Town Woodhouse milling machine re-commission and cnc conversion

[Agroshield]

...would I lose the versatility of the machine once converted?

If you include three MPGs (manual pulse generators) in your CNC setup, you can drive the machine in the same way as a manual machine. They are a lot cheaper now (about £20 from a GoodBang near you) than they used to be. In the old days, when they were pricey, normally only one was used, but then you had to remember to set it to command the correct axis - cue much profanity and broken tools when you thought you were moving X- and you actually moved Z.

 

[Fastdruid]

Inch and a quarter 5tpi would tie in with my measurements, not sure about the angle, but I expect that would be standard as well?
Desired end result? Well I bought it to fix, and cnc it. I am having second thoughts - would I lose the versatility of the machine once converted?
As for automating the z axis, I was planning to drive the 4 inch quill for that.
I may temporarily repair this, with a view to using the table lead screw once I've finally decided to go cnc.
I'm happy to hear views of experienced machinists on here though.

Acme is a standard thread form so IIRC about 12.5deg. Certainly the page on them says they're acme http://www.lathes.co.uk/townwoodhouse396/

IMO a good conversion will give best of all worlds, I intend to have mine with individual mpg's.

Obviously cant see much of what is there so I'm going off the previously mentioned images at lathes.co.uk but X and Y look tbh to be pretty straight forward to convert. The normal issue is having the room to fit in a ball nut and it looks like there is plenty of space there. The real question is going to be round Z, you're either going to be very limited in depth *or* you're going to have to move the knee.

If you're going to drive the quill then you need to have a think about how it's going to be done, I believe a number of people have converted bridgeports using a mount on the indicator on the front and then run into issues because its designed for "light" use rather than to drive the quill. You can use the "original" mechanism (I don't know how that operates) but that will really depend on the backlash in it, although you can account for it and use a glass scale or similar to work round it.

If you drive the knee then again on the plus side there is plenty of space for a ball nut on the Z, hell worst case you could replace the nut mount in its entirety. The question will be how easy it is to drive the screw. Although you could also consider a driven nut. Due to the design there is plenty of room.

 

[minimutly]

Acme is a standard thread form so IIRC about 12.5deg. Certainly the page on them says they're acme http://www.lathes.co.uk/townwoodhouse396/

IMO a good conversion will give best of all worlds, I intend to have mine with individual mpg's.

Obviously cant see much of what is there so I'm going off the previously mentioned images at lathes.co.uk but X and Y look tbh to be pretty straight forward to convert. The normal issue is having the room to fit in a ball nut and it looks like there is plenty of space there. The real question is going to be round Z, you're either going to be very limited in depth *or* you're going to have to move the knee.

If you're going to drive the quill then you need to have a think about how it's going to be done, I believe a number of people have converted bridgeports using a mount on the indicator on the front and then run into issues because its designed for "light" use rather than to drive the quill. You can use the "original" mechanism (I don't know how that operates) but that will really depend on the backlash in it, although you can account for it and use a glass scale or similar to work round it.

If you drive the knee then again on the plus side there is plenty of space for a ball nut on the Z, hell worst case you could replace the nut mount in its entirety. The question will be how easy it is to drive the screw. Although you could also consider a driven nut. Due to the design there is plenty of room.

Hmmm, some good answers there. The knee mounted kit is going to be heavy, couple of hundred kgs plus I reckon. Motorised nut would be good though.

 

[minimutly]

Hmmm, some good answers there. The knee mounted kit is going to be heavy, couple of hundred kgs plus I reckon. Motorised nut would be good though.

Here's a Bridgeport that was on ebay this week - sold cheap too...

 

[minimutly]

The quill drive is a rack cut into it, driven by this arrangement:-

 

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Mr Woodhouse, please meet Mr Werner. Some success tonight, knee is on my fritz werner, and the broken fasterer removed. Now to mount it squarely.

 

[fizzy]

Not hanging about are you

 

[Fastdruid]

The quill drive is a rack cut into it, driven by this arrangement:-

View attachment 279540

How much backlash is there going to be if for example you replace that handwheel with a motor? There appears to be at least two gears there plus of course the actual gear driving the quill.

