Bridgeport Manual to CNC conversion...

[V8kid]

I dunno about komatias but I got mine from AliExpress, machined to my own drawings for a nominal cost, like $5 per ballscrew - that saved a lot of messing about and they did a good job. The std ballnut diameter is slightly bigger than the std leadscrew nut, so I machined my yoke out on the (Bantam) lathe. Unless your machine is in almost new condition, you may struggle to justify expensive ballscrews, as there will be slop and backlash even once the gibs are adjusted. The ballscrews account for only a part of the total.

Using an MPG means I don't need the X & Y handwheels, which (in my case) added a lot of unhelpful moment of inertia to the servos so I removed them and have a better servo response. I still have the original DRO I fitted years ago but this simply tells me how worn the machine is. If you can achieve backlash below 20-30um (0.001"), I'd say you are doing well. However, the actual accuracy will be worse when you are machining as opposed to cutting air, as there will be additional forces at play - but the willy wavers tend not to get into measuring that, as it spoils the fun.

Thanks for the info I have already converted my Series 1 to CNC using the standard acme screws and also removed the handles 'cos I was getting fed up with getting the occasional thump. By the way I'm with you on appropriate accuracy for outlay I paid £130 for my Bridgeport as it literally had fallen off a lorry and the head casting was cracked. Another £150 in used spares got me going and I've spent another £700 on the CNC conversion. So value.

One thing that puzzles me is the wall thickness on the standard Bridgeport yolk. I can see there is enough meat to machine out from 32mm to 40 mm but there does not appear to be enough wall thickness for the securing bolts without breaking through the wall. How did you manage?

I guess I could buy a 2505 ballnut and dismantle the X axis to see better but I'm pushed for time as we are moving house in the spring so any info would be highly appreciated.

Cheers
David

[Muzzer]

Not sure where you get the 32mm figure from. The yoke bores are designed to take bronze cylindrical leadscrew nuts, as opposed to being directly threaded. On my Series 1 BP clone, the original X&Y leadscrews were 1-1/4" ie ~32mm OD. The actual leadscrew nuts themselves were pretty much 39mm OD and the DFU2505 ballnuts are 40mm OD, so I only had to bore the yoke bores out about 1mm on their diameters.

As for fixing, there's really no need to use all 6 holes. I only fitted two M6 screws and there was enough meat for me to drill and tap the holes. If you look at the tensile load that would cause an M6 bolt to fail, you'll see that this wouldn't be the first link in the chain to break.

Here's a bit about the actual boring: https://mightyshiz.blogspot.com/2018...or-larger.html

And fitting the thing back on the machine. Note that the Y axis ballnut may foul something (I forget - possibly the knee lift?), which required some angle grinder action on mine.
https://mightyshiz.blogspot.com/2018...allscrews.html

Murray

[V8kid]

Not sure where you get the 32mm figure from. The yoke bores are designed to take bronze cylindrical leadscrew nuts, as opposed to being directly threaded. On my Series 1 BP clone, the original X&Y leadscrews were 1-1/4" ie ~32mm OD. The actual leadscrew nuts themselves were pretty much 39mm OD and the DFU2505 ballnuts are 40mm OD, so I only had to bore the yoke bores out about 1mm on their diameters.

As for fixing, there's really no need to use all 6 holes. I only fitted two M6 screws and there was enough meat for me to drill and tap the holes. If you look at the tensile load that would cause an M6 bolt to fail, you'll see that this wouldn't be the first link in the chain to break.

Here's a bit about the actual boring: https://mightyshiz.blogspot.com/2018...or-larger.html

And fitting the thing back on the machine. Note that the Y axis ballnut may foul something (I forget - possibly the knee lift?), which required some angle grinder action on mine.
https://mightyshiz.blogspot.com/2018...allscrews.html

Murray

Excellent thanks Murray I appreciate the work you put into documenting that. I'll spec the end machining myself as my drive arrangements are a little different to yours and whilst I'm waiting on China Post will bore out yoke.

Yup 32mm was a mistake on my part.

Never used a faceplate before fortunately one came with my Triumph lathe so it promises to be good fun.

Cheers
David

[Muzzer]

FYI, I used Machine Parts Store on AliExpress https://www.aliexpress.com/item/8537...27424c4dIg5u4J
Service was good, the machining worked for me and the price was right.

Let us know how you get on!

[V8kid]

Thanks for all the advice I have now ordered from an eBay supplier in China as I did not get any replies to my request for quotes from AliExpress and am going ahead with C7 spec 25mm ballscrews and double ballnuts. Cost was £179 inclusive of eBay fees of £24 which the vendor detailed separately and was clearly upset about!

