[dvirb]

Hi everyone!
I have a question regarding the cutting of small gears (each less then 0.5inch diameter)

How to go about crossing out the spokes / cavities ?

First mill out the cavities and then cut the teeth ? Or vice versa?

It is brass and the gears are pretty thin (around 1mm thick)

I was also thinking about taking a bar machined to diameter, cut the teeth for about the length of 5 gears, then CNC the cavities using a turntable and indexer, and finally slicing it to individual gears - any good?

I am worried about metal deflection since it is thin and small.

Any help is so much appreciated!

Thanks


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[coherent]

If I were you I'd do some searching on the web. There are a number of ways to accomplish the task. It kind of depends on what machine(s) you have available and what tooling/accessories. If this is all new stuff to you, then get ready to spend a few bucks especially if you need a rotary table/indexer. A manual or cnc indexer is normally used for hobbing gears on a mill or a lathe, but there are some diy jigs and manual fixturing methods that you may find info on within these forums or elsewhere on the web. I'd imagine manufacturers who make small gears out of thin material just mass punch them out. For a small shop you would most likely grind a custom single point (or multiple point) cutter or bit and use an indexer. Milling, grinding or drilling away excess material prior to the final milling is an option, it all depends on if the extra step(s) save time or not. Cutting a solid bar and slicing it into multiple gears is an option. If you don't have an indexer/4th axis on your mill but your mill is CNC, you could mount say 6 or 12 brass discs at a time of the correct thickness and diameter in a custom fixture plate (for example holding each down with a button head socket screw at the center of each) and using a small (tiny in this case) end mill profile cut each gear.

All that said and done (and while I'm all for making things yourself), you may be able to buy the gears in quanity for less than you can make them for and just modify the bore etc to fit your needs and the spend time saved making other parts! Check ebay and online watch part suppliers.Also they sell spur gear bar stock that you can "slice & bore" your own gears from. Not real cheap but an option. Search "pinion wire".

[cmnewcomer]

My experience with parting off a pre cut (i.e. teeth formed) length of material, even if it's pinion rod, is that it leaves a small burr on the teeth. Cleanup may be a problem, at least for me, in this scenario.

Fortunately, I'm working with Grandfather clocks so I don't have gears that small but the pinions are as small as 32DP with 6 leaves and yes, I have a lot of cleanup on these.

I usually stack several blanks on an arbor and cut them at once. I have crossed them out before hand but I make sure to have a strong backing plate.

Good luck.

Best Regards.

Carl

[handlewanker]

Hi, I have cut some small racks using hand ground end mill type cutters for one off projects some years ago.

The cutters were hand ground using an optical projector to get the profile right......they only had a 29 deg included angle to cater for as this is the rack profile.......gears need to have an involute profile to mesh correctly

Optical projectors are costly, and some years later I used a simple modified Hanimex film slide projector to project a cutter profile at a 10 metre distance onto a sketched gear form profile on the garage wall.

In this case the gears were made from plastic for some missing 20 tooth change wheels on my lathe.

The cutters are just flat form tools with 2 cutting edges and were ground up from old HSS centre drills.

The projected profile was about 20 times the size when projected onto a drawing of the gear tooth 20 times the size of the cutter.

I used a 3mm diam drill shank to calibrate the projected distance to give a 60 mm wide shadow of the drill shank.

Cutting thin brass blanks for gears needs to have at least a few spare blanks clamped together with aluminium end support plates, as the last one in the stack always gets bent over at the ends of the cut........the aluminium end backing plates "should" keep the brass from deflecting.

The cutter needs to be running at about 3,000 rpm with some coolant with many light cuts both forward and backwards to prevent cutter deflection.....the cutter has a tendency to deflect away from the side of the cut when going forward so the cutter needs to be moved back at the end of the cut as well at the same setting each time.

I've had success by moving the table a few thou sideways to make the cutter cut on each tooth flank when going forwards and then back which makes the cutter cut only on one face at a time......the last cut to depth needs to be run back and forth a number of times to get the final tooth width correct.

