Jump to content

Entry for August 2020's Guitar Of The Month is open - ENTER HERE!

curtisa

Routing Body Shapes Without Tearout

Recommended Posts

After doing several bodies in the last couple of years and researching the topic a bit, I think I've finally settled upon a method for routing body shapes on the router table that appears to work everytime with next to no tearout. I'd seen mentioned in several places to always "route downhill", but it was never entirely clear to me at the time what exactly "downhill" was in relation to a body curve. Hopefully the following post(s) will clear that up for anyone else who, like me was confused by this terminology, and also give a pictorial guide to shaping a body blank to a template without the router destroying your work.

The bit I use is one of these bad boys:

20130623_153532.jpg

It's a 3/4" diameter CMT flush trim bit with 2" long downshear blades. I've modified it with the addition of an extra bearing on the shank to make it a dual bearing unit. You can probably buy these things with the dual bearings already installed. I just had the extra bearings in my toolkit and fitted it. The dual bearings are necessary, and will become clearer on their function further on. As an alternative you can use two separate bits - one with the bearing on top and one with the bearing underneath. It will have the same effect, only you'll need to swap bits halfway through the procedure. Whatever bit(s) you end up getting, make sure they're quality ones with enough blade length to cut the full thickness of your body blank. The size and duty of the bit probably necessitates the use of 1/2" shanks - a 1/4" shank bit will likely break when used in the following application.

In this example I'm trimming a figured top wood down to a precut blank, but the procedure is the same if you're trimming a blank down to your template (in fact this is exactly what I had done earlier with my blank in the pictures). The body in this one is Tas blackwood (unfortunately easy to burn, so please 'scuse the visible scorch marks!) and the top is Tas myrtle burl with a fair bit of figure and knotty sections:

20130623_153427.jpg

20130623_153648.jpg

I've cut the top down with about a 1/4" overhang, but in practice the smaller this oversize cut is the better - less timber to route, easier on the bits, less chance of tearout. Nevertheless, trimming the overhang down from 1/4" works OK with a soft timber like myrtle with only 15mm of timber thickness to deal with, especially in multiple small passes:

20130623_153458.jpg

The bit is installed in the router and cranked up so that the top bearing rides on the template (or in this case the body). If you were to use the two-bit method you'd install the bit with the bearing on top in the router. With the bit at this height we're going to route half the body curves in a particular order:

20130623_153735.jpg

The next trick is to work out and mark which curves shall be tackled with this bit. The idea is to have the bit attacking the workpiece in such a way that the rotating blade wants to "push" the grain of the wood back onto itself as it cuts, rather than try to lift the grain away. This "lifting while cutting" is what causes the tearout in the first place. Imagine routing an edge across the endgrain of a block of timber from right to left with the workpiece facing away from you. As the router starts the cut on the righthand edge everything starts fine, but as the bit finishes the cut on the left it breaks out a piece off the edge perpendicular to the cut. A similar thing happens when routing curves depending on which direction you attack it from, and (what I now realise) is what is referred to as "downhill routing".

The next series of shots hopefully explains this better. I've marked in pencil the start and end of the cuts. With the bit rotating in an anti-clockwise direction this satisfies the "slice-away" cutting action for each curve.

From the bottom of the control cavity to the waist:

20130623_154147.jpg

From the top of the arch where the players arm rests to the strap button position:

20130623_154127.jpg

From the outer edge of the bass-side horn to the waist:

20130623_154107.jpg

From the middle of the treble-side horn to the tip of the treble-side horn:

20130623_154006.jpg

From the bottom of the treble-side cutaway to the middle of the heel:

20130623_154027.jpg

From the outer edge of the treble-side horn to the tip:

20130623_154042.jpg

Edited by curtisa
Typos and restore pictures

Share this post


Link to post
Share on other sites

After firing up the router and cutting only the marked sections, you should end up with something like this. Each cut only goes as far as halfway around the curve:

20130623_154716.jpg

20130623_154728.jpg

20130623_154746.jpg

20130623_154759.jpg

20130623_154817.jpg

20130623_154832.jpg

Share this post


Link to post
Share on other sites

The body is flipped over and the bit is then cranked up so that the shank-side bearing rides against template. If using the two-bit method you'd swap for the bearing-on-shank bit at this stage:

20130623_155101.jpg

With the body flipped over it is now possible to trim the remaining curves while maintaining the "slice away" cutting rule. Marking the sections to cut shows the following. Essentially all you're doing is just routing away the leftover bits so that all the previously routed sections are all joined together.

