Bridgeport Mill as "main" machine

iusedtoberich

iusedtoberich

AzB Silver Member
Silver Member
I'm buying a Bridgeport mill hopefully next week with the intention of converting it to a CNC. I was planning to use it to build dedicated cue building equipment, but the more I think about, I might be better off using IT to build cues. A 9x42 table size model has exactly 30 inches of table travel, just enough to machine a cue. If I find a 9x49 model I will have about 36 inches of travel. There will also be plenty of room on the ends of the table for the cue holding fixtures. I could mount a saw blade to the spindle and use it for turning shafts and butts. I could mount small diameter end mills and use it for inlay work. The only downside I see is the relatively slow spindle speed for doing inlay work with small diameter tooling. I could mount a high speed spindle to the quill, however, if the cuts weren't satisfactory.

Does anyone see any possible problems with this approach? I realize it is best to have dedicated machines for each operation, but this way I can get started with a minimal investment. Plus I figure in the start I will have much more time than customers, negating the time advantage of dedicated machinery. I already have a 12x36 metal lathe and a full woodworking shop.
 
I am not a cuemaker, so take this with a grain of salt.

I haven't priced the equipment out, but it seems like you could easily spend more retrofitting your Bridgeport than a CNC inlay machine would cost.

As far as turning cues on the mill, why not use the lathe? By the time you mount a 4th axis to the mill table, your x axis travel will be reduced quite a bit I think, so the 49" table would probably be necessary for this approach.

Even so, I am a big Bridgeport fan, and I think no shop should be without one, even if it is just used to build tooling and drill holes.

-CM
 
Thanks for the response. I estimate the cnc conversion will cost 115 per axis for the drivers, stepper motors will be free (friend is giving them to me), power supply will be free (I have many transformers and electronic components lying around), cheap pc + monitor is 200, linux is free, software is free (EMC). So I'm looking at about 700 in my case. If I want ballscrews, it will be a few hundred more, but I will try it first without them.

There should be enough travel with a 42 inch table, but of course I would prefer a 49 or even 54 inch table if I can find one local to me. But even with the 42 inch table with 30 inches of travel, I will have 6 inches on each end, which is plenty of room for the 4th axis and the centers to hold the cues. I would make several "tables" that would index in the T-slots of the mil's table. One table for each of the major operations...turning, inlaying, point cutting, etc. That way, I can change jobs just by removing the entire table and not removing the cue.

I could turn the cues on my lathe, but I would have to do it between centers, as there is not enough room when the chuck is in place. And I would have to make a taper bar for the lathe or put a cnc control on it. I also would need a separate motor (router) for the turning, where on the mill I can mount a saw blade in its spindle. So I would prefer to do it on the mill, as that would also work for inlaying and point cutting, which the lathe would not as easily.

Finally, like you said, every shop should have a mill. You can make nearly anything with just a lathe and a mill. I plan to do quite a bit of metal work also, so the mill will always be useful to me.
 
I would guess you will have a backlash problem without ballscrews, unless you program to back up and come back in from the same direction on each cut. The older CNC Bridgeports (Bandits) didn't really have smooth 2-axis motion, they would move .001 in x and then .001 in y.

It is a good idea to build fixtures to go on the table for each operation, that way you can hang tooling outside the machining envelope. If you want to save time dialing in each fixture, you can drill the table (!!! I know...) and pin the fixtures. That way they will drop in the same way every time, and you don't have to indicate them in, just pick up your zeros and go.

-CM
 
Conventional leadscrews in CNC Bridgeport?

Hi Nick,
I didn't want to keep hijacking Willee's thread, so I am taking this back here. You stated in your reply to Blud that
iusedtoberich said:
"As far as my bridgeport mill question, I used a standard Bridgeport knee mill that was factory converted to a cnc mill (it was ordered new that way from bridgeport). This mill did not have ballscrews, it had conventional leadscrews. "
Click to expand...
Are you sure? I have never seen a factory CNC Bridgeport, even the old Bandits, that did not have ballscrews.

When I think about the backlash in a conventional acme leadscrew, even on a new machine, I don't see how it can be CNC capable. We're talking about .005" to .010", even when new. I've run older machines where this was more like .030" or .040". Thats why you have to back off and come back up on your position when you are cranking the handle by hand, whether you're using the dial or a mag-base indicator on the ways, it doesn't matter. All conventional leadscrews have to be operated this way to be accurate. This is easy enough to check, just wiggle each crank until you feel the screw start to turn. Look at the dial, and see how much backlash you have.

The other thing you will probably need to do is get rid of the "fishtail" in the table. Put an indicator on each side of the knee (on the Y axis ways), and grab the end of the table and give it a shake. Watch the indicators. To fix this, you'll have to scrape the ways and put in new gibs. No big deal, but if you don't, your repeatability with CNC control will suffer, because the table will be moving around (in Y axis) during x axis travel. It's not as important when you are cranking the handle by hand, because you are watching your indicators and you can bump the crank as you go. Older mills can also float up and down in Z during x axis travel, not uncommon at all, but locking the knee will help minimize this.

I assume your CNC conversion will use encoders on the feed motors. They will not be able to compensate for backlash (unless you can write it into the PLC program) and fishtail, since they just count revs. If it will be controlled by glass scales, the backlash will be better accounted for, but fishtail will not. But fishtail is easier to fix than backlash, because backlash can only be eliminated with ballscrews.

Not trying to talk you out of your plan! Like I said before, all shops need a Bridgeport, regardless. Just trying to give you some food for thought.

-CM
 
CM,

I no longer work at this place, but I distinctly remember looking under the table and seeing square or acme thread lead screws. The backlash was minimal (for conventional screws). It was about .003 inches if memory serves me correctly (this was in 2001). This was measured just by looking at the handwheels and feeling the change in direction. The mills were the regular series one and were new in about 1997. I just checked Hardinge's site (they now own the bridgeport name) and all their series ones use ballscrews. Maybe it was an option in the past, or maybe I'm wrong about the mills I used having conventional screws. I just sent an email to a guy that still works there to see if he can confirm what screws it has.

All the cnc controller software has backlash compensation. So when changing directions, it will turn the screw the amount of the backlash + the desired increment. And as long as I conventional mill instead of climb cut, I don't think there will be a big problem with the backlash (this is not based on experience, just an educated guess).

I've never worked "under the hood" of a mill, but I know a few people at work who have and have converted them to cnc models. I will have some good help available to me. You guys might want to check out CNC Zone as well. Look under the DIY wood routers and you will see guys build nice cnc machines ranging from MDF framed models to nice and heavy steal framed and almost industrial looking models.

Thanks for the heads up on the table slop. I don't know anyone who has rescraped the ways so I might be asking you for some guidance if I need to go that route. I will be going to a local tool place to get the mill this week (hopefully!). I was planning on bringing a last word indicator with me to check the spindle runout and also placing it against the quill and then shaking the quill to see if it moves. I figure I can take up as much backlash as I can by adjusting the spacing between the nuts (they use 2 nuts like in a lathe, right). And I was planning to just try to rock the table from corner to corner to try to pick the tightest machine available. Of course, I will have to settle with whatever he has, which might be the best of only a few mills he might have in stock.

Do you have any other suggestions as to what I should check for when buying a used mill (that I can check while at the tool supplier)?

Thanks,

Nick
 
Check the table, knee, quill and spindle bearings for movement. Feel up inside the spindle for burrs or wear where the collet goes in. Take a flashlight and mirror with you and do a visual inspection of the inside of the spindle. Make sure the drawbar threads are good. Listen to the motor and spindle at highest RPM. If it's a belt drive head, check the belts and pulleys for wear. Check that all the oilers are working, and that the ways are wet.

Take an indicol with you and loosen the head, crank it all the way over in every direction, and then sweep it back in. Loosen the ram and move it in and out, and spin it around to make sure it moves smoothly. Drop the knee and extend the quill all the way and see how square it stays. With the quill all the way extended, wiggle it with your indicator on the table and see how much slop there is.

