Cutting BA and imperial threads on a Cowells

[BillTodd]

This may be an eye opener to some of you:

Note the bit about number of threads V. strength

http://www.gizmology.net/nutsbolts.htm

[Raggle]

Depending on the degree of accuracy required a 38T wheel (or any multiple of 19) is useful.

This from Davis Urwick's own description of his Metalmaster/Impetus multi-purpose machine tool.




The above shows an imperial leadscrew using the 38T as a driven gear. Unsurprisingly, a metric machine using one as a driver offers the same degree of accuracy to produce an imperial thread.

My own Exe (shown far too many times on the posting pics thread) has a permanent 18T gear on the spindle and 12 TPI leadscrew so the Urwick formula is 3/2*desired thread. A 38T wheel on the leadscrew with suitable idlers produces a 1mm pitch thread to the same small error.

I'm on the lookout for a suitable gear 38T- 18DP- 14.5 pa along with 39T and perhaps 60T. Acetal would do as I don't want to endanger those lovely bronze ones I have. I haven't contacted HPC yet, but their lists are for 20 deg p.a.

Maybe I've rambled a little here but I wanted to show that I have matured into posting pictures Thanks Jo.

Ray

[Dan Rowe]

Jo,
I have been working on upgrading my Cowells to cut more threads. I stumbled with the 3/4-16 thread for the Sherline spindle adapter. I am thinking a 37 and 47 gear is needed for the set but I have not made a final choice. I do not have any 32 DP cutters so the additional combination can be made with a single cutter is a key point for me.

The change stud that comes with the lathe will not work as both gears idle on the stud. I made new 5mm studs and nuts for a change stud and the power feed gears. I changed it a bit from a simple bolt so I only need a single wrench to tighten the gear stud. This also fixed the fouling problem with the bolts that secure the bearing plate for the lead screw.

I made a bushing with a key to have a stud with a driven gear instead of a simple idle gear like the original. The part shown is long enough for two studs. I left extra length for a file guide on both ends. The section where the key looks different was a senior moment when I changed lathes.


Dan

[Jo]

Thanks Dan,

I could easily add a 37 and 47 tooth gear whilst I am setup. Luckily I have a full set of 32DP cutters which is what I think the Cowell's gears are.

Do we need driven studs if it is a single chain

Jo

[Jo]

Jo

Are you intending screw cutting right down to 16BA ?

It is all about having the maximum number of possibilities, from the smallest number of change wheels. I don't rule out the possibilities of cutting my own taps and dies : I have seen the price of these small ones And I will be needing to cut internal 32 and 40 TPI threads on some jobs.

I would very much like a coventry die head, at the right price with a full set of all the cutters for all the threads I have listed, in the meantime I have a lathe .


Bill: The reason I used TPI was that the online calculation tool Bob pointed me at worked in TPI, The threads I calculated have less than 0.5% error in the pitch for the cost of cutting the two gears. Whilst I was set up to cut the 63 tooth gear for the imperial threads cutting the 31 tooth is nothing and yet the additional accuracy .

Jo

[Jo]

New gear blank sizes (I know you will need them Chris )

31 tooth = 26.2mm
63 tooth = 51.6mm

37 tooth = 30.96mm
47 tooth = 34.13mm

All blanks 6.35mm thick, 9.5mm bore, 2.35mm wide key. The originals were steel.

Edit: depth of cut for teeth = 1.71mm

Jo

[Dan Rowe]

Jo,
Here is a drawing of the new parts for my gear set up.



It is all in inches. I decided not to use the bushings for the power feed for the compound gear bushing , because it would have been a bit thin.

Dan

[Jo]

Thanks Dan,

Jo

[Dan Rowe]

Do we need driven studs if it is a single chain

Jo,
With a simple train there is not much room with out making a new gear quadrant. I think only a single keyed bushing is needed but I made two just in case while I was set up. The original simple stud can be used for the idle gear for the left/right hand switch.

The other reason I favor the 37 and 47 tooth conversion is the gear quadrant is short and my new gear studs made it a bit shorter so my thinking is having the metric transposition gears nearly the same size gives the best chance of a workable solution in the available space.

Dan

[Jo]

With a simple train there is not much room without making a new gear quadrant.

Dan

I had been wondering about that but until I drew it out, I wasn't sure. Ideally the biggest gear needs to be on the lathe spindle to minimize the length needed on the quadrant.

