exact knife. Totally unsuitable for this job in my opinion. 
 

My own procedure is to mark lightly but visibly with a two bladed marker with sharp points. Then I use a fairly stout knife to trace along the lines deepening the line over any difficult top grains and finishing the line into the corners. At that point I make one pass all the way around on each line a second pass as deep as I can make it then a third pass with a knife with a thinner profile which gets me to full depth. The limiting factor in cutting to depth is the friction of the sides of the knife compressing the wood and the thinner profiled knife on the third pass alleviates that. 

[nathan slobodkin]

On 8/20/2024 at 9:00 AM, David Burgess said:

Single blade with shim as a depth stop, used to cut to uniform full depth. Bevel on blade goes toward inside of groove, leaving outside wall straight and uncompressed. Stabbing the blade in a bar of soap periodically provides lubrication, helping it cut much more easily.

 

So this has a rounded blade to cut in either direction?

[JonGeo]

20 hours ago, nathan slobodkin said:

Glad to hear you abandoned the exact knife

lol I had for the most part.. but the last pass or 2 I used it. It was the only blade I had that was thin enough to go deep without binding as much. once the groves were deep enough I didn’t wander as much. Plus I needed it in the corners. I think I was able to do one without mangling it up. When one cuts the corner do you cut the c-bout lines first or top/bottom bout line first?
 

Roy’s book recommends using the curve of a gouge to mark the outer purfling line in the corners.. what gouge? None of my gouges have an appropriate profile for the curve. As I understand it the purfling comes in from the edge a little making the edge look wider? So using the purfling marker in the corners won’t work?

this might be getting off topic lol

[David Burgess]

9 hours ago, JonGeo said:

When one cuts the corner do you cut the c-bout lines first or top/bottom bout line first?

I don't think it matters, as long as you get the lines where you want them.

9 hours ago, JonGeo said:

Roy’s book recommends using the curve of a gouge to mark the outer purfling line in the corners..

I never had much luck with that, and one reason is that my purfling corners are not a constant radius (nor were those of Stradivari or Guarneri, to the best of my knowledge). I prefer to use a knife to cut the lines exactly where I want them.

[FiddleMkr]

On 8/23/2024 at 7:20 AM, JonGeo said:

 

Roy’s book recommends using the curve of a gouge to mark the outer purfling line in the corners.. what gouge? None of my gouges have an appropriate profile for the curve. As I understand it the purfling comes in from the edge a little making the edge look wider? So using the purfling marker in the corners won’t work?

If you have a gouge that fits then it’s a good idea. I often use the purfling marker and continue the C to the point on each side. Then the I draw the upper and lower bout lines freehand (with aid of a french curve, or circle template, etc). You just have to make sure that the outer edge is correct in the C’s.
           Alternatively, make the outer edges true in the upper and lower bouts and let the purfling follow them, and freehand the C’s.