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First CNC Bridge
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10106&t=42199		 Page 1 of 1
Author:  Blain [ Sun Dec 15, 2013 7:45 pm ]
Post subject:  First CNC Bridge
I've done a couple practice runs about a year ago and then took a long break away from CNC and guitar building (doing housework....)

So I finally got back to it and cut my first actual bridge on the CNC using Vetric's Cut3D.

The pin holes were marked by the CNC, but drilled on the drill press afterwards.

The last picture also has already been cleaned up with 150 and then 400 grit sandpaper (very minimal sanding done which I loved).

I also haven't radius'd the bottom yet, and am still contemplating doing that on the CNC or just doing it by hand.....

Attachment:
Machining Bridge.jpg


Attachment:
Cutout Bridge.jpg


Attachment:
Finished Bridge.jpg
Author:  arie [ Tue Dec 17, 2013 10:47 am ]
Post subject:  Re: First CNC Bridge
very nice so far!

may i suggest?
-no need for a tapered cutter for that work. you could use a 3/8"~1/2" dia. ball or square endmill (the same tool as your outer profile even) and program with a larger stepover. the work will go by much faster.
-do the saddle slot while you're at it. you already know the angle and dimensions.
-drill the pin holes in the same setup.
-no need to cut the wing transitions at an angle. cut them parallel to the length of the part. 3 axis moves take more time and more code.
-way too much tool overhand on that second tool. loose the long flute length and chuck it up closer and you'll get a better finish.

keep at it! :)
Author:  Blain [ Tue Dec 17, 2013 8:25 pm ]
Post subject:  Re: First CNC Bridge
Hey Arie,
Thanks for the feedback.
I didn't realize a square endmill would do ok for this. I'll give it a try because the tapered ball that I used took a long time (1 hour 45 min to 2 hours) But it was the only tapered ball nose (or ball nose for that matter) that I had, so that's why I used it (again, I thought I needed a ball nose for this operation).

When you say larger stepover you're saying like 50% - 100% or so? Meaning less overlap?
I was doing this probably 100% wrong. I was using a 1/16 ball nose with a 10% stepover. No wonder it took so long. I just figured all that was needed.

Thanks again for the comments. This is exactly what I need as I haven't done a whole lot with the CNC yet.
Author:  Andy Birko [ Tue Dec 17, 2013 11:15 pm ]
Post subject:  Re: First CNC Bridge
For something like that, I'd use a 1/2" ball nose bit with a step over of about .008" to .012" and it would give really good results. Probably smoother than what you have because the scallops would be smaller. The absolute stopover I'm using is only a little bit more than you're doing but the radius of a 1/2" ball is so much greater that the end product comes out smoother. Typically speaking, you want to use the largest bit that will get the job done. I have a couple bridge designs that need a 1/4" bit but so far, that's the smallest I've needed to go to get the detail I want.

It's possible that I could go to an even larger stopover but something like that would take less than 10min so I haven't experimented that much. I'm feeding at 300ipm for something like that but I could probably safely bump it up to 400.

Going the long way instead of angle will speed up the process as arie mentioned. You wan't the longest passes without changing directions to get the quickest time.
Author:  arie [ Wed Dec 18, 2013 11:10 am ]
Post subject:  Re: First CNC Bridge
Blain wrote:
Hey Arie,
Thanks for the feedback.
I didn't realize a square endmill would do ok for this. I'll give it a try because the tapered ball that I used took a long time (1 hour 45 min to 2 hours) But it was the only tapered ball nose (or ball nose for that matter) that I had, so that's why I used it (again, I thought I needed a ball nose for this operation).

When you say larger stepover you're saying like 50% - 100% or so? Meaning less overlap?
I was doing this probably 100% wrong. I was using a 1/16 ball nose with a 10% stepover. No wonder it took so long. I just figured all that was needed.

Thanks again for the comments. This is exactly what I need as I haven't done a whole lot with the CNC yet.


no problem. it's certainly not the only way, but yep, good old flat endmills will work for your lofted features. a good way to visualize this is to take a finished part and hold a cutter in your hand, hold the cutter perpendicular to the bottom of the workpiece and move the tool across the surfaces as if you were the machine. observe how the tool makes contact and it'll quickly make sense.

Andy's figures are appropriate for stepover (the amount the tool cuts into the work with each pass) amounts. .0075 to .015 with a 1/2" ball or flat would be where i'd start from. scale up or down from there. it kinda depends upon how fast you want to make your part vs. how much sanding you want to do. wood being very easy to sand, you could pick up the pace a bit whereas were it metal, sanding would be more wasteful. too much sanding and you loose a certain crispness to the features. only you can decide this one.

were it mine also i'd probably just run some screws straight down through the scrap areas of the bridge blank and into a subplate and loose all the blocks, and etc, of the set-up. imo a bit cleaner, visable, and more secure.

you're doing fine. if you want you could look at some tube videos of carvin or taylor machining their guitar parts for ideas.

https://www.youtube.com/watch?v=fnUKqRqQbao
https://www.youtube.com/watch?v=vVhApvkq9Ao
Author:  Blain [ Wed Dec 18, 2013 8:48 pm ]
Post subject:  Re: First CNC Bridge
Thanks Andy and Arie. I appreciate your willingness to help.

I'm going to re-try this and see what I get.
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