Top of the players' armrest to the middle of the waist:

20130623_155236.jpg

Outer edge of the bass-side horn to the tip:

20130623_155300.jpg

Middle of the bass-side cutaway to the middle of the heel:

20130623_155311.jpg

Middle of the treble-side cutaway to the tip:

20130623_155332.jpg

Outer edge of the treble-side horn to the waist:

20130623_155407.jpg

Outer edge of body curve to strap button position:

20130623_155440.jpg

Share this post


Link to post
Share on other sites

The final pass with the router gives the following results:

20130623_160708.jpg

20130623_160717.jpg

20130623_160735.jpg

20130623_160744.jpg

20130623_160803.jpg

20130623_160751.jpg

In the last shot you can see where I proceeded slightly past the middle of the cut on the blackwood body and some of the grain has fuzzed-up. If I had continued around further this would have undoubtedly resulted in destructive tearout because the cutting action of the bit at that point would have been violating the "slice-cut" or "downhill-cut" rule and was starting to move into the "tearaway-cut" or "uphill-cut" zone. However, the myrtle top at that point where there is a lot of knots and inclusions shows no sign of tearout.

Edited by curtisa
Restore pictures

Share this post


Link to post
Share on other sites

I've messed around with various methods, straight flush trim bits. spiral cut bits, bits I've had custom made etc etc - I've gone back to simply bandsawing close and then rasping back. I find is actually quicker and completely eliminates tearout and massively reduces risk of injury.

As far as Chris' video, I have a friend who is missing a finger due to a router table accident. A professional wood machinist, not just a hobby hack. Every time Chris posts that video it reminds me of my friend. I excercise extra caution when using router tables, just google images "router accident" and you'll see why.

Share this post


Link to post
Share on other sites

Yes, router table pose risk. But understanding your tools and taking the proper safety precautions should always be observed. All tools are dangerous, but if you respect them, use them carefully and for purposes they are capable of, you can easily minimize this risk. I use my router a TON, and there are definitely tools in the shop I'm more fearful of that it!

Chris

Share this post


Link to post
Share on other sites

understanding your tools and taking the proper safety precautions

------------------------------------------------------

Mate, in the video, you're not even wearing safety glasses! It's very scary! I've spent enough time on a router table (and spindle moulders which are even more freakin scary) over the years to have more than one router bit explode, one of those times flying at least 10 - 12 meters across my own workshop. Imagine if that hit you in the face.

I know my "safety" talk might seem to be a bit over the top to some people, but after working in a factory environment for so long and seeing so many industrial accidents... it's for a bloody good reason I preach safety. I work with so many people who are missing fingers, eyes, extreme chemical burns, oil burns, crushed in machinery or forklifts, caught between belts and having flesh ripped from their body etc etc. I work in this environment every damn day and see **** I wish I never had to.

I myself have had so many "near misses" that its a ****ing big wake up call to not just take things for granted. I have a grip/push paddle I use these days for routing that is missing a chunk out of it AND I route much more cautiously than you demonstrate in the clip. I can photo this if you dont believe. What I'm saying is dont get too attached to your digits as it's going to happen one day unless you change your ways.

Share this post


Link to post
Share on other sites

My router scares the crap out of me more that any other tool I own. I'm in Searl's camp. I try to cut super close to the template so I'm only routing off a sixteenth or so of material. Following that method I've never really had any serious tearout yet.

Watching Chris's vid and how he was holding that body made me uneasy. I try and keep my hands as far away from that blade as possible while still having a grip on the piece.

Some poor guy just posted over on the luthier talk forum that he just screwed up a digit pretty badly on his router and it was his first build.

Oh and thanks for suggesting we google "router accident'. I just did and I'm going to be even more nervous when I use my router now.

  • Like 1

Share this post


Link to post
Share on other sites

No worries sd, glad to be of service.

I was just trying to think of other workplace near misses I've bene involved in. A couple years ago I was on a site, up on a ladder whilst a guy was about 3 meters in front of me on a ladder with a framing gun. First day he'd ever used one and he shot a full length framing nail through a 35mm piece of timber. The nail came so close to my head I heard it whoosh past my ear! I can assure you he copped a verbal beating.

