Preventing material chipping when using bandsaw?

[sixmillionways]

I am trying to cut small disks of plastic with a bandsaw but am getting quite serious chipping on the exit side of the plastic. The bandsaw blade makes a perfect smooth cut on entry into the material but I cannot replicate this on the exit edge.

Can anyone suggest any ways to improve matters? Is it the sharpness of the blade? The speed of blade? The speed of feedrate? Number of teeth per inch? Removal of swarf?

Any help much appreciated because it is driving me mad!

 

[MintJulep]

Yep, all of those things.

You might also try a backing plate on the exit side.

 

[CoryPad]

What is the material? Is it a brittle polymer like acrylic? Slowing the feedrate towards the end, and using a backing plate, would be very helpful in this case.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[strokersix]

Certain materials, notably brass and acrylic, like negative rake angle cutting edges. Not sure if this geometry is available on a bandsaw blade but you can alter single point tools and twist drills accordingly. It makes a huge difference in cutting performance. No chipping and a smooth cut.

 

[sixmillionways]

Thanks for the advice, excuse my ignorance but what is a 'backing plate'?

 

[mog69]

A backing plate is simply a piece of similar material to that which you're cutting placed on the exit side of the working piece. The idea is to support the exit face of the work piece at the cut edge to stop it breaking out as the band saw blade pulls through - the exit face of the backing piece breaks out instead.
regards
mog69

 

[sixmillionways]

Ok, I see, unfortunately the plastic is a disc shape and therefore would only touch a backing plate at one point.

 

[mog69]

I'm assuming that the discs are "penny" sized / shaped?
So long as the backing plate is larger than the the piece you're cutting out, the band saw will pull through both the work and the backing pieces at the same time as the cut is made. The backing piece is still supporting the work piece at the point of cut. In effect you are laminating the plastic sheet by laying the backing piece on top of the work piece, so you'll end up with a finished disc cut from the work piece, hopefully with clean entry and exits, and a similar disc simultaneously cut from the backing piece with a clean entry and a torn exit which would be scrap.
regards
mog69

 

[sixmillionways]

Apologies, I did not make myself clear in my original post.

What I am doing is cutting small discs in half lenthways to create two thinner discs. Therefore the discs are standing on one point of contact during the cutting process. This means I cannot use a backing plate in the way that you suggest (thank you for the information regarding backing plates though - I'm sure it will come in useful at some stage!)

 

[mog69]

I see. Well depending on what sort of conversion rate from thick to thin you could accept, could you consider linishing or milling each disc down to the required thickness? This would only give you one thin from one thick instead of two thin from one thick dics, but you'd get a better control of surface finish and wouldn't get the break out problem at the circumference.

 

[sixmillionways]

That would be a much better option than using a bandsaw but I'm afraid the purpose of the exercise is to create 2 thinner discs rather than 1...

 

[mog69]

Bummer! Well I guess you're down to looking at the geometry of the bandsaw (higher tpi blade, slower feed, coolant(?), etc) or explore other techniques. If the discs are small enough diameter it might be possible to cut through with laser and maintain acceptable flatness / parallelism of faces - sub-con laser services are quite widely available these days.

 

[sixmillionways]

Precisely! I will look into these things and try them out. I will also investigate lasers, I had not considered that before. Thanks for your help.

 

[Compositepro]

We have to cut carbon fiber composite tubes all the time which are very sensitive to fiber pull-out on the backside of the cut. The trick is to rotate the tube as you cut so that you first score a clean-edged groove all the way around. You can continue rotating for the entire cut or cut without rotating after the groove is made. If you use a fence on the saw to control disc thickness you could even make this kind of cut manually.

 

[RobWard]

I don't know what size your discs are, but, personally, I'd be looking to part them off on a lathe. Clean, repeatable cuts, but depends on the size of the disc as to whether it's a) possible, or b) economic.


Rob

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[sixmillionways]

The discs are only 13mm in diameter and I want to process quite high volumes (ie 1000's a week) therefore parting off on a lathe is not a viable option I'm afraid.

 

[Compositepro]

With the right fixtures and tooling a lathe can be as productive as a bandsaw and the finish will be better.

 

[klaas2]

well old trick tape the both ends
and cut in the midle of the tape to prevent any material
chip out

 

[SMF1964]

"Disks are 13mm in diameter and you are looking at high volumes..."

If you are dead set on using a bandsaw, you could fabricate a backing plate made from a piece of square bar stock that is at least 20 mm thick. Mill a 13mm diameter round channel in the bar such that it can support the entire disk on edge. This will support the workpiece as you cut through it. If you make the backing piece out of wood or plastic, you should be able to cut through everything. If you don't want to keep manufacturing the backing piece, you might be successful with first cutting the backing piece with the bandsaw, then anchor the two pieces on either side of the blade so that they are separated by the saw kerf.