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Discussion Starter · #1 ·
So I have a bit of a question about using a CNC to clean up the outer contour of a specific shape. The first image is a bunch of the test runs I did and what the profile is.

Now the second image is what my boss wants to be the finish quality of it. Now we just bought a very cheap as CNC from someone who got it from an auction. Now one of the thing that my boss want the CNC to be able to do are these profiles which are wall brackets, there is another profile like this that he wants me to do but is more complicated. The way we usually finish these wall brackets is sanding it and then routing the edge with a cordless router. The other wall bracket profile my boss had a custom made 5-6" diameter bit for a shaper/router table that finishes the surface and routing the edges in one pass. My boss wants to see if I can do the same surface finish using the CNC just for faster production and if you see how he has to set up the shaper and the jig he using is very sketchy and no one in the entire shop wants to do it so my boss is the only one that does it and he wants it so he doesn't have to do it.

Ok back to the question. Now the second picture is what it looks like once its been sanded and routed. The third picture is what I'm getting with the CNC. There are some part of it where the bit chatters creating bumps and ridges on the surface almost like bit is bouncing on the surface. Last picture is the bit I'm using which is a straight bit. It is a clockwise turn and I have it running at about 12-14000 rpm and roughly 100in/min. The wood is poplar. On the first image is the path that I take on my test runs. The bottom left curve usually is smooth-ish but once it starts doing the curve of the second path around the center of the curve where it moves down the curve is usually where the chattering and vibrations start.

Now I have tried to change some settings on Mach3 like CV feedrate making sure Constant Velocity is one and all that kind of stuff. I've been going through multiple forums seeing if anyone my have an answer for why it starts to vibrate when it does a curve. My boss now suggested for me to try flipping the profile and going the opposite way I was going. I don't know if I should speed up the bit or lower it or speed up the bit and lower the feed rate. I'm planning on buy this bit which is a compression bit, same diameter and length but it is expensive and I'm buying this on behalf of the company on my own card so before I buy it to see if it will give me a much cleaner and smoother finish I want to know what your guys' thoughts if I can make the straight cut finish it like the sanded down version.

A little note: I an not cutting this wall bracket piece. I am only running the bit along the contour of it and only taking out 1/16 - 1/18th of an inch or 1 - 3 mms of material to try and smooth out the surface from the band saw and I am using the entire length of the bit along the contours of the bracket.

TLDR: I want to use the CNC to create a similar surface finish to the sanded down version of the wall bracket with a straight bit but encountering problems with the bit vibrating and chattering parts of the wall bracket. Boss said I should try flipping/mirroring it on the y-axis to see if the problem is "climb cutting". I want to buy an expensive compression bit on my behalf of the company but holding off to see what your guy's recommendations are.


Thanks,




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I would use a spiral downcut bit, not a straight bit. Also, since you are not using the CNC to cut out the parts, I would not pay for a compression bit. All that said, since you are not cutting these on the CNC, how are you positioning them after you have cut them out on the bandsaw? I would think that it must be a bear to set X0Y0 on a piece like that already cut out.
 

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Mike
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The chatter might be caused by looseness in the CNC itself. Make sure all the bolts and screws are tight, check to make sure the drive screw nuts aren't worn and need to be replaced, make sure the drive rails are clean and properly lubricated, and check the bearings in the spindle to make sure they are not creating excessive runout, tram the spindle, and skim the spoil board to make sure it is flat to the spindle.

I would also consider cutting out the parts using the CNC instead of using the band saw. You could be cutting several parts with a single toolpath, or use 2 sided carving to do the whole project except the cleanup and sanding.

If the boss wants the roundover done on the CNC then I would set up the file so it is a 2 sided project. Cut the roundover on one side using a point cutting roundover bit, then change bits to a spiral endmill, cut alignment/index holes for dowels, then remove the board. Cut alignment/index holes in the spoil board, locate the board on the spoil board face down using dowels. Then change back to the point cutting roundover bit and run the toolpath for that side. Change to the spiral endmill and run the cutout toolpath. Sand the parts. Now that you have the alignment/index hols in the spoil board you can pull the dowels, locate the board, cut the alignment holes in the board, change bits, flip the board and locate using the spoil board alignment holes and dowels, cut the roundover toolpath, change bits and run the cutout toolpath. You are now ready for the next board .......

