Frustration with getting clean cuts in MDF

[dlcw]

Hi All. I know this has been hashed out many, many times but for the life of me I cannot get non-scalloped cuts going around arches in MDF. They are small scallops but to much for the customer. If I can't solve this problem I will loose a $2500 per month customer. Personally, I think I'm stuck with the inherent slop in the machine and probably won't ever solve the problem.

2009 PRSAlpha 96x60

What I'm cutting are letters in 1/2" MDF. The Aspire file is attached. They are simple letters with 1 or 2 holes in them to hang them on a wall. Certainly not rocket science.
Here are the steps I've taken so far:

- Removed my air drill to lighten the weight and reduce whiplash
- Tightened every nut, bolt and screw on the machine
- Clean the rails and v-bearings and reseated the drive motors
- Cleaned the racks and pinions to make sure there was no grit in them
- Changed the USB interface board in the controller to the newest version from Shopbot
- Have version 3.6.44 of the software

Have tried the following bits with the following configurations. The bit needs to be 1/4" for the letters to come out right. The cut process is to rapidly cut my first line in a climb direction (thousands of letters need to be cut) then conventional cut to the final line slower to get a good finish.

- 1/4" downcut spiral (factory new). 9500rpm at 4ips climb on first pass. Have tried from 9000rpm - 11000rpm at 2ips for second cut. Got scallops

- 1/4" Super O single-flute upcut bit (factory new) at 9500rpm at 4ips climb on first pass. Have tried from 9000rpm - 11000rpm at 2ips for second cut. Got scallops

- 1/4" 2-flute straight bit (factory new). at 9500rpm at 4ips climb on first pass. Have tried from 9000rpm - 11000rpm at 2ips for second cut. Got scallops

The letters are going to be painted and they have to look really good or they are rejected by the end customer.

Can the level of finish required be delivered by this machine or am I shooting for the moon and will only end up in low-earth orbit?

[pkirby]

Don,
I'm sure you've thought of this, but just in case you didn't... Check that the actual CAM file has arcs / bezier curves in it instead of extremely small line segments. I tried opening up your attached file but it won't pull up in my partworks.

Also, if it comes down to losing a customer, then I would buy an oscillating spindle sander to cleanup the curves.

[dlcw]

Sorry Paul. The file I attached is an Aspire file. I tried to go in and node edit to make sure that everything was smooth points and Aspire won't let me convert any of the points to smooth points. I got a DXF file from the customer and the lines seem to be very clean.

Thanks Paul. I do have a spindle sander but the smallest diameter drum is bigger then some of the curves I have to get in to. Also, if I have to do a bunch of sanding, I will loose money on each job because of sanding labor.

One of the reasons I got a CNC was to be able to do this type of work and have the machine make me some money, not put me in a position to loose money. Ughhhh. :(

[Brady Watson]

The vectors look like they could be cleaned up a bit. If you ungroup and go into node editing mode, you'll see a lot of extra nodes and arcs that could be defined as one big smooth bezier curve. You'll have to take some time to 'groom' the artwork to get a truly optimized production file. If this is a repetitive job - as in the same artwork every month, then this will pay for itself quickly. Otherwise, you'll have to charge them to clean up their artwork.

-B

[frever]

Check cutting parameters of the bit, maybe you are cutting too fast

[tmerrill]

If you want smooth nodes you need to convert the vectors to Beziers using the Fit Curves to Vectors tool. Then all nodes on each end of a Bezier segment will have the Smooth Node attribute selected and blend with the next segment.

Tim

[Chuck Keysor]

I guess everybody else was looking at this while I was.....

I zoomed in on the part of the letter that was pictured, and noted that the curve did look really smooth. But upon going into node edit mode, I found this curve was made up of 7 straight line segments and 6 arcs.

In maybe a bit of over-kill, I deleted everything except the end points of the subject curve, and converted the remaining arc to a Bezier curve, and got it to fit pretty close without any effort. So that would eliminate a lot of nodes/segments. It would seem that this would be the first thing to experiment with before going back to re-cut a sample. Chuck

[michael_schwartz]

There are so many variables, but I don't think the inherent flex found in the machine helps much.

