production methods for routing items from sheet (repeat batches)

[jimbo_cnc]

I will want to make (or have made) some delrin parts. It could be ongoing production if the product sells.

I'm new to CNC so working this out from scratch! As the part has features both sides, and some of those features are 4x M4 screw threads, I was thinking this:

Say the part is 58x58x25. So, get a 250x250mmx25mm sheet and clamp that. Then drill all the holes for threads and cut the features on that side. Remove from table and press fit the inserts (if that's the method). Then use those threaded holes to fix the sheet to a jig (just a sheet with matching holes) . 16 items times 4 screws each (or maybe 3 or 2 ??). Clamp the jig and set zero somehow. Cut the 2nd face features and cut the parts free.

Is that right?

[jimbo_cnc]

Also, if you are cutting smallish (say 50x50x6mm) one sided parts from a delrin sheet. Do you have to prevent them moving as you cut them free? or is it easy to arrange so that they don't fly of or whatever.

[EddyCurrent]

Also, if you are cutting smallish (say 50x50x6mm) one sided parts from a delrin sheet. Do you have to prevent them moving as you cut them free? or is it easy to arrange so that they don't fly of or whatever.

You can use 'tabs' such as these in Cambam, just cut them free with a knife etc.


http://www.youtube.com/watch?v=DQrOSpJE7wA

[Boyan Silyavski]

You have to have precise machine to do many parts both side at once, successfully i mean. Most probably you will spend long time figuring how to do one part right and then try many parts at once.
Inserting things will or can lead to further difficulties.

The correct way as i see it is to Zero All, surface the bed, or some sacrificial piece fixed over the bed. Zero Z. Then cut on the now flat bed the working sheet in some rectangular form. Zero Z. Then cut the same form recessed in the bed so the previous shapes have tight fit. Then you can fit the working sheet in the recessed hole. Zero Z. Then do the one sided job, then flip and do the same at the other side. And so on.

So no second zeroing all and don't use the reset button or turn off the motors so they can keep position/assuming steppers/. It would help also if you check again the Z0 when flipping the sheet. It would help further id at the XY Zero you make the program to drill a hole, so you can see eventual mismatch before you started destroying material.

Thats how i do it on a cheap machine.

[jimbo_cnc]

Thats a good method silyavski. I would have to put external radius on my part so that they will fit onto the milled hole but that is no problem. My items will be 58x58x24 and roughly octagonal.

How to hold the pieces down? double sided sticky tape?

It will be expensive to mill a new 'jig' each time out of plastic. I wonder if MDF or chipboard will work.

or I could try to re-use the same jig again, I would need to get it very square on the table. I could drill and tap 4 holes in the table bed to mount the jig in the same place ?

[EddyCurrent]

Fixtures, have a read at this to see if it's applicable.
http://www.practicalmachinist.com/vb/cnc-machining/how-i-design-build-fixture-246485/index2.html

Also do a search for "two sided machining", Google brings up several interesting reads.

[Boyan Silyavski]

How to hold the pieces down? double sided sticky tape?

Once you recessed the surface of the jig so the actual work sheet lays there with almost tight fit/ tight fit at least 2 perpendicular sides, you can use hot glue gun. thats what i use mostly. Its cheap and easy fix. very fast also. You can use screws with washers also. If the sheet is bigger than say 100x100mm, very thin double sided stick tape at the middle also. If the machine passes pass through the tape, the last mm should be done very slowly or you will brake bits in the goo. Alcohol drops later on the hot glue plastic make removing very easy.Double stick tape is removed only with acetone.



It will be expensive to mill a new 'jig' each time out of plastic. I wonder if MDF or chipboard will work.

Probably no more than one time. Use hard plastic or aluminum when you know what you are doing.



or I could try to re-use the same jig again, I would need to get it very square on the table. I could drill and tap 4 holes in the table bed to mount the jig in the same place ?
yes the same jig, but if you mean accuracy, pins will not be enough. you should learn to align material and zero properly.

Mach3 2010 Screenset (http://home.comcast.net/~cncwoodworker/2010.html) is my recommendation here.Very capable and handy. Probably you can program your own buttons but will waste a lot of time figuring how all is done.

Now using the before mentioned screenset+ fixed plate and ring cut from tube, that all permanently fixed on the jig, there will be no problem aligning again and again.

PS. The guy at the nearest scrap yard or eco center, i dont know how is called where you live must be your new best friend. A lot of expensive materials for free. I am using 8 old McDonalds plastic doors the last 2 years, which i obtained for free there.

[TonyD]

I found watching the Grimsmo channel on youtube interesting, he deals with some of these issues, not that the solutions above wont work.
https://www.youtube.com/user/JohnGrimsmo

[jimbo_cnc]

PS. The guy at the nearest scrap yard or eco center, i dont know how is called where you live must be your new best friend. A lot of expensive materials for free. I am using 8 old McDonalds plastic doors the last 2 years, which i obtained for free there.

Ah yes! I've been costig everything with fresh stock, but I will make enquiries about scrap plastic. Unfortunately local scrap is all council run and bureaucratic now. Also I don't know how to recognise different plastics or what will machine OK. Guess I will learn soon enough.