Clean cuts on clear pine

[jseiler]

I'm through cutting 3d parts with tabs with a benchtop PR32 and having problems with the edges having a bit of fuzz. I'm hoping to get these cut with little need for sanding, but that's not the way its working out. The wood is 3/4" thick and I'm cutting in two passes. The first pass cuts down .65 inches @ .4 in/s. The second pass moves in 50/1000 and finish cuts and adds in the tabs. It does not appear the parts are moving while cutting the last pass. I'm using a bosch 1/4" spiral upcut and the board being cut is screwed to the table and clamped on the edges. Any clues on how to get a finish quality cut? Ultimately I'd love to be able to do it in one pass (2 passes takes twice the time on my slow PR32).

[jseiler]

Opps...that should be 2d parts.

John

[Brady Watson]

John,
You are taking too much off in one pass at .65 deep. Try speeding up the move speed and cutting only .25" deep per pass down to .65 or so with .03" allowance on the profile. Then have it go back full depth to clean up the waterline/stepdown marks.

If you are moving the tool that slowly and that deep...I can tell you that you are killing the bit and quickly working with a dull tool. Keep the bit cool and the tool moving by taking a smaller bite at a shallower depth.

-Brady

[jseiler]

Thanks for the quick response. One would expect that if I were running too deep, I'd be getting dust, burning and horrible sounds. I get shavings, no burning, sounds smooth. Wierd. I do see that my cleanup pass needs to take off more than 50/1000", so I will try that. I didn't check to see if the bit was getting real hot. I do know that router (PC 890) wasn't bogging down.

A while back before I cleaned up the mechanicals, I was only able to jog at .7in/s and cut at .4in/s without missing steps. I haven't tried a max speed test since. I emailed shopbot about what the max speed should be on a PR32 (it seemed low to me), but I got no response. I then posted here to find out if anyone knew how fast a PR32 should be able to go, but I didn't get a response. I'm interested in upgrading to a 24V PS, but I need to make sure the machine is running properly with its 12V PS before I try going higher. I don't want to compound problems.

I'll have another try using your suggestions and see what happens. I'll also see if I dulled my bit, but since I didn't run it very much like this, I'm probably ok.

thanks again,
John

[mrdovey]

John...

You might also try cutting a bit of scrap with straight and down-spiral bits. The up-spiral lifts the wood fibers at the edge of the cut.

Bit sharpness also has a noticable effect on cut quality - and the fact that a bit is new doesn't necessarily mean that it's really sharp.

...Morris

[elcruisr]

I can sometimes get better cuts in problem woods with one of the polished spiral "O" flutes originally meant for plastic. Try it with a single flute tool, deeper gullet and sharper rake angle will sometimes make the difference.

Eric

[ron brown]

John,

At one time all ShopBots were 12V. The move to 24V gave no problems, better speed and fewer lost steps.

Of course this was in the daze of threadded rod "Z" axis and cable-drive when the first part of any day was tuning the cables and "Z" screw, manual rezeroing the X and Y and setting the Z with a piece of paper, saying a quick prayer, running the part and then start adjusting again.

Ron

[jseiler]

I spent the last 4 days working with my design. I moved the machine speed up to .6in/s, conventional cutting direction, 3 passes @.255in, no cleanup pass. The pieces came out pretty nice. I continued to use the upcut bit. When the parts are done machining, I sand the tops with an orbital sander which cleans up the fuzz on the top caused by the 1/4 up spiral. After I pop the tabs, I can sand off the machine marks in about 5 minutes on a belt sander. Not bad. When I get the other bits in, I'll give those a try and see if I can do better. If it weren't for the tabs, I'd not even have to sand. Time for some vacuum holddown???

Thanks for the advise,

John

[jseiler]

I'm still having some problems with losing some edges on a part. The part is a 3 in circle with 8 half circles (1/2" diameter) cut into the edge (it looks like a cog). I've tried cutting it all at once in both directions. I've tried cutting out the small half circles first then cutting the profile of the large circle. (when I do this, I get clean holes, but the profile pass kills the edges where the grain is weakest). It just seems like the profile pass knocks off the edges where the half circle joins the larger profile. I've tried straight double flute bits and a spiral upcut bit. The straight bit was a little better. I'm at a loss here because I know people can cut accurate cogs on a shopbot. I'm going to try to rotate the whole thing by 22.5 degress to see if I can get a little more support.

The part is part of a larger assembly I'm cutting from a 2 ft blank. I'm also going to try to use a 1/2" bit a make just cogs on my blank instead of using the 1/4" bits I'm currently using for the other parts.

Any other ideas? Maybe its just the nature of pine?

regards,
John

[paco]

Hi John!

I'd say the lighter the woods the more tear out (grain end) one get (White Pine, cedar and such)... I have had acceptable/better (less tear out) results from climb cutting but with more sanding to do after (fuzz)... Cimb cut will prevent knock off corners too where conventionnal break 'em most of the time...

