THK rail retrofit on a BP series 1 manual?

[Sparky_NY]

I have seen and read many posts on converting a series 1 bridgeport to cnc as well as similar knee mills but have never seen anyone retrofitting a BP to linear rails (THK HSR type).

The reason for my interest is my BP has the 42" table and suffers from the standard sagging of the table resulting in the travel getting tight at the extremes. The routine fix for this is regrind the table and rescrape the ways in BUT I wonder why not grind the bottom ways flat again and install a set of THK type rails? This would increase precision I would think, eliminate the necessity to rescrape the ways back in and greatly benefit a future conversion of the machine to cnc control.

I realize retrofitting a set of rails would raise the table height and require something be done to reattach the screw mounting under the now-higher table. I would guess a simple spacer block would accomplish that task, besides, ballscrews would be in the future retrofit plan anyways. I would further guess that the machine, if retrofitted with rails, would require much less motor torque and thus cheaper motors. My guess would be 35 size rails.

I'd love to hear others thoughts on this and especially any links to rail conversions done.

[sp1nm0nkey]

Seems like a cool idea. I wonder if the lack of resistance would cause backfeeding though (wouldn't be a problem with CNC)... I'm not sure why but something doesn't feel right about doing it on the Z... so I'd just do the X and Y and drive the quill. There may be no problem with it, I'm not sure.

[contractdesign]

I think you would get a lot of chatter when milling. The tables would definately slide extremely easy and smooth. However the energy created from milling would not be absorbed into the machine due to the design of the ball bearing tracks in the linear bearing assemblies. Might be a good idea on a cnc drilling machine though. These are just my thoerys based on machine design and building experience

[Sparky_NY]

Originally Posted by contractdesign I think you would get a lot of chatter when milling. The tables would definately slide extremely easy and smooth. However the energy created from milling would not be absorbed into the machine due to the design of the ball bearing tracks in the linear bearing assemblies. Might be a good idea on a cnc drilling machine though. These are just my thoerys based on machine design and building experience

I would think the opposite would be true, less chatter. I believe high end cnc mills use linear rails due to their accuracy, lack of play, low lube needs and ease of movement. "Cheaper" cnc's use conventional ways and the high end stuff often use linear ways I believe.

[contractdesign]

I don't know. I guess I base my ideas on the fact that the bearings and rail are hardened surfaces. To me the cast material of the ways and guides would absorb more energy, similar to the meanite castings for the machine frame, resulting in less vibration

[Sparky_NY]

Originally Posted by contractdesign I don't know. I guess I base my ideas on the fact that the bearings and rail are hardened surfaces. To me the cast material of the ways and guides would absorb more energy, similar to the meanite castings for the machine frame, resulting in less vibration

I have been surfing, searching and reading up on commercial cnc mills on the net tonight. It appears we are both right. Linear rails are commonly used on machining centers but box ways are also common. My reading found box ways are more common in high horsepower machines with linear rails preferred in machines under 10 hp. Chatter is the primary reason for the box rails in the high HP machines. Haas machining centers use linear rails, the reason for their high rapid speeds. Other manufacturers use linear rails in their smaller machines and box ways in their larger machines.

My intended project is a bridgeport, series 1 variable speed machine, a whole 1.5 HP so linear rails should be fine.

I am thinking something like THK HSR series, about 25mm size.

[CarbideBob]

I'm surprised that there are no replies from someone here who has tried this.

I've seen this done several times over the years. On the ones I've seen the table assembly is machined back to keep close to the original mounting heights. The resulting machine is basically unusable as a manual machine mainly due to the lack of friction because you no longer have any gib locks.

Linear way machines need high preload zero backlash screws and rigid bearing mounts to control the tables when climb milling. Attention should also be paid to the stiffness of the motor/leadscrew coupling.

High end machines use linear ways for high speed and low cost designs. Heavy roughing machines are mostly box ways which have much better damping characteristics.
Machines designed for hard metal machining are also usually box ways.
No one uses dovetail ways in a full size cnc machine design due to stiction, wear, and tracking problems.
Linear motor machines must use linear roller ways due to the high attraction forces between the magnet and coil which will "lock up" a box way design.

Linear ways are available that use rollers rather than balls. These provide much better damping than the ball type.
You also may want to consider mounting 3 bearing trucks per rail rather than the normal 2 to improve stability.
Bob

[Sparky_NY]

Originally Posted by CarbideBob I'm surprised that there are no replies from someone here who has tried this. I've seen this done several times over the years. On the ones I've seen the table assembly is machined back to keep close to the original mounting heights. The resulting machine is basically unusable as a manual machine mainly due to the lack of friction because you no longer have any gib locks. Linear way machines need high preload zero backlash screws and rigid bearing mounts to control the tables when climb milling. Attention should also be paid to the stiffness of the motor/leadscrew coupling. High end machines use linear ways for high speed and low cost designs. Heavy roughing machines are mostly box ways which have much better damping characteristics. Machines designed for hard metal machining are also usually box ways. No one uses dovetail ways in a full size cnc machine design due to stiction, wear, and tracking problems. Linear motor machines must use linear roller ways due to the high attraction forces between the magnet and coil which will "lock up" a box way design. Linear ways are available that use rollers rather than balls. These provide much better damping than the ball type. You also may want to consider mounting 3 bearing trucks per rail rather than the normal 2 to improve stability. Bob

It was a toss to post it here in the BP section or in the area on linear ways. I would love to get more info on how it was done such as how the table was cut back to lower the profile, what size ways were used and so on.

