New 12 x 37, Working on Technique. Have Surface Finish Question

[Steve M]

Semi new machinist, picked up a new Smithy 12x37 (usual 12x clone) and have been seeing what I could do with it. Had a piece of 3/4" steel, probably CRS that I was using to see how fine a finish I could get with sharp carbide and HSS, both 1/2". I basically was making cuts and checking the finish at various depths and speeds. When I went for fine cuts I noticed a "banding" effect. These bands were spaced about 1/8" apart circumstantially around the part, about 1/16" wide and darker than the areas next to them, basically like stripes.

I was using 700 RPM for the HSS and up to 1250 for the carbide at the slowest longitudinal feed rate into the chuck. To get the fine cuts I set the compound at 5 degrees and advanced it rather than the cross slide. My math told me doing it that way gave me 0.0000174" per .001" of compound advance. I tried the same thing on my 6 x 19 3 in 1 with the same bits and approx the same speeds and feeds and didn't see them.

I made sure the gears were well meshed, tightened the drive belts, tightened the drag on the cross slide and saddle, and tried again using the tailstock. Helped a little. Funny thing is that the banding doesn't seem to be speed sensitive but I need to recheck that.

Using the fingernail roughness check, i couldn't pick up the bands nor could I with a sharp scribe.

Using the technique above, I seemed to be able to take off 0.0002 without problem so I am happy about that. I am posting a picture below. Best I could do, hope you can see - you may have to download the pic and blow it up n a new tab. There are three turned areas. The one in the center is where the banding is. the others were turned at deeper cuts for comparison.

Can anyone give me any clues as to what is going on? A friend suggested stuck chips on the cutter point but these are very regular. Thanks for any help.

Steve in Central TX

 

[george wilson]

Something in your power feed train is off. The lead screw could be a little bent,for example,causing an echo to regularly appear on your work. Run the lead screw with no half nuts engaged,and see if it is bent. Sometimes people get lead screws bent by improperly putting a strap around the lathe,including the lead screw,when they are lifting the machine. The lead screw should never be squeezed by the strap when the machine is lifted.

The lead screw,if bent,can cause the carriage to lift a little with each revolution. Check the carriage where it meets the bed as it runs along,to see if there is a little lift in a spot as it moves.

Also check the carriage to make sure the gibs have been properly adjusted to hold the carriage DOWN as well as SIDEWAYS(in and out,if you have a FLAT bed with no V ways) against the bed. Disengage the lead screw and adjust the gibs so that the carriage moves smoothly,but without any slop. With those old fashioned multiple screws that Smiths probably use,it can be tricky to figure out which screw you have tightened too much. I have them on my 16" lathe's carriage too,though the crossfeed and compound have tapered gibs with a single screw at the end.

Beyond the leadscrew,look at the gear train to see if all the gears are running true,and if all the teeth on the gears look normal.

Many things can be a little off on a Smithy type machine.

P.S.: I just noticed you "WELL MESHED" your gears. How? They are not supposed to run snugged. Take a piece of THICK paper. Place it between your gears. pull the gears together WITH the paper between them. Tighten down the quadrant and leave the gears to run a bit loosely.

 

[SteveM]

Wrong Steve M:think1:

 

[Steve M]

Something in your power feed train is off. The lead screw could be a little bent,for example,causing an echo to regularly appear on your work. Run the lead screw with no half nuts engaged,and see if it is bent. Sometimes people get lead screws bent by improperly putting a strap around the lathe,including the lead screw,when they are lifting the machine. The lead screw should never be squeezed by the strap when the machine is lifted.

The lead screw,if bent,can cause the carriage to lift a little with each revolution. Check the carriage where it meets the bed as it runs along,to see if there is a little lift in a spot as it moves.