Sorry for hijacking the thread

  • Like 1

Share this post


Link to post
Share on other sites

I also try and take off 1/16" or less. That's what allows for me to use that big bit and not grab. Understanding grain and bit directionality, etc. also helps be safe.

I'm of the opinion that fearing your tools is a BAD idea. Respect them, sure. But fear does not bode well. Makes you hold with flimsy hands, not think well, etc. you're asking your router to throw things around then.

You want a nice sturdy grip that is placed optimally for torque. Also understanding WHERE its going to pull or throw if it does keeps you safe too.

I HAVE had close calls too. I don't anymore because I know how to use my tool.

Chris

Share this post


Link to post
Share on other sites

I also try and take off 1/16" or less. That's what allows for me to use that big bit and not grab. Understanding grain and bit directionality, etc. also helps be safe.

I'm of the opinion that fearing your tools is a BAD idea. Respect them, sure. But fear does not bode well. Makes you hold with flimsy hands, not think well, etc. you're asking your router to throw things around then.

You want a nice sturdy grip that is placed optimally for torque. Also understanding WHERE its going to pull or throw if it does keeps you safe too.

I HAVE had close calls too. I don't anymore because I know how to use my tool.

Chris

I hear ya. I agree being terrified of your tools is bad to a point. Take my saying my router scares me as me having a tremendous amount of respect for it. I know if I screw up its going to have some very serious consequences. I really like my fingers and I take every precaution to avoid that. I do try and keep a firm grip on my piece. I'm constantly thinking about and anticipating a potential kickback and where my hand will be if it happens and where the piece will potentially go. I always wear a dust mask, eye protection as well as ear muffs to boot

Not trying to hijack your thread either curtisa but I think safety is always worth discussing. Great thread by the way.

Share this post


Link to post
Share on other sites

I also try and take off 1/16" or less. That's what allows for me to use that big bit and not grab. Understanding grain and bit directionality, etc. also helps be safe.

I'm of the opinion that fearing your tools is a BAD idea. Respect them, sure. But fear does not bode well. Makes you hold with flimsy hands, not think well, etc. you're asking your router to throw things around then.

You want a nice sturdy grip that is placed optimally for torque. Also understanding WHERE its going to pull or throw if it does keeps you safe too.

I HAVE had close calls too. I don't anymore because I know how to use my tool.

Chris

I agree Chris. Fearing a tool is a bad thing. I hate when people say you need to be afraid of it. I learned at a young age, and have always felt extremely in control of my tools.

I have been building 8 years non stop, and have probably cut and planed more board feet of wood than most people will in their lives.

I have done all my guitar bodies with a router and bearing bit. The only time i have ever gotten tearout is when i go too fast or dont cut it close enough to the line. I havnt gotten tearout in about 3 years now.

And for the record, i have never had a power tool accident. I did hit my knuckle on an upright belt sander a couple times, but that was because i was in a hurry and pushing too hard and trying to make it cut faster.

But i used a router table and router on every build. And Chris is right, Respecting the tool and being in control of it is what prevents injuries.

Share this post


Link to post
Share on other sites

Not trying to hijack your thread either curtisa but I think safety is always worth discussing. Great thread by the way.

If it encourages constructive discussion, then all is well in the world. There is no doubt that the router is one of the most dangerous pieces of kit in our workshops, and talk of safety around them should be encouraged. When I'm using them it's safety specs, earmuffs and dust mask time. And unless I can see an air gap between the plug and wall socket, my hands don't go anywhere near the collet to change the bit.

In regards to Chris' video, I had seen it earlier and a lot of what he discusses is what I've used in my methods too. The thing that strikes me as different is that it appears he cuts the body in one direction only. At 2:04 for example, he's cuting a section in a direction that is flowing uphill against the grain, wheras I would flip the body over and cut downhill into the "valley". IME depending on the timber you're working with, the cut that Chris does in that video increases the chance of tearout, especially in areas where the curve is even tighter such as inside cutaways and tips of horns.

Share this post


Link to post
Share on other sites

Not to contradict or nullify anything said previously (excellent video BTW Chris, thanks for doing it!) however I'll just brainblah how I approach edge routing. Most of it is probably the same as already said, just adding more fuel to the fire.

I mostly rout on a table now. Like Chris I also start on the outermost convex curves, moving the bit clockwise around the body creating "exit points" so that cuts don't end where material can be pulled out or grabbed and any subsequent climb cuts can exit gracefully. These initial cuts end in either the bottom of a convex curve or where the grain starts to lead into the direction of cut. It is useful to visualise this.

