bore hole/knot diameter

[ahoiberg]

Question for you brush makers (I haven't made one for ages and can't recall what my thoughts were surrounding this back then):

How close do you match the knot diameter and the hole bored into the handle to get optimal splay of the knot hairs? If you have a 24mm knot, do you shoot for a 26mm bore? 25? 27? Any hard and fast rules?

I set up a block template and drilled a hole with each of my available forstner bits, but it either seems too tight or too loose depending on the knot size/bore hole combo I'm using and I suppose that's because my bits aren't metric.

Any thoughts on the matter would be appreciated! Thanks

 

[global_dev]

does it matter how deep the hole?

i thought i read a mm larger than knot

 

[ahoiberg]

does it matter how deep the hole?

i thought i read a mm larger than knot

for sure, curious on both but moreso on the width of the hole as I don't have a good feel for how that would affect the performance of the knot. I think I have somewhat of a feeling on how the depth would affect it but certainly would appreciate any thoughts on either.

 

[GAW9576]

I'm still learning, and I don't want to misinform you so, this thread is very informative and it should have the answers you are looking for. It talks about depth and diameter.
Brush Knot Depth?

 

[ahoiberg]

Stellar, thanks!

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[CigarSmoka]

The “correct” diameter and depth of the hole is purely subjective and depends solely on how much backbone/splay the user wants, and both will affect the backbone/splay. I like about 1.5mm over the size of the plug, not more than two, so I’ve become picky about my knots. Ones with glue bumps above the plug just won’t do because the diameter will be larger causing more splay than I like. This won’t be noticeable right away, but after some use it will become obvious. As for depth, a lot of my 24mm knots have been 67mm tip to tip, so for finest/two band I drill 18 - 20mm deep and curve the top of the cup over a bit until I get a hole 17mm deep for a loft of 50mm. For my two silvertip knots I went with a final depth of 19mm for a 48mm loft, which still allows plenty of splay from the softer hairs. I’ve settled on these numbers after a few years of making brushes, and they’re not what I originally started with. I thinks it’s a good place to start, you can always tweak it more to your liking on the next brush. There will ALWAYS be a next brush... Hope this helps.

 

[ahoiberg]

Awesome, great info, thanks much!

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[Big Jim]

I always anticipate matching the hole to the knot because knots are not all that consistent, even from the same batch. The diameter of the bottom disc is often a little smaller than advertised, usually about .5 mm. A bit above the disc, the knot is larger. I call this the ''swell". How high up the swell is and how much it swells is also very variable. It tends to be more and higher if the knot has a high glue bump. You need to allow for the swell as it is in the area that ends up in the hole. Inexperienced brush makers sometimes don't get the bottom of the knot set all the way down the the shelf or bottom of the hole. I usually use a Dremel Drum Sander to finish the hole diameter with the knot in hand until I get the fit I want.

Personally, I like a brush with a lot of loft, so I set them higher than most that post here. I would suggest that you initially set the knot with silicon so it can easily be removed and you can experiment with different lofts. When you find the sweet spot for that particular brush then set it with epoxy.

Enjoy your projects. You will have many more.

 

[GAW9576]

One suggestion I really liked in the post I linked was to drill a bit deep and stack plastic washers under the knot, setting with silicon so you can easily adjust several times with one knot to experiment. I made two handles just for this and drilled quite deep, so I have plenty of room to experiment. I can't afford to buy a bunch of knots so this is a way to try multiple settings for the price of one or two knots.

 

[Graydog]

I like to have the knot to size it all up .
in The past I have drilled 2 holes , it was for a 24mm knot + glue bump
the first hole was 1 inch at 15mm the second hole was 1 1/8 at 13mm allowing for
the glue bump and splay with a nice fit for the base of the plug

 

[CigarSmoka]

I always anticipate matching the hole to the knot because knots are not all that consistent, even from the same batch. The diameter of the bottom disc is often a little smaller than advertised, usually about .5 mm.

Thanks for bringing this up. I didn’t elaborate on it at all because I felt like I already wrote a short story. I always measure the plug with vernier calipers and many 24mm have actually been 23.5. Same for the full loft, just because it says 67mm doesn’t mean it is. It’s always best to measure rather than trust the so called specs.

I like to have the knot to size it all up .
in The past I have drilled 2 holes , it was for a 24mm knot + glue bump
the first hole was 1 inch at 15mm the second hole was 1 1/8 at 13mm allowing for
the glue bump and splay with a nice fit for the base of the plug

Great minds think alike... and so do ours. I’ve done the same thing when using a knot with a stupid large glue bump. My 23.5mm VS needed the hole increased to 28mm. I made up for it by setting the loft lower like I do with silvertips and it’s actually one of my favorite brushes. Not as firm and more splay than most two band finests, but less splay and not as soft as a silvertip.

 

[oscar11]

I'm with CigarSmoka.

 

[ahoiberg]

Again - thanks for all the knowledge, much appreciated.

I've heard the 'glue bump' term several times, but the way you are describing it is different that what I thought it was. Does it affect the diameter of the 'plug' of the knot or is it 'inside' the knot at the base of the hair where it is actually glued into the knot?

 

[Big Jim]

The glue bump protrudes up from the base. All knots have some, but some have way more and many people consider it to be too much. When the glue bump is significantly higher than the top of the handle it is usually considered too much. That is why some people say that the glue bump affects the "effective" loft. The glue bump can also affect the "swell" I talked about earlier, making it necessary to open the hole more than you may have anticipated. I don't know how factual it is, but I have heard that the reason we are seeing more knots with a higher than before glue bump was an attempt to better control shedding.

 

[Graydog]

The glue bump protrudes up from the base. All knots have some, but some have way more and many people consider it to be too much. When the glue bump is significantly higher than the top of the handle it is usually considered too much. That is why some people say that the glue bump affects the "effective" loft. The glue bump can also affect the "swell" I talked about earlier, making it necessary to open the hole more than you may have anticipated. I don't know how factual it is, but I have heard that the reason we are seeing more knots with a higher than before glue bump was an attempt to better control shedding.

I have found that the less expensive knots have bigger bumps.

 

[CigarSmoka]

I have found that the less expensive knots have bigger bumps.

+1 That great deal from a company I won’t name but is the highest card in Spades isn’t so great when you factor in things like the glue bump, smaller than specified, and underfilled. It’s always worth the few extra bucks to buy from a reputable company. If the knot lasts 10 years, it adds up to a whopping $1/yr over the life of the brush. Don’t skimp.

 

[Big Jim]

I buy a lot of knots from ACE and haven't run into a high glue bump from them yet. I can't say as much about several other Chinese suppliers. It doesn't seem to be as common as it was a couple of years ago.

One of my favorite knots is a 26 mm Silvertip that ACE had custom made for me with high loft.

 

[Rudy Vey]

I have had knots that swelled so much just above the glue, they needed 4-5 mm larger hole than the nominal size.
But, normally I would say most knots I use need 1.5 mm larger holes. Depth depends on the loft you want to set and shape of knot. I think that for a fan shaped knot, the loft should be about double the size at exit ( for bulbs 2 mm more).