Help with square thread

[old martParticipant @oldmart]

I would first think hard about repairing the broken end, that would seem by far the easiest option. As the thread is closer to ACME than square, Acme would be better. From the pictures, the depth of the thread on the shaft is greater than the depth on the worm wheel. So the core is smaller than it needs to be. If the thread depth was made the same as the depth of the matching threads and they did not quite fit, then extra cuts on one or other flank would widen the thread and this could be done while cutting the thread using the wheel as a gauge.

[duncan webster 1Participant @duncanwebster1]

Isn't it more likely to be 5 tpi and a 0.2" circular pitch gear. Cutting a thread to match a 16 tpi gear would be an odd tpi.

[Anonymous]

The calculations by Nick are wrong. For a start the gear looks to be helical. The pitch diameter D is:

D = N/(Pn x cos (A))

where Pn = normal diametrical pitch, ie, perpendicular to the teeth, N = number of teeth, A = helix angle of the teeth

The helix angle is going to be quite small so the cosine will be very close to unity. So we can simplify to:

D = N/Pn

We also know that the outside diameter, O, is:

O = D + 2/Pn since the addendum is 1/Pn

If we ignore the helix angle we get the formula for spur gears:

O = (N + 2)/Pn

or Pn = (N + 2)/O

if N is 66 and the OD is 4.1" then the diametrical pitch is:

Pn = (66 + 2)/4.1 equals ~16.6

Since pi is involved either the TPI of the worm, or the diametrical pitch of the gear, must be irrational. For convenience of making the gear the diametrical pitch of the gear is usually a standard value giving an irrational number for the TPI of the worm.

Industrial lathes, including mine, will cut the irrational TPI values corresponding to integer values of DP allowing worms to be made. Of course the TPI values on the lathe use an approximation for pi.

Andrew

[JasonBModerator @jasonb]

It would seem unusual to use an irrational number tpi on a milling machine feed screw (worm), a look at the engravings on the handwheel dial would confirm if it is a convenient 0.200" per turn as anything else would be a right PITA

[old martParticipant @oldmart]

I have a very good 1" leadscrew for a Smart & Brown model A which would work very well as a mill leadscrew, but unfortunately it is 6tpi which is such a poor choice of pitch.

[Anonymous]

Looking at the picture posted by the OP the machine doesn't seem to have dials. Despite discussion of a nut to move the table I don't think it has one. It is stated that turning the screw thread, with the gear locked, moves the table, so the locked gear is in effect the nut. With the gear unlocked rotating the gear uses the screw thread as a rack.

It's a mistake to assume this is a hobby style manual mill, it isn't. It is a production mill, probably for a basic second op. It would have been set up by a tool setter and used by semi-skilled labour moving levers with distances, possibly only in X, set by stops.

Andrew

[JasonBModerator @jasonb]

Yes I'd started to come to the same conclusion about no traditional nut but the fixed wheel being the nut. Was hoping it may have been a way to confirm if it is truely 5tpi

I suppose it depends what the OP intends to use it for, if simply working to the stops than stick with the existing screw but if he hopes to use it as a manual mill then a more useful say 10tpi screw and suitable wheel may be a better option

Edited By JasonB on 15/07/2022 21:28:25

[Nigel Graham 2Participant @nigelgraham2]

I am part-way through re-commisioning a Denbigh Model H4 horizontol milling machine, which looks of similar size to that discussed here.

Although mine has screw-feeds throughout, the less sophisticated H1 – H3 had lever-action feeds. It seems this was common for production-machines intended for routine batch work – cutting screwdriver slots, spanner-flats, short key-ways, stiffening-flutes and the like.

As far as I can make out the assumption was that the bulk of horizontal-milling is right-through cuts or to stops, where the length of cut was not critical. The important dimensions are side-to-side and depth. This is parallel to the shaper and planer actions, cutting from fresh air to fresh air.

Consequently, lever-feeds and peculiar lead-screw pitches are not a problem. My Denbigh uses 8tpi for the cross-feed (normally the important one, for regular, binary series dimensions) but 6tpi for the less-critical, long and knee travels. I have prepared a spread-sheet chart of decimal equivalents for their dials.

My advice to Nicholas:

– Repair the broken screw as others have described (by grafting a new end-piece).

– Restore the mill to as near original as you can.

– Don't scrap any of it.

By all means make a vertical head for it, but if I were to be doing so I would make one that gives the machine minimum modifications.

My example: There's much useful life left in my Denbigh mill, and there very likely is in your machine.

I have to replace the missing or never-fitted cardan-shaft and associated parts, using the lathes.co photographs as a guide. Advice here about a replacement worm for the existing lead-screw wheel, agreed on replacing both worm and wheel with a matched pair, although at cost of either a very expensive left-hand version or fitting a 2-wheel hence reverse, gear drive. I have some old, possibly-Myford, change-wheels ideal for that, giving a feed-rate choice. (Fixed centres so constant tooth-count sum.)

The drop-bracket for the single cylindrical over-arm is missing; instead the machine came with a broken, badly-repaired, casting clearly "re-purposed" from Goodness-knows-what. I will probably fabricate one, have established how to bore its bearing housing in-situ. I may though give the arm a keyway to maintain angular position.

I need arrange a drive to the fully-framed spindle, originally intended for line-shaft drive; but which is fairly compact for a cramped workshop. Denbigh offered a motor and chain-drive option, so that would be appropriate; with a sprocket on a register fitted to the spindle's original 3-step, flat-belt pulley. That heavy, cast-iron pulley may give a valuable flywheel effect helping smooth cutting.

Unlike my other machine-tools, this one will use a single-phase motor already waiting patiently. A horizontal-mill is a slow-and-steady metal-muncher not needing the fancy 3ph sets that do complement and enhance my lathes' and vertical mill's mechanical transmissions.

[HopperParticipant @hopper]

^^^^ Agreed. I would not waste time cutting a new thread for this job when repairing the broken end is so much quicker and easier. Part of being a good machinist is deciding the quickest and most effective ways of doing things.

And you will really struggle to cut a 5TPI thread on a Myford with an 8TPI leadscrew due to the forces in the high gear ratio required.

[Howard LewisParticipant @howardlewis46836]

I have a 10 tpi leadscrew, and matching nut, which you can have if you want to try adapting it to your machine.

PM me if you are interested.

I am in East Anglia., UK

Howard

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