Hi Gang:
I would really like to see a new design for a Bridgeport head, made to work with CNC. All the converted head are Kludges of the standard casting, with massive add-on for quill control. A new casting might incorporate a proper Z axis drive.
Just wishing. Carl.
Do not just wish nor wait for someone else to do it. If you want that then DIY. That has always been my approach whenever I have been unhappy with some feature.Originally Posted by carl blum
Today I noticed that wiggle in knee got worse, I thought it's time to tighten the Z axis gib and it went to the end of its motion range. Absolutely catastrophe. I was fully aware that steel on steel guides will not last very long but I wasn't expected that happen that quickly.
X and Y axis guides looks still very good. Only Z axis got literally plowed. I don't know what to do. Buying HGR linear guide sounds like only reasonable option because even after remachining all this will probably repeat quickly.
I did some calculation in HIWIN linear guide configurator software and HGR 20 should be enough.
Several months back I changed the X axis dovetails on my lathe to linear ball rails. I would highly recommend it. I wish my BP mill had them.
There are some videos in this playlist https://www.youtube.com/playlist?lis...JVwwFANxyTc2Kj
SteelCraft (Nov 10, 2024)
I converted my 3D model to new guides. Only one probleam appeared. Dove tails of table collide with carts.
This is where it collides:
Cut them off?
tonyfoale (Nov 10, 2024)
"Cut them off" Yes if they are in the way then get rid of them. I think that if you make your proposed changes then your mill will be at least 1 or 2 levels higher. Good luck. I shall follow with interest and great anticipation. One day I would love to visit Poland.
One thing to remember is that if your lift screw for the knee is under the centre of gravity of the knee/table assembly then there will be very little load on the Knee rails and even with the table at the extremes of movement the load on the knee slides is never very high.
One thing to consider is that to get the best from linear ball slides you also need to use ball screws for the motion as well. When I converted my lathe that was even more important than the linear ball slides.
One tricky aspect to using linear ball rails, unless you have CNC or electronic control, is how to lock the different axis.
Last edited by tonyfoale; Nov 10, 2024 at 08:30 AM.
SteelCraft (Nov 10, 2024)
Lack of "self locking" properties of ball screws is disqualifying them from manual machines. Anyway I still have to find a way to lock all axis. I think about steel block with close fit to the rail which will act as a clamp when needed.
I always make calculations for the worst scenario for example: fully loaded table on extreme end position where generated torque load on rails is the biggest possible.
Last edited by SteelCraft; Nov 10, 2024 at 11:07 AM.
No it doesn't. That is a popular myth. When I first fitted one to the cross slide on my lathe when still manual, all the arm chair experts told me that it would break my wrist and I'd suffer all sorts of dire consequences. The solution is very simple. First ask why it doesn't happen with a similar pitch ACME screw. It is friction, so the answer is to add some friction to the ball screw. I just made a little spring loaded face brake lined with oiled felt. Simple but it worked perfectly, I never had any backing off at all, yet the action was as smooth as silk.
My Bridgeport mill was supplied from new as a CNC, (not to me) and so it had ball screws but it was also fitted with manual winding handles to enable it to be used as a manual mill. There has never been any problem with the table, saddle or quill backing out. The friction of the belt and servo motor is enough, yet the torque needed to wind the handles is still way less than a mill with ACME threads. It does not need much friction torque to stop any kick back.
SteelCraft (Nov 10, 2024)
That's interesting. I had a picture in my head of machine knee going down on it's own weight. Changing to ballscrews will be considered in the future.
Honestly I have some worries that those guides will not meet my expectations (chatter on surface finish) and there will be no turning back after cutting of dovetails from table.
Last edited by SteelCraft; Nov 10, 2024 at 01:33 PM.
Yes the knee would be the ultimate test, but maybe you could kill two birds with one stone. If you put a clamp on the screw it would stop the screw from turning under the weight and also it would solve the problem of putting a lock on the ball rails. A few months ago I had a bad fall and damage my left shoulder. This makes using the BP knee handle very painful so I have been planning to fit some form of powered lift. Last week I did some calculations to see what torque I would need. It was easy to measure the torque on the manual handle but it was much higher that it would need just to lift the weight. So I wanted to see what was going on. Much to my surprise I found that the friction on the lift screw overwhelmed the friction on the dovetails and weight of the knee assembly. I did not think of replacing it with a ball screw but that would reduce the handle torque considerably. Too late now I am only 1/2 day away from having the power feed operational.That's interesting. I had a picture in my head of machine knee going down on it's own weight
Honestly I have some worries that those guides will not meet my expectations (chatter on surface finish) and there will be no turning back after cutting of dovetails from table.
When I put the linear rails on the cross slide of my lathe, my only concern was the same as yours - vibration and chatter. In fact surface finish improved. I can only guess why but with normal slides there has to be some clearance or you can't move them. Whereas the linear ball rails are slightly preloaded and there is no movement.
SteelCraft (Yesterday)
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