Capabilities: Is a 1 hp PM mill enough for 18-8 stainless steel? Or 2 hp?

[Larry$]

David's discussion of mills is excellent. I've never used either type he discusses, I'm a hobbyist. I do think you should not translate the information about the Knuckle, tilt & nod, ram movements to a full sized knee mill. At least in my experience those connections are quite ridged. But my mill weighs about twice what the baby knee mill does.

When it comes to tramming the head, tilt is relatively easy but nod was a PIA for me. Both directions are driven be a gear system. Slack the locking bolts, use a wrench to rotate the worm screw and tighten, Simple, in theory but when you tighten the locking bolts things move. Especially the nod. It is very easy to move the ram & lock. Rotating the upper assembly so you can reach over the back edge of the table to work on the end of a long part that has been clamped to the side of the table is also easy but limited.

The back of the ram has mountings for accessories that I have no experience with. Just rotate the upper works and put that end over the table. Rotation is manual. At least in theory by using the rotation you could extend the maximum length of part you could surface. Same goes for moving the ram in/out. I don't know how much that would affect the over all accuracy.

The size of the table, used in the descriptions, 833, 8" X 33" or in my case 9 X 49 is not the usable machining area. The available X movement is considerably less. The knee & table add up to a lot of weight. so moving them up/down takes a lot of turns of the crank making a power Z very appealing. Ideally I suppose locking the knee when machining should be done. But any slack in the gibs will slightly change the angle of the knee when locked. I think that in practice the knee & table are so heavy that it would take pretty aggressive cuts to make them move. Take that into consideration before tramming.

Ceiling height. My mill has the Reeves variable speed belt drive. If you need to replace the belt it takes space above the motor! Rare but!
Moving a 2000+# machine. Once on the concrete using pipes won't work on my machine due to cutouts in the bottom. Rental machine skates & toe jacks are available. My machine has a lifting eye & I have access to a 5000# forklift & experience. The head can be rotated to get under garage door openings. The base is narrow and the machine is tall, so stability is lacking when moving it by the base. Be careful!

 

[davidpbest]

David's discussion of mills is excellent. I've never used either type he discusses, I'm a hobbyist. I do think you should not translate the information about the Knuckle, tilt & nod, ram movements to a full sized knee mill. At least in my experience those connections are quite ridged. But my mill weighs about twice what the baby knee mill does.

Based on my use and inspection of various knee mills, the rigidity of the machine is only partially related to weight. Specifically, the robustness of the knuckle and ram castings, and the bolts/mechanisms that secure the head, knuckle, ram, and turret together are all factors that vary from one machine brand to another - even among the 2500-3000 pound mills. I know for instance that @Larry$ has a Jet 9x42 which has a ram and knuckle casting that is far beefier than many of the lower cost Bridgeport clones. His machine also has a Reeves variable-speed drive that adds considerable mass and thus rigidity to the J-head. Some of the most stout and rigid mills I've seen in this category are the Acra LCM-42 and LCM-50, the Sharp mills, and the Index 847 mills. I'm sure there are others.

The size of the table, used in the descriptions, 833, 8" X 33" or in my case 9 X 49 is not the usable machining area. The available X movement is considerably less. The knee & table add up to a lot of weight. so moving them up/down takes a lot of turns of the crank making a power Z very appealing. Ideally I suppose locking the knee when machining should be done. But any slack in the gibs will slightly change the angle of the knee when locked. I think that in practice the knee & table are so heavy that it would take pretty aggressive cuts to make them move. Take that into consideration before tramming.

Excellent observations, and table travel is often further restricted by power feeder attachments. For instance, my PM-935 has a 9 X 35" XY table, but the X-axis travel is only 22" with the power feeder installed, which is about the same as the 833. The maximum spindle nose to XY table dimension on the 935 is 17" with the knee fully lowered, and there is no riser ring available to extend that range like there is on many of the larger knee mills. In contrast, the same maximum Z-height dimension on the 833 is 3" greater, or 20.5".

 

[Chewy]

Blondihacks just posted a video today on the PM 728 which is one size smaller then you are looking at. She was able to go .075 in 1018 with a 1/2" endmill. I have noticed that the quill DRO sucks just like on the other models. Mine is all over the place +- .0015, sometimes .002. Not very accurate. Also My DRO ( 5years old ) is light years brighter.

 

[davidpbest]

Blondihacks just posted a video today on the PM 728 which is one size smaller then you are looking at. She was able to go .075 in 1018 with a 1/2" endmill. I have noticed that the quill DRO sucks just like on the other models. Mine is all over the place +- .0015, sometimes .002. Not very accurate. Also My DRO ( 5years old ) is light years brighter.

I watch Blondihacks 728 test video earlier. The test she put that mill through was certainly worst case by doing a full-width slotting operation. My guess is she could have gotten at least to 0.100" depth of cut with only 3/4 of the end mill engaged - like you would normally do when squaring up raw stock.

 

[Larry$]

Stefan Gotteswinter did a video on tramming a square column mill. Worth watching if you need to do one.
I also watched Blonihack's new video. I avoid slotting if possible. Recutting chips causes too many problems. If you have to slot mill use an air stream to clear chips as much as possible. Unless you are in a race is it really worth maxing out the machine's capabilities?

 

[Gladiataar]

If you're considering a knee mill and are pressed for space, you might want to look for a Burke Millrite. They're about 2/3 the size of the Bridgeport and very capable for their size. They seem to be popular with hobbyists because of their compact size and number of features.

