Worm & Wormwheel Advice Please

[Nigel Graham 2Participant @nigelgraham2]

[Nigel Graham 2Participant @nigelgraham2]

Overhauling a Denbigh H4 Horizontal Mill, the one significant loss is of all the power-feed components except the worm-wheel that rotates the lead-screw.

I would like, eventually at least, to fit a new feed mechanism similar to original; i.e. driven by a Cardan shaft from pulleys on the rearwards projection of the spindle.

I have identified the wheel reasonably confidently as of 12DP (22T, pcd approx. 1.8&quot.

Although my Harrison L5 might manage the very cautious over-driving necessary for an inch diameter, deep, left-hand, 3.18TPI thread, I am not sure the available change-wheels and lead-screw gearbox – and its operator! – would give that pitch to sufficient accuracy. I would use free-cutting mild-steel as likely to give a sufficiently good finish; and this is not going to be transmitting heavy loads at high speed.

So, buy a worm?

12DP worms are available, with right-hand thread as standard so I'd need add a reversing gear-pair, but all of 20º pressure-angle.

I can measure the wheel's helix angle sufficiently to help identify the vital statistics, but not its PA, which might be the older 14.5º standard.

A rather costly alternative is fitting a new worm-wheel on a manufactured stub-axle; or possibly to the existing wheel's face, losing some table travel. I don't want to alter the original significantly (a circle of 4BA tapped holes is allowable).

My questions are…

– Is there a simple way to measure the pressure-angle of a worm-wheel (or spur wheel) without gear-verniers etc?

– Would a 20ºPA worm work on a 14.5º wheel effectively, or would one soon ruin the other / each other?

All help gratefully received!

[DC31kParticipant @dc31k]

The challenges of making something fit an existing wheel, of unknown specification and unknown wear history are great.

What is so sacred about the old one? Machine the teeth off the old one and shrink or loctite or Dutch key a bored-out new one in their place. Fit a plain sleeve and cut new teeth into it if necessary.

If the worm is on the end of two universal joints, that might give some scope for altering centre distance (eccentric bush) and could open up the full gamut of gear manufacturer's catalogues, both in imperial and metric.

On measuring PA of spur gears: https://www.geartechnology.com/issues/0992x/janninck.pdf How and whether this applies to a worm wheel, I do not know.

[Anonymous]

There's a typo in the worm tpi, it should be approximately 3.81 tpi, or more accurately 3.82 tpi.

Theory says that different PA gears cannot be run together. In practice there are ways of fudging it, but that means changing centre distances which probably isn't practical in this instance.

My lathe will screwcut 12DP threads if that's of any interest.

Andrew

[Nigel Graham 2Participant @nigelgraham2]

Thank you for those pointers;

DC31k –

No, the machine and its components are not so sacrosanct that I cannot replace as you suggest, and it would be the most reliable solution. Usually I prefer to make new parts and especially any accessories fit existing mounting-points, edges, etc., rather than make the machine fit the new parts; but this would be replacing like for like with no significant change.

The wheel and its stub-axle / leadscrew carrier are cut in a single piece of bronze but there is enough metal available to mount a replacement on a shoulder.

The worm-wheel is not heavily worn, and there is a clue beyond missing drive, that it might never been used – the table lacks the T-slot carrying the feed off-trigger block. This is clearly visible in the lathes.co illustrations apparently taken from the Denbigh catalogues. Denbigh's other H-series mills had lever-operated tables, but the table on mine also lacks anything for lever-feed components. Perhaps it is an early version assuming through-milling or manual control analogous to turning to a shoulder.

Hence, since appropriate worms and wheels are readily available, I think your suggestion probably the easiest and certainly most reliable. After all, I would have to make all the other parts to my own design and with a reversed drive, reasonably in keeping but not original.

Imperial or Metric pair? Well, inch sizes would match the machine but not matter functionally, and any universal joints available now are likely to be metric only. It would not be noticeable and anyway the main drive to the spindle, with a massive reduction around 24:1, will probably have to use metric components!

The actual worm-wheel tooth-count is not critical within reason, and I wonder if changing it from 22 to 24, assuming clearance for it, may be an advantage because the lead-screw is of 6TPI (lead = 0.16r inch or 4.23mm). Or would that lose any "hunting" effect?

'

Andrew –

Thank-you for correcting my typo.

And for your machining offer, but given the difficulty in identifying properly a worm-wheel likely also to be a bit worn anyway, I think DC31k's suggestion of machining the component to carry an appropriate replacement, paired with its correct worm, better than trying to make a worm.

++++

Many moons ago I rescued a very old Hure (not Huron) Universal Mill from a local scrapyard, and installed it in the workshop our society rented at the time. One of the teeth of the bevel gear operating the knee-screw was broken, and I managed vaguely serviceable dentistry by welding pieces of steel rod in its place and filing them to shape! The mill, probably 1880s-1890s vintage, was later sold to Weston Zoyland Pumping Station Museum, and subsequently into private preservation.

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