Cutting gears - CNC vs laser

I have a job that requires cutting some spur gears (50-150mm dia) out of 10mm delrin/acetal/nylon sheet and I'm trying to decide between the two processes.

I have the DXF file in 2D, plus I can offset the path to suit the machine and tool bit, so that's not a problem.

On one hand, I can laser cut them and achieve (maybe!) a nice 0.03mm tolerance on the top plane, but it has that dreaded taper/cone and given the thickness of the sheet - I'm afraid the taper will ultimately dictate the final tolerance. I'm not sure if I can keep it under 0.1mm, which will make the gears just about functional.

On the other hand, they could be CNC milled but I'm having trouble getting quotes for the job. An end mill of 1.5mm dia is needed to cut the profile, and the problem is a standard end mill of 1.5mm only has a length of cut of about 5mm, then it tapers into a larger diameter for better support. So a special 'long neck' bit is needed to cut through 10mm sheet, and 4-5 passes with the right bit. I understand that, but is it so much of a problem that shops would refuse the job? Is there something else I'm missing?

For what it's worth, the original steel gears were also CNC milled, I can count 7 passes on the outside edge of the gear.

personally I would mill it. I cant see it being a problem using a long necked cutter, tooling cost would be added to the job. Rib processing cutters are long necked and can be found quite cheaply, plus its only plastic sheet so wont be hard on cutters anyway. put a rfq up on here. I'm sure someone will help you out

If HPC Gears ( http://www.hpcgears.com/n/products/1...spur_gears.php £3 to £10 each ;-) ) didn't have something suitable in stock I'd look into 3D printed parts as all the plastics you mention can be 3D printed and yield durable parts, what's the gear specification and application?

I think most CNC shops, even small ones, will be turning this kind of work away rather than giving you a "Don't Want The Job" price, just in case you agree, they probably know you should be able to buy much cheaper than having them made, if you go RFQ include all the info, sizes, number of teeth, MOD or DP etc. etc.
Regards,
Nick

What's so special about the these gears, that you can't just buy some of the shelf ones, and modify them?

Anything steel is more likely to of been hobbed, which can still leave marks.

Nobody is quoting because it sounds like a quite time consuming and fiddly job, so it's probably not even worth their time quoting.
Rather than use a long bit, depending on quantity, you could make a jig, so you machine one side, then flip it, and machine the other side.

Buying off the shelf is not practical, gears have an odd bore and some of them odd tooth number.

Thanks for the suggestion johngoodrich, I will put up a RFQ. I found 1.5mm long neck bits for £15 at shop-apt.co.uk, but only 1.2mm flute length. That is 9 passes for 10mm stock, I can see why a shop couldn't be bothered at that. But I'm sure I can find better.

Wonder if you had a CNC with 4th axis (i.e stepper driving a horizontal chuck bolted to the bed) and created a form tool to go in the CNC collet you could do it?

Or a manual dividing head for that matter.

So work piece is a round blank, 10mm thick, but held vertically say on a mandrel. CNC machine a valley between 2 teeth at the 12 o'clock position above the work using the form tool, then index the work around one tooth worth and continue until all teeth are cut? Then you are only reaching in by the depth of the tooth and not the width.

Or come in at the 3 o'clock position with a sideways form tool?

More ideas here [08:17]
https://www.youtube.com/watch?v=3wd8wE_VxPo

You could stack up a few parts and cut them all at once, maybe using a tailstock for support?

I made custom gears before. I milled them top down with a 1mm endmill (13 mm flute).

I guess we can all wait for the RFQ to see whether they are MOD or DP and in what size, with what bore/boss etc.

I do love it when the specification of what is required is kept secret, failing to provide a good specification for quote is a major reason why companies refuse to quote :D

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Originally Posted by magicniner

I guess we can all wait for the RFQ to see whether they are MOD or DP and in what size, with what bore/boss etc.

I do love it when the specification of what is required is kept secret, failing to provide a good specification for quote is a major reason why companies refuse to quote :D

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You're being rather presumptuous. Did you express an interest in the job and you didn't get what you need?

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Originally Posted by Miller

You're being rather presumptuous. Did you express an interest in the job and you didn't get what you need?

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I think it's what would be classed as prior experience. In business time = money. So if the person wanting the quote can't be bothered supplying all the details required upfront, that costs time. Now as somebody who occasionally quotes for jobs, I want to see the information up front, so I can make a decision on if I'm able to do it, not waste time sending PMs to get that information.

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Originally Posted by m_c

I think it's what would be classed as prior experience. In business time = money. So if the person wanting the quote can't be bothered supplying all the details required upfront, that costs time. Now as somebody who occasionally quotes for jobs, I want to see the information up front, so I can make a decision on if I'm able to do it, not waste time sending PMs to get that information.

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Let's get this straight: all you need to know in order to see if you're interested in the job or not is in my first post. Just ask yourself few simple questions:

Am I happy milling 10mm delrin with a 1.5mm bit?
Am I happy milling 14 x 50-150mm shapes out of delrin?

If the answer is yes, you get the dxf file and that's that. You don't need to know gear profile, you don't need to know bore. What's with all this hand-holding?

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Originally Posted by Chaz

I made custom gears before. I milled them top down with a 1mm endmill (13 mm flute).

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I've seen Chaz's gear's and they were spot on, too much work for me.

Make a mold and nylon cast them is another possibility.

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Originally Posted by Miller

Let's get this straight: all you need to know in order to see if you're interested in the job or not is in my first post. Just ask yourself few simple questions:

Am I happy milling 10mm delrin with a 1.5mm bit?
Am I happy milling 14 x 50-150mm shapes out of delrin?

If the answer is yes, you get the dxf file and that's that. You don't need to know gear profile, you don't need to know bore. What's with all this hand-holding?

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Yes.
Yes.
I can see a dummy lying on the ground, but no hand-holding.

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Originally Posted by Miller

You're being rather presumptuous. Did you express an interest in the job and you didn't get what you need?

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Er, No, Forum etiquette is to quote only in response to RFQ, you hadn't made one at that point.

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Originally Posted by Miller

If the answer is yes, you get the dxf file and that's that. You don't need to know gear profile, you don't need to know bore. What's with all this hand-holding?

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Your answer as to why professional shops will not touch this is in there, no one wants to make scrap and if you can't tell them and it's not specified on the drawing then they probably assumed you were a somewhat clueless Model Engineer and filed it under B 1 N :hysterical:
P.S. I just realised that you don't know if they're MOD, DP, something else or a hash-up :congratulatory:

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