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I'm just starting to make my own bridges and am struggling with a slotting fixture design.
What shop-made fixture(s) do you use to cut bridge slots for Martin style bridges, off the guitar?

I'm low tech and don't CNC or have a CAD package. I'm using plywood, routers, bandsaw and hand tools. I do however have a nice-looking bridge WITHOUT A SLOT.

I have a model done up in foam board (Ted Woodford style, for on-the-body slotting) but I still have to have a way to fix the bridge while routing it. I've seen a number of on-the-guitar fixtures, but I want to slot on the bench as well as on the body, with the same fixture. I'm not a fan of Beau's bench fixture. Need to be able to cut a slot on the guitar as well. I dont need plans, but a pic would be helpful. Thanks for reading this far.....

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Measure Twice,

Karl Borum

Karl--

A pretty low tech way to do it off the guitar is to have a base of a single layer of plywood, with another layer of plywood on top. The second layer has a hole in it a bit larger than the bridge. You can either double stick tape the bridge down in the hole, or screw it down with drywall screws through two bridge pin holes. On top of these two layers, you have two pieces of plywood making a third layer. They are adjustable rails, against which you register the sides of a router base. You make them adjustable by having studs in the second layer of plywood that stick up through oversized holes in the third layer, with washers and nuts to secure the rails once you have them where you want them. This is all pretty fiddly, but you can get it to work with enough fussing. You mark out where you want the slot to be, you get the end mill down to almost touching the top of the bridge, then adjust the rails until the end mill follows the lines you marked out. It's important to have end stops, or you could overshoot the slot. I don't have photos, because I use a really nice handmade tool I bought off another builder getting out of the hobby. But I remember using something like what I describe above back in the day. Good luck with the project.

Here’s my high tech setup. It’s non adjustable but I make the bridges the same way all the time anyway. The pic shows a dummy bridge shaped with holes, but in practice, I slot while it’s just a blank of correct thickness…

Sorry my bad, this one only slots off the guitar. Maybe some ideas in there anyway…

If you’re slotting on the guitar, wouldn’t the bridge be glued down?

If you build a jig for slotting on the guitar, I think all you'd need for bench slotting is a piece of plywood shaped such that the jig can clamp to it, and a couple of screws through the pin holes to hold the bridge down.

I do the slot before shaping the bridge, since it's easier to hold onto a rectangular block of wood (my "jig" is a few cam clamps and a straightedge). Plus then I can taper the thickness from the underside to tilt the slot backward a bit, to counteract forward rotation under tension and improve downward pressure on UST pickups (not that I typically use them...)

This one cuts on and off the guitar. The plywood base keeps the router path flat so that the bottom of the slot is flat. The bottom is auto store cork gasket material so as not to mar a guitar top. I also cut my slots before I form the bridge. I screw the blank down to the table to keep it in place and clamp the guide around it. I sometimes prop the slot side up with a shim to cut the slot at a slight angle. The adjusting screws go into T-nuts and allow the slot length to change and adjustments for different routers. A plunge router is convenient but not necessary.

I did find that double-sided tape is not sufficient to hold the bridge while routing. That’s why I screw the blank down and rout before shaping.

Here's my steel string off-the-guitar jig. Wedges hold the bridge in place. It's a crude prototype that I meant to upgrade, but it just keeps on ticking.

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formerly known around here as burbank
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http://www.patfosterguitars.com

OK then! I'll make a baseplate that clamps in the vise.
Should have a fixture, and a slot, by tomorrow.
Thanks for all the replies.
Any reason NOT to cut .125 slot (standard, modern Martin style belly bridge)? The bridges I've been buying are .090 and I would like the extra real estate.

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Measure Twice,

Karl Borum

I use 1/8” saddle slots, but I’m not trying to recreate any specific familiar model of guitar.

I don’t know how to intonate with a 3/32” saddle. I can just barely make it work with 1/8”.

Another fan of 1/8” saddles here too.

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Steve Smith
"Music is what feelings sound like"

Here's ours, works great. Laser guided, vacuum clamped permits us to slot after the bridge is glued in place and on the guitar. I can never get the Youtube post utility here to work that shows a preview so if someone would be so kind as to repost with the preview working it would be much appreciated.
https://youtu.be/d3tmkLVEAQo?si=T3gLwtQiMZr4sq6X

I have a pin router, so I guess most people don't have that, but this is, how I do it…



Sent from my iPhone using Tapatalk

Here is mine. I have used it for years after seeing Tony Karol post it a long time ago. The bridge is held in with a wedge of wood as shown in other posts. The really cool thing is the pin hole drilling jig Tony posted. The jig is positioned properly and clamped to the drill press table and five spacers for whatever pin hole spacing you want to use are made. then you just take out one spacer for each hole. Cool thing is that it positions the pin holes in line with the saddle slot so they are all equal distant which gives symmetrical break angles and does not put them all on one grain line. It's easy to get the spacers perfect with a thickness sander and micrometer.