I understand how frustrating things like this can be when it comes time to make money. 3/4 the time I love my Shopbot. Each project has its moments though where It would make sense to have a more robust machine, and I will probably go that route in the future.

Of course you could cut this on big iron, and get worse results.

I would test this on some larger parts that you can hold nice and tight to rule out hold down. I have noticed this before on occasion.

[dlcw]

Wow guys!! Thank you for all the feedback. I looked a lot closer at the letters and you were right. The curves are just a series of straight lines linked together. I will be going through to cleanup all the artwork prior to cutting a couple more test letters. I've let the customer know about was found on the artwork. In the interest of developing a long term relationship with this customer (potentially taking over all US manufacturing and distribution) I've told them that I would do the cleanup for free so I could produce a good quality product for them.

I'll keep you all posted on how things turn out after the artwork cleanup.

Thanks again!!!! :)

[jerry_stanek]

when I blow it way up there is an extra line on the outside letter not quite a duplicate vector but almost it weaves in and out

[adrianm]

Pretty much all I do is cut sheets of MDF day in and day out. The only time I've seen edges like that is when I've had rough vectors.

I use a 1/4" two flute down spiral running at 13k with a feedrate of 5ips/2ips. I use a pass depth of 1/4" as well.

I tried doing the two pass method for a while but to be honest I couldn't really notice any difference in edge quality although I did on ply and wood.

[dlcw]

Thanks Adrian and everyone else for your wonderful responses. It turns out that the problems seems to be artwork received from the customer. I need to go through and get it cleaned up and then I think I will be back in business.

At least the CNC got one heck of a thorough tuneup because of this exercise. :D

[adrianm]

I noticed that you seem to be doing the 1st and 2nd passes using offset vectors. It's probably better to use the allowance offset in the toolpath itself for this rather than creating another vector.

Once you've got nice clean bezier vectors and offset them the new vector isn't as nice and clean as the original one...

I use a toolpath template that is set with the correct depths, offset and climb/convential settings for this type of thing. Makes life a lot easier as I'm always forgetting which one should be climb and which way the offset should go!

[dlcw]

Ok. I've spent some time re-entering all the parts as text (Aspire generated vectors) instead of a bunch of converted vectors. I then cut them on the machine and I am still getting little scallops on the letters. Is this just inherent in the machine slop and not correctable? :confused:

[nat_wheatley]

Is the customer really complaining about that?

[SterlingDevelop]

WOW!

I wish our PRT cut that nice.

[dlcw]

Actually guys, it looks worse in person then what the camera can detail. I would have thought I could get really clean cuts with no scallops. But maybe I'm dreaming with this machine.

It might require a big-iron machine to get cuts with no scallops. Don't know. Just discouraged right now. I hate the prospect of loosing a $2500 a month client.

[bleeth]

Don:
I will make the assumption that the letters are not going to be used raw, but rather will be painted. Since they are mdf they will need some pretty good sealing on the edges and then sanding prior to top coat. The letters you now have look ready to seal to me. That minimal amount of edge imperfection will be gone after sealer sanding.
If your client thinks he needs better quality than that then he is less knowledgable then he thinks he is.

Dave

[pkirby]

Don,
What method are you using to hold down the letters and what size are they?

I've cut some 8" letters before and I had to hold them down with a vacuum table and tabs. I have an idea on to tell if your bad edges are a result from hold down...Run a test piece with the same parameters for your first and second cuts but do NOT cut all the way thru on the final cut. If the letter edges come out good, then you know hold down is not the issue.

[Brady Watson]

If you haven't already replaced the pinions on the X&Y, do so. They really influence cut quality, especially around curves ~ as do the rails themselves. Just because they are hardened, doesn't mean that they won't get some schmutz build-up on them and the wheels, causing little vibrations that get transferred to the cut. A scotch pad will make short work of the rails, you'll have to get creative cleaning the wheels. The PRS tools without rail wipers just mash chips into the rail.

