Metric Screwcutting on Super 7B with 127 Tooth gear

There are indeed combinations that give results closer than standard leadscrew maufacturing tolerances.

Posted by Pete Rimmer 02/11/2021 16:36:19

My lathe uses 16DP gears except for the metric set which utilises 127 tooth.

They are 32DP. I expect that a Myford 7 would get away with 40DP conversion

gears but 32DP would do just as well being ever so slightly finer than 0.8MOD.

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The gear chart for the 127/60 gears i find much
easier to use that the pair of 21 tooth gears that
i have had for as long as i have had the Myford.
Since the 60 and 127 gears are always in the same
places setting up only requires changing perhaps
one or two gears,for instance to change from
.075 to 1.00 and1.25 mm pitch only requires
a change on the tumbler from 30 to 40 and 50 tooth
and reposition the banjo,i only use a few pitches
anyway mostly 20 tpi 1 mm and .75 mm and since
the half nuts have to remain closed for metric cutting
i use a mandrel handle so it is useful to be able to
keep the gear change door closed.

I nearly made a 20 dp 127 gear but realised quite
early on that fitting and using it may turn out to be
a real problem ,i doubt if you would be able to do the
full range of metric pitches using one , if you know different
i am sure you will let me know.

Anyway the chart is there for you to use if you wish, it
would only suit .8 mod or 32 dp .All of the pitches
have been set up on my machine so they will fit in
some are a tight squeeze even with .8 mod gears.

John

As a general observation (i.e. I have seen it enough now to be bothered to ask the question), why is it that you produce a metric threading chart that includes pitches that are never used and are entirely non-standard (e.g. 2.25mm, 3.75mm, 1.3mm, 1.1mm)? I have seen it with metric thread dial indicator charts as well.

How many imperial threading charts do we see that include 6 1/2 tpi or 15tpi or 9 1/2 tpi?

Posted by DC31k 04/11/2021 15:03:01

As a general observation (i.e. I have seen it enough now to be bothered to ask

the question), why is it that you produce a metric threading chart that includes

pitches that are never used and are entirely non-standard (e.g. 2.25mm,

3.75mm, 1.3mm, 1.1mm)? I have seen it with metric thread dial indicator charts

as well.

How many imperial threading charts do we see that include 6 1/2 tpi or 15tpi or

9 1/2 tpi?
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The chart replicates all of the metric pitches in the Myford
handbook and a few extra , the Myford chart only goes to 3.5 mm.

I suppose if i had missed any of these out someone else
would have pointed out that the chart would have been incomplete.

The 3.75 came out as i had transposed some gears when setting
up ,as it came up as exactly 3.75 i just left it in the chart.

As a matter of interest my Warco lathe has on its screwcutting chart
4 3/4, 5 1/2, 5 3/4 , 9 1/2, 11 1/2, 13 1/2 and 15 tpi and also includes
2.25 mm and 0.313, 0.625, 0.875 module.

As i said here ,
Anyway the chart is there for you to use if you wish,
if it is of no use to you ignore it.

John

At barely half a thou per pitch you would never notice, in fact it's probably less error than you would get from low-end gears and less difference than a perfectly acceptable profile shift would produce.

One of my nephews brought a stripped plastic gear to me from his RC car because it was the third set he had stripped. He asked me if I could make stronger gears for him. When I measured the gears the problem wasn't that they were not strong enough - the shop was selling a pair of gears one of which was 48DP the other was 60DP.

Thanks, Pete

My post was really just a light-hearted ‘reality-check’ on your

“ever so slightly finer than 0.8MOD”

MichaelG.

Here is the gearbox chart from my Willson slantbed which offers both 6 1/2 and 9 1/2 TPI. Some of the metric pitches are non-standard, too.

Incidentally, the lathe has an interesting change gear arrangement using a 125/127T pair with the same OD thus permitting rapid changes between metric and English. The system is covered in detail on lathes.co.uk.

9 1/2 qnd 6 1/2 are only there because they are half of 19 and 13. It's either have them on the gear chart or have a space.

There are 9.5tpi threads but they are so uncommon you wouldn't deliberately add that pitch to a selection if it weren't for the above.

Of course, but I was responding to DC31k's (possibly rhetorical) question!

I will pose one of my own: how many gearboxes provide standard pitches only?

No idea, not many I expect.

The HLV offers different imperial pitches depending if it's a UK or USA machine, and non-standards are incidental.

Of course, screwcutting isn't the only reason for using the leadscrew. Spring winding or coil winding might use odd pitches. I used my lathe gearbox to wind a coil with the pitch set to the wire diameter.