I use the StewMac jig for slotting on the guitar but Hesh's is the goods.

IMG_1319 by Terence Kennedy, on Flickr

IMG_3844 by Terence Kennedy, on Flickr

IMG_3852 by Terence Kennedy, on Flickr

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I have no idea what you mean. All my jigs are living their best life…time.

Same. All of my jigs are crude prototypes that continue to work so they have never been fancified. Mine is similar to several of the ones above. It's very simple and allows adjustment for bridges of different sizes, different saddle angles, and saddle location. However, after looking at the jigs in this thread and then thinking about my jig, it occurred to me that I can make a small functional upgrade that will be worth doing. Good timing since I'll be slotting a bridge in a couple weeks.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I use the StewMac slotting jig but it has gotten pretty expensive. It would probably be pretty easy to build something similar.

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Steve Smith
"Music is what feelings sound like"

I've posted before, but here it is.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I recently upgraded my bridge jigs.

I wanted to have an easy way to uniformly radius the top of my bridges so the first pic is the bridge "holder" with an acrylic router sled. Pretty simple, the bridge gets double stick taped down and the router in the sled just slides back and forth to radius the top of the bridge.



The second pic is the slotting jig. Again basic stuff the 4 thumb screws in the corners of the acrylic jig adjust for height, the bridge is double side taped to a flat surface, the jig is positioned over the bridge and lightly clamped in position. (The height adjust screws have rubber tips on them.) Though I haven't tried to slot a bridge on a guitar, I made the jig long enough to span a guitar top and be lightly clamped for that also.



I love jigs out of lexan just because you can see thru it. I pick up cut offs from a glass shop just for jigs.

I'm suffering from severe jig envy Hesh! I wonder if there would be interest in a pure favorite jig thread. This place is loaded with great ideas and jigs. If there was a thread of jigs I would have a ball browsing thru it. Any interest??

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Brian R, Wood Mechanic
N8ZED

Hesh, that's one heck of a jig.
I gave your link the ol' one-two but its looking like I can't get the preview either, but thanks for the link
https://youtu.be/d3tmkLVEAQo?feature=shared
https://www.youtube.com/watch?v=d3tmkLVEAQo

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Measure Twice,

Karl Borum

The idea for a very high precision, rock solid saddle mill with repeatable results and high tolerances was dictated by function. Dave worked on his saddle mill for over 10 years with a few prototypes that we used along the way. This was also how Dan E. developed the neck jig, necessity is the mother of invention.

With this said and using Six Sigma speak what is "critical to quality" for a good saddle and slot?

The saddle should fill the slot everywhere, including the ends with no gaps. There should be no lean, no slop, the slot and saddle should be uniform the entire length and height (no dovetail saddle slots...). 0.003" of slop it too much and I'll throw this out there for the next statement.

I had the SM jig and gave it as a present to an OLFer about ten years ago. Can't remember who but I hope it has served you well. Dave's jig came online back then for us and I had the use of that one and did not need the SM jig.

What's wrong with the SM jig is slop and a design spec around a Dremel or Bosch Colt. We tested all lam trimmers and found the Bosch Colt to be the sloppiest one on the market with runout commonly exceeding 0.005". Not to breath our own air Link Van Cleave a former member here and master woodworker who had his own TV show on the west coast also tested Colts. Link was also Jim Krenov's apprentice at the Redwood School, taught at the school and was given Jim's hand made planes after Jim passed. But I digress with one last addition here Link is a close friend and we were lucky enough to have him be our apprentice for two summers.

Link bought 5 Colts and tested them all with the thought that he would keep the best one and return the rest. The best one in his batch had 0.005" of runout making them all unsuitable and he returned them all and went with DeWalt.

Link's experiences mirrored our own testing.

So the SM jig and many others depend on the fitment of parts to slide, effortlessly forward and aft. There by definition has to be some slop in so much as the parts need space to move. With the SM jig I had I could wiggle the slide ever so slightly and I would guess the slop to be around 0.003". This is a guess though.

Saddle slots need to be accurate and consistent the length of the slot.

Now add the slop of a Dr*mel (more than a Colt) or a Colt to the slop in the SM jig and we may be approaching 0.010" of slop. That's awful and not suitable for a good saddle mill. This is why Dave developed his own.

Our mill also used the DeWalt lam trimmer which we measured at around 0.0025" of runout. So Dave searched and found special collets that dial in the runout even further and our mill these days produces around 0.0005" in runout. And that works for us.

So why does it matter?

Craftsmanship dictates that our saddles fit well with zero gaps everywhere. No space seen in the ends and the curve for the ends is matched to the saddle and slot. Many guitars will receive a UST (under saddle transducer aka pick-up) and that means the saddle need to move up and down with very little resistance for the UST to balance or have a shot at a good balance correctly.