A sharp bit using an appropriate speed should allow cutting across end grain without incident up to a few degrees out without tearing. Beyond that the grain may well start parting and/or fibres being pulled up and out, especially if you bit is dragging rather than slicing the fibres. Usually my first cut on a Strat-ish doublecut shape is clockwise from the apex near the controls (right hand instrument, top facing upwards, router bit rotating anticlockwise) all the way around to the end grain on the butt of the body, maybe just beyond the "flat" of the back. Beyond there the grain reverses into dangerous climb cut territory.

I will climb cut where it is appropriate and where I can guarantee control. This is not always easy to discern and often I draw my cutting paths on the inner face of a cutting template in advance to reduce having to think on the fly during operations. This is never safe since operations should be fully premeditated. I then copy this plan onto the top of the body so I have a plan of action right in front of me. I don't however use a spiral bit which Chris is slowly convincing me that I should. I use a shallow-ish (12mm flute length, ~1/2") bearing guided cutter which doesn't take too much at once. In that respect this makes climb cuts safer since there is less material at any one time for the cutter to grab. My templates are usually about 10mm thick so I can make my initial passes shallow and protect the surface from losing fibres and chips where they are least supported. With a normal cut I might take anything up to the full 12mm however for climb cuts I prefer to do these in two passes. For fragile parts (think pointy or acute corners like an SG) there is nothing wrong with taking small feathered passes. Even applying several strips of masking tape on the template bearing surface to gradually move into the final shape helps.

Climb cutting is not too much of an issue providing that the operator has the workpiece under control. The bit catches and climbs into the cut when the opposing force of the workholding (or toolholding in the case of a handheld router) is exceeded. If you can't hold onto the workpiece (or the router) then control is lost and you have a climb. Not good. CNC machines perform climb cuts as a matter of course on the basis that the workholding and toolholding (normally) exceed the strength of the wood fibres. The wood will move first. Reducing cut depth or the amount of material being removed during a climb cut operation moves this balance into your favour. As Roy Underwood wisely points out, the key to woodworking is understanding and taking advantage of where wood is strongest and also where it is weakest. Routing is no different.

Another point which is rarely mentioned is the consistency of the bandsawn shape before you even take it to the router. Any nicks, grooves, slots, corners, thicker sections, etc. are fodder for the router bit "catching" and taking more material in the form of a torn chip. If you have the ability to pop in a larger bearing or have insert bushings on your table router, these help immensely. Having a consistent routed shape 2-3mm (1/16" - 1/8") from the final shaped edge makes the final operations that much more predictable.

Apologies for the wall of text. Hopefully some of that will be intelligible and maybe even useful. Maybe not.

Share this post


Link to post
Share on other sites

I'm going to add two more things to what Prostheta said:

1- I don't use a SPIRAL bit. I use an angled cutter bit. The blades are attached with a few degrees of angle. You do not NEED a spiral for cuts like this. The angled bit is enough for one main reason: when a route without an angled blade spins it hits the wood every couple milliseconds TWICE, with the whole edge smacking the wood at once. An angled blade on the cutter makes it so the hit is gradual, thus reducing the risk of grabbing, tear out, etc. the actual fact it's cutting the fibers at an angle isn't the key here, which a lot of people think it is.

2- curtisa is right, I do downhill cuts starting at the tops of curves, and then do UPHILL cuts to finish the additional sections. As Prostheta pointed out this allows me to get rid of pieces of short grain wood at these apex points that would later be prone to grabbing and flying off in chunks. Once that risk is gone, I'm no longer fearful or tear out on the cut, and can go up hill. I have a couple reasons why I do this:

- most of my guitars are carved tops that are carved before routing to final shape. I don't often have the option of flipping over the body and doing all downhill cuts. So I got comfortable with a method that allows uphill ones that are clean.

- rotation of the bit is VERY important. I'm of the opinion that at all times one should do their best to PUSH WOOD INTO THE CUTTING SURFACE. If I did my second cut from the apex down, I would be cutting WITH the cutter rotation. I have found that this is just asking for grabs, throws, tear out, etc. if you think about it, all your tools want you to push wood into the cutter, not with it... Jointers, shapers, all of 'um except CNCs really.