Hope this helps.....it really is quite easy once you give it a go.

BTW, I used a dividing head on a Bridgeport manual mill, and you could use a 4th axis if you mount a dividing plate on the spindle end and index each tooth separately instead of driving the gear blank with the stepper.

The gear blank does need to be held firmly and prevented from rotating when the cutter is working or the blank will deflect sideways, so some form of brake is needed to prevent rotation for each tooth cut.....trust me, this is extremely important.
Ian.

[brass00]

you should also consider heat treating your gears

[dvirb]

you should also consider heat treating your gears

Thanks for the reply!

The gears are mainly from brass, heat treat them?

I am not doing production, only small quantity of gears. But willing to avoid hand filing the spokes of each (they are also small).

Another thought - clamp a 1mm brass plate and cnc mill the gear blanks (mill 2-4 blanks, leaving tiny spots around them to hold them in place), then saw them out of the plate by hand, grind and finish them, finally cut them on a lathe. makes sense or will it deflect ?


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[handlewanker]

Hi.....just as a thought .....a 1 ton lever press costs about A$100 on EBAY.........as the blanks are so thin and not many of them I would think in terms of a single point tool held in the ram of the press and a simple indexing plate to rotate the gear blanks under the tool to cut each tooth.

The cutter is a single tooth hand ground profile of the shape to suit the gear DP or Module....whatever....and is used in the press like a press tool with a similar shape to a boring bar used in the lathe.

It would take only one downward cut to produce each tooth in the blank, indexing round for the number of teeth required.

As the material is brass the cutters can be hand made from silver steel which is a high carbon steel available in short 350mm lengths of various diams. on EBAY, and can be heat treated with a simple gas torch etc.

Silver steel can be hand worked with files like mild steel in the soft state so hand filing or machining is possible with the heat treatment afterwards.

To get the cutter profile you can use an existing gear and make the cutter to suite using a 10X magnifier to check the cutter profile between the gear teeth.

The cost outlay per cutter is cents plus your labour, and it only needs 8 cutter profiles to cut any number of gear teeth from 10 teeth to a rack.

Personally, I would grind the cutters from old HSS centre drills as they maintain a sharp cutting edge for a long time.
Ian.

[dvirb]

Hi.....just as a thought .....a 1 ton lever press costs about A$100 on EBAY.........as the blanks are so thin and not many of them I would think in terms of a single point tool held in the ram of the press and a simple indexing plate to rotate the gear blanks under the tool to cut each tooth.

The cutter is a single tooth hand ground profile of the shape to suit the gear DP or Module....whatever....and is used in the press like a press tool with a similar shape to a boring bar used in the lathe.

It would take only one downward cut to produce each tooth in the blank, indexing round for the number of teeth required.

As the material is brass the cutters can be hand made from silver steel which is a high carbon steel available in short 350mm lengths of various diams. on EBAY, and can be heat treated with a simple gas torch etc.

Silver steel can be hand worked with files like mild steel in the soft state so hand filing or machining is possible with the heat treatment afterwards.

To get the cutter profile you can use an existing gear and make the cutter to suite using a 10X magnifier to check the cutter profile between the gear teeth.

The cost outlay per cutter is cents plus your labour, and it only needs 8 cutter profiles to cut any number of gear teeth from 10 teeth to a rack.

Personally, I would grind the cutters from old HSS centre drills as they maintain a sharp cutting edge for a long time.
Ian.

I am going to cut the teeth on a lathe, you have me an idea regarding the cavities of the gear, what if I make the gear blanks with cavities by punching them? Would that be a good idea? (If brass is punched out does the metal gets weaker cause of the punching process?)

Many thanks for all the replies!

[handlewanker]

Hi......most clock makers cut their gears on a dedicated gear cutting machine and some of the machines are quite ornate and very expensive collectors items......the cutters revolve at quite high speeds to prevent the formation of heavy burring etc.

