13DP Gears or Gear Cutter

Hi all,

I am looking for gears for one of our weaving looms, which control the number of threads per inch. There is a gap in what came with the loom, so I don't have the exact gears I want to replace, but I do have ones with more or less teeth. Using these gears I have calculated that the gears are 13DP.

For example I have:

38T at 3.175" (38+2/3.175 = 12.59)
36T at 3.02" (36+2/3.02 = 12.58)

The load on the gearing is light to medium light and it runs very slow, around 6rpm. The bore is 15/16" with a 5/32" keyway.

12DP & 14DP are obviously fairly standard items, but 13DP is pretty rare and so I am having to explore all the options, including getting an involute gear cutter and making our own. However, this is also a non-standard item, so would have to be a specially made cutter (I have been quoted £145 nett at the moment), but with the advantage that I can make a range of gears and using a No. 3 can actually fill in the gap I have.

Few questions really:

1) Anyone happen to know where I can get 13DP gears or involute gear cutter off the shelf?

2) Would Delrin be a suitable material for the job?

3) The keyway is the bit that concerns me the most, as I don't have a broach or press. How successfully can this be done on a lathe?

All thoughts and suggestions very welcome!

James

Hi John,

Thanks for your reply. It's a British loom, made up in Keighly (pic below of the cog in place for interest). I'm assuming I wouldn't get a way with a 2 MOD? I did look at that. A 38T in 2 Mod is 3.149" and my gear measures 3.175"

Sadly they weren't adverse to using slightly odd parts, the nut that holds it on (that is also missing) is 15/16" with 16tpi! Again, I cannot find one of those.

James

Edited By James Jenkins 1 on 26/04/2017 13:38:10

Edited By John Stevenson on 26/04/2017 15:16:39

What is the thread profile? There is a 15/16" x 16tpi Unified thread

JS. If you look to the left of the "green" gear there appears to be a slotted  slider/possible quadrant?

Edited By KWIL on 26/04/2017 15:19:43

An effective way to produce singe square keyways in a one or few off gear is to drill a hole at the keyway position before making the main bore and finish off with a file. The hole takes out most of the material and makes it much easier to keep the sides square. Obviously use a pillar drill or mill the ensure the hole is parallel to the bore and in the right place. My practice is to use a drill next fraction size down from the keyway width, or equivalent if the key isn't an inch fraction size. I set the radial position of the hole such that the top of the hole is in line with or a touch below where the top of the finished keyway will be. Usually that means the centre of the hole will be on the diameter of the finished bore. Other folk suggest a smaller hole with its centre a bit outside the final bore diameter. Doing it that way means more accurate filing, with a deal of cut'n try, but gives more leeway to accommodate small errors in the original hole position. A starter hole is still worthwhile even if the bore has already been fitted. Significantly reduces the amount to be filed out and does make it easier to keep everything on line.

If you use delrin I'd be unsurprised to discover that a very accurate keyway could be produced by finishing off using a single tooth broach. Such a broach could quite easily be made from a piece of tool steel affixed crossways into a carrier rod whose diameter is a close sliding fit in the bore. Probably best done using the lathe shaper style. Maybe taking 10 thou or so per edge left. Similar technique can be used with metal gears but the depth of cut has to be adjustable so the keyway can be cut in multiple small bites. Either adjust the bit or use a smaller support bar and put the cut on with the lathe slide. Something of a balance between number needed and the faff to do individually or the faff to make up a tool. Moi! After the third or fourth time I bought a broaching set from that auction site for £ very reasonable due to being the only guy who wanted one that week. Usually prices are too rich for my wallet.

If you have access to a shaper decent gears can be cut by simultaneously traversing and rotating the blank under a simple rack form tool. A rack form tool has straight sides making it quite easy to grind. A flexible wire fixed to the machine body and running round a drum of gear PCD diameter minus half the wire diameter is the usually advised method for generating the necessary gear rotation. Obviously the gear blank and drum must be centred on a common shaft. I see no reason why a rack with a gear having the right number of teeth running on it would not be equally effective at generating the rotation movement should such be available.

Clive.

You could well be right, only the whole of the loom though is constructed with Whitworth N&B and it was made in 1950/60's in Yorkshire. Could it really be metric? I guess the answer would be to get a M24 x 1.5 nut and try it. I have been offered a used No 4 & 5 13DP cutter for £10 each, which seems good value. I just need to factor in the time getting my head around gear making (I have bought the Workshop Series book).

