Member postings for Andrew Johnston

Not necessarily!

If the relay fails closed there might be problems, but the resistor overheating isn't one of them. If the relay fails open the following would happen. After the capacitors are charged the current drops to a small value; just enough to run the control electronics. This current will be much lower than the design inrush current so the resistor will be fine. If the VFD is activated and starts drawing a current higher than the design inrush current from the DC link capacitors the DC link voltage will fall. That will trigger an undervoltage and the system will turn off. It's quite likely the resistor will survive.

Andrew

The problem with assumptions is that they can lead one up the garden path. The resistor is there to limit the inrush current due to input filter and DC link capacitors when the unit is first switched on. It is not intended to carry full load current. Once the capacitors are charged the relay is closed and the resistor plays no further part.

Apart from value, the important resistor characteristic is its overload capability. Wirewound resistors are very good at taking short term overloads without going pop. The resistor value will be set to give a maximum inrush current and the power rating is determined by the charge characteristics. These types of resistors are designed to run hot, up to several hundred degrees!

I suspect that the value will be rather higher than 12 ohms. On a practical note these type of resistors are often only available in the E6 series, which does not include 12.

Andrew

Component value codes are normally printed on the green body, which doesn't look to be damaged? The PCB that the resistor is attached to contains a relay. So almost certainly the resistor is part of a current inrush limiter. If the resistor has failed it is quite possible that the cause was a relay failure. So it would be worth checking the relay and drive circuit.

Andrew

It's a wirewound power resistor. Probably around 5 to 10W, and 5 or 10%. Often used as an inrush current limiter.

Andrew

If that is correct, I'm not sure why one would want to do that? Much simpler, and better, to leave the rods as they are and case harden the pins.

Andrew

It's not as simple as simply choosing the correct cutter. It is not possible to cut proper straight tooth bevel gears using involute cutters. However, there are two approximate methods.

Straight tooth bevel gears are normally designed using the diametrical pitch on the outer edge. An approximation to the tooth form is created in three passes, one normal pass and a two passes offset slightly in either direction. The involute cutter will correctly form the curve on the outer end, but the curve at the inner end is incorrect. This can be corrected with a file, or by running in with a mating gear. For a straight tooth bevel gear the equivalent number of teeth, used to select the cutter number, is the actual number of teeth divided by the cosine of the pitch cone angle. This tends to make the teeth shape more rack like. The problem with this method is that the involute cutter needs to have the correct form for the selected DP, but needs to be narrower than standard so it will pass through the small end. Cutters of this type used to be available commercially, but are now obsolete and pretty much impossible to source.

The alternative method, getting around the problem of a special cutter, is the parallel depth method. This still requires three passes of the cutter, but the gear is designed using the diametrical pitch at the inner edge of the gear. So a standard involute cutter can be used. The problem with this method, for an existing application, is that the diametrical pitch at the outer edge is unlikely to be a standard value. In which case the gears will not fit without a lot of faffing about with DP and tooth counts. Even then some changes to the design will be needed.

Andrew

Do you mean the bevel gear pinions?

Andrew

Looking at the bigger picture I use ER20 on both my R8 taper mills for cutters, although not that often. Everyday milling is done with 6mm and 10mm endmills in sidelock holders. i've never used ER collets for work holding; I use Burnerd EC collets on the lathe.

Andrew

Value is a microfarad, rather low for a metal can electrolytic, and the voltage rating is given as 400VAC, so it isn't polarised. The MPP stands for metalized polypropylene, so most likely self-healing.

Andrew

it's odd that the metric TDI for the Harrison M300 lists some pitches that are integer factors of the leadscrew, albeit any number can be used but with a specific gear. But other pitches, such as 0.5mm, are not listed although the lathe will cut them. Some pitches that are not factors, such as 6.5 and 11.5mm are also not listed on the TDI. Presumably one has to keep the half nuts engaged. I suspect that the inconsistency is due to lack of space on the front of the TDI.

The imperial TDI from my lathe may have the same inconsistencies, but it's not fitted as I don't use it and I'm not going to climb up to the shelf it is sitting on to have a look.

Andrew

I buy standard metric taps from industrial suppliers, mostly Dormer brand. Other thread form taps are bought from Drill Service. The exception being for ME theads which are from Tap & Die, and are carbon steel, cut thread not ground. I no longer buy from Tracy Tools. Most of my larger taps, up to 1-1/2", have been bought secondhand, or NOS, on Ebay. Taps for one off use are normally bought on Ebay, the most recent being M3 x 0.6mm.

When buying hand taps I normally only get taper and bottom. I prefer spiral flute taps as I do a lot of machine tapping on the vertical mill and repetition lathe. However, spiral flute taps work fine by hand.

Split dies come from the same sources, although I don't use dies that much. Most external threads are screwcut or done with Coventry dieheads.

I normally drill for ~70% thread engagement prior to tapping. A bit more for fine pitch threads, less for coarse pitch and/or tough materials like stainless steel. I rarely use cutting lubricant, simply not needed, and is a pain to clear up. When I do use lubricant (coarse pitch/large diameter/tough material) I use Rocol RTD.

For tapping by hand I don't use a guide, just line the tap up by eye. This is easier with hand taps than for spiral flute taps as the long taper on a hand tap aligns the tap to start with.