Also check the carriage to make sure the gibs have been properly adjusted to hold the carriage DOWN as well as SIDEWAYS(in and out,if you have a FLAT bed with no V ways) against the bed. Disengage the lead screw and adjust the gibs so that the carriage moves smoothly,but without any slop. With those old fashioned multiple screws that Smiths probably use,it can be tricky to figure out which screw you have tightened too much. I have them on my 16" lathe's carriage too,though the crossfeed and compound have tapered gibs with a single screw at the end.

Beyond the leadscrew,look at the gear train to see if all the gears are running true,and if all the teeth on the gears look normal.

Many things can be a little off on a Smithy type machine.

P.S.: I just noticed you "WELL MESHED" your gears. How? They are not supposed to run snugged. Take a piece of THICK paper. Place it between your gears. pull the gears together WITH the paper between them. Tighten down the quadrant and leave the gears to run a bit loosely.

Thanks. I'll check the gears. By well meshed I meant no noticeable backlash but will run the paper check and report back. I had the gears out a couple days ago and didn't see any noticeable irregularities. The machine has tapered gibs and I did adjust them. It also has a lead screw and a slotted driveshaft that turns gears in the saddle for the longitudinal and cross feeds. I'll try making some cuts with the leadscrew instead and see if the issue is there and with manual feed as well. The feed rates are a little slower with the driveshaft and the manual recommends that for turning, and the leadscrew for threading.

Since the rings seem to be harmonic, it may well be in one of the shafts though. I used a strap to lift the machine when I unpacked it but I was careful to run it behind the shafts and now over then so I was lifting off the casting. Didn't look like the straps were putting pressure on the shafts but can't rule that out.

Off to the shop to do the checks you've recommended. Thanks again.

Steve

 

[george wilson]

Be sure to use THICK paper. Maybe fold typing paper 4 times and put it between the gears.

The lathe could have been lifted wrong at the factory.

 

[Steve M]

Just came back to the house after checking the 12x over. Gear backlash looked good using thick paper as a gage. I tightened up the tapered gibs on the slides, set the drag a little tighter on the longitudinal ways in the slide, and took some more cuts. This time I took cuts with the leadscrew, the power longitudinal feed, and manually. With the power feed, the issue was still there, it lessened a little manually when the feed was locked to the driveshaft, and with the leadscrew it disappeared. I didn't try it manually locked to the feedscrew but I suspect the results would have been nearly as good with the leadscrew drive off. I didn't notice any bent spots in the power feed shaft or the leadscrew. The power drive actually has a little slop where it rotates in the saddle so it would take a pretty good bump to upset the cutter point. I am wondering if excessive backlash in the power feed gears would be the cause of the problem. I am thinking my next step should be to try the power crossfeed and see what the finish looks like. That might tell me if the problem was in both power feeds or just the longitudinal drive.

Can't remember if I mentioned it but I noted the same thing on an older Enco 12x I had. Maybe this is a characteristic of that family of machines.

For the record, the leadscrew feed rate is double that of the power feed shaft. 0.0089"/rev vs 0.0047"/rev. My 3 in 1 is leadscrew only and that explains why I was getting a better finish on it. Guess the workaround is do finish turning using the leadscrew and hogging with the power feed.

Steve

 

[azgoldust]

Just came back to the house after checking the 12x over. Gear backlash looked good using thick paper as a gage. I tightened up the tapered gibs on the slides, set the drag a little tighter on the longitudinal ways in the slide, and took some more cuts. This time I took cuts with the leadscrew, the power longitudinal feed, and manually. With the power feed, the issue was still there, it lessened a little manually when the feed was locked to the driveshaft, and with the leadscrew it disappeared. I didn't try it manually locked to the feedscrew but I suspect the results would have been nearly as good with the leadscrew drive off. I didn't notice any bent spots in the power feed shaft or the leadscrew. The power drive actually has a little slop where it rotates in the saddle so it would take a pretty good bump to upset the cutter point. I am wondering if excessive backlash in the power feed gears would be the cause of the problem. I am thinking my next step should be to try the power crossfeed and see what the finish looks like. That might tell me if the problem was in both power feeds or just the longitudinal drive.