The number of parts you can cut at one time depends on how big your CNC work area is and how creative you are. You could actually set up several boards at one time and just use the four toolpaths, board alignment, roundover, roundover, cutout, and have a lot of parts cut per run. Cleanup can be done while the machine is culling more parts.
 

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Discussion Starter · #4 ·
I would use a spiral downcut bit, not a straight bit. Also, since you are not using the CNC to cut out the parts, I would not pay for a compression bit. All that said, since you are not cutting these on the CNC, how are you positioning them after you have cut them out on the bandsaw? I would think that it must be a bear to set X0Y0 on a piece like that already cut out.
So what I did was I traced out a finished bracket on pen and paper, scanned it, and then traced it on fusion 360 and from there I have made a small ghetto jig with clamps just for test from a scrap piece of 3/4 MDF and then routed out what the contours of the bracket is so I know how to position it. It is close to where to zero positions are, I have a work offset for this particular test run job.
 

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Discussion Starter · #5 ·
The chatter might be caused by looseness in the CNC itself. Make sure all the bolts and screws are tight, check to make sure the drive screw nuts aren't worn and need to be replaced, make sure the drive rails are clean and properly lubricated, and check the bearings in the spindle to make sure they are not creating excessive runout, tram the spindle, and skim the spoil board to make sure it is flat to the spindle.

I would also consider cutting out the parts using the CNC instead of using the band saw. You could be cutting several parts with a single toolpath, or use 2 sided carving to do the whole project except the cleanup and sanding.

If the boss wants the roundover done on the CNC then I would set up the file so it is a 2 sided project. Cut the roundover on one side using a point cutting roundover bit, then change bits to a spiral endmill, cut alignment/index holes for dowels, then remove the board. Cut alignment/index holes in the spoil board, locate the board on the spoil board face down using dowels. Then change back to the point cutting roundover bit and run the toolpath for that side. Change to the spiral endmill and run the cutout toolpath. Sand the parts. Now that you have the alignment/index hols in the spoil board you can pull the dowels, locate the board, cut the alignment holes in the board, change bits, flip the board and locate using the spoil board alignment holes and dowels, cut the roundover toolpath, change bits and run the cutout toolpath. You are now ready for the next board .......

The number of parts you can cut at one time depends on how big your CNC work area is and how creative you are. You could actually set up several boards at one time and just use the four toolpaths, board alignment, roundover, roundover, cutout, and have a lot of parts cut per run. Cleanup can be done while the machine is culling more parts.
See thats where the problem is, thats way too many steps and will take way way too long. The purpose as to why my boss asked me to try to clean out the brackets using the CNC is for production purposes. If it takes roughly 30-40secs for someone to use a belt sander and sand it and then router the edges, while this method might take minutes to do. On the shaper/router table he does it in one pass and the bit he is using is not different from my straight bit for the CNC. Imagine a straight bit that is 5-6" in diameter and has three knives with a round over part on the top and bottom and it cleans out the bracket in one go, some bracket does have to be sanded a little bit but after it goes through the shaper/router table its good to go and doesnt need further cleaning/finishing.

The CNC is solid I've tighten all available screws even added locktite on some of them, replaced the bearings on the x-axis the y axis is a little harder to replace since the entire y axis gantry weights 150-200 lbs or more. Lubed everything and I tried tramming the spindle the best I can since this is some cheap as chinese CNC and there is no eccentric nut to tram it.

Cutting the bracket from a solid piece of poplar will take a long time and alot of wasted wood. Ill try and use a 1/2" shank 3/4" diameter down cut 2" long bit. Ill still try to use the straight bit but flip the bracket and go the other way and see if what he asked me to try will work.
 

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Chatter can also be caused by inadequate clamping if the part is not held tight.

Remember you are cutting these out using a band saw, then putting them on the CNC to be trimmed to the finished size, then using the shaper to add the bullnose profile, and finally finish sanding. All of the parts produced like this are individual parts.

