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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: What to drive a J & S with ?
19/09/2021 13:15:12

Drive sizing should be based on the VA rating of the motor - how much it will consume. Output power is less than this because of various inefficiencies.

3 phase motors can be wound for different voltages, and in different ways. In general the difference between a 240v 3 phase and a 415v phase is that the star point of the windings is connected on one of them (can never remember which way round) - known as star or delta winding.. The star point may be available in the motor connection box, or it may be buried in the windings. You can often get to it, but with some amount of faff.

If the star point is not available (depending on age quite likely for an industrial machine spec’d for 415) you need to put 415v into the machine. If it’s available then reconfiguration will mean it can run on 240v 3 phase.

I think tthe lack of VFD in 240 in to 415 out is that there is not much demand for them. Modern motors can be configured to 240, and not having the increase the voltage in the drive makes it cheaper.

When I examined this originally (15 ish years ago) my Mill has a Dhalander wound 415v motor, and a number of other 415v motors, so it was simplest to go Rotary converter. Since then I have added more ex industrial machines and I just plug them in. Might not be the most economical way to run them, but the only time I notice larger electric bills is if I’ve been doing a lot of welding - the machine shop doesn’t seem to use that much (although I haven’t actually quantified how much)

Dave

Thread: Lathe dro readings wrong.
18/09/2021 15:55:55

I assumed the 0.5mm difference was down to slightly dodgy measuring technique with the digital very-near

Dave

18/09/2021 13:20:07

35mm would be the diameter reduction for 17.5mm of movement.

Sounds like your DRO is telling you how much smaller the bar will be vs how far the cross slide has moved

Dave

Thread: cutting spur gears on a mill
18/09/2021 10:51:08

Maybe because it’s easy to set the angle using a sin bar?

Dave

Thread: A Couple of Questions about Pipe?
17/09/2021 19:47:02
Posted by Swarf, Mostly! on 17/09/2021 19:41:14:

Engineering history can be obscure but interesting. For instance, why is the outside diameter of nominal 1½" pipe actually 1 and 29/32 inches?

I think this one is because pipe sizes are measured for the bore.

Makes sense if you need to know how much stuff will flow down a pipe I think

Dave

Thread: cutting spur gears on a mill
17/09/2021 11:53:42

Pressure angle and flank angle of an involute rack are the same. the thread is (to all intents) a rack wrapped around a cylinder. (squint a bit and ignore the helix for that approximation).

Thread angle is the included angle, but pressure angle is half that - so a 60 degree thread form should make a 30 degree pressure angle involute.

Dave

Thread: I need to cut chamfers into x64 pieces of mild steel - any advice?
17/09/2021 07:20:25

And why I suggested a belt. If you get the direction of travel wrong you might still get some rounding - you want the belt travelling:

\
\
-> this way - off the thin end

Dave

Thread: Lapping Brass?
16/09/2021 20:05:18

These are involute not cycloidal profile, 32dp IIRC. Largest tooth count is 144.
They were cut using the Sunderland method, so inevitably have a small number of facets on each tooth.

Being involute the rack is straight sided, so I was thinking of “just” making a length a bit longer than the max number of teeth and running them one way on it - using the rack a bit like a file if that makes sense.

Is timesaver one of the “none embedding” compounds? Would you run it brass on brass?

Dave

Thread: Leadscrews and other features of Asian milling machines
16/09/2021 17:27:18

I never look at the dials on my mill, only the DRO.
For what it’s worth just looked and there is .055” backlash on the Y and just under 0.03” on the X.

Still makes accurate parts as the DRO measures where the table is, not where the screws think it is

Dave

Thread: Lapping Brass?
16/09/2021 17:24:10

Evening all,

I want to improve the form of some brass gear teeth.
They are involute profile, so I think running them against a rack form with some sort of cutting compound would do this.

However I very much do not want the compound to embed in the gears, or when they are in service they will lap the mating pinions, probably leading to premature wear / failure.

Any idea on what to make the lap from, what cutting compound to use or other tips?

Dave

Thread: Workholding Problem
12/09/2021 19:52:21

Are those disks in the middle where the diff is?

<memories of childhood >

Always good to see an end result

Dave

Thread: CNC Knurling
12/09/2021 16:55:44

Looks good. Only real difference I can think of is knurling doesn’t “sink” below the original surface - which may or may not matter. At least with this you can guarantee a good knurl and not worry about it not tracking in.