Chris

  • Like 1

Share this post


Link to post
Share on other sites

the two points in Chris's statement above I think are most important-as far as a clean cut is concerned- at least in the numerous FAILURES I have had- and reading in this forum-and other wood working forums- I used to use almost the exact bit Curtisa used- changed to an angled bit and found that the fuzz and lift/shattered edges went away.Pulling the wood away (rather than back tracking a cut)- downhill cuts- all that is the way to do it.

I think router tables are 20 times safer than holding a power tool spinning at 1000x of rpms. If you are about to sneeze- you can pull the object away from the blade- if you are holding a router- different story. Every cut should be "mentally planned" ahead of time- my router is spinning this way- my push is this way- my hands will be here while I make the cut. I will stop here, I will approach there. etc. I do this everytime- and I can atleast say I still have all my digits still. Doesnt mean though I wont encounter something at some point- but if I think it thru each and every time- the risk is minimized.

I must admit defeat when it comes to maple though. I still use a robo sander on horns as I just cannot seem to win- or rather- I am not willing to chance it once I have that much work and or cost into a project to risk tear out. I admit it openly. I am chicken in that respect.

I dont want to harp on Chris's video- but if any noobs are watching/reading- please wear safety glasses whenever you are using a power tool.

Share this post


Link to post
Share on other sites

Couldn't agree more. Thank you for clarifying also, Chris.

Yes, horns are quite a challenge. That short unsupported end grain just begs to split up or throw chunks. You've just got to go for it confidently and most importantly, safely.

To add onto my previous missive, here is the first "introduction cut" from my last proper build. Although not the same approach method as a table router, the idea still applies. Cutting the first couple of mm in a light cut which is kinder to the unsupported wood on the surface. Also, that inset part in the scroll was a PITA to do, reminding myself which way to go each time! Note that despite the bit rotation being clearly marked on the machine I drew a circular arrow on the blank anyway. I didn't do the whole "smoothed consistent edge" on this blank since the spindle speed of the router is CNC fast compared to table router fast.

bodywork3.jpg

Share this post


Link to post
Share on other sites

I agree. When I release the full video I plan to do a safety introduction that will include a 'do as I say, not as I do comment.' Folks need to remember that this video has been building over a half decade so far. I was worse with safety when I started, better now. This I a fairly old video now... I was still living in SC! Also, I often leave glasses off while filming, for operations I normally would wear glasses for, because I'm doing other things, need to be able to see the view finder of the camera well, etc.

Chris

Share this post


Link to post
Share on other sites

Good points and comments, all.

I have destroyed several horn tips because I was attempting to go around the body in one direction only. At best I was able to salvage what I had done by sanding the affected area down. At worst I had to trash the blank (and change my trousers!). The most common offenders are the tips of the horns around to the outer edge of the horns.

I should probably clarify that at no point in my method am I employing a climbing cut. I have never felt safe or comfortable using climbing cuts on the table with such large bits. Even though I am always routing downhill, by flipping the body over for each alternate curve I ensure that the direction of cut is always into the bit rather than away from, and at the same time the cutting blade only removes material in such a way that doesn't try the push the grain back up into itself and cause a grab.

Share this post


Link to post
Share on other sites

True. It all hinges on whether you cut full/large depth or multiple light passes as I do. Tips and long thin horns such as on the build pictured benefit from progressively deeper cuts to that the grain is well-supported.

Share this post


Link to post
Share on other sites

I often leave glasses off while filming, for operations I normally would wear glasses for, because I'm doing other things, need to be able to see the view finder of the camera well, etc.

I sincerely hope your tools realise you're filming and are on their best behaviour. ;)

Personally I don't plug a power tool in unless I have the right kit on to protect my eyes, ears and lungs.

I'm also unconvinced that a router table is inherently safer than a handheld - there are arguments both ways. A handheld router will often jog away from the workpiece if you ask too much of it; a router table will often try to do what you asked it, even if that results in the router bit biting deep into the workpiece, picking it up and throwing it at you. My router is "only" 1400W - not much compared to many out there, but that's still nearly two horsepower. Having seen the damage one horse can do to a person, I'd rather not give my router any opportunity to do anything silly. I've used it handheld and in a table setup and the worst damage has always been in the latter. Also worth remembering that it's a lot harder to get your hands near the bit operating a handheld router than a table router.

Without wanting to sound like some sort of awful workshop calendar, the most dangerous tool in the workshop is the one you're using. Well, assuming that you're not leaving chisels stuck in holes in the floor.

  • Like 1

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...