Tapping blind holes doesn't bother me, the vast majority of tapped holes on my traction engine cylinders castings are blind. Spiral flute taps really come into their own as they will thread to within a pitch or two of the bottom.

Andrew

It's usual for metric thread dial indicators to have 5 gears. By chance i have just bought a metric thread dial indicator for a Harrison M300, as part of a job lot where what I really wanted was the multi-position saddle stop. I'll be selling it on as my M300 is imperial.

The metric M300 TDI has five gears; 14, 16, 18, 20 and 22 teeth and the lathe has a 6mm pitch leadscrew. It is relatively poor for pitches below 1mm, doing 0.225, 0.25 and 0.75mm. I suspect your lathe would do similar and it certainly ought to be able to do 1mm pitch, but you may need different gears.

Andrew

I've had trouble using HSS milling cutters, too but carbide is no problem. It's almost as if the material hardens while being cut. A recent job was to make two simple blocks, with clearance thru holes, to act as drive dogs on the Int40 vertical attachment for my horizontal mill. The screws were 1/4" BSW. After spot drilling, and failing with a 6.4mm HSS drill, I drilled through 6mm using the carbide spot drill without a problem. Should be easy to open out to 6.4mm, right? Three fudged HSS drills later......

The tips of the drills are blunt, and blue, indicating that they are simply not cutting, just rubbing. Next time I'll take the advice and slow the speed right down.

Andrew

Assuming the engine design is imperial I wouldn't change to Mod gears. 1.75 Mod is not exactly the same as 16DP. The Mod gears will have different centre to centre differences, even if the tooth counts are tweaked. Before you know it the change will turn into a major redesign exercise.

Using the incorrect gear cutter will result in the wrong curvature on the teeth, so the gears will not mesh smoothly. If the incorrect cutter is to be used better to use one with a lower tooth count than the gear, as the tooth will end up more curved than they need to be. They will mesh with another gear. if the tooth is less curved than it needs to be it may not mesh at all.

Andrew

I'm not sure either, seems a bit over-complicated?

My everyday drills, 1 to 10mm by 0.1mm, are Dormer 4-facet. Imperial, number and letter drills are conventional, but don't get used that often. I expect drilled holes to be within 1 to 2 thou, usually oversize. For better accuracy I bore or ream depending on size and machine in use.

For general work 4-facet drills start, on the vertical mill, without needing a centre, provided the work is smooth and flat. If I need to be assured of positional accuracy I use a carbide spot drill first. Never got on with using centre drills for spotting. I only use centre drills in the lathe.

Stoning drill edges never worked for me so I have a small set of slow helix drills for use on copper alloys. Bronze and gunmetal are worse than brass for pulling in. I also have a few fast helix drills, but I don't machine much aluminium these days.

Carbide drills are used for tough materials, like tungsten, and are also stiffer than HSS. So small carbide drills are less prone to wander than HSS.

I would agree that drilling is a good way of removing metal. I have a good selection of Morse taper drills (all secondhand) for opening up holes before boring. Biggest drill is 1-7/8".

Although hand sharpening is easy, and I have a Clarkson T&C grinder for conventional and 4-facet drills, by the time my drills need sharpening they're not in great overall shape. It's much simpler and cheaper to buy new drills.

I also have a good selection of LH drills, and centre drills, for use on the repetition lathe.

My only problem drilling is with gauge plate. In it's supplied state it ought to be fine, but it seems to eat my HSS drills.

Andrew

I suppose it could be done but seems a complicated solution to a simple problem. A chaser only cuts on the front chamfer. The remaining "teeth" are forward (on an external thread) of the cutting line and are used to pull the diehead forward. On an internal thread those "teeth" would interfere with the already cut thread. So they would need to be ground back. Tilting the chaser will not work as it will mess up the helix angle.

Andrew

In the text for the US patent it clearly states that the unit can be used for simple slotting. Essentially the two slides are locked together so that rotation of the tool does not occur towards the end of the stroke.

Andrew

That's the art of patent writing; include all the information but in a form that is difficult to follow.

With regards to slotting internal keyways I don't think tooling is readily available to buy. I just grind the shape on the end of a short length of HSS, or use a push broach.

Andrew

Jason is correct, tooth depth has no relation to the number of teeth and hence OD.

The OD of a gear is the tooth count (N) plus 2 all divided by the diametric pitch (P):

OD = (N + 2)/P

For diametric pitch gears the addendum (J) is the inverse of the DP:

J = 1/P

In theory the dedendum is equal to the addendum, but in practice some clearance is needed, so the dedendum is greater than the addendum. The clearance is arbitrary but the dedendum is normally 1.157 divided by the diametric pitch:

K = 1.157/P

For smaller tooth count gears the clearance can be increased so the dedendum becomes:

K = 1.25/P

The total depth of cut should be marked on the involute cutters, often defined as Df, or D+F. For the cases above it will be 2.157 or 2.25 divided by P.

Andrew

The chart can never be correct. There is not a one to one relationship between spindle speed versus material and diameter. There are simply too many variables, not least due to variations in material. Generally cutting speed, and hence spindle speed, is a fairly broad curve and not that critical. I never use charts, just do a quick estimate in my head based on some broad rules of thumb, what sort of tooling I am using (carbon steel, HSS or carbide) and experience.

Andrew