Got to love the forum smilies in code

Dave

Thread: General sharpening direction help
12/09/2021 11:46:30

Depends if you want the burr or not. I was taught to do chisels so the burr was there, it giving a razor edge.

Similarly cabinet scrapers actually use the burr as the edge (although generally deliberately hooked over)

The burr edge is of course not very strong, so I guess less frequent sharpening with the compromise of slightly less sharp is a way to think of it.

Scissors of course cut by shearing, so require a fairly blunt angled edge, but with a good corner - a burr on that would not work.

Dave

Thread: Workholding Problem
12/09/2021 09:21:45

Soft jaws are how I would do that.

bore them to hold the larger diameter x2 mm deep.

plenty of grip as they should match the curve perfectly, and repeatable setup ( I assume you have at least 4 to do)

Dave

Thread: What to drive a J & S with ?
10/09/2021 19:07:47

Robert - you seem to know about the subject- can modern drives take 240v single phase and output 415 three phase, or do they still require a 240v three phase motor?

Dave

09/09/2021 20:55:09

The 540 motors drive the spindle and the motion - the secon motor runs a hydraulic pump which also supplies the ways with lube as well as drive. Not sure slowing that down is a good idea.

I am well out of touch with VFD tech, but I have not seen any evidence of torque ripple on my machines.
The three phase is well balanced according to my measurements (taken some time ago admittedly, but back then I worked for a large generator manufacturer in Loughborough) It’s also at the correct voltage.
Do VFD drives now do step up conversion as well?

Dave

09/09/2021 19:44:13

I thinking you have 4 options, and the fifth would be to get “real 3phase” installed

firstly re-motor to single phase. Whilst the hydraulic pump won’t care I believe the spindle motor is balanced and smooth running and single phase motors of a reasonable price are neither. There is conjecture that single phase also produces torque ripple that is visible in the finish.

secondly use the capacitor static converter trick to start the motors, which then run on 2 out of the 3 legs, with the back emf generating the third phase. You will need to either convert the motors to 240v or get a 240 to 415 step up transformer. Also not considered a great way to run a grinder. You lose some motor torque - probably not an issue, but the unbalanced leg can be an issue for surface finish.

Third option is a VFD. This will create “proper” 3 phase, but often relies on the motor inductance to smooth the chopped waveform. Older motors and VFDs may not play nicely, meaning you may need a new motor… I do not have much experience with more modern drives, so there may be a solution that works well. You probably don’t need the variable speed, so it’s possible that will simplify things. You will need an appropriate sized one and again possibly need to step up the voltage to give 415 out ( iirc the mains gets rectified to dc then chopped back to ac so it’s possible that a vfd will also step up but I don’t remember seeing one that does)

fourth is how I run my 540 - a rotary converter. This uses the capacitor trick on an idler motor to generate “proper” 3 phase. Boxed ones also usually incorporate a step up transformer. I have 3 industrial machines (TOS FNK25 mill, CVA lathe and the 540) and have been running them with a Boost Energy unit for many (10 or 15) years.

I would talk to Newton Tesla and see if they would recommend a unit and what the limitations may be - you need to run 2 motors and its possible that might be a bad thing to do from a vfd, but I would also consider a rotary. If you get more 3 phase machines you can just wire them in to you rotary.

Dave

Thread: CNC Knurling
09/09/2021 17:17:23

w is write, and will trash an existing file of the same name, a is append - so open if the file is there and write to the end, otherwise create a new one.

It makes little difference if you are creating the whole file at once.

My earlier example was for a rolling log file, where you actually want to keep the existing data - just the handiest example I had around.

edited to say I think in Python there is an implicit newline in a print

 

Dave

 

Edited By Dave S on 09/09/2021 17:18:11

09/09/2021 12:59:16

This is microPython, rather than Python, but the file handling is the same IIRC.

file writing on a pico

The code snippet (culled from that link):

#This writes whatever is passed to it to the file     
def WriteFile(passed):
    log = open(LogFileName,"a" #open in append - creates if not existing, will append if it exists
    log.write(passed)
    log.close()

should help 

 

Dave

Edited By Dave S on 09/09/2021 13:01:19

Thread: It would be nice to know........
09/09/2021 12:53:31

Last time I used mine in reverse was when I accidentally made a left handed cutting tool to create the spaces on an escape wheel.

Didn't realise I had it reversed until I got the whole thing setup in the mil and ready to run - then a bit of a 'hang on... something is not right here' moment.

Glad to have reverse for that - the cutter took a while to make.

Dave

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