Member postings for Andrew Johnston

Agreed when turning, high surface speed often cures the problem. Strangely though the same isn't true when threading. Here's an M16 thread in EN8 I made earlier:

It's not a super smooth finish but works fine as a drawbar on the horizontal mill. The thread was cut on an imperial lathe, so I couldn't use my high speed threading attachment. Instead I was slow 'n' steady with the half nuts always engaged, 85rpm I think.

Incidentally H=0.866P is the theoretical depth of the 60° triangle, it's not the same as the actual thread depth, that's 5/8H, unless one uses Zeus.

Andrew

To expand on the reply by Tony; this is a 16ER-AG60 insert:

It is an external insert intended to cut a range of ISO metric pitches. The nose radius will be set for the smallest pitch in the range covered. For larger pitches the radius will be too small and the depth of cut needed will be larger than theoretical to compensate. However the outer diameter will remain the same; for internal threads the core diameter will remain the same.

In contrast this is an 16ER 2.0ISO insert, full profile intended to cut only 2mm pitch external threads:

Note the larger nose radius. I will be using this insert later today as I have some M16 threads to screwcut.

Andrew

When single point threading I ignore tapping tables and percentage thread engagement. For internal threads I turn the bore to the theoretical core diameter, ie, assuming 100% thread depth. When screwcutting I aim for the theoretical thread depth, checking fit with a mating part. Once near the theoretical depth I take fine cuts, may be a couple of thou off the diameter. When checking fits it is important to do a spring pass and clean the thread with a fine brush. Almost invisible dust or swarf can be the difference between a nice fit and loose. Most of my single point threading is done with full profile inserts rather than HSS, so I can assume that the thread profile is correct.

Andrew

The accuracy limits for my lathe gives tolerances for the parallelism of the tailstock barrel (with barrel extended and locked) compared to carriage movement that are biased upwards and forwards. Although the tolerances are small, 0.015mm and 0.02mm per 100mm in horizontal and vertical respectively. So if the tailstock barrel isn't perfectly parallel to the carriage movement it should be pointing slightly up and/or to the front.

Andrew

Not specifically as I'm not familiar with the mill, but offer the following points for consideration:

• Get a bigger rotary table than you think you need, a lot of room can be taken up with clamps
• A vertical/horizontal mounting table is useful - I use vertical as much, or more, than horizontal
• My rotary table has a 1" parallel hole in the centre; to my mind that's more useful than a Morse taper, as it's simple to make location spigots and fixtures

To illustrate the points, here's a horizontal setup:

Note the central rod; simply stock bar with a 1" spigot turned on the end. The clamps under the gear only just fit on the table. And a vertical setup:

Andrew

That makes you one up on me; I've never made parallel tooth bevel gears.

Andrew

Given that the sides of the wedges are radial lines all that is needed is to offset the cutter by half its diameter, away from the wedge, while indexing round each tooth with the rotary table and the wedge shapes will appear. Opposing sides of the wedges need equal, but opposite, offsets. Each cut only goes to the centre, not across the diameter.

I expect Jason will knock out a quick Alibre drawing to show the method.

Andrew

The existing parts don't look like they've got a taper on the teeth. Machining straight-sided teeth is a simple operation involving a rotary table to machine the sides of each tooth in turn. If a taper really is needed, simply tilt the head of the mill, or the rotary table. Aluminium seems an odd choice of material. Steel would be better wearing and less prone to fatigue.

Andrew

I'd start with the drawings. Understand how the parts fit together, how each part will be machined and how it will be held for each operation. Identify which dimensions are important and which not. Note tools that will be needed but not avaiable, or change the design. Check for drawing errors, especially on important fits between parts.

I expect my parts to fit together even if they are machined months apart, so I make parts in the order than interests me. Early on I made the spur gears for my traction engines even though the engines didn't come together for several years afterwards. If machining in a conventional order I'd start with the base.

