CN / small parts / tabs

[Matt Snowball]

Hi ,

I am a brand new CNC user and we have just bought a new CS41 . Our application is a little different in that we are using ecabs to design & cut roadcases for musical equipment. We have previously been manufacturing these by hand for three years now.

We are running into problems with hold-down of small parts. I've searched the forums and have looked at the various workarounds - double sided tape , leaving a skin, reduced extraction, resurfacing spoil board, etc . Given the choice , I'd leave a skin , but since we are cutting HPL (Formica)-faced ply, face down, this is not practical.

The Tabs function in CN could be a great answer to my problems, but seems not to function as well as it could. Can anyone with more knowledge and experience help me with the following queries?

1) The main problem I can see is that only one tab is applied per part. If you're cutting a long thin part, and a single tab is placed right at one end , it simply does not function to stop the part moving.
If - say - four tabs could be placed evenly around the perimeter, then the tabs would work just fine.

2) The machine can sometimes cut completely through its tabs on a second pass . I know I could work around this , but it would also be nice if the tabs were left in place whatever.

Thanks in advance

Matt

[Daniel Vonderheide]

Matt,

You are correct. CN will only apply one tab per part. You cannot apply more than one to any given part and that one tab will be placed at the beginning of the part. If the part is placed next to another part, the next part will ignore the original tab and cut through it.

[Todd Miller]

Matt,
As of now you can only run one tab per part using Control Nesting (CN). Are you nesting the parts? Or are you running the same program over and over again.

OPTIONs....
You can get a bigger pump to increase vacuum.
Use weather stripping to create a vacuum chamber (thus increasing vacuum). This would only be applicable if you were running the same program over and over again)
Use a Double pass when cutting parts. (reduce the double pass feed speed to reduce the load on the tool).

[Damon Nabors]

Considering the HPL is only around 1/16 thick. I would suggest using dbl sided tape. Another thing you could use that we used back in my body shop days of working for the father in law. We used a PSA (pressure sensative adhesive) to spray on the pad of our orbital sanders to help the sanding disc stick to the sander. It was sticky but unlike contact adhesive, you could still peel it off when finished. I have never tried it but wonder if you could spray that stuff on a sacrificial waste board and then apply the HPL. You would probably have to flycut the waste board each time you changed sheets due to the dust sticking to the adhesive.

Just a thought.

[Neville Bastian]

Hi Matt,
I'd say getting a second pump would be the answer as you will spend the time and money in messing around with work arounds anyway.

If you want another way to hold items down is the hot glue stick gun. This is cheap and quick. You would need to know where the small parts are which may be easy. Once you apply to glue where required it then could be removed by applying a heat gun.
Another thought is when optimising those small parts the material used could be called small parts material then they would be grouped together instead of randomly over a big sheet so it would be quick to apply the glue idea or whatever else you find to use. You may be able to focus the suction in a very small area to enhance hold down. This may require some modification to your plumbing though.
One last thing. A used sacrifical sheet is your weak link. Maybe a area of your bed could be little used. You can change you 0,0 point just for the small part issue.
I've run out of idea's. Your turn Damon.
Neville

[jerry johnson]

Matt, maybe you could use a compression bit and cut the material with the hpl facing up and then leave an onion skin on the plywood. I don't know if you are performing any other operations to the parts like boring holes or creating pockets that necessitates the material being placed on the machine hpl side down though. Keep trying.

[Nick M Singer]

Matt, before you run out and buy a bigger vac pump, just bear the following in mind (and this applies to anyone thinking the same) No matter what size vacuum pump you buy the laws of physics apply and in this case if you were to have perfect vacuum (which you dont) you only get 15lbs per square inch at sea level. If the parts are very small you dont get much surface area and hence little hold down pressure to counteract the forces of the router. We run an 800 cubic meter pump which is way over spec and it makes no difference to parts under a certain size. Matt in your case why dont you turn the sheet material around ,HPL side up, and run an onion skin, it is really the simplest solution.

[Neville Bastian]

That's a bummer. Nicks pump is bigger than mine

Neville

[Nick M Singer]

Ja it just sucks!!!

[Neville Bastian]

Hey Nick,
If the recession hits hard in South Africa you could get into face lifts as a side line with that size pump.

[Nick M Singer]

I was wondering if it could lift the Rand, it sure has taken a hammering over the last week. Looks like the yanks are dumping our currency so now Thermwood products are more expensive.

[Dale Wills]

Matt,

Does this material you are cutting have a plastic protective film? If so I may have an option for you.
In the past I have had to machine a heap of small acrylic parts (letters, sunglass display arms etc) and had problems with them moving. We successfully machined these small parts by glueing a sheet of 3mm MDF to the protective film on the acrylic. We used minimal sprayable laminex adhesive to join them together and then loaded the 3mm MDF down on the machine table with the acrylic facing up. Once we tweaked our daylight values (3mm to account for the MDF), machining these small parts was no problem. Since we were now only cutting into our 3mm MDF by our 'cut through' depth, all we needed to do was just peel the acrylic piece from the protective film which was of course left glued to the MDF.
I can potentially see this approach being used with any material with a protective film.

Dale

[DanEpps]

I was wondering if it could lift the Rand, it sure has taken a hammering over the last week. Looks like the yanks are dumping our currency so now Thermwood products are more expensive.

The Yanks? You haven't been following very closely--the Yankees didn't even make the playoffs (baseball).

[Matt Snowball]

Hi ,

I wanted to say thanks for the replies & suggestions. I haven't had much time to devote to figuring this out yet , what with learning how to use the machine and having to keep cases in production as well. Just wanted to reply on a few points:

The parts we make are different and are nested differently every time - most of the cases we make are one-offs

1)The double sided tape is a good idea , but time consuming to locate and apply every time a small part appears in the nest.

2) Increasing vacuum - The CS41 has the pump built in and attached, and changing for a larger one would be a last resort for me. My spoil board is in great condition, so no huge improvement to be had there .

3) Double -pass cutting. I really like this now I've figured out how to make it work - I think I had to update my CN soft ware to make it function. I think it's going to help a lot.

4) I would love to cut the ply HPL-side up and leave a ply onion skin, but then details on the parts would be mirror-imaged, and internal dadoes would become flip-ops. Not practically workable for me.

5) Dale - thanks for the idea. the HPL doesn't have a protective film, but I will be sure to use this if I get any suitable specialised routing work to do. I like it.

6) Neville - I like the 'small parts material' trick, but It would be a last resort for me - I want the small parts in the main nest for economic material usage.

I'm going to try out a combination of tabbing, double passes, and leaving larger gaps between the nested pieces - I think that could be promising.

Matt

[Brad McIntosh]

One problem with tabs, even if they are properly distributed around a part, is that if they do not line up with the tabs on the adjacent parts.... well they get "disconnected" when the next part is machined. Now that may not be a problem for the part that was previously machined, but the current part being machined has one less tab holding it to anything. AS more parts on a sheet get cut out, the "detachment" gets worse.

I have seen some CAD/CAM systems with tabbing options that let you specify where the tabs are to be located once the parts are nested. This way you can insure that the tabs are located such that they are aligned with those tabs on the adjacent parts. The only thing is, is that this sort of manual input is more for nesting layouts that will be run over and over since there is a time investment for the manual tab referencing.