Can't remember if I mentioned it but I noted the same thing on an older Enco 12x I had. Maybe this is a characteristic of that family of machines.

For the record, the leadscrew feed rate is double that of the power feed shaft. 0.0089"/rev vs 0.0047"/rev. My 3 in 1 is leadscrew only and that explains why I was getting a better finish on it. Guess the workaround is do finish turning using the leadscrew and hogging with the power feed.

Steve

I would check the drive train to the leadscrew to see if any of the gears are eccentric,
happened to me on my Shop Fox.

 

[Steve M]

I would check the drive train to the leadscrew to see if any of the gears are eccentric,
happened to me on my Shop Fox.

Too early to be out in the shop now but I am thinking even more that the problem lies in the power longitudinal carriage feed gearing. I'll try to post a pic. The 12x machines two lengthwise power feeds and three shafts in front below the ways. Top is the leadscrew, middle is the power drive shaft for cross and longitudinal feed that I am talking about, and the bottom works the forward reverse switch for the motor. As I previously mentioned, using the leadscrew instead brings good results. It occurred to me last night that the feed rack under the front way has close to the same number of teeth per inch as the "bands" on the workpiece. That seems to point to an eccentric gear.

Steve

- - - Updated - - -

Be sure to use THICK paper. Maybe fold typing paper 4 times and put it between the gears.

The lathe could have been lifted wrong at the factory.

Thanks again, George. Good advice, hadn't heard that before.

 

[michiganbuck]

73597[/ATTACH] Steve in Central TX

What was the part length and how was it held? Chuck and center? Tool bit held as short as possible? What does Smithy say about the condition?
Do you get anything like that with turning a larger diameter part? Perhaps 2 inch diameter?
Do you feel vibration in the machine..And does it change when this condition occurs?
I was looking at the 1237 and 1340 and expect to drive to their shop and have a look soon.
Do post more information good or bad.
How do the Smithy chucks run for run out and wobble??

Would like to here a report on the 1340 from some one having that machine.

 

[Steve M]

What was the part length and how was it held? Chuck and center? Tool bit held as short as possible? What does Smithy say about the condition?
Do you get anything like that with turning a larger diameter part? Perhaps 2 inch diameter?
Do you feel vibration in the machine..And does it change when this condition occurs?
I was looking at the 1237 and 1340 and expect to drive to their shop and have a look soon.
Do post more information good or bad.
How do the Smithy chucks run for run out and wobble??

Would like to here a report on the 1340 from some one having that machine.

The part was a CRS shaft that started out at 1" with multiple cuts down to 0.5" or so held in a thre jaw chuck and tailstock. May have had the tool bit extended too far. Don't have any 2" stock so all I have done was the 1" steel. Did do 1.5" brass and didn't see the bands. I can feel a little vibration in the machine when it runs but mainly just low Freq stuff. I did tighten down the drive belts and that actually helps. I have read that those linked belts work well so I may pick up some tomorrow. The vibration was random and probably related to the loose belts. I had the belt tensioning bolt set up wrong and corrected it to be able to get the belts a little tighter. At some point I want to pull the motor pully off, The cast, unmachined rim of the pulley doesn't seem very concentric to the bore but I haven't checked the grooves. I've checked the chuck for runout and it was under 0.001 but haven't checked for wobble.

There is another 1236 thread here and the OP reported exactly the same issue. Yet another found an out of round drive pulley. Both did say the linked belts made a big difference. I don't think the 12x37 is quite up to the finish standards ad Smithy's 3 in 1s, Seems pretty much like all the other 12x machines I have looked at though. Rough castings, burrs here and there, and so on. Not much backlash, however and the controls all work smoothly. My only real beef so far is the difficulty I see ahead in using a spider on the spindle bore away from the chuck. If you google 12x36 lathe issues you will see that what I have going on isn't uncommon for this class. Haven't talked to Smithy yet, though.

All I have been messing with had been CRS. I am going to pick up some drill rod and see how it works out.

I will say I was very impressed with Smithy's service.

Steve