WM280v PCD

[DalboyParticipant @dalboy]

I have spent last evening trying to work out the PCD of the holes on my WM280v lathe. I have done a search with no luck unless I missed it.

I have measured the stud distance on the chuck backplate I have. and by working with that I seem to have worked out that the PCD is 74.70mm

I just need someone who may already know the correct measurement to confirm whether that is correct

It has been a very long time since I have done any trigonometry

[DalboyParticipant @dalboy]

[Michael GilliganParticipant @michaelgilligan61133]

My guess is that the nominal PCD might be 75mm

… if you share your raw measurements, everyone can have a go at working from those

MichaelG.

[DalboyParticipant @dalboy]

Posted by Michael Gilligan on 15/06/2022 08:39:50:

My guess is that the nominal PCD might be 75mm

… if you share your raw measurements, everyone can have a go at working from those

MichaelG.

The outside measurement of bolt to bolt is 72.8mm and the studs range from 7.4mm to 7.8mm so there is a little flexibility in measureing as they are classed as 8mm. These measurements are from the 4 jaw chuck for the machine. I have measure at least three time to come up with these

I have a spare piece of material that I could do a test drill on

[John HaineParticipant @johnhaine32865]

Why not use the cross slide micrometer dial? Put a point in the tool holder at centre height, align it to a centre in the h/s taper, then wind it out until it is aligned with the centre of a hole. Read off the PC radius from the dial (or PCD depending on how its calibrated). the BS eyeball is surprisingly accurate at judging relative position.

[DalboyParticipant @dalboy]

Posted by John Haine on 15/06/2022 08:59:09:

Why not use the cross slide micrometer dial? Put a point in the tool holder at centre height, align it to a centre in the h/s taper, then wind it out until it is aligned with the centre of a hole. Read off the PC radius from the dial (or PCD depending on how its calibrated). the BS eyeball is surprisingly accurate at judging relative position.

A good idea but it did help blow the cobwebs off of my trigonometry whether it was accurate is another thing

[SillyOldDufferModerator @sillyoldduffer]

Seemed like a bit of fun before I hit the road.

Measured the inside and outside distances between all three studs, averaged them and did the maths to get average centre to centre distance.

Also measured perceived centre to centre distances by eye.

Finally applied 3-hole PCD formula: PCD = centre to centre distance / 0.866 (At least one motoring website has the divisor wrong: I used this maths teaching site.)

Results:

So, measuring carefully suggests PCD is 75.11, and eyeballing the centres gives 74.77mm. Most likely the actual PCD is 75mm. I used an inexpensive digital caliper and don't know how accurate it is at 75mm. Digital calipers aren't brilliant : this one's no better than ±0.02mm on a good day, and I wasn't super careful. The worst error comes from the inner measurement, where the sharp jaws sank into the threads: I should have padded them with an accurate parallel and compensated.

Dave

[DalboyParticipant @dalboy]

Thank you guys I have done a test drill in a piece of 1/8" steel and the first two attempts went totally wrong not through calculations but trying the circle function on the DRO so cracked that part of it.

I first tried 74.7mm which I had worked out and it was slightly off so went for the 75mm and it came out perfect.

All the information given was certainly very helpful.

I have now written this in the booklet that came with the lathe for further reference

[not done it yetParticipant @notdoneityet]

If one is really stuck, there are internet calculators for finding this sort of thing.

Used a lot by those who want to check wheel fitment on vehicles. Measure between bolt holes , plug in the number of studs and out comes the result.

[Howard LewisParticipant @howardlewis46836]

A practicl way to find out is to take the make a number of short studs with sharply pointed ends (Or buy them fro Arc Euro mused to offer them ).

Screw the studs into the chuck, blue a plate (Fit a central pillar if you like, and rotate the chuck around it. The pointed studs should mark a circle which t you can then measure.

The PCD is unlikely to be anything other than a whole number of mm, on a Metric machine, (AS the OP found ) or a sensible fraction or decimal part of an inch on an Imperial machine.

Howard

[SillyOldDufferModerator @sillyoldduffer]

The actual distance between two studs on a 3-hole 75mm PCD is 64.951905mm

My averaged measurement gave 65.04 which is an error of 0.088mm, about 0.0035", or 0.136%

Derek's measurement must have given him 64.692098, an error of about 0.260mm ( 0.010" ), or 0.4%. Pretty good considering threads make it hard to identify a stud's centre, and 8mm bolts never are! For many purposes, being a little over a quarter of a millimetre out on a 75mm PCD wouldn't matter. The usual practice is to drill PCD holes distinctly oversize to allow for such inaccuracies.

Curiously none of my Maths for Engineers books cover this question, not in theory or by providing a formula for 'n' studs. Closest is this question from a Technical School textbook:

Three holes A,B and C are to be bored in plate as shown. Calculate the distance between B and C, and the diameter of the pitch circle.

Hope the class can all answer this one: the textbook is aimed at 15 year olds. Did anyone go to a UK Technical School? There was one in my home town, but Wikipedia says they were rare beasts. Pity, I thought they were a good idea. Another opportunity missed.

Dave

 

Edited By SillyOldDuffer on 16/06/2022 14:53:29

[Tim HammondParticipant @timhammond72264]

I attended the James Watt Secondary Technical school in Smethwick (West Midlands) between 1956 and 1961. It was given the name "James Watt" because it was located about a couple of miles from the old Boulton and Watt works. I received an excellent education there, biased towards science and technology, which has served me very well indeed throughout the rest of my life.

[roy entwistleParticipant @royentwistle24699]

Double post

Edited By roy entwistle on 16/06/2022 16:14:20

[roy entwistleParticipant @royentwistle24699]

I attended  Technical College 1948 to 1950, then one day a week day release then three nights a week.

The first day there, we were lined up in the hall and the teacher went down the line picking us out for Pit , Mill, and Engineering. We decided which course to follow after about three months.

I understand that the local industries paid for the Techs. and that was were they got their apprentices from.

Edited By roy entwistle on 16/06/2022 16:12:39

[Author]

Posts