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Member postings for Dave S

Here is a list of all the postings Dave S has made in our forums. Click on a thread name to jump to the thread.

Thread: Painted granite surface plate
18/10/2021 18:58:49

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

Thread: Hi all, newbie with first lathe, rare one i think.
17/10/2021 19:15:21

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

Dave

Thread: Flexispeed Lathe
12/10/2021 17:55:06

Unimat 3 and 4 are 14x1

Dave

Thread: cutting spur gears on a mill
11/10/2021 17:05:08

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

11/10/2021 13:15:36

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

Tonys nice picture

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

11/10/2021 12:53:17

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.

11/10/2021 10:48:54

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

11/10/2021 09:31:21

I've had a poke about and I haven't found the article I thought I was looking for.

However here are a few, from different authors which all state (in summary) that the generation of an involute by a hob is limited in its accuracy by the number of gashes in the hob, and these gashes are each responsible for a facet on the tooth form.
I hadn't picked up the also scalloped effect of the hob, but that also makes sense, and is not present in planed gears.

This (**LINK**) is a comparision of Planing (MAAG is another name for the Sunderland planing process) and hobbing:

End of Page 12:
An appreciable factor in the tooth profile accuracy of
coarse pitch gears with small numbers of teeth is the number
of enveloping cuts available for generatin~ the tooth
profile (Fig. 10). With hobbing, this depends on the number
of gashes on the hob and the number of teeth on the workpiece.
The larger the number of gashes, the larger will be the
series of tangential cuts and therefore the better the
approximation to the ideal involute. The hob outer diameter
however necessarily increases as the number of gashes is
increased, on the assumption that the length of the tooth
does not drop below the economic limit which allows a
. sufficient number of regrinds.

Both these articles from Gear Technology archives also mention it early on:

**LINK**
**LINK**

Incidentally there is a wealth of gear related information in the Gear Technology back issues.

And this paper (**LINK**) is about the process of hobbing on hardened gears - not a common thing as most hardened gears are ground finished.

Pg13 has a nice set of illustrations.

I stand by my original statement.


Dave

11/10/2021 07:27:27

I think a better definition of a generating process is: “The shape cut by the tool is not the inverse of shape of the tool.”

In other words the cut surface is created by a series of tool / workpiece movements.

Sunderland planing is a generating process - a rack form cutter is applied to a workpiece to create a gear.

Zero helix “hobbing” is also a generating process - one that without any relative rolling movement produces a (quite rough approximation of) an involute.

I’ll see if I can find the reference- I’ve read a lot of gear manufacture articles and books. I’m fairly sure it was a technical paper by a manufacturer of hobs ( not the previously linked one) about why their jobs were superior- because they had more tooth gashes.

It maybe angels dancing in a pin head stuff, but it is no less interesting for that.

Dave

10/10/2021 18:24:04

Hobbed gears have the same number of facets as the number of tooth gashes in the hob iirc.

So they are an approximation to the involute which usually have many facets.

Dave

Thread: Being nice to a vise
08/10/2021 15:38:25

Never worried about it with my metalworking vices.

A none issue in a workmate - there are 2 screws just balance the loading.

Dave

Thread: cutting spur gears on a mill
07/10/2021 19:23:43

Over on homeshopmachinist there is a thread where a chaps leadscrew appears to be the wrong pitch.

In the middle of that is :Plastic gear making

Spur gears free moulded install a lathe…

[Nomex Donned]
Dave

Edited By Dave S on 07/10/2021 19:25:29

Thread: Brazing/high temp silver soldering with 999 Ag wire?
07/10/2021 13:56:21

Copper to copper is certainly possible with fine silver - that's how I attach things I'm going to enamel together.

I use plain borax for the flux.

I suspect brass will be 'tricky' as IIRC the melting point of silver is above the melting point of brass.

Can't say about the price comparison for sure, but a reel of fine sliver wire I bought was about £30 and I expect it to last me for a good few years yet.

Dave

Thread: gr 8.8 ht bolt steel which carbide tip
06/10/2021 18:52:08

For what it’s worth I don’t think these are made of chinesium or cheese, but the modified quite easily with a quality HSS endmill

Tools1.jpeg

Tools2.jpeg

Dave

06/10/2021 17:38:52

Don’t forget that tool holders are just metal, and can be modified.

My 2 mainly used ones are both larger than ideal for my tool holders, so I chucked them in the mill and removed a chunk from the underside.

Dave

Thread: Shim stock
03/10/2021 16:25:14

I have used razor blades for small shim stock - the double edge type, not the ones moulded into plastic.

Dave

Thread: I need to cut chamfers into x64 pieces of mild steel - any advice?
29/09/2021 15:59:18

“Coolant” with carbide is mostly to flush the chips away.
The need to cool the tool, especially with the limited power of your Proxxon is minimal.

Intermittent coolant on carbide tends to thermally shock the tool, and is more likely to lead to poor tool life.
Carbide coatings have been developed to increase tool life without coolant.

I mostly use coolant when I want to be able to measure a part soon after cutting - it helps to keep the part at near to ambient temp - so my measurements are more likely to be accurate…

I would trust MSC and Ketan for info more that Axminister.

Dave

28/09/2021 18:44:10

Coated carbide on mild steel and a machine that isn’t setup for full flood I would try no coolant at all.

Also go look at MSC direct for *much* better choice and prices.

Dave

Thread: PM Research#1 Cylinder Can it be saved ?
26/09/2021 13:05:45

I would bore slightly oversized and sleeve the cylinder.

If you haven’t made the valve yet you can make it oversized to cover your ports.
The limit on flow will be the (presumably to do) drilled holes from the ports to the ends of the cylinder, so the imbalance in slot sizes shouldn’t affect the running much.

Dave

Thread: cutting spur gears on a mill
24/09/2021 17:30:05

Pretty sure you can calculate PCD from Mod and tooth count, and can do Mod from tooth count and outer diameter.

From that you can layout the centre distances of a pair of gears.

PCD for a single gear is not a very useful number anyway

Dave

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