Yes, there is a slider, to allow gears of different diameters to be used.

Thanks for your help with the keyways, very helpful!

Buy the 13DP cutters if you have the chance, if they are a better fit than the metric jobs. It is possible to cut gears with more teeth or less teeth than the cutter was designed for. EG, the number 4 cutter could do the gears normally done by a number 3. The profile is not perfect but should be good enough for low rpm low power applications, specially if you use Delrin, which is quite commonly used for lathe change gears of a similar nature and application.

Yes you can cut a keyway in the lathe using a boring bar with the toolbit turned sideways and run the carriage back and forth like a shaper. I have done cast iron Myfrod changegears like this, with a 5/32 keyway IIRC. Tedious but do-able. Easy peasy in Delrin I should imagine.

"

For example I have:

38T at 3.175" (38+2/3.175 = 12.59)
36T at 3.02" (36+2/3.02 = 12.58)"

When you made those measurements, what did you measure exactly?

Hi John,it was the outside diameter, at the widest point. I think the calculation is right? I have measured the PA as 14.5, rather than 20.

Thanks, Hopper - I think that's the reassurance I needed. They have a No. 4, 5 & 7 so perhaps with this lot I could do 3 to 8?

James

James,

I have sent you a PM .The green inbox link at the top of the page should be flashing.

Do not discount mixed metric and imprial threads.

I have two virtually identical machines, made a hundred, or more, years ago. One has metric fixings and one imperial.

Parts, as in castings, were likely purchased for both from the same supplier and one was assembled on the continent while the other by a UK manufacturer.

So there is the chance that some metric assemblies for a large machine could have been bought in by the company in Yorkshire. Especially as businesses from that part of the UK were well known for buying in parts at the lowest cost, or rather 'best value for money'! is the keyway actually 5/32" or 4mm?

At a pinch, yes. You'll need to get yourself a copy of Ivan Law's book Gears and Gear Cutting too if you have not done this before.

Hi James,

I have been thinking about this. Considering the age of the machine it is highly likely that rather than a DP series cutter being used to produce the gears, they were cut with a 0.25 CIRCULAR PITCH cutter. This gives dimensions very close to those that you have measured.

The tolerance on the OD of a gear blank is usually +0.0 to -0.002 but can easily be a bit smaller, it does not matter provided you allow for it and cut to the correct depth of tooth.

It would be possible to check the gear geometry further if you could measure the BASE TANGENT SPAN. This would eliminate variations in the OD and also confirm your pressure angle.

Good logic, Neil

... Perhaps James could put one of the available gears on a flatbed scanner, and post a couple of images.

1. The whole gear
2. A detail of [say] a group of three teeth.

[a sheet of transparency film will protect the glass, and a sheet of white paper will protect the pressure-plate]

MichaelG.

Thanks all.

Yes, I have a copy of Law's book in order, hopefully here today. I have ordered a M24 x 1.5 half nut to see if that's correct.

Neil, I reckon you are right. 0.25 CP is 12.56 DP, which is closer than 13DP or 2 MOD. Now where on earth do I get a 0.25 CP cutters or gears from?

James

TBH at 6 rpm and made from Delrin a 13 DP, 0.25" CP or 2 Mod in either 14.5 degrees PA or 20 degrees PA will work in this application.

You only have to look at some of the early gears where they were just cut with a 3 cornered file to accept this.

Ok not geometrically correct but it won't be racing at Silverstone this weekend.

I'm afraid circular pitch cutters are not too common these days. You may find it easier and cheaper to make your own cutter. The book you have ordered may have information on making them, they are not difficult.

I have cut some small gears in aluminium using a single point flycutterin the past, producing the approximate involute in the lathe on round silver steel then filing down to half the diameter and hardening rather than make a complete circular cutter.

You could make it as a disc in a similar fashion with just one cutting edge, and mount it eccentric to give the cutter clearance. It would be more robust than my way.

If push comes to shove, Ivan Laws' book will tell you how to make flycutter for such gears as 0.25 CP.

(Have never tried it, lacking both the courage and the absolute need)

As John S says, at low speeds and with a bit of backlash, (I feed a sheet of paper through the mesh, to set it, aiming for about 0.003" you probably won't have a great problem, with something which is theoretically incorrect, such Mod 2.

If possible, some thick gear lubricant grease will result in quiet running.

Hope that all goes well.

Howard