 

[minimutly]

Roughed out. Not sure if I need to make a block to fill behind the "ramp"..
I don't see why not?

 

[minimutly]

How much backlash is there going to be if for example you replace that handwheel with a motor? There appears to be at least two gears there plus of course the actual gear driving the quill.

I wouldn't fit the motor to the handwheel, I'd fit it to the fast feed shaft (the one at the bottom right of my pic of the greasy gearbox). It would need gearing, I guess.
Decisions decisions...

 

[Fastdruid]

Hmmm, some good answers there. The knee mounted kit is going to be heavy, couple of hundred kgs plus I reckon. Motorised nut would be good though.

Weight really is just a matter of gearing. My Harrison is maybe a little bit less hefty but still it's got to be 150+Kg in the knee and table.

Bit hard to tell but I think it's somewhere around a 16t worm (hard to tell as the only photo I have is from the top) and 30t worm gear....actually thinking about things it's probably a 15t because one full turn is 2mm of travel and it looks to be a 4mm pitch (1965 but a metric mill!) acme so 2:1.

You can fairly easily work out the effort required (and hence what motor you will need) with a torque wrench or spring scale.

For my Harrison it's (very very) roughly about 6Kg of force on a 5"/12.7cm lever to lift the knee. Convert that to Kg-cm (multiply together) to get 76Kg-cm which is 7.45Nm, multiply by two (because gearing) and you get the required torque to drive it directly of 14.9Nm.

There is however a substantial amount of friction in that system (and that's ignoring the friction of the knee on the column), if you look at it from the perspective of an almost frictionless ballscrew and consider the knee/table assembly to be 200Kg using a 5mm pitch ballscrew... you only need about 1.8Nm. Equally you can "remove" some of the weight by using counter balances or gas struts.

If you want to play with things there is a nice calculator here: https://www.servotak.eu/tools/engineering_calculator

 

[minimutly]

Motorising the knee has it's advantages - keeping the flexibility of the head being a big one, longer range another. Downside I see is loss of rigidity from running with a "slack" knee, but I guess this can be adjusted down to a degree. You also have to factor in any additional weight in the job you might put on the table....
Gas struts, hmmmm

 

[Fastdruid]

Obviously you have to consider the weight on the table...but there personally I took a weight of 200Kg (on top of the knee/table itself) to be a realistic "safe" figure on the basis that I'm unlikely to ever actually put something up there I can't lift up there...and 200Kg is far over what I could get on there!

That's still only ~3.6Nm to lift it (ie a total of 400Kg) with a direct drive on a ballscrew. I went with an ~8Nm (can't remember off the top of my head if 8 or 8.5) motor on mine on the basis that it was the biggest I could fit inside the knee and a decent ~4.5Nm over and above what I should need (and if that isn't enough I can gear it down). It's hard to tell how you could do yours but as I said a driven nut would very much be possible as there is plenty of open space (the Harrison goes into the cabinet and clearance is tight, you can do a driven nut but it's harder).

Rigidity, well yes, although no different to not locking the Z when using it normally. I wouldn't have thought it was an issue normally due to the mass as long as your Z ways are in decent condition. If they're bad enough that you need to lock them then that's an issue in itself. With mine I have no quill so if I need to plunge I'm using the knee anyway.

In an ideal world I would personally do both the knee Z and the quill Z, as that gives you a 4th axis (albeit one that may be tricky to use without CNC control of the head angle).

 

[minimutly]

Obviously you have to consider the weight on the table...but there personally I took a weight of 200Kg (on top of the knee/table itself) to be a realistic "safe" figure on the basis that I'm unlikely to ever actually put something up there I can't lift up there...and 200Kg is far over what I could get on there!

That's still only ~3.6Nm to lift it (ie a total of 400Kg) with a direct drive on a ballscrew. I went with an ~8Nm (can't remember off the top of my head if 8 or 8.5) motor on mine on the basis that it was the biggest I could fit inside the knee and a decent ~4.5Nm over and above what I should need (and if that isn't enough I can gear it down). It's hard to tell how you could do yours but as I said a driven nut would very much be possible as there is plenty of open space (the Harrison goes into the cabinet and clearance is tight, you can do a driven nut but it's harder).