Punching out cavities in brass only work hardens the immediate area where the punch pierces the material....but a cavity has area and a punch tonnage requirement goes up as the size of the punching area increases.

It can be done in one hit......not really practical for one off or small volume gear production as the cost of the press....need a bigger one......starts to rise etc.....but the punch and die also needs tool making experience to successfully make one

You could probably cut the cavities by hand with a fret saw or a motorised jig saw if the volume is very small.

I have a 9 ton fly press........the manual type with the handle and a ball on the top of a screw that rotates to drive the ram down.....they appear on EBAY in various tonnages and are worth their weight in gold for lots of small and even larger press work.

Youi could also invest in a hydraulic shop press as your volume is quite small.....a 6 ton bench top model sells at A$140 approx. on EBAY.......making the punches and dies is another expense.

If you have a simple 3020 CNC gantry router the cavities can be cut out with a 2mm end mill..........outlay for a 3020 CNC router on EBAY approx. A$700.....skill and some experience is needed.
Ian.

[dvirb]

Hi......most clock makers cut their gears on a dedicated gear cutting machine and some of the machines are quite ornate and very expensive collectors items......the cutters revolve at quite high speeds to prevent the formation of heavy burring etc.

Punching out cavities in brass only work hardens the immediate area where the punch pierces the material....but a cavity has area and a punch tonnage requirement goes up as the size of the punching area increases.

It can be done in one hit......not really practical for one off or small volume gear production as the cost of the press....need a bigger one......starts to rise etc.....but the punch and die also needs tool making experience to successfully make one

You could probably cut the cavities by hand with a fret saw or a motorised jig saw if the volume is very small.

I have a 9 ton fly press........the manual type with the handle and a ball on the top of a screw that rotates to drive the ram down.....they appear on EBAY in various tonnages and are worth their weight in gold for lots of small and even larger press work.

Youi could also invest in a hydraulic shop press as your volume is quite small.....a 6 ton bench top model sells at A$140 approx. on EBAY.......making the punches and dies is another expense.

If you have a simple 3020 CNC gantry router the cavities can be cut out with a 2mm end mill..........outlay for a 3020 CNC router on EBAY approx. A$700.....skill and some experience is needed.
Ian.

Thank you so much for your tips and info! I highly appreciate it.

Regarding the press punching, what if my brass piece is 1-2mm thick and shape to be punched area is around 5-8mm ? Would 1-2 ton pressure should punch that shape like butter ?

[handlewanker]

Hi....a 2 ton lever or rack and pinion type press should do OK....bigger is better.........the punch tonnage increases as the circumference and thickness of the blank increases and 2mm thick won't be like butter as brass is quite tough.

For instance, steel has a shear strength of about 28 tons per square inch..........a bar of steel 1" square would shear at 28 tons pressure.....or if the steel was a piece of plate 1/8" thick the pressure to shear it is determined by the cross section of the shear face.....IE 8" wide by 1/8" thick is still 1 square inch of material and would take 28 tons to shear it.

Punching is where the punch cuts the whole periphery of the blank on a die face in one hit, whereas a bench type hand shear progressively cuts the metal on an extended plane.....this is the same as a guillotine where the sheet is sheared progressively along the cut line as the blade descends.

For your purpose the press is the weak link as it can only press within it's frame strength and once you start using press tools the need usually get bigger, mainly when bending forming and punching happen.

A fly press or a lever type is a quick action one, good for punching and shearing etc...... whereas the larger and less expensive hydraulic shop type press is slower, as it has to be hand pumped, but the tonnage is far greater which makes it good for bending and forming work.

A shop type press also needs dedicated die sets to mount the tools on as the hydraulic ram of the press is not all that well guided on it's own.....the die set is a complete stand alone tool and has vertical guide bars to ensure the punch does not "nip" the edge of the die due to misalignment.........it "can" be used in a hydraulic press but the cutting action works better and cleaner if it's quick.
Ian.