Andrew

There's an almost identical thread on 'practicalmachinist' with the predictable blunt responses. That, and mention of 'littlemachineshop', implies that the OP is based in the US?

Andrew

Confuse would have been a better word.

Andrew

Mine too.

Andrew

Depending on material I'd use 3.9 or 4mm, with 75% or 60% engagement respectively.

Andrew

Probably running at 1000rpm. I don't have any swarf left (that I can find) but thickess is likely around 8-10 thou. Without measuring I don't know the exact feedrate, as the lathe in question only has an uncalibrated hydraulic feed - you adjust it to give the required finish.

Andrew

My horizontal is 5hp in the high speed range, 4hp in low speed. That's why I've never made it cough whereas I've stalled the other mills (and lathe) by being over-ambitious.

Andrew

It would be better to take one cut rather than lots of small cuts. The 2" bolts at the bottom of this photo were turned from 1/2" square steel down to 1/4" diameter in one pass with no tailstock support:

And the resultant swarf:

To be fair the work was held in a 1/2" square collet which doesn't work loose, but still no support needed.

Andrew

Do everything up tight, and then tighten some more, and ensure the cutter is well supported. Make sure the work is firmly held; I've had a slab mill lift work out of a Kurt vice. Spindle speed the same as any other cutter of the same diameter. Feeds based on number of teeth and number of thou per tooth. I usually start at 4 thou per tooth. From a practical viewpoint I suspect the lack of arbor keyway and lack of power will be the limiting factors. A slab mill can remove serious amounts of metal; this is pussyfooting as the mill didn't even notice as the cut started:

Andrew

When it came to machining the 6-sided splines on my Burrell SCC crankshafts I considered the slot 'n' key method, or using an offset endmill to form the sides of the spines and then nibbling out the space in between the splines. Neither seemed satisfactory, or prototypical. It is possible to buy commercial spline cutters, but none were available in the size needed. So I designed and made my own, from gauge plate which was hardened and tempered:

The splines were cut on a horizontal mill plus dividing head, the same setup as for gear cutting. Finished splines:

The internal splines were cut using a HSS toolbit and a slotting head with rotary table:

A length of HSS was ground on a cylindrical grinder to get the correct end radius (by measuring the diameter) and then cut in half and clearances added by hand:

The internal HSS toolbit is a couple of thou under width so final fitting of the gears to the splines was done with files. I made a test piece on scrap bar, and a messed up gear blank, to check out the process beforehand:

These are the finished splines, and outer gear:

Note that there is clearance on the inner and outer diameters, so the splines drive purely on the sides. The gears fit in all six orientations, although the shake varies from nothing to a little in the different orientations.

I made a few mistakes along the way. I didn't support the middle of the crankshaft when cutting the splines. Consequently the splines vary in width by a thou or two along their length. Each gear only works on half the spline length, so they are hand fitted to that length. The inner gear goes all the way along the splines, while the outer gear will only go half way down the spline. Originally i made a set of gears with a lot of inner clearance and with a straight ended slotting tool. But they didn't look right, so I binned them and made another set. In addition I hadn't got to grips with machining EN8 at the time, so the finish on the first set of gears was poor - another reason to recycle them.

Andrew

No idea. The ISO standard doesn't distinguish between internal and external thread depths. Clearance is achieved by a tolerance class which specifies the amount above, or below, a reference line for the internal and external threads. The thread depth is the same for internal and external, simply moved up, or down, a bit.

I've got a set of Zeus tables but haven't used them in over 40 years because they don't explain where their numbers come from, or why, so I don't trust them.

Andrew

All the information needed is in Machinery's Handbook. The theoretical depth of the full triangle (H) for a given pitch (P) is:

H=0.866P aka H=sin(60)P

By definition the thread depth of an ISO metric thread is:

0.625H or 0.541P

So for a 1mm pitch thread the thread depth will be 0.541mm or 21.3 thou. Simples!

Andrew