correcting a poorly cut bridge saddle slot

[stoneaxe]

First, the problem. The contour gauge shows the shape of the bottom of the saddle slot, the original saddle is shown below. There is a distinct arch to the bottom of the slot. The saddle was rounded and poorly fit.


So after looking everywhere for both a 3/32 end mill and a collet to hold it and what to fit it in, I got a little impatient and made a scratch stock to cut it it, using a 1/8" x 1/8 HSS lathe blank. I ground it to 3/32" (a little less, actually, .090) and made a carriage and ways out of wood. Like this. if you ever do this it is much easier to cut the carriage groove first then plane the ways to fit, as most planers have much better control of depth of cut than a table-saw. Or you could use a shaper.
The bit is captured in a 1/8" slot, and locked by a 1/4-20 machine screw. The saddle slot needs to be cut a little at a time. Hardest thing was keeping the ways from sliding on the body. if I ever expected to do this again I would use a screw to adjust the height on the cutter.
and provide some thin non-skid to the bottom of the ways.

The business end-

The ends of the slot were finished with a small 1/16" chisel, a scraper and this little dremel bit held in a pin vise to round the ends of the slot.

[Frank Ford]

That's some fine improvising and classic technique!

[stoneaxe]

Frank, thanks!

Careful grinding of the cutter proved to be very important- a parallel sided bit was not sharp enough, the sides of it had to be ground back to form a more acute angle. A "T" style parting off bit with a hollow in the top edge would work well too, perhaps.
The ability to reverse the bit and cut from the other direction is useful to get close to the end of the slot.

[MC5C]

I am very impressed with the thinking on this tool. I would have used a depth stop on a chisel and scraped it out - same kind of idea, but yours is a lot cooler and a lot more accurate.

Brian

[Halcyon/Tinker]

A novel solution!

[LouieAtienza]

Cool idea! I probably would have done something similar with a router, but this way is a lot quieter...

Sometimes, it may have not been a badly machined slot, I've had a couple guitars where the entire bridge had warped, causing the hump in the slot.

[stoneaxe]

A router would have been an easier solution, but the combination of needing a 3/32" or .090" bit with a long enough cutting length, and a collet reducer to hold a 1/8" shank, meant waiting for an order. I looked through an entire selection of dremel bits at the store, ,sure there would be one that would work, without success. Too short, wrong diameter, not an end cutting bit, etc.

Finally realized my focus had shifted from getting the job done, to finding some arcane tool.

[LouieAtienza]

Check out PreciseBits.com and see if they have a collet and collet nut for your router. They have 1/8" collets with way better runout than the standard collets; basically ER collets and nuts made for hand-held routers. The nice thing is you can run a variety of endmills not normally accessible. I've used reducer bushings and they're OK but never like the way they seat into the collet. I've used Amana's and they're pretty good.

[stoneaxe]

Thank you Louie! Great site.

[redir]

Very clever indeed. I've always just used a chisel as a scraper and then finished off with short straight edge with sand paper glued to it but that is bullet proof and well thought out in it's simplicity design.

[Bax Burgess]

These also show/correct the uneven slot floors.

http://www.stewmac.com/Luthier_Tools..._Levelers.html

[Hot Vibrato]

Originally Posted by stoneaxe

A router would have been an easier solution, but the combination of needing a 3/32" or .090" bit with a long enough cutting length, and a collet reducer to hold a 1/8" shank, meant waiting for an order. I looked through an entire selection of dremel bits at the store, ,sure there would be one that would work, without success. Too short, wrong diameter, not an end cutting bit, etc.

Finally realized my focus had shifted from getting the job done, to finding some arcane tool.

I'm usually not comfortable truing a 3/32" slot with 3/32" bit using my existing saddle slotting rig. I sometimes just fill the slot first and start over (which is sometimes the best option, especially if intonation is off). I'm using the stew mac dremel base with their slotting jig: http://www.stewmac.com/Luthier_Tools...cjAaArNT8P8HAQ

The best way to get the dremel rig and Stew Mac slotting jig to line up with an existing slot is to insert a 3/32" rod in place of the bit and use it to align the jig. Then lock the adjustments in place and then replace the rod with the 3/32" bit and cut the slot. This method usually works pretty well.

If the slot is exactly 3/32 and the slot is too, then there is no margin for error, and I'm not confident that I can be that accurate with the dremel rig. If maintaining originality is not a concern, then enlarging the slot to 1/8" is an option. You can fudge it in one direction or another to help correct intonation.

If cost were not an issue, this would be my saddle slotting rig: https://youtu.be/eY9_RRyxByg

...but at $2700, it would take a lot of bridge repairs and replacements before it would pay for itself. Here is the manufacturer's website if anyone's interested: http://www.annarborguitars.com/styled-5/index.html

[stoneaxe]

That saddle slot mill is impressive, but it would take a LOT of work to make it worth while. A very nice set up could be made from MDF, some plastic for guides, a DeWalt plungerouter and some collets and bits from the place Louie mentioned.

[Hot Vibrato]

Originally Posted by stoneaxe

That saddle slot mill is impressive, but it would take a LOT of work to make it worth while. A very nice set up could be made from MDF, some plastic for guides, a DeWalt plungerouter and some collets and bits from the place Louie mentioned.

I agree. I've never been entirely happy with the Stew Mac Dremel base and slotting jig so I bought a Bosch Colt (laminate trimmer sized) router with the intention of building a slotting rig around it. It just hasn't made it to the top of my to-do list... If you ever make one, I'd love to see your design.