Hello, I’m new to CNC machining and would love to get advice from those who are more experienced. I built a 4x8 CNC router using the openbuilds platform, and it is not as rigid as I would like. I would like to purchase linear rails for it as an upgrade. Looks like it would cost around $750. I am quite interested in the possibility of buying a Grizzly G704 (7" x 27" 1 HP Mill/Drill with Stand at Grizzly.com), converting it to CNC, and using it to mill my own linear rails. It will cost me more money in the short term but give my shop more capabilities in the long run. I know that I may not be able to achieve the same accuracy with the grizzly as some precision milled rails, but would the results be acceptable? TLDR: is the Grizzly mill precise enough to mill my own rails with acceptable results? Thanks!
Technically possible, but I think you'd be disappointed. Rails are ground, not milled, both for tolerances and surface finish. You also really don't want to use it in sections if you can avoid it, and the G0704 can't cut long enough for 4ft and 8ft sections. It'll also take about five times as long and three times as much money to convert the mill as you think it will. That's not to say you shouldn't do both... They each do different things. I have both. I built the mill first, currently working on the router using the mill- you get way more options when you can machine your own parts. Also, rigidity on large machines is a function of geometry and materials- adding linear rails will just make a faster, almost equally floppy machine. You have to bend rails into straightness when you install them, they're not structural.
Thanks, Rob. You’ve given me the information I need to know. I will not be buying the Grizzly, at least not for the purpose of making rails. I have to disagree that the V-wheels or linear rails are not structural parts of the machine. I don’t mean to be snarky, but the top half of my machine is resting on the v-wheels in the y-axis v-wheel gantry assemblies. I would think that upgrading to linear rails would increase rigidity versus the v-slot wheels. I know that the lack of rigidity on my machine is in part due to geometry and deflection in the aluminum, and to some degree the table itself, but the largest culprit seems to be the v-wheels. Not necessarily deflection in the v-wheels themselves, but definitely in the gantry assemblies. I may be mis-understanding you. Do you know where I can buy 4ft sections of linear rail? The largest I can find online is 700mm. I have the parts to make a mini-mill right now, but I watched a YouTube video where the author ended up having to replace the aluminum extrusions and V-slot because it wasn’t able to mill aluminum billets very well in stock configuration. I need a mill with a low RPM motor that is rigid enough to mill 6xxx series aluminum faceplates for amplifier chassis’s. And it looks like the best way to do that is modifying a Grizzly. The minI mill may technically be able to do it, but it seems that it won’t be able to do it with a great surface finish. I am not intimidated by big projects, but I have not researched it extensively. I’m not sure if I would need to purchase custom milled conversion parts or not. I will do more research before pestering the forums with questions about the Grizzly conversion.
You can find 1000mm to 1500mm linear rail for about 75 to 100 a piece. They will be fine for your purposes, it's not like you need aerospace tolerances. If you want to add rigidity, you can throw some 40x40mm steel tubing into the center of the c beams. Also switching from lead screw to rqcknand pinion will help. There is no magic to making these machines rigid. You do it in steps, a bunch of little tweaks make an overall difference. These machines stock however will mill aluminum and brass just fine, but you need to adjust speeds and feeds. Not like a proper mill where you can take off larger amounts of material.
These seem... Self-contradictory. I'd agree that moving from wheels to rail would significantly rigidify the moving parts, wheels are soft and squidgy and you could spaghettify your machine before the rail carriages would even think about deflecting. It wouldn't, however, significantly rigidify the whole machine. V-Slot/C-Beam is, at any size over about 1000mm, for the purposes of high-performance machine building... Jello. As I demonstrate in my (ongoing) router build here: M4: 1510SS Heavy Mk.I Readily available all over eBay in lengths up to 2500+mm? You'll might have to import. I would agree with this. Aluminum machines are generally not great at machining aluminum themselves. There are exceptions to this rule, I've seen people get "actually pretty good" finishes from careful cutting strategies, but I've never seen genuinely excellent results from basic extrusion machines for demanding aesthetic requirements. Also I'd disagree about the "low RPM motor" part. You just need an actual spindle with multiple pairs of angular contact bearings- aluminum has no meaningful limit on SFM, usually the faster the better. G0704s have off-the-shelf conversion kits people make. It's definitely easier than the G0758 that I converted: Grizzly G0758 Benchtop Mill Conversion
You could use the PM25, similar in price to the G0704, but you don't have to take a die grinder to the base to convert to CNC.
Okay thanks for the tips guys. I’ve been lurking in the shadows for a little while and have learned from your other posts as well as from others in the community. I will check into the PM25. As for the linear rails on the 4x8 router, I’m not expecting an order of magnitude difference, just a marginal improvement. Right now, the machine has a little trouble face milling hard wood because when the router leads into or out of the workpiece, there is a smaller area for the force of the router to be applied to and the router pushes down further. When the whole area of the face mill is engaged, there is more resistance against the downward force of the machine. Also, there is less friction when only part of the face mill is engaged, so the router RPM is a little higher. I think the two ways around this would be 1. A more rigid machine, and 2. A constant rpm motor.