Its good to hear it has been done. My searching produced zip results which wasn't encouraging. My intent would be cnc only so the lack of manual use wouldn't be a issue. I've seen HSR type rails frequently on fleabay in lengths that would work nicely on a BP, fairly cheap too.

Hopefully someone will chime in with info/details/links or even pictures (dreaming now) of a retrofit that was done. How the finished product performed would be interesting as well.

[LeeWay]

My experience with my little mill build may just help in this case. At least to support what has been suggested in this thread.
Here is a link to the build.
http://www.cnczone.com/forums/showthread.php?t=32609

At any rate, I did use THK HSR 25's on the X table. About 24" long. I used THK HRW 27's for the Y. I am doing an upgrade and redisign of the table now, but I did have it running with some smaller 269 oz steppers on a 24 VDC PS and 3 Gecko 203V's. Also fitted on it is NSK 15 mm 10 mm pitch ground ball screws. Standard single ball nuts. These are used, so I don't know if they have the alternating ball configuration to preload them or not, but I can see no backlash. I will check it eventually.

Now what I noticed on these undersized motors running with this lower voltage is when I went to mill an edge of a piece of steel by simply jogging the X, the Y initially backed off the cut to about half of what I had it set to cut. It continued to cut that though. I was using a 3/4" end mill. Normally I would claim backlash! This just isn't the case here and it is back driving instead. The crews do have a rather high helix. I was wondering about the back driving possibility at the start of the build. I decide to throw bigger PS, nearly max for the Gecko's and larger steppers at the problem. I figured I needed those if I planned to mill much steel anyway. I hope to start installing them soon. If these don't overcome this issue then I will be forced to use timing gears and switch from direct drive. Not all that much work, but you see that back driving can be an issue even with some stepper motors attached. Not sure what the screw pitch is on a BP, but the smaller the pitch, the better it would hold up against BDing.

[Sparky_NY]

Originally Posted by LeeWay My experience with my little mill build may just help in this case. At least to support what has been suggested in this thread. Here is a link to the build. http://www.cnczone.com/forums/showthread.php?t=32609

Thanks much, a very interesting thread on your build! The problem you are having is a important consideration. I believe your problem is ultimately the force of the cutter is pushing the table, back-turning the ballscrews and overcoming the holding torque of the stepper motors resulting in movement. Its a combination of extremely free table movement because of the linear ways, a very coarse ballscrew pitch, and finally stepper holding torque. In your case, the coarse pitch screw is the big offender. A finer pitch screw would resist being backdriven much more and also multiply the torque of the stepper motor more.
The most common screw pitch is .200 or put another way, 5 turns per inch travel. That is what I used on my x/y table (cheap rolled screws). I also used 270oz steppers at 24v. I have no complaints of stepper torque or table being pushed BUT it does have dovetail ways which do not travel nearly as free as your linear ways.

Presently, I am considering between cnc'ing the bridgeport table with linear ways and ballscrews or doing a rebuild of my x/y table with linear ways. I just make small parts so either would suffice as far as travel is concerned. The cost to retro the bridgeport table would be much more naturally but the end result being much better. Decisions Decisions.....

So far, the info from this thread is giving me a lot of meat and potatoes to consider.

[davo727]

Hi, Riser blocks are available to raise the head of the mill, if losing Z clearance is an issue. Sounds like some good info posted on this thread already. If it were me I think I would try to just mill the dovetail off the bottom of the table to clear the rails and blocks and just barely surface the origional table ways to get them flat and bolt the rails to them, keeping as much of the table mass as possible. Do you have any connections that have a mill or surface grinder with 42 inches of travel to work your table over? Sounds like a cool project, post pics please if you get going on it Dave

[Sparky_NY]

Originally Posted by davo727 Hi, Riser blocks are available to raise the head of the mill, if losing Z clearance is an issue. Sounds like some good info posted on this thread already. If it were me I think I would try to just mill the dovetail off the bottom of the table to clear the rails and blocks and just barely surface the origional table ways to get them flat and bolt the rails to them, keeping as much of the table mass as possible. Do you have any connections that have a mill or surface grinder with 42 inches of travel to work your table over? Sounds like a cool project, post pics please if you get going on it Dave

Rails and their blocks are pretty low profile so the loss of Z travel would only be something like 3-4 inches which I wouldn't mind at all. As for the physical mounting of rails/blocks, I would guess the table could be milled as you say. Accurately milled on a good machine, I think grinding could be skipped.

Personally, I don't need the 42 inch size table my hobby BP has. If (BIG IF!) I could find a 36 inch table from a junk machine that would be easier and cheaper to retrofit and take up less room in the garage. Due to rail installation, no need for a table matched to the machine.

Still hoping someone shows up with a completed project link to something like this.