Jo

[craynerd]

Hi Jo, my first post on here.

Only had my Cowells two days but I`m in love with it. I just wish to understand the screwcutting gearing a bit more, since I`ve still not fully got my head around it. I currently only have the standard train: 20t on headstock spindle-driving double gear 16/32, Intermediate double gear- 20/56, 64t gear on leadscrew extension.  I`ve never really done any screwcutting, read a lot about it and done a bit on my boxford. I`ve read and understood how to put it in backgear and now I just need to figure what gears are actually changed. I know this table was posted on the yahoo group which was helpful (just tried to post it here but it won`t keep the format so will copy to excel later and attach). I just need to figure which is driver 1 and driver 2 and then make myself a set of wheels.

Jo, what material are you intending using? Steel? 

I have had confirmation that the wheels are 32DP

Chris

[Jo]

Dan: I think you have it a 37 and 47 it is here are the gear chains:

0BA     Driven gears  16  50  Driver gears  20  40   Pitch 1    error %  0
1BA   Driven gears  35  47  Driver gears  40  37   Pitch 0.899    error %  0.033
2BA     Driven gears  64  25  Driver gears  35  37   Pitch 0.809    error %  0.077
3BA     Driven gears  47  35  Driver gears  30  40   Pitch 0.729    error %  0.070
4BA     Driven gears  35  32  Driver gears  20  37   Pitch 0.660    error %  0.108   
5BA     Driven gears  47  40  Driver gears  30  37   Pitch 0.590    error %  0.072
6BA     Driven gear  47   Driver gear  25   Pitch 0.531    error %  0.360  (Simple)
7BA     Driven gears  30  64  Driver gears  25  37   Pitch 0.481    error %  0.367
8BA     Driven gears  50  35  Driver gears  16  47   Pitch 0.429    error %  0.066
9BA     Driven gears  40  64  Driver gears  20  50   Pitch 0.390    error %  0.159
10BA   Driven gears  50  32  Driver gears  35  16   Pitch 0.35    error %  0
11BA   Driven gears  35  37  Driver gears  25  16   Pitch 0.308    error %  0.362
12BA   Driven gears  50  40  Driver gears  35  16   Pitch 0.28    error %  0
14BA   Driven gears  47  37  Driver gears  25  16   Pitch 0.230    error %  0.007
16BA    Driven gears  64  40  Driver gears  30  16   Pitch 0.187    error %  1.333


26TPI   Driven gears  40  30  Driver gears  25  47   Pitch 0.979    error %  0.221
32TPI   Driven gears  64  37  Driver gears  40  47   Pitch 0.793    error %  0.010
40TPI   Driven gears  32  37  Driver gears  16  47   Pitch 0.635    error %  0.021
60TPI   Driven gears  50  35  Driver gears  20  37   Pitch 0.422    error %  0.033

 

Jo

[Dan Rowe]

Chris,
The gears you have are the auto-traverse kit. They can be used as one of the idle gears for the table in the book. I just joined the yahoo group but it is simpler to post images with a thread on this type of forum.

The screw cutting kit has the additional gears in the table and mine came with a BA bolt that is close to M5. That always bugged me a bit, but the new gear studs fixed that.

Here is a drawing of the bushing included with the screw cutting kit for the simple gear train in the book.


Jo, yes I have been playing with the calculator that Bob mentioned and it looks good for me but I did not check any BA threads as I rarely use them.

Dan

[Jo]

Hi Jo, my first post on here.

Only had my Cowells two days but I`m in love with it .......  I just need to figure which is driver 1 and driver 2 and then make myself a set of wheels.

Jo, what material are you intending using? Steel? 

I have had confirmation that the wheels are 32DP

Chris

  Chris

Driver 1 is on the lathe spindle, driver 2 for  the threads on the original chart is the one on the leadscrew. Dan and I are now looking at playing with the idlers that you use between these two gears to " make up the gap between the spindle and the leadscrew" which normally are set up to not change the ratio.

For the gears: Myford use CI, Cowell's use Steel. The CI will be easier on the gear cutters but it all depends what comes to hand at the time .

I did check the size against my gear cutters, yes 32DP and Dan also confirmed the 32DP size.

Jo

[tel]

Brass would be OK for the gears as well - they are not under a lot of strain.