Another threading question?

bob531 · Nov 24, 2016

Hi , If this is a stupid question then please tell me, but is it possible to thread 4.456 TPI on a Southbend 9A.Or as close as possible. For a worm which will engage with a 60t 14dp gear.(i think my calculations are correct.)
Thanks Bob

johnoder · Nov 24, 2016

You just need to gear up to cut DP leads.

The LEAD of the worm will be the same as the CIRCULAR PITCH of the 14 DP wheel

This is 0.2243995" on 14 DP

Related prior thread:

http://www.practicalmachinist.com/v...ing-make-hobs-245474/?highlight=Model+gearing

bob531 · Nov 24, 2016

Im not sure i understand but according to a spreadsheet i have for calculating metric threads if i put a 45t gear on the stud and select A for the left tumbler and select for 5 tpi it should cut 5.715mm which is very close to 0.2243995" or 4.456TPI.
I`ll read the link.

iwananew10K · Nov 24, 2016

You have 4.5 TPI with the standard gear set(.22222..." lead)

Maybe close enough depending,and certainly so to drive a standard spur gear.

The above was edited because I got my numbers mixed up, your 5.715mm is a little closer.

bob531 · Nov 24, 2016

Im getting confused ,it has to be pretty accurate . I worked out using the 45t stud, it gave a 0.00058" pitch error , is using the standard 4 1/2tpi more accurate?

4.456tpi in metric would be 25.4 / 4.456= 5.7002mm. Using a 45stud and select A-5tpi should give 5.715mm lead.

Am i doing this completely wrong.
Thanks Bob

iwananew10K · Nov 24, 2016

LOL,I am certain I am more confused than you- I edited my initial post.

nt1953 · Nov 24, 2016

If you use a 45 tooth stud gear on a double tumbler box you will cut a 5.715mm pitch thread. 5.715mm x 1in./25.4mm = .225in. 1/.225 = 4.444TPI.

I have a spreadsheet for calculating what you need for spur gears and worm wheels. If you want it, let me know.

bob531 · Nov 24, 2016

NT1953, thanks it was the spread sheet you sent me a couple of weeks back that i used to figure this out. I wasnt sure whether i was doing it right! (the thread about the 2mm metric thread and the 79teeth gear).
I assume the thread dial cant be used as it will be cutting a metric thread??
Can anyone tell me what outside diameter the worm would likely be to match a 60T 14dp spur gear ,i believe the pcd of the spur gear is 108.86mm?
Thanks

kitno455 · Nov 24, 2016

Bob, you have asked one of my favorite questions, again

I think we should verify what you are doing, however. A worm is not going to mesh with a spur gear, unless you offset the worm axis to the helix angle. Some of the lathe-based dividing attachments do that.

allan

johnoder · Nov 24, 2016

Controlled by center distance in whatever housing the pair of gears has to fit in

The worm wheel (it is not a spur gear) has a pitch diameter and the worm has a pitch diameter

The two added together and divided by TWO has to match the center distance

Good info starts on page 310:

Internet Archive: Error

bob531 said:
Can anyone tell me what outside diameter the worm would likely be to match a 60T 14dp spur gear ,i believe the pcd of the spur gear is 108.86mm?
Thanks

bob531 · Nov 25, 2016

The centre distances are not set. It is for a makeshift dividing attachment and yes i know there are spur gears ,helical gears and worm wheels etc...
Im just wanting an idea of worm diameter to see if its workable using the 60T 14DP spur gear i have .

It will have to be offset probably and i think the wider the diameter of the worm the less the helix angle???
Thanks

kitno455 · Nov 25, 2016

bob531 said:
The centre distances are not set. It is for a makeshift dividing attachment and yes i know there are spur gears ,helical gears and worm wheels etc...
Im just wanting an idea of worm diameter to see if its workable using the 60T 14DP spur gear i have .

It will have to be offset probably and i think the wider the diameter of the worm the less the helix angle???
Thanks

Correct on all counts. If the worm was the size of the earth, you would not have to offset it at all, and you would need that 4.456 TPI. But, with reasonable stock sizes, you actually have to increase the TPI. In the end, what matters is the distance between the threads at the pitch diameter, as measured perpendicular to the helix line. The pitch (or lead) is measured parallel with the thread axis.

Do you think you have space for a 1 inch diameter worm? We should be able to iterate toward a solution in that general size class.

allan

bob531 · Nov 25, 2016

Yes i should have room for a 1" worm . The 60T spur gear is also 1" thick at the moment as its off an old Harrison lathe but cant be thinned if need be.
I`ll have to think again as the 45 t southbend change gear i thought i had is a 55T so wont work for cutting that thread . :confused:

nt1953 · Nov 25, 2016

Check out this video to see how to use your thread dial when making metric threads: Metric Threading with an Inch Lead Screw - YouTube

bob531 · Nov 25, 2016

Nt1953, thanks for the video ,i watched it and wondered what the advantage of doing it that way is rather than just leaving the half nut engaged?
Last time i tried it i left the half nut engaged and backed out the crossfeed /reversed the motor using the inverter drive at the same time. Then repeated with a deeper cut.

nt1953 · Nov 25, 2016

The advantage to using the half nuts is apparent when threading up to a shoulder. It may not matter too much if you are fast enough, have a low inertia lathe, and have a good three phase motor, but for most folks, the half nuts make life easier. The process shown in the video is actually quite easy.

bob531 · Nov 25, 2016

I`ll try it and see the next time i thread. I do use the half nuts all the time when imperial threading.

kitno455 · Nov 25, 2016

ok, I did a bit of figuring this afternoon. Starting with SOH and TOA, and working backwards, I come up with the following formulas. PD is pitch diameter of the worm (not the OD), and DP is diametral pitch of the spur:

worm pitch = 1 / (tan(asin(1/(DP*PD))) * pi * PD)

offset angle = asin(1/(DP*PD)) * 180/pi

Assuming I did that right, I ran some numbers for you. With your 14 DP spur gear, here are some possible worms:

PD TPI Angle
0.750 4.436 5.465
0.875 4.441 4.682
1.000 4.445 4.096
1.125 4.447 3.640
1.250 4.449 3.276
10.00 4.456 0.409

The last one is crazy of course (10 inch diameter worm), but it shows that we are on the right track, since it approaches the DP/pi value you would use for a rack. So, I think you need a thread around 4.445 TPI. Based on the pitch you can make with your gears, we adjust the PD of the worm to match.

allan

bob531 · Nov 26, 2016

Hi |Allan,
Thanks for working that out for me ,gives a good idea what to aim for.Just need to find a 45t gear !Found some but 44T and 46T. Apart from Boxford to any other lathe makes work with South Bends. Myfords obviously dont.
Bob
p.s i think ill try and cut a delrin one on my small rotary table, i dont have the correct 18dp involute gear cutter but a 14dp no7 i have seems to be an ok fit as long as i dont cut too deep. It should be a 2.5" blank,is that correct for a 45t?

johnoder · Nov 26, 2016

It should be a 2.5" blank,is that correct for a 45t?

47 divided by 18 = 2.611 OD

It does have a 2.500 pitch dia.

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