Bridgeport Manual to CNC conversion...

[John S]

Dave, look for the head off a BOSS 1 to 5 but you will need the ram as well as that head is a really solid fixed head that doesn't suffer from the nodding and endless tramming that the rubber Bridgeports suffer from.

Best thing with the BOSS head is the quill is the ball screw. The balls run round the outside of the massive quill so no bolts and no trying to tip to one side.

This is one reason I posted in post #3 to look for one and not do the conversion. Don't know what the conversion has cost but got to be loads more than upgrading an existing CNC.

Hell for a grand you could have had my 10 x 54 CNC Beaver in fully working order with 4th axis, 30 odd BT40 toolholders. 6:1 speed increaser and 8" vise if you were prepared to strip it to get it out.

[Davek0974]

The head from a BOSS would be nice but not seen anything from a BOSS listed for some time now, seem to be getting very rare.

Whereabouts are you BTW ? ;)

Yes i decided way back in the beginning to go ahead because I just cant fit anything bigger than a manual BP in the shop - no head room at all. Yes it cost a lot but I have had a blast doing the conversion, learnt a lot and still have a usable machine to play with.

The Quill connection I was not happy with from the outset - its a terrible weak spot, unless a much taller connection is made to the quill to spread the load - this then reduces Z travel respectively and I'm already seeing the bad side of poor Z travel and i only lost 5mm from standard!

I think there is some redemption in using the quill rack/pinion - it's the strongest method and as long as backlash is dealt with should be just as accurate.

[Colin Barron]

Future thoughts...

Machine is working very nicely so far. However I still have big doubts about the weakest link in the system - the Z-axis drive, its a pretty tortured setup, the connection to the quill is poor being just one bolt, the block face is only around 15-20mm square where it bolts on, there is around 75mm from the quill to the ball-screw so there is a massive twisting moment acting on the joint and the ball-nut.

There is already movement visible between the nut and the quill - you can see the connecting arm twisting when nudging the ball-screw pulley by hand so it would be pretty rough under drilling or plunge-milling forces. I can see a very short life for the ball-nut as they don't like twisting moments, axial or radial but not twisting.

It was assembled with loctite but with only the one bolt I doubt that would do much.

I have a feeling it would have been better to go with the drive idea in the pictures below, the quill rack is designed to take all the Bridgeport can do, there is not much backlash and what there is could be helped with a gas-spring maybe to keep it under positive pressure.

No idea what the pinion gearing ratio is but likely something like 3" per turn so would use a bigger reduction from motor.

Just looking ahead, but as i said, I can see issues here already and thats only after a few jobs....

Attachment 19486
Attachment 19487

Hi Dave,
This company has managed to fit the Z drive on the other side of the shaft and have more mounting plate bolts. The z drive is almost at the end of the video.
https://www.youtube.com/watch?v=6b0Jf-xzE0s

[Davek0974]

Thanks, yes I've watched that vid a few times ;) Its the same deal really, same shaft in head just different connection styles. Maybe the way to go.

[Colin Barron]

Dave, look for the head off a BOSS 1 to 5 but you will need the ram as well as that head is a really solid fixed head that doesn't suffer from the nodding and endless tramming that the rubber Bridgeports suffer from.

Best thing with the BOSS head is the quill is the ball screw. The balls run round the outside of the massive quill so no bolts and no trying to tip to one side.

This is one reason I posted in post #3 to look for one and not do the conversion. Don't know what the conversion has cost but got to be loads more than upgrading an existing CNC.

Hell for a grand you could have had my 10 x 54 CNC Beaver in fully working order with 4th axis, 30 odd BT40 toolholders. 6:1 speed increaser and 8" vise if you were prepared to strip it to get it out.

The size and weight is why bigger machines are only a grand. For most garage workshops the 875Kg series 1 manual Bridgeport is probably bigger and heavier than they had wanted to go but they are cheap, lots of info, plenty of spares and the conversions are well documented.

[Davek0974]

Great if you can find one i guess :)

[Davek0974]

Thinking out loud...

With the drive to the quill rack, probably from the left side as done in the FlashCut kits etc, without measuring i think the quill pinion does 2 full turns for full travel so about 75mm/turn, with a 4:1 or maybe 5:1 belt reduction from servo to pinion shaft, would that give enough resolution/torque - the motor would only ever do 8 or 10 full revs??

I could probably make a bracket pair which could take one or two layshafts which could offer much higher reduction ratios.

Maybe even look into worm-gear reduction.

Just working things through...

[Colin Barron]

Is there much backlash on the quill gear, i have seen a drive with a ballscrew on the front of the head.

[Davek0974]

No the quill pinion is pretty firm. Yes the screw on the front is a popular method because its the only place you can get a direct connection to the quill, trouble is its an awkward connection and i'm not happy with it yet.

[Davek0974]

Finally scraped up the cash for proper thrust bearings on the X & Y axes. Got Nachi P4 matched pairs from BearingBoys Ltd


I fitted them in DB or back-to-back style, seemed the best option here - the preload is taken up by the screw nut rather than the bearing retainer.

What a difference, smooth as silk now and no shims needed :)

Will measure backlash again to see what change it has made.