As for accuracy; I believe you can get accurate parts but you might not be much happy with the finished edges...?

Have you give a try at Eric suggestion about a O-flute tool?

Does that help?

[jseiler]

Haven't tried the oflute yet. I just got it a few days ago and haven't had a chance. I look forward to giving that a whirl. The one thing I haven't tried yet is cutting the profile with thick skin on the backside first, cutting the small circles into the edge next, then a very light profile cleanup pass.

The project is for a small rubber band gatling gun. The "cogs" hold 1/2" dowels in place to form the rotating barrel assembly. All the other parts come out beautiful. The one that requires complex curves, shaqrp edges and tight tolerances is coming out poorly. Right now, I just use a little extra glue in that area and sand off the excess to fill, but that sucks.
I'd much rather have clean parts and reduced assembly time.

I'll report back after I try out the oflute.

I'm also thinking that using a 1/2" bit in a direct profile pass might work out better, but I haven't tried this out yet either.

regards,
John

[mrdovey]

John...

Can you post a drawing or photo of the problem part (or post a link) so we can see what we're talking about?

...Morris

[jseiler]

I can't post a picture until later, but maybe I could describe it better. It looks like a 3" cookie with 8 evenly spaced half circle "bites" taken out of the side. The half circles are .5" diameter. It looks like a cog. I formed this shape by drawing a 3" circle and .5 Inch circle and wrapping 8 evenly spaced copies of the .5" circle with the centers on the 3" circle such that they are 45degrees apart. Its really simple (and a picture would make it really clear).

I problem is where the small "bites" form an approximately 90degree edge with the outside of the large circle. The router likes to knock those edges off. Of course, its not consistant, it tends to do it on 3 oclock and 9 oclock sides, so its likely a grain issue.

I tried through cutting this shape directly (single pass and multiple pass with tabs).

I tried "drilling" the 8 .5" circles first (which came out clean) then profile cutting around the 3" circle (multiple .25" passes with tabs, tried both directions). And it never fails to fail. Its just frustrating.

I can post a picture tonight if I can figure out how to post a picture.

John Seiler

[paco]

Hey John!

You wrote you've tried both cutting direction; was'nt it better with climb as for thoses knock off?

If I can help with the image(s)/picture(s), let me know...

[jseiler]

The overall finish was better on the conventional, but it tended to knock those edges off a little more regularly. i had searched the archives and found posts that suggested looking at the finish of the waste piece and the cutout piece to see which direction to cut (helpful advise).

I'll have to cut some new ones in a test file so I can keep track of how each one was cut and its orientation. I took all my failed attempts and threw them in a box so I'm not sure which one is which and which direction they were oriented when they were cut. When I do my new test cuts, I'll mark the cutting direction and orientation better.

[srwtlc]

John,

With the design that your cutting and because of it being pine, it's going to be hard to get away from what is happening (the nature of the material). Rotation of part or file or direction of cut will be of minimal help.

I know what shape your talking about, as I've used the same design for knobs on jigs and such.

If you can, you could try a slight radius on those corners. Or, if actual finished diameter isn't critical and if you have a stationary belt sander, drill press, or a lathe, you could cut the outer diameter larger and then chuck them up on a homemade mandrel and sand them to the finished diameter. That would be quick work on a stationary belt sander and a hand drill with a 100-120 grit belt. That way you would sand past the chipout giving a clean crisp edge.

Scott

[gerald_d]

A couple of things you could think about.....

1. Use some penetrative, hard-drying paint/sealer/varnish to bind the fibres together before cutting.

2. Cut anti-clockwise around the part (conventional) for the least breaking out. Don't do a cleanup pass.

3. Start with a very shallow slow cut and take progressively larger faster bites after that.

4. Make move speed very slow, use the sharpest bit, spiral DOWN, high spindle speed (tiny chipload).

5. Stick a "mask" over the job.

6. Pull a piece of scrap ply down on top of the job.

7. etc.

Rubber band Gatling (http://www.backyardartillery.com/machinegun/)

[srwtlc]

Ohhhh! I see a winter project coming on!

[Brady Watson]

Hey Gerald,
Bill Young made one of those a few years ago and brought it to the Jamboree...It was really cool!

-Brady

[gerald_d]

Oops, I didn't mean to drift the thread, honest. In the pics I think we can see the part that John is talking about. It might save him the trouble of posting a pic.

[jseiler]

Actually Gerald, its a really fun problem. My little one is no where near the complexity of the one you linked.