When I install a UST if the saddle is tight I sand and fit, sand and fit until it can be removed with only my fingers but has no wiggle room and God knows does not rock...

The slot needs to be perfectly flat as well as the saddle bottom needs to be perfectly flat as well also for response and good balance.

Just in this one example it is not possible to meet all these criteria with the SM jig or jigs of this type that do not use precision slides or bearings. Our saddle slots are uniform the entire length, SM style jigs may produce less than uniform over the length results meaning the saddle has to be less than uniform to fill the slot perfectly.

There are other applications as well such as relocating the saddle slots on the many Martins in the early 70's that had the saddle located incorrectly in the manufacturing process. We eat these for lunch when we see them and the mill makes it very easy for us to mill the slot uniform, plug with matching wood, mill slot in the correct position and make a new saddle (maybe if the old one sucks) that matches the precision of the new slot. In the past other shops might have to make a new, larger bridge or move the existing bridge often leaving a visible sign that the instrument had the saddle slot addressed.

On a valuable instrument not having to replace a bridge is huge.

A great repair should not be visible.

So we don't produce these mills anymore we made the mistake of engineering a product for at its price point.... had a world wide global market of say 20. We did sell them to a few manufacturers who use them and we did sell them to some of the finest, best known and largest repair shops in the world. A few quality conscious builders bought one too. But at the ending price of around $4K it was not exactly an Amazon item nor was it the business we are in.

Is several extra thou of accuracy and precision worthy of our spend on the mill? Yup and I would do it again. We have capabilities that no one else has such as a saddle slot with a pocket in the slot for the UST making a very neat installation.

We can slot for a leaning saddle that some of you like to do too with a platform added and our stability is rock solid. In general with our vacuum clamping you can lift the entire guitar and mill assembly. You don't want your cam clamps to come lose on a SM jig when repairing the slot in a pre-war Martin now do we.... And the clamps on the SM style jigs are in the way, can be bumped, can come loose and can damage an instrument it attached incorrectly.

With all this said there are things you can do to these style jigs above to tighten them up. Strategic placement of tape and such, replacing parts and then making them fit snug with trial fitting and ever so slight adjustments, etc.

Dremel's are junk for high precision work and lam trimmers need to be tightened up also for high precision work. I've never done it but I can seem many of the SM style jigs that slide above benefitting from a few hours spent tightening up the fit of the slide and the selection of a DeWalt Lam trimmer with the after market addition from a company called Precise bits of the special, high precision collet for these trimmers. IIRC the collet is around $100ish.

My bet is you could at the very least reduce the slop on these jig by half or more and that's pretty good.

I have 1 jig that will allow me to slot the bridge on the guitar, or off the guitar. The jig clamps to the guitar, with 4 spool clamps, and is set up for my Dewalt laminate trimmer. It is my $25 shop made version of the $4000 one that Hesh referenced above. It is a little crude looking, since I am still using the prototype. It works great for the volume that I do. After is is clamped onto the guitar, I can adjust the tilt, the angle of the saddle, the depth at each end, I can do a thru slot and I have adjustable stops for routing for a drop in saddle.
If I need to slot a saddle before it is installed, I have a platform that I can attach the bridge to, and clamp the jig onto the platform. The platform just serves as a dummy guitar.
I can't imagine not having this jig in my shop. Mine can't hold a candle to the one Hesh uses, but it works well enough for me.

After reading Hesh's awesome post of Thu Feb 01, 2024 6:56 am, about runout, I went to the Precise Bits website and searched for Colt collets, none found. I did a google search for PRECISION COLT ROUTER COLLETS and got a hit, back to Precise Bits website:

https://www.precisebits.com/lab_reports ... lt_TIR.htm .

The Colt collet was discontinued in 2015 because of crappy router bore geometry. Its a good read... Had a long look at their products and articles too. That led me to measure the runout on the Colt in my shop:
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MEASURING and CORRECTING the Total Indicated Runout on my Colt PR20EVS Router:
Starting TIR: .006" (using commodity 1/4" drill bit and Chinese dial indicator, on stand, on granite plate)

I cleaned the collet, the bore and the bit with iso alcohol and Q-Tip, and wiped dry with paper towel (as recommended by Precisebits.com).

Ending TIR: .001" ISYN! Wish I could post a video of the method and results.

Thanks for the link Hesh and the detailed essay on router slot cutting. My bridge will have a better slot to show for it.

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Measure Twice,

Karl Borum

Good going that's a lot of capability so your jig sounds very cool to me, nice job!

Very cool Karl that's what this forum was created for to be a place were we all help each other up our Lutherie game. Good going and thanks for the links too I had not seen these.