Using the CNC to do the roundovers and cutouts on several parts at the same time only leaves the finished sanding to do. Someone would have to set up the boards on the machine, change out bits when needed, flip the boards, and remove the parts when finished, but that person can be doing the final sanding while the machine is running cutting more parts. Once it is set up and the operator gets used to running the CNC I think it would more productive because it would free up some workers to work on other jobs.

I guess it all depends on what size work are your CNC machine has and the size of the boards and part size to how many you can cut at one time. You never gave that information.
 

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What Mike said!! Do it all on the CNC.
 

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G'day @triablemilk09 , welcome to the forum.

I cannot see the benefit of doing production type work, in many steps, on different machines?
 

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Discussion Starter · #10 ·
Chatter can also be caused by inadequate clamping if the part is not held tight.

Remember you are cutting these out using a band saw, then putting them on the CNC to be trimmed to the finished size, then using the shaper to add the bullnose profile, and finally finish sanding. All of the parts produced like this are individual parts.

Using the CNC to do the roundovers and cutouts on several parts at the same time only leaves the finished sanding to do. Someone would have to set up the boards on the machine, change out bits when needed, flip the boards, and remove the parts when finished, but that person can be doing the final sanding while the machine is running cutting more parts. Once it is set up and the operator gets used to running the CNC I think it would more productive because it would free up some workers to work on other jobs.

I guess it all depends on what size work are your CNC machine has and the size of the boards and part size to how many you can cut at one time. You never gave that information.
I guess that could work. My table is 4'x8'. If I cant do it all in one pass then I guess do it on 2 passes.
 

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Ok, so I ran a CNC department that was doing very similar things when I took over the department. The part was the inside arch of wooden doors. They were sized and coped, sent to us, bandsawed and then cnc’ed (trim and profile)…then sent back to machining to be pinned with dowels. It was ridiculous. It was changed to sized, coped and pinned by machining, and then sent to use for cutting and profiling with two heads of the CNC. Handing time dropped incredibly, and that is what you are after.

we had a vacuum table, but toggle clamps and fixtures work very well. Even if it requires a bit change, trimming all the parts and profiling separately will still save you machining and handling.
 

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Ok, so I ran a CNC department that was doing very similar things when I took over the department. The part was the inside arch of wooden doors. They were sized and coped, sent to us, bandsawed and then cnc’ed (trim and profile)…then sent back to machining to be pinned with dowels. It was ridiculous. It was changed to sized, coped and pinned by machining, and then sent to use for cutting and profiling with two heads of the CNC. Handing time dropped incredibly, and that is what you are after.

we had a vacuum table, but toggle clamps and fixtures work very well. Even if it requires a bit change, trimming all the parts and profiling separately will still save you machining and handling.
One caveat, this can certainly be done more quickly and easier on a CNC…but there are plenty of other processes that are faster on other machines. Just because it can be cnc’ed doesn’t mean it should. Afterwards our backwards boss wanted everything Cnc’ed which led to delays, bottlenecks and ultimately the closure of the company. It is about saving time, not just running the machine…which is how he thought it worked.
 

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As usual, I am late to the game here. Chatter can also be caused by taking too big a bite. I would create a final pass that takes no more than .05", maybe less, as the clean up pass. Another thought is using a 3/4" straight bit seems awfully big. What does your chip load come out to be with it?

I agree with suggestions to do as much as possible on the machine. Minimize tool changes and do as many as you can in a single run. By cutting a lot of them from a single piece of wood, you can minimize clamping issues and spread out tool change time over many pieces.

I would try starting with a shallow depth of cut and increase that until you get the chattering. Then back off from that for your standard DOC.

Finally, what is your machine? Brand? Spindle size? Maybe some photos? Especially of the supports, rails, screws and gantry. How rigid is it? You can't judge that by trying to manually flex it. It doesn't need to flex much to cause problems. Also, resonance can be a big issue.
 

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Discussion Starter · #14 ·
As usual, I am late to the game here. Chatter can also be caused by taking too big a bite. I would create a final pass that takes no more than .05", maybe less, as the clean up pass. Another thought is using a 3/4" straight bit seems awfully big. What does your chip load come out to be with it?