As can been seen from the linked picture, the barrel holder wheels are not the same diameter. On mine, they are 2.5" for the front and 3" for the back. On the first iteration, I cut the half circle cutouts straight down and when the barrels were glued into place that small bit of left little 20/1000 gaps at the back of the large and small barrel holders. To get proper fitting, one must cut almost straight down to the bottom of the wood, move out 20/1000 or so and finish the cut so everything lines up nicely. Of course, almost noone would see this misalignment unless they scrutenized the assembly very carefully (especially if you allow a little glue to ooze there), but when you own CNC equipment and live in a world of constant perfectionist revision, one must try to resolve this. There are many small things which really make assembling something like this hard to do perfectly. I've spent probably 200+ hours designing and redesigning parts so they fit together perfectly and can be assembled rapidly and produce repeatable accuracy. I plan to sell these things assembled for $20/ea at swap meets. Including machining and assembly time, there isn't a huge amount of margin here, so its important that when I take these into miniproduction, the parts come out clean and accurate with minimal finishing, sanding, made for rapid assembly. I thought this would be trival when I bought my used PR32...guess what...its harder than it looks and its a huge amount of fun if you like tinkering.

regards,
John

[bill.young]

Hey John,

Have you thought about using plywood for those parts? Baltic Birch ply is stable and pretty cheap.

Bill

[btk]

John,

You may have tried already, however can you
cut out the large circle first and then go and drill
the 0.5" holes as the final step?

Brian

[mrdovey]

John...

Recently I was having difficulties with tearout in hard maple when cutting cross-grain dados - which I think is closely related to your problem.

I tried a number of techniques to eliminate the tearout and the most effective turned out to be plunging a straight bit so that the point of the tearout was actually drilled, rather than routed. Once the tearout points had been taken care of, I routed the dado normally with no problem.

This technique might help you to avoid tearing out the difficult side of each of the "bites".

...Morris

[jseiler]

Bill-I'd love to use plywood because I could lay down a sheet and cut a boatload of parts at once, but for this project, the edges show and I believe it wouldn't convey the handcrafted toy look I'm looking for.

BTK-Haven't tried that yet, but I probably will before this all over. I just have this feeling that I'd have to make the tabs numerous and substantial to take the final hole drilling process after the profiling. Maybe I really need to look into vacuum.

Morris- This may be a winner. If I use a drilling motion to cut the edges that are giving me the problems, I could profile the rest and the bit wouldn't touch the problem area again.

[bill.young]

Hey John,

I can certainly understand the "look and feel" issues and agree with your choice of solid wood to give a hand-crafted feel.

My experience with building things with moving parts, though, has been less than satisfactory when I've tried using pine and other softwoods. They tend to move a lot with temp and moisture changes...just the conditions that a toy faces...causing binding or slop in the mechanism.

I don't know what your design is like but what about a hybrid solution...ply for the mechanical parts and solid word for the rest?

Just a thought,
Bill

[jseiler]

Bill--My design is quite simple, which is the beauty of it. The stuff that requires good tolerances doesn't move and is just glued into place. The moving parts can be a bit sloppy for several reasons. I've considered cutting recesses for nylon bushings, but so far, I haven't needed to do that. If I were building something as complicated as what gerald posted in his link, I'd most certainly have to build the mechanicals from ply. Heck, that thing looks like a clock. Mine is cut from a blank of dimensional clear pine (1"*6"* 2ft). That gets everything but the dowels. I'm using tabs to hold everything in place, and when i'm done, I can pop the tabs, round some edges and assemble it in about 15 minutes (it took forever to figure out how to "use" the accuracy of the cnc to insure rapid assembly). I'm working on some glueup fixturing to speed the assembly up even more (I hope to keep up with the machine). With my slow machine, I can route 2 blanks/hr. I hope to speed that up to 4-5 with a power supply upgrade, hand optimizing the .sbp to eliminate a good bit of unnecessary moves, and mounting multiple blanks for cutting sequentially.

I came up with a fixture to cut the dowels for the barrels, but I can cut them faster by hand with jigs, unless I can find a way to improve that. The design requires the dowels be rotated to cut a v shaped point at one end and a notch at the other 90 degrees rotated. I'm wondering if I could use a keyhole cutter with multiple passes at the pointed end, then cut the notch with the same cutter. Haven't tried that yet. Right now, those dowels get cut with a 1/4 upspiral, then I rotate the dowels 90 degrees and use the 1/4 spiral to make a notch on the other end. Gotta be a better way. The other dowels get gang cut on the miter saw with jigs. I can make a box of those in no time.

I've got a couple of other projects in the queue before I can get back to work on the gun, but I should have some more info after the weekend.

thanks everyone,
John

[bill.young]

Hey John,

Sounds very well thought out...I'm looking forward to seeing some pictures.

Bill

[jseiler]

I tried the o-flute 1/4" bit, it did a little better, at least keeping the stuff broken off attached so I could push it back into place for glueing. I've got some other things to try too (cutting profile first, drilling problem areas etc), but I'm stuck in another project on a short fuse. (Kids play table. 2x2in oak legs, red oak skirt, 3/4 red oak veneer plywood top painted with blackboard paint. Must be finished by tonight to be used tommorrow. I'd hate to think how long it would have taken me without a shopbot).

I found a more local source for baltic birch, so I'm going to try some of that too. That would actually lower my production cost a bit. Its $55 for a 5ftx5ft sheet in 3/4 in.

Thanks everyone for all the help,
John