Run the indicator down the table top full length of the travel and check for z axis variation. When you get to the end, make a Y axis move and travel back to the starting point. Look at the ways and quill for wear and galling. Of course, check the backlash in the screws and make sure they travel smoothly in all directions without sticky spots. Verify that the quill feed works, the stop works, and the spindle brake works. Make sure all the locks work, and the x-axis power feed and rapid button, if equipped. If it has a DRO, clamp something to the table to dial in and move off and back to check for positional repeatability.

When you are done with all this, you can tell the sales guy it's a piece of junk and offer him $500 less than he is asking if he'll throw in a clamping kit, an anglelock vice, and a collet set. He may take it just to get you out of the building. :) :)

-CM
 
Well, I went and saw the mills today. They had a nice selection of about 20 mills. Most were Bridgeports and a few were imports. They all had 42 inch tables. Most were in nice condition, but two of the brideports were emmaculate. No dings at all in the table. The ways looked brand new, they still had all of their hand scraping marks. I couldn't get the tables to "fishtail" when pushing them, and the backlash only seemed to be about .004.

Unfortuanntely, the "owner" of the place was not in, so I will have to call him later this week to see what he wants for the good ones. His "helper" said he normally gets between 1500 and 3000, depending on condion. I would definietly pay 3000 for that one, as it had a DRO, powerfeed, and vari-speed head.

I'll update later on...
 
Joseph Cues said:
That's great news Nick.
I hope you get a great one for a great price.
Click to expand...
Or better yet, at that price, get three of them, i'll let Joey pic his first... :p

Thanks,

Jon
 
Update

Hey guys. I finally got the mill:) Closed the deal last week. They are going to deliver it this week sometime. I got the Bridgeport and not a copy. It is a nice tight machine. Relatively new for a Bridgeport, the serial number places it as a 1981 machine. It is a 1.5HP, vari-speed head, 9x42 table with powerfeed. Everything works very well, and seems tight. It does have one problem though; The head makes a thump sound. I was going to swap heads with another machine because of that. They powered up about 5 more heads for me and they all made the same noise. So I suspect it is a common problem, probably the belt. When they deliver it, I will have to take a look at it. I also bought the service manual, so I should be ok. Now I just have to clear out the shop for it;). It's going to be a tight fit. I'll post some pictures when they deliver it.

Nick
 
Bridgeport machines

> It seems to me that using a Bridgeport mill as an inlay machine would be rather limited,without some kind of serious modification to be able to cut on an angle,unless you just want to cut rectangular windows and box type points,or plan on using a swivel vise or clamping fixture for this. The ones I use at school have all kinds of backlash,forcing you to go back the same way you came,in an effort to keep all the slack going the same way,and vibrate enough to possibly cause problems milling delicate pockets for inlays. If you can indeed do this,and make it work well enough to make a decent product,by all means contact me with pics and part numbers,as I have access to a CHEAP local Akro-Mill. Tommy D.
 
Thanks Jon. I will have a lot of work to do once it gets here. I will have to run electrical service for it and reorgainize the shop so I can move around the machine. I have masking tape on the floor now to try to find its optimal position. It takes up a lot of space when you account for its table travel.

Tommy-D, for cue work, I would convert it to a cnc machine. That way it could cut any shape I program. For now, it will just be a general shop machine. Nearly anything can be made with a lathe and a mill.

Hmmm... I just thought of something else. Rather than convert the mill to cnc, I could add an auxiliary XY table on top of the mill's table. This auxiliary table would be cnc controlled and would be just for making cues. This should save me a lot of money because I could use smaller ballscrews and servo/stepper motors than if I were converting the full mill for metal work. It would probably be more precise as well because the auxiliary table would use linear rails instead of the mill's dovetail way system. And this way I could still use the mill's spindle to attach the saw blade onto, which is probably more rigid than anything I could easily make.

The only downside to this is that I won't have cnc capability for metal work, which is something I would like to have.

Anyway, this is going to be a long term project. I probably won't even start it until the middle of september, as I will be quite busy until then.

Nick
 
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