-B

[dlcw]

Paul, hold down is a Gary Campbell 4-motor vacuum box. I get over 7" of hold down vacuum with the air flow of 4 motors.

In addition, I am cutting in one pass conventional with speeds of 4ips at 11000rpm. I cut .375" per pass so 2 passes total on each letter. With the down spiral 2-flute bit, the kerfs are packed VERY tight with chips and dust. I REALLY have to pry hard to even get the letters off the table after the vacuum is turned off. The letters are about 8" tall.

I will try your test tomorrow. Graduations and parties to go to today.

[garyc]

Don...
A couple thoughts.....
1) Agree with Brady on the pinions and rails. For the ultimate quality they (pinions) should be replaced every few hundred hours. Scotchbrite the rails, brass toothbrush for the wheels and install felt wipers.

2) The minor chatter that shows in your pics could be in part to bit deflection of the downspiral as it pushes thru packed chips.

Like Dave says, you are getting pretty close to what primer can cover/fill. You are also approaching the max quality that can be attained with a rack and pinion machine.

[dhunt]

Is the customer really complaining about that?That's what I'm thinking!! :cool:

[dhunt]

WOW!

I wish our PRT cut that nice.Same here! :cool:

[dlcw]

The customer was really impressed with the quality of the cutting. However she had problems with the quality of the MDF. I've got standard 48lbs density MDF from my supplier. It's all any of my suppliers carry. She said it wasn't good enough MDF (for painting).

She apparently gets MDF from somewhere in South America that has a lightly tempered face on both sides (harder then standard 48lbs density MDF). I've never heard of of seen anything like this. Neither have any of my suppliers.

Anyone ever heard of MDF with a lightly tempered face (kind of like tempered hard board, just not as hard) on both sides?

[jerry_stanek]

We use Plum Creek MDF if we need a quality paint job. It cuts a whole lot different than standard MDF.

[dlcw]

I will have to check with my suppliers on Monday to see if they carry or can get that brand. Jerry, are you in the eastern or western US?

[Ajcoholic]

I know I can get different grades in Canada... some are darker and have a harder face. The cheap stuff is lighter brown and you can tell the surface is no where near as hard/temepered.

AJC

[myxpykalix]

Why not go back to the customer and tell her to give you some specifics of where she gets it, ect so maybe you can find out direct from her source? :eek:

don your email bounced back to me sent you a PM:)

[wberminio]

TruPan is from Chile.Plum Creek is finer grain.Ranger board is another.

[steve_g]

Plum Creek holds detail very well, it's what I use... however I would not describe it as having a harder face. It's very consistent top to bottom, Just what I like for my purposes.

SG

[jerry_stanek]

We use it for display tool boards because it paints up really nice and you only need to primer the sides and not seal them before you primer it.

[pkirby]

Don,
I've had great success with specifying "Door Core" grade MDF with my suppliers. It's typically carried in 3/4" but it is available in other sizes. Come to think of it, the door core I have looks lightly temepered on both sides. 3/4"x4x8 Door Core typically costs me $25-$30 sheet from a wholesale lumber yard.

[steve_g]

Door core... http://www.uniboard.com/door-core,1,1
a name I'd not heard. looks like a very interesting product... even a HDF version.
Sadly, no Texas distributors. :(

SG

[pkirby]

Door core... http://www.uniboard.com/door-core,1,1
a name I'd not heard. looks like a very interesting product... even a HDF version.
Sadly, no Texas distributors. :(

SG

The product I use is under the brand name Langboard Elite. Most of the suppliers in my area (Atlanta, GA) refer to "door core" as a grade of MDF rather than a brand.

[dlcw]

Looks like Door Core is strictly a northeast product with nothing out west. Bummer....

[beacon14]

Ranger Board is another brand I've heard of. When I ask a supplier for "A" grade MDF they usually know what I'm talking about.