By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Allendale July 23rd

Gear spec for threading dial

All Topics | Latest Posts

Search for:  in Thread Title in  
David Standing 123/07/2019 17:00:49
1241 forum posts
45 photos
Posted by Michael Gilligan on 23/07/2019 16:53:40:

Oh well ... There wasn't any interesting argument after all.

MichaelG.

There's still time Michael wink

DC31k23/07/2019 19:15:50
42 forum posts
Posted by Michael Cross 4 on 23/07/2019 15:04:41:

For future reference for anyone who might need it, I received this in a helpful PM (many thanks):

...in theory the gears should be helical with the teeth at the helix angle of the thread....

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.

old mart23/07/2019 19:40:29
319 forum posts
26 photos

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.

JasonB23/07/2019 19:48:44
avatar
Moderator
15773 forum posts
1650 photos
1 articles

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

Michael Cross 423/07/2019 20:00:17
35 forum posts
1 photos
Posted by JasonB on 23/07/2019 19:48:44:

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

JasonB23/07/2019 20:04:13
avatar
Moderator
15773 forum posts
1650 photos
1 articles

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

Michael Cross 423/07/2019 20:20:15
35 forum posts
1 photos

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

JasonB23/07/2019 20:42:43
avatar
Moderator
15773 forum posts
1650 photos
1 articles

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.

20t dial gear.jpg

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

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

Michael Cross 423/07/2019 20:46:06
35 forum posts
1 photos

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

JasonB23/07/2019 20:52:13
avatar
Moderator
15773 forum posts
1650 photos
1 articles

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

Michael Cross 423/07/2019 21:01:29
35 forum posts
1 photos
Posted by JasonB on 23/07/2019 20:52:13:

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

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

Michael Cross 423/07/2019 21:03:46
35 forum posts
1 photos

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

jacques maurel23/07/2019 22:19:10
avatar
67 forum posts
16 photos

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.

Michael Cross 424/07/2019 14:48:11
35 forum posts
1 photos

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.

jacques maurel25/07/2019 11:04:27
avatar
67 forum posts
16 photos

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

JM

JasonB25/07/2019 13:05:40
avatar
Moderator
15773 forum posts
1650 photos
1 articles
Posted by jacques maurel on 25/07/2019 11:04:27:

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.

DC31k25/07/2019 19:07:23
42 forum posts
Posted by JasonB on 25/07/2019 13:05:40:
Posted by jacques maurel on 25/07/2019 11:04:27:

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).

JasonB25/07/2019 20:01:23
avatar
Moderator
15773 forum posts
1650 photos
1 articles

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

Michael Cross 425/07/2019 21:55:31
35 forum posts
1 photos
Posted by jacques maurel on 25/07/2019 11:04:27:

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

JM

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.

Michael Cross 425/07/2019 21:59:55
35 forum posts
1 photos

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

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Support Our Partners
TRANSWAVE Converters
Ausee.com.au
Warco
Advertise With Us
Eccentric July 5 2018
Meridienne Sept 2019
Eccentric Engineering
ChesterUK
Allendale Electronics
emcomachinetools
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest