Member postings for Dave S

Looking good. Biggest improvement I made to my Unimat 4 was fitting brass gibs.

Dave

This:

is a roller filing jig for a lathe.

the 2 rollers guide the file in a linear manner. Traditionally they are used with a head stock division plate to file squares and other polygons onto the end of round stock.

Squint a bit and refer to my sketch and you will see how to make such a thing.

However Robert is correct - use a machine to do the job

Dave

2 or 3 angle plates and a pair of rollers. Set the whole lot on a flat surface. Adjust the rail to give required size, push up against angle plate to give squareness.

Sketch appears to be at 90 degrees to the intention.

Dave

Nice job.

Did you take a minimal skim of the headstock and tailstock? (Assume you did)

Dave

I’m not sure I agree with “Tricky Stuff”.

Possibly “narrow specialism, niche application stuff”

Magnetic properties of materials is a specific area of materials science, and a well studied one at that.

The information is available, it’s just not actually that critical to most applications.

Dave

? That’s as illuminating as a blown 10w bulb.

A sketch would be simple, but I think you are attempting to face the end of the bar to it’s long axis?

In which case a lathe will just do that, and a roller filing jig will also with appropriate fixture to hold the plate.

Dave

Are you trying to do a 90 degree end - i.e. square across from long side to short side, or a 90 degree included angle at some angle to the long side?

Picture (simple sketch) would probably resolve the ambiguity.

Look up “Filing rest” with a pair of rollers.

Dave

Which is why the cover I made for mine is this shape:

Dave

If you can tilt the head but don’t have a quill feed you can still only do vertical movements

Dave

Seems reasonable to me.
It costs money to stock a thing, pick it, pack it, arrange postage and deal with any customer enquiries. I would be surprised if they actually make more than a couple of quid (end profit after all costs and taxes) on a minimum price order.

It also helps to keep time wasters away, the sort who as a load of questions and then buy a tiny amount never to be seen again.
Whilst it may seem an impediment to small businesses actually if a business can’t afford a £35 order it really isn’t much of a business.

Machining is a hobby for me, but I will buy from trade suppliers by making sure I order a good selection of both the things I need now and the stuff I’m likely to need before I can make another order upto the minimum amounts. That way I can access stuff the hobby supplier doesn’t stock, and often at a better price.

Dave

The bbs has had a software update and a lot of content is no longer where it was, even assuming it still exists.

The way back machine will probably be able to find it on John’s old site.

IIRC it involved using a cone drill to make the cutter as it already had relief angle on it.

there was also a method of offset turning to create relief behind a “circular” tool - it ends up looking quartic for those who remember the allegro

Dave

Steel shim stock is widely available, and precise in thickness

Google will find you a supplier

Dave

I’m currently building a “freestyle” regulator clock based on the book by Peter Heinmann.

Ive pretty much only used the book to understand the hows - like the gear ratio and escapement design.

Point is that I’ve not used the measurements really - my clock is 32dp involute geared, not cycloïdal with lantern pinions.

So maybe getting a couple of good books on builds and then going ahead in your own measurements is a viable thing to do.

Dave

I suspect the cheaper plates are dyed to make them look black.
Because everyone “knows” black granite makes the best plates (unless it’s pink of course)

I wouldn’t advocate used of scotch brite - an abrasive- on a precision flat - it’s likely you haven’t done much damage to the precision, but it’s not easy to tell without proper measurements.

Dave

My CVA has 3 v belts from motor gearbox output to the headstock. I guess it’s for torque transmission.

Dave

Unimat 3 and 4 are 14x1

Dave

Indeed more cuts is a bit like a finer lathe feed, but the difference is that the lathe will still turn round (form) with either.

The hob with “courser feed” (1 gash in extremity) creates a different, more approximate form.

The other difference is that the hobs “feed” is set at manufacture but the number of gashes.

The thing to remember is that even a couple of gashes gets you a very close approximation of the correct form, and by the time you have a typical number of gashes in a commercial hob the form is essentially perfect, and other factors in machining are likely to have at least as big an effect on the perfection of form.

Dave

As a thought experiment imagine a hob with a single gash.
We will have to assume that the formed cutting tooth is smalle enough to escape fromt he nascent gear during the hobbing rotation, to line it up with the next tooth.
As a basis for this Tony Jeffrey has a nice picture on his site on the page about rackform hobs. http://www.jeffree.co.uk/pages/multi-tooth-gear-cutter.htm

If you visualise the hob as having a single gash then the top of the picture shows all the places that you are cutting on a single hob rotation.
Then the blank rotates in the helix and you make another cut. With a single gash the blank has to rotate a full tooth space before the next cut is made.
The end tooth form you will achieve is that of the tooth form detail - an obviously faceted shape.

Further down the page Tony illustrates how by 4 cuts per tooth (in other words a hob with 4 gashes) gets much closer to the correct form - to the point that although the tooth must have flats on it - it can be no other way with a straight sided cutter - the approximation to a curve is such that it would be easy to miss, and for all practical purposes is an involute.

Dave

Firstly I was mostly calling myself a pedant - for insisting that the hobbed profile is actually a series of facets.

Apologies if that came across as insulting to anyone.

In the case of a Lathe turned cylinder, assuming the lathe is in good order the cross section of that cylinder will be circular. (the intended form). There is a helical groove running down the cylinder, which does affect the cylindrical size - crests are larger than troughs. but it does not make the cylinder none circular.* Hence the helix can be described as surface roughness - now close to size (not form) is the cylinder.

The hobbed gear has both surface roughness issues (its a cutting process and will leave tooth marks after all) AND form issues. The faceting is a departure from the ideal surface form, and hence is more appropriately termed an approximation, just as using a circular form tool cutter is a departure from the idea from, and hence an approximation.

Gear grinding has 2 benefits. Firstly the cutting tools are capable of taking a much smaller cut - so can leave the surface roughness at a lower level, and secondly there are millions of cutting tools operating all the time - equivalent to a hob with millions of gashes - so the form produced is much more perfect than it would be with the limited number of cuts a hob makes.

Dave

*actually it does if you cross section you get an oval with the DOC as you go peak to trough.

From a pendants point of view Hobs approximate (closely or not depending on the hob parameters) the involute curve with a series of straight lines.

The number of gashes on the hob directly relates to the number of straight lines.

This is not a surface roughness thing, its an approximation thing.

From a practical POV even 5 facets is perfectly adequate for most gearing in non critical applications, and the approximation (circular in the case of form cutters or faceted) make no difference. That doesn't mean its is not an approximation.

Dave