Thread: Selecting Gear form cutter

Hi at the Forum,
DSCF0124.jpg Pictured is a fibre gear that I've just been quoted $500.00 to cut in bronze. ( thought that a bit steep ) I have the equipment to do it myself ie. horizontal mill, indexing head ect, but not the expertise to select an appropriate form cutter. The gear is 35.35 mm OD 14 mm wide 53 teeth and a helix angle of approx 21 deg. ( hope that's the right term ). The gear is from a photo copier gearbox the only gear in fibre. (shear pin ? ). its the second one to let go. In my wood working side of things I have motorised the fences on my spindle moulder to drop in and out as required. The gearbox is part of the setup.

I could go with a single point fly cutter but would prefer a form cutter , any help would be appreciated.

Bit of an odd one that. I'm assuming that as it was in a photocopier, it is metric.

I have a spreadsheet that I use to calculate these things, and the closest I can get with those numbers is a 0.6 module gear with a cutter normally used for 80 teeth.
When you say you have a dividing head and horizontal mill, is it a universal mill and (universal) dividing head? (Necessary for helicals)

Michael

The gear pressure angle will also need to be determined.

Originally Posted by jcatcmc

Hi at the Forum,
DSCF0124.jpg Pictured is a fibre gear that I've just been quoted $500.00 to cut in bronze. ( thought that a bit steep ) I have the equipment to do it myself ie. horizontal mill, indexing head ect, but not the expertise to select an appropriate form cutter. The gear is 35.35 mm OD 14 mm wide 53 teeth and a helix angle of approx 21 deg. ( hope that's the right term ). The gear is from a photo copier gearbox the only gear in fibre. (shear pin ? ). its the second one to let go. In my wood working side of things I have motorised the fences on my spindle moulder to drop in and out as required. The gearbox is part of the setup.

I could go with a single point fly cutter but would prefer a form cutter , any help would be appreciated.

My suggestion is to buy a 3D printer and print the gear. All up it'll probably cost you less than paying for a bronze gear, you can print as many as you need and the gear will still be the weak link in the system.

As Michael said, you need a mill with a swiveling table and a DH you can gear so the rotation is timed with the table travel. A semi-universal BS-0 type DH won't do.

I've got all the machine tool bits, think I'd still go down the 3D print road myself for something like this.

PDW

Michael and PDW.
Why would a universal dividing head be needed?

Originally Posted by shedhappens

Michael and PDW.
Why would a universal dividing head be needed?

To cut a helical gear, you need to do 2 things.

1. Swivel the table to the angle of the helix. This is the most common way though it is possible to use a vertical mill & tilt the head to the required angle.

2. The blank has to rotate as the table travels to present to the cutter. This is done by a gear train between the table screw and the DH spindle. This rotation is what creates the helix. The Victoria U2 dividing head I recently got off of caskwarrior came with a booklet listing the gear combinations and settings. Michael has a spreadsheet, a much more modern approach.

It's been over 20 years since I did a gear like this myself but IIRC you also needed to use a smaller cutter than the norm for spur gears due to the side-cutting action.

Michael can expand, he's done far more of this sort of gear cutting than I have.

Bottom line, unless you can get your DH spindle to rotate in time with table travel, you can't cut a helix. The BS-0 type heads have no provision for this.

OTOH they're a lot lighter & cheaper and mine gets far more use than the bloody heavy but gorgeous K&T universal DH I've got. The Vicky universal DH is somewhere in the middle WRT mass/size.

PDW

Originally Posted by PDW

2. The blank has to rotate as the table travels to present to the cutter. This is done by a gear train between the table screw and the DH spindle. This rotation is what creates the helix. The Victoria U2 dividing head I recently got off of caskwarrior came with a booklet listing the gear combinations and settings. Michael has a spreadsheet, a much more modern approach.

PDW

Ok thanks, I think the penny has dropped?
So the need for the gear blank to rotate is so that the depth of cut is equal from one side to the other. If the blank didn't rotate the the depth of cut would be shallower either side of the tooth centreline.

cheers, shed

What's the bore size? I might be able to hob one for you at work if we have an arbor that will fit (and if we can work out exactly what module/DP it is).

Originally Posted by PDW

... you also needed to use a smaller cutter than the norm for spur gears due to the side-cutting action.

Michael can expand, he's done far more of this sort of gear cutting than I have.

Probably the easiest way to explain it is that gear tooth profiles are effectively measured square to the axis (think a spur gear). When a helix angle intrudes, the tooth profile gets narrowed by the trig involved. Therefore to counter this, the cutter is changed.

In this particular case the tooth count is 53 (no. 3 cutter if the Chinese would use the same system as everyone else) but the helix means the best cutter to use is a no.2 as that flattens the flanks of the tooth. From the limited information given, my best guess is a 0.6 Module cutter. I omitted it, but usually they are 20 degree pressure angle. Unless you have the gear in your hand, there are usually some assumptions you have to make when working out gear specifications. (Bit like identifying non metric threads)

As Peter said, the universal dividing head is geared to the mill to roll the blank over while being cut. You might just be able to cut a gear without that rolling motion but the helix angle would have to be low and the tooth shape would be shallow at the ends if the proper depth in the middle - so would likely not mesh well.

Originally Posted by elanjacobs

What's the bore size?

My estimate is 10mm bore

Hi again, Thanks so far for the replies, the origin of the gear is Japanese so it would probably be metric spec. both the mill and DH are universal; which I use mainly for cutting splines , not much gear experience.
Micheal G was pretty close on the bore size ( 10.4 mm ) not really important at this time as I would chuck mount a bit of hollow or solid bar on the DH and finish later ( that way I will have all the potential stuff ups covered ). With regards the pressure angle that's related to the fact I`m asuming its a Metric gear ??

regards, John

Older gears (DP) have a 14.5 degree pressure angle, most module (metric) specified gears are 20 degrees.

Michael

Originally Posted by jcatcmc

Hi again, Thanks so far for the replies, the origin of the gear is Japanese so it would probably be metric spec. both the mill and DH are universal; which I use mainly for cutting splines , not much gear experience.
Micheal G was pretty close on the bore size ( 10.4 mm ) not really important at this time as I would chuck mount a bit of hollow or solid bar on the DH and finish later ( that way I will have all the potential stuff ups covered ). With regards the pressure angle that's related to the fact I`m asuming its a Metric gear ??

regards, John

In that case, seeing you've got the correct tooling, I'd have a go. I strongly suggest a trial piece using something like HDPE or aluminium because you're pretty much bound to bugger up the first one or 2. When you've got the settings all good, make the bronze one.

PDW