As for moving machinery a little planning will go a long way. I've moved dozens of machines by myself using a drop deck trailer and some 1" rod as rollers. Drop deck trailers lower the deck to ground level for loading and unloading. To move the machine, you can lift one end with a prybar or "Johnson bar" just enough to slide a 1" diameter rod under it something like this.

Here's a link to some information and pictures on the Lathes UK website:

Page Title

That Millrite looks like one solid piece of cast iron. If one comes up on the used market around me, I'll definitely give it some consideration. 2/3 BP may just be workable in my limited garage space too.

It's funny you posted that picture of a drop deck trailer. I didn't even know those things existed until about 3 weeks ago when I was watching a video of a guy deliver a BP all by himself using one.

 

[Gladiataar]

I encourage you to read the attached. Hope this helps.

I can't thank you enough for these two write-ups! Not only did they explain a lot about mills in general, but one of them even compared some of the EXACT PM models I've been considering.

 

[Gladiataar]

@Gladiataar comments agree with my own sentiments. The Bridgeport-type mills offer tremendous flexibility (and powered quill downfeed for boring), but at a sacrifice in rigidity. They are also easier to tram than a square column benchtop mill, but realistically, once the benchtop column is trammed, it doesn't go out of alignment with heavy milling like the nod on the Bridgeport.

As far as stainless steel goes, the stuff is gummy but not excessively hard compared to something like 1018 steel. I machine 304 and 316 all the time, have done some very precise work in the stuff with both my RF-45 benchtop and my PM-935 baby Bridgeport, but you have to know what you're doing. Quality of tooling, precise and consistent feed rates, rigidity, and some form of chip evacuation are critical aspects of machining stainless on any type of machine. Additional HP is not the key to SS milling, technique is. You can see some tests I did on 304 here, and some examples of what my benchtop RF-45 was capable of here in 304 (read the descriptions that go with the still images). You might also have a look at some deep face milling operations in both aluminum and 304 using the PM-935 and Hass tooling at this link (watch the videos toward the end). Those videos will give you some idea of removal rate limitations in 304.

These videos are exactly what I was hoping to see at some point-- milling 304 stainless on a moderately sized bench mill. I'm quite happy with what I saw regarding it's capabilities. The size of SS material block I'd be starting with would be nowhere near that thickness. Most of the SS I've hacked up in the past using my primitive tools has been 1/4" thick or less, so I'm seeing now that, with multiple smaller passes and use of roughing mills (forgot about those!) , a PM-833 will do what I need it to do.

 

[ErichKeane]

I second the Millrite. I had one for years, it is a very capable machine! I'd suggest making sure you get an R8 one (about 1/3 of them are B&S#9spindles), but otherwise they are a big step up from any benchtop ones and quite a managable size.

 

[Gladiataar]

Thank you again everyone for all the super helpful info and support to the new guy. So much to read and so many videos to watch!

Here's where I'm at:

As far as PM bench mills go, the PM-833 is looking more and more like good fit for me. After seeing and hearing all of your feedback regarding working with 304 or 18-8 stainless, I think it would be best to stay in the 2hp camp no matter what style I end up choosing, and take the 1hp PM-728VT out of the running. Besides, the PM-833 seems to have just so much more to offer at a price increase that's within my budget. That being said, some of the drawbacks/limitations of this machine that I can see are:

1) Lack of a power down feed on the quill. I'd definitely opt for the power Z lift at time of order, but from the literature you've been providing, not having that quill down feed essentially takes boring operations off the table, at least not without some serious modification.

2) Lack of a power Y feed option. Not having a proper/steady feed rate could introduce difficulties when dealing with harder materials beyond aluminum. Not even having this as a future add-on option seems like a serious limitation to me.

3) I noticed in one of the videos that the y-direction table dimension on the PM-833 is 8-1/4 inches, compared to a common knee mill's 9+ inches. Correct me if I'm wrong, but I think this smaller dimension wouldn't allow me to mount a 6" vise horizontally, running the long way in x-direction. Is this a major disadvantage?

4) Nod adjustment will be a bit more challenging. I was hoping this to be a one-time set-and-forget thing, but I'll need to get my shims ready.


Regarding the knee mill option, it seems like just about everyone either ends up upgrading to one anyway, or highly suggests just biting the bullet right from the start and passing on the smaller machines. And to be honest, I can see why. Bridgeports and their clones have so much flexibility in what they can do, how easy it is to find replacement parts for them, and how well they hold their value. As a lowly woodworker and hobbyist though, I'm still not fully convinced that they're a perfect fit for everyone. As far as overall cubic volume and weight, many bench mills sit at about half their counterparts in both categories. For lighterm users and space-challenged individuals like myself, these are legitimate concerns when deciding on how much machine is really needed and how much machine we want hanging around our shop. What's a MAJOR bummer though is that these smaller, less capable bench mills are not half the price of the better used machines (i.e. BPs & clones)! From my shopping research, dollar for dollar, a used BP clone is equal to or less than a new bench mill. I think it's worth pointing out though, there is one key word difference here -- "used". As a newbie, knowing very little about metalworking, having the ability to differentiate between what's a "good" or "bad" machine in the world of used equipment is a serious gamble. Having no clue what I'm doing, then plopping down $6k on a piece of used equipment with absolutely no warranty or recourse of any kind, can be a scary proposition. If you have someone experienced enough to go shopping around with, that's great, but not everyone has access to that resource. Just sayin', there may be a method to all that "rookie madness".

It'll be fun looking back at this post 10 years from now. I'll probably cringe and smack myself in the head!