I guess machining the yoke will be fun as in order to align the bore I will have to centre both the tailstock and headstock ends of the existing bore and I can envisage them fighting with each other for alignment. Anyhow it keeps us out the Pub.
Cheers
David

[Muzzer]

Wasn't too difficult to set it up in my case. There was a machined surface that I bolted against the face plate and I got the bore dialled in quite accurately with a bit of care.

[V8kid]

Whoa! Have 3 months gone by? After finally getting the house ready for sale it was time for me today to have a bit of time in the man cave and Muzzer was mostly right in that it was easy to bore out the yoke on the faceplate - here a tap and there a little tap and with patience it dialled in spot on. The original Bridgeport acme bush had a 3 thou clearance but I decided to make the ballscrew 1 thou clearance and its a nice snug fit

However I should have read Muzzer's post more carefully as his machine is slightly different to mine and sods law the casting walls on the yoke were thinner on my series1 Bridgeport making the fixing holes problematic.

The X axis was no problem as the casting thickened around the securing flange allowing 3 fixings but the Y axis was iffy. I managed to offset grind one of the ballscrew ( V hard material would not drill) fixing holes and just manage to get 2 off diametrically opposite 6x1mm fixings with a 0.5mm wall thickness on the outside but I held my breath whilst I did it.

Only have stainless fixings in stock so will have to order HT Allen screws before assembling.

Cheers and ta for the pioneering work Muzzer

[Muzzer]

Heheh, good to hear it went well! You don't actually need a lot of screws. If you look at the yield strength of an M6 machine screw, you'd break something else before 2 of them failed.

What's next? Have you got the rest of the parts for the X and Y drives? I forget what your plan is for the Z axis - I might have to look earlier in the thread to see what you planned....

[V8kid]

Already converted to X&Y CNC using existing Acme screws and Steppers on Mach4 with Ethernet smoothstepper. Works OK but..... Hoping the ballscrews will give more speed and accuracy.

For the Z axis I obtained a spare head casting and machined the front flat so I can get the ballscrew closer to the quill and extended the slot downwards so I can get 2 fixings on to the quill. Need the X&Y axis up and running to machine the necessary Z axis slipper, Not sure how well this will cope with drilling large holes may have to do that manually.

How did you reinstate the oil feeds to the yoke? For the X axis I drilled into the old bush keyway which should distribute the length of the yoke but for Y axis just put a drip onto the ballscrew at the back side. Not sure how much lube is required.

Cheers

[Muzzer]

IIRC, there were oil pipes for each of the X and Y acme nuts and I was able to reinsert them into the ballnuts. The X can be fitted when the table is still off but I fitted the Y with the table and saddle in place (forget why) and I recall a little bit of messing about inserting the pipe into its ballnut, via the hole in the front of the knee.

Nice solution, getting a spare head casting. I wasn't impressed by the crude approach used by Elrod, South West Industries etc that relied on a Z axis yoke sticking way out from the quill to clear the features on the standard head. When combined their massive castings (you could have kept a rabbit inside the housing Elrod "designed" and supplied for his conversions), the end result is enormous and expensive - but not even very robust around the ballnut and quill.

I designed my own Z axis drive using a 16mm ballscrew that replaces the threaded feed adjustment rod. This allows a ballnut yoke with a minimal overhang and the installation is compact and neat.

Unless you plan to do a lot of heavy drilling, you don't need a big motor for the Z. As a starting point, if you ignore friction, the torque and force are related like this:
Torque x angle = force x distance where angle is number of radians in one turn (2*pi), distance is the ballscrew pitch (metres) and the torque (Nm) / force (N) are your input and output.

So for my 2Nm Leadshine closed loop stepper and 4mm ballscrew, the stall force would be ~2 x 2 x 3.14 / 0.004 = ~3000N, which is equivalent to the weight of about 300kg (or three fatties). That's a lot of force and would be a test for most ballnut installations. Any pulley reduction ratio would increase that further - in fact mine is 1.8:1, resulting in about 1/2 tonne of force.

In practice, you lose a fair bit of your motor torque along the way, especially if you don't align your ballscrew carefully, but even so, you can see that a lot of these conversions you read about are stupidly overspecified in terms of stall torque, reduction ratio etc. There's one good reason for fitting limit switches...

Keep us updated on your progress!

EDIT - missed out a factor of 2 on the left side of the fag packet sums. Answer was right, though.