Gear spec for threading dial

There's still time Michael

For fullness of answer, one should qualify this by saying it is true assuming you want the axis of the gear perpendicular to the axis of the leadscrew. If you skew the gear to the helix angle of the thread, a standard spur gear can be used. See GHT's headstock dividing attachment for Myford lathes where a skewed worm wheel (the leadscrew) drives the spur bull gear.

If you read the HSM link (and I also think it is in Cleeve's book), many of the common metric pitches do not require a threading dial at all as you can engage the halfnuts anywhere you like.

I would try to get a plastic gear and thin it down to about 4mm, so the helix angle of the leadscrew which is acting as a worm gear has little affect.

Set of 3 gears on e-bay for £20, hardly worth bothering with anything else.

I saw - but unless you've found a different listing to my they're the odd sizes, I need the common ones - 16, 18, 20

yes, think they are for another harrison.

Looks like the M250 uses a double gear with 20T and 21T and 6 divisions on the dial, most drop in on 1 or4, couple of others need a different number such as 1,3&5

Thanks - that's simpler than 14, 16, 18 etc - with 8 graduations. Hadn't occurred to me to do it that way.

I suppose if you can draw out a 1.91 MOD gear in CAD then all you need is to find someone who can print, laser, water jet or CNC them.

This is a 20T 1.91MOD, 14.5pa with a rounded root to make milling easier.

Edited By JasonB on 23/07/2019 20:43:28

Edited By JasonB on 23/07/2019 20:47:08

That's a terrific idea - you're making me feel stupid. Thanks!

taking it one step further you could do a helical extrude of the profile and print that.

Or if going the CNC route then mount the blank at an angle then index it round cutting one tooth gap at a time assuming you don't have 4th axis and all day to do the helix for you. That's if a helix is really needed.

Edited By JasonB on 23/07/2019 20:53:25

I was just thinking that - no reason not to when you're going that route.

I find I very rarely need to use a 3d printer but there are times when it really comes up trumps.

Note: the number of divisions on the disc must be a submultiple of the number of teeth.

20 teeth is possible with 4 divisions (5 teeth between divisions so 30mm) for the pitches submultiple of 30mm. 6 divisions for 30 teeth. The number of teeth (20 or 30) is determined by the TDI braket position.

20 teeth with 5 divisions for pitches sub of 24 (12, 8 and 4mm)

21 teeth is possible with 3 divisions (7 teeth between divisions so 42mm) for the pitches submultiple of 42. 6 divisions for 42 teeth.

22 teeth with 2 divisions for pitches sub of 66.

26 teeth with 2 divisions for pitches sub of 78.

Thanks to all who have helped on this - my final decision was to use 20 teeth and 8 markings on the dial. Although this doesn't quite cover all scenarios I think it will suit me best most of the time as it means that for a lot of threads (anything that goes into 15) I can hit any number, whereas with 6 divisions you've got to wait for the dial to come round to 1 or 4 and that'll irritate me. Anything that goes into 30 I can hit quarter revolutions of the dial, 60 is half revolutions and 120 you just pick a number and don't change your mind.

That really only leaves the 7s - multiples of 0.35. I very rarely need those so if they come up I'll either just leave the nut engaged and reverse or switch the gear for 21 teeth and pick a number.

Be careful as the number of divisions must be a submultiple of the number of teeth!

JM

Why would that be? The factory gear is 20/21T and the dial divided into six yet from the chart I posted above it will work with a large range of pitches, infact all that the gearbox can produce.

The dial is divided into six but the usable divisions of the dial with any particular gear have to be a submultiple of the gear's teeth. For the 21t gear, you can only use 1, 3 & 5, which would be 7 teeth apart. For the 20t gear, you can only use 1 & 4, which are ten teeth apart. Poor 2 and 6 never get a look in with these gears.

Could you amend your 3D model to the correct pressure angle? Think about it: is any module gear in the world likely to have a 14.5 degree PA?

The 14.5 degree PA corresponds with the 29 degree thread angle of imperial ACME. Metric trapezoidal has 30 degree thread angle, which would be 15 degree PA (I wrongly stated that it would be 30 degrees in a post above but cannot find any way to correct my error).

I based my sketch on Andrews suggestion as it seemed the 14.5 pa would give a longert thinner gear which looks more like the actual Harrison ones than your 30degree pa (not 20) suggested angles being far closer to 15deg.

As it is actually a 6mm CP gear we could just as easily say it can be cut to 13.299DP in which case 14.5pa would have been quite common at one time. Infact a 14DP, 14.5PA gear may be a better option than a 2MOD, 20pa one if substituting nearest standard gears, only 0.01MOD further out on size from 6mm CP but a lot closer on pa.

I've deleted the file now but happy for you to post a corrected one.

Edited By JasonB on 25/07/2019 20:46:55

I don't think that's right.

1/8 of 20 teeth is 2.5 teeth, on a 6mm ledscrew that's 15mm of travel so any thread where you'll find threads exactly 15mm apart will be fine. That covers 0.2, 0.25, 0.5, 0.75, 1, 2.5, 3, 5 and 7.5

2 divisions on the dial is 30 mm so that adds 2 and 6 to the list and so on.

doh - you can't hit half a tooth. Has to be 4 divisions.