I agree with suggestions to do as much as possible on the machine. Minimize tool changes and do as many as you can in a single run. By cutting a lot of them from a single piece of wood, you can minimize clamping issues and spread out tool change time over many pieces.

I would try starting with a shallow depth of cut and increase that until you get the chattering. Then back off from that for your standard DOC.

Finally, what is your machine? Brand? Spindle size? Maybe some photos? Especially of the supports, rails, screws and gantry. How rigid is it? You can't judge that by trying to manually flex it. It doesn't need to flex much to cause problems. Also, resonance can be a big issue.
We bought this cnc for like dirt cheap C$3500 from a cabinet shop that one of their friend got from an auction. Its called a Industrial Cnc Artisan 408, spindle 2.2kw. Y axis bearings need to be replaced but entire gantry weights 3-400 lbs. Pretty rigid. Only complain is that tramming the spindle is not the easiest because it doesnt have an eccentric nut and only have machine holes which had threads of which the previous owner and I had the same idea of filing it to make a more oval hole so we can adjust the plumbness or levelness of the spindle.
 

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So I have a bit of a question about using a CNC to clean up the outer contour of a specific shape. The first image is a bunch of the test runs I did and what the profile is.

Now the second image is what my boss wants to be the finish quality of it. Now we just bought a very cheap as CNC from someone who got it from an auction. Now one of the thing that my boss want the CNC to be able to do are these profiles which are wall brackets, there is another profile like this that he wants me to do but is more complicated. The way we usually finish these wall brackets is sanding it and then routing the edge with a cordless router. The other wall bracket profile my boss had a custom made 5-6" diameter bit for a shaper/router table that finishes the surface and routing the edges in one pass. My boss wants to see if I can do the same surface finish using the CNC just for faster production and if you see how he has to set up the shaper and the jig he using is very sketchy and no one in the entire shop wants to do it so my boss is the only one that does it and he wants it so he doesn't have to do it.

Ok back to the question. Now the second picture is what it looks like once its been sanded and routed. The third picture is what I'm getting with the CNC. There are some part of it where the bit chatters creating bumps and ridges on the surface almost like bit is bouncing on the surface. Last picture is the bit I'm using which is a straight bit. It is a clockwise turn and I have it running at about 12-14000 rpm and roughly 100in/min. The wood is poplar. On the first image is the path that I take on my test runs. The bottom left curve usually is smooth-ish but once it starts doing the curve of the second path around the center of the curve where it moves down the curve is usually where the chattering and vibrations start.

Now I have tried to change some settings on Mach3 like CV feedrate making sure Constant Velocity is one and all that kind of stuff. I've been going through multiple forums seeing if anyone my have an answer for why it starts to vibrate when it does a curve. My boss now suggested for me to try flipping the profile and going the opposite way I was going. I don't know if I should speed up the bit or lower it or speed up the bit and lower the feed rate. I'm planning on buy this bit which is a compression bit, same diameter and length but it is expensive and I'm buying this on behalf of the company on my own card so before I buy it to see if it will give me a much cleaner and smoother finish I want to know what your guys' thoughts if I can make the straight cut finish it like the sanded down version.

A little note: I an not cutting this wall bracket piece. I am only running the bit along the contour of it and only taking out 1/16 - 1/18th of an inch or 1 - 3 mms of material to try and smooth out the surface from the band saw and I am using the entire length of the bit along the contours of the bracket.

TLDR: I want to use the CNC to create a similar surface finish to the sanded down version of the wall bracket with a straight bit but encountering problems with the bit vibrating and chattering parts of the wall bracket. Boss said I should try flipping/mirroring it on the y-axis to see if the problem is "climb cutting". I want to buy an expensive compression bit on my behalf of the company but holding off to see what your guy's recommendations are.


Thanks,




View attachment 400652 View attachment 400649 View attachment 400650 View attachment 400651
Hi. Have you thought about breaking your profile vector up so you can do climb milling on some parts, and conventional milling on others. If you use the same technique throughout on a part like you picture, you will on some curves be picking the grain up.
 
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