Rigidity, well yes, although no different to not locking the Z when using it normally. I wouldn't have thought it was an issue normally due to the mass as long as your Z ways are in decent condition. If they're bad enough that you need to lock them then that's an issue in itself. With mine I have no quill so if I need to plunge I'm using the knee anyway.

In an ideal world I would personally do both the knee Z and the quill Z, as that gives you a 4th axis (albeit one that may be tricky to use without CNC control of the head angle).

Well I couldn't lift the Woodhouse knee onto my Fritz Werner. But that said, I shan't be getting rid of it so hopefully anything heavy will go on that. (Skimmed both banks of a Rover v8 on it before now).

 

[Fastdruid]

Well I couldn't lift the Woodhouse knee onto my Fritz Werner. But that said, I shan't be getting rid of it so hopefully anything heavy will go on that. (Skimmed both banks of a Rover v8 on it before now).

Yeah, the Harrison knee is about 30Kg and the Woodhouse looks more substantial. I can lift more than that but I'm pretty sure I shouldn't!

I'm sure its far short of 200Kg though, I mean 200Kg is a motorbike parked on top basically...Or me, my wife both kids and the dog!

 

[minimutly]

Maybe it's not 200, but I couldn't get it off the ground, so it's 150 plus - the cast walls are over half an inch thick. Will have to weigh it.
Anyway, have sort of made a decision on what I'd like to do. I would like to motorise the knee, this keeps the turret manual, and retains the flexibility of the machine for small, weird angle machining jobs, as well as helping with the notcher. I'd also like to start at the top and work down - that is the table x and y axis - but this really would be a silly way of doing work twice - except that I need to fit the knee to check and set up after the repair.
Hmm, maybe I'll straighten the leadscrew for now, but then I'll still have to modify it to fit the new ballscrew "nut".

 

[Fastdruid]

It kind of depends on how accessible the knee lift mechanism is on it. On the Harrison you _need_ to remove the entirety of the table, ie both X and Y to be able to remove the leadscrew for the Z because you cannot get in to the nut on the top of it without the Y slide being removed.

This is partly why I haven't fitted my motor etc to mine despite it being "ready", I don't want to dismantle it twice (it's less the time and more the weight and how awkward it is).

If you can remove the Z screw and trust the locking mechanism on the knee to hold it up (and use a brace as well!) then it would be far easier. You have the advantage with yours as well because the hollow "base" for the Z bolts on, if you can drop it down enough you could remove that entire section.

One option might be to cut the top section off the Z leadscrew and make a coupler between that and the leadscrew. You'd lose a bit of capacity but can then relatively easily swap out the leadscrew for a ballscrew. You'd have to consider how you'd stop it rotating if you're going to use a rotating nut of course but that may be fairly easy.

 

[minimutly]

Not very accessible at all...
Which means trying to complete 2 jobs at once (and I'm not very good at finishing one job these days). I think maybe refit it with the lead screw for now, check and set up with the spirit level then remove and fit the ballscrew kit once I've got it and a controller here.
One thing about ballscrews - do they suffer from brinelling - due to the point contact of the balls being higher than a lead screw? Just a thought.

 

[Fastdruid]

Well yes...and no as long as they are correctly specified. See page 34 here https://www.hiwin.com/pdf/ballscrews.pdf

> The static load is the load which will cause the balltrack to have a plastic deformation exceeding 0.0001x ball diameter. To calculate the maximum static load of a ballscrew, the static safety factor Sf of the application condition should be considered.

If again we take 400Kg to be a reasonable load with a single FSC ballnut https://www.hiwin.com/pdf/ballscrews.pdf

A 25mm 5mm lead ballscrew has a Static Load Co of 4612 (Kgf), so we can see that actually 400Kgf x 2.5 =1000Kgf which is < 1/4 of the Static Load Co.

 

[fizzy]

Forgive me for being stupid but why does everyone use ballscrews for cnc?
Why does accuracy matter? Can't a cnc device read off DRO type scales and move everything related to the readout? That way play and backlash etc wouldn't matter or would it?