Member postings for Andrew Johnston

I've never noticed much difference in hardness when grinding a wide range of HSS toolbits. I'd be surprised if the cold hardness varies by more than a few points on the Rc scale. Cobalt is added to increase hot hardness, at temperatures far above those at which one can hold the toolbit.

When grinding HSS I have a water dip simply to cool the toolbit so I can hold it. The whole point of HSS is that it retains hardness at high temperatures. So, unlike straight carbon steels, it is unlikely to be annealed when hand grinding. After grinding a toolbit I give the cutting edges a lick with a diamond hone to remove any small burrs.

If I need to remove a large amount of material I use the milling machine. A carbide cutter will mill HSS with no problem, like the involute shape on this 1/4" square HSS:

Andrew

There is no mention of 7/32"-40 UNS in any of my professional engineering books. I would agree with JB that 12-40 is the closest listed thread.

From a practical point of view it should be easy enough to screwcut. Alternatively the difference in width at the base of the flank between the hypothetical 7/32"-40UNS and 7/32"-40ME is less than a thou. So unless the application requires precision fits a 7/32"-40ME die will be fine.

Andrew

Flexo Springs stock a wide range that can be ordered online:

Flexo Springs

Andrew

I used my Bridgeport to cut a 1/8" keyway with a broach, and was less than happy. Never used the mill with a broach since. I now have a slotting head on the back of the Bridgeport and a proper arbor press.

Andrew

I won't be visiting today as I am duty tug pilot this afternoon. However I expect to be exhibiting there at the Model Engineers Day on Sunday the 2nd of October.

Andrew

Can you mark the silver steel with a file? If yes then it isn't hardened, if not then it is hardened. To anneal heat to 800C and let it cool naturally.

I suspect that the silver steel isn't hardened and that the problem is with the drilling technique. Trying to drill a cross hole in 1/16" rod is an invitation for the drill to wander. The hole needs to be started with a stiff drill, preferable stub and carbide. My go to supplier for drills is Drill Service.

Andrew

Reamers need to cut, 0.05mm is far too small an allowance, even if the drill cuts to size. i'd use an allowance of 0.2mm assuming a machine reamer, or 0.15mm for a hand reamer.

When reaming gunmetal and bronze the material can move slightly rather than cut if the allowance is to small. That results in fudged cutting edges and the materilal gripping the reamer tightly.

Andrew

I don't know the answer, as I never use telescopic gauges. Up to 1/2" I use ball gauges, as suggested by Hopper. From 1" to 32" I use internal micrometers. For the awkward 1/2" to 1" range I use test pieces, either from stock material or ground to a specific size as needed.

Andrew

Use a loft from the sketch to a point on the rotational axis to create the tooth shape?

Andrew

PS: In reality I expect the teeth are cut straight during manfacture with no taper

Edited By Andrew Johnston on 27/08/2022 22:43:57

Grind the top of the toolbit down until it is on centre height.

Andrew

Early Bridgeport heads, for use on horizontals, offered a choice of Morse or B&S tapers. I think the R8 taper was introduced in 1938 during the design of a completely new milling machine, which became the Series 1. All three rely on the taper to transmit power, and so are limited in the power that can be transmitted. The concept of the 7 in 24 steep tapers, such as the ISO series, for milling machines was first patented in 1927. With these the taper is for location, power is transmitted via seperate drive dogs.

Industry moved away from self-holding tapers in the 1930s as milling machines became more powerful. The R8 taper is only popular due to the success of the Bridgeport Series 1. Two of my milling machines use R8, and the third is ISO40.

Andrew

That's one minor reason I stopped writing artlcles, and also why I no longer post here about my traction engine build.

Andrew

The R8 taper was developed for milling machines and is self-releasing, the Morse taper wasn't, and isn't.

Andrew

I must admit I missed all the content complained about; I only skimmed the bevel gear CNC article to see if i could learn anything from it, as in the past I have CNC machined bevel gears. Still, I suppose an article about roofing a shed is better than one about a shed shedding a roof.

The stock answer to those who complain is to write an article for publication and enlighten us. I have written articles for ME and MEW, athough nothing recent for a variety of reasons.

So the challenge for Peter Bryce and Iain Hogg is to put pen to paper!

Andrew

Repetitive taps on opposing sides with a hammer; doesn't need to be hard but you may need a lot of taps. Repetitive, and oscillatory, side loads are a classic way of loosening a taper. Which is why it isn't recommended to mill without a drawbar.

Andrew

I do sometimes buy steel from that source, but only for fixtures and formers. I don't expect it to be to any particular tolerance or to be consistent in machining so wouldn't want to incorporate it into a model. This lump of steel was bought via Ebay to make a former for a rolled chimney:

As you can see the finish is inconsistent:

Acceptable for a former but not for a model part.

Andrew

As I understand it that could be true, in the early days of centreless grinding when the process wasn't always well controlled. Now I would expect silver steel from any reputable suppler to be round to a high degree of accuracy. Lacking a proper roundness meter (*) I think the best way to check for roundness is to rotate the stock in a V-block on a surface plate using a DTI or indicator to measure out of roundness. I don't think that rotating between centres will give results that are easy to interpret.

Just tried the V-block and DTI method on a length of 25mm silver steel. Barely a needles width variation on a 0.01mm dial, so around 0.001mm? The DTI needs to be close to the V-block or one can get more than a 0.01mm variation just my varying the force pushing the rod into the V-block.

Andrew

(*) If I ever win Euromillions big time a proper metrology laboratory will be part of the new workshop

I'd use GR17, easy to machine and to get a good finish. I would only use SG iron where strength in tension is important, ie, for the crankshafts and front axles for my traction engines. I've also found SG tricker to machine, and it has a propensity to chatter badly.

Andrew

It is well within the looser tolerances for drawn bar. For diameters from 10mm to 18mm my supplier normally supplies h10 tolerance, ie, a maximum of 0.07mm. Tolerances are always negative, mostly for economic reasons. I've just measured some offcuts of EN3:

1/4" diameter - undersize by 1 thou

1/2" diameter - undersize by 1.5 thou

1" diameter - undersize by 2 thou

I would normally expect drawn bar stock to be 1 to 2 thou undersize. So 4 thou is pushing it a bit, even if still within the limits. Who was the supplier?

Same as ordinary steel silver steel is not always the same. I've bought 1/2" silver steel that was 1 thou undersize. In contrast Coventry Grinders quote a tolerance of ±0.00025" for 1" diameter and smaller.

Andrew

I do something similar, but mounted in parallel with the main milling head, for small cutters on my CNC mill. The high speed spindle is mounted on the main spindle nose using a home made bracket:

Andrew