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I thought it might be beneficial to share information & experiences on this subject because it seems to come up often. The typical reason behind grinding in general is to achieve a high degree of accuracy as well as surface finish. Personally, I am interested in what you have achieved with classic 'lathe based' TPG (tool post grinders) like Dumore, Themac or maybe other brands out there. Specifically any example projects, metals, what kinds of abrasives, speeds, feeds, measurement controls.... Or maybe you built a custom accessory for grinding specialized work – bearing shafts, crankshaft journals, cylinder liners or other engine/tooling gadgets.
(I hope this link comes across) but this is one of my favorite posts of some of Jim Allen’s prior model engine work.
.90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
2 popular commercial TPGs
Themac Themac | Tool Post Grinders | Lathe Grinders
Dumore Tools - Dumore Corp
I’ll kick things off with my own (very limited) experience on the TPG front & some random pictures. Maybe like others I initially cobbled a Dremel type tool in a holder. If the parts were tiny, at best one could say they came out shiny as long as the magic smoke stayed in the motor. But dimensional accuracy was….hit & miss. What was I expecting with $10 bearings sitting in a plastic housing? Something approaching a die grinder was a bit better, but not really. Most suffered by lack of clamping rigidity IMO. I pondered some offshore integrated motor/spindles, some with handy ER collet shaft ends & variable speed typically used on homebrew cnc routers. But generally low-ish watts and/or rpm, seemed a bit more like a speculation buy & looking more & more like a 'real' grinder.
I hummed & hawed about building my own spindle. And soon recognized I knew little about specialized bearings, temperature compensation, specific spindle oils, motors or VFD’s, requisite lightweight belts to achieve low vibration shaft speeds between say 10-40K rpm, … Eventually I decided to get a Themac J35 when the $ opportunity presented. On the plus side it seems like high quality spindle, longstanding industry history, USA manufacture, variable spindle height positioning to suit different lathe swings, a range of OD & ID accessories, relatively easy mount to lathe compound with T-plate adapter.
On the negative side - cost! I suspect commensurate with what is involved to manufacture them. Although there are some used deals out there. The Themac also has kind of an oddball proprietary shank taper that inserts into the spindle cartridge & must be an accurate fit. Eventually I figured out dimensions & made a few arbors of my own. Grinding wheels of specific size & grit & abrasive are not exactly common either, at east in my part of the world.
The bigger rub with TPG’s in general (and maybe this is more my own naivety) is they don’t seem to play well with applications where work is supported by a tailstock (TS) center. The TPG spindle wants to be pretty much exactly where the TS is. And I would think TS support represents many if not the majority of typical precision grinding applications. One can extend the TS quill to full length which helps by presenting the smaller OD, but not really enough & also loss of support. Theoretically you could mount a large diameter wheel which would bring the spindle back out of the way of TS. But I run out of table extension if towards operator. One could mount the TPG at an angle & dress the wheel at an angle, but now you might be limited in terms of approaching a raised boss on the shaft & certainly couldn’t grind in between a small gap like crankshaft journals between neighboring counterweights. I had this idea to mount the TPG on my cross slide on the opposite slide to give more room & would also help stabilize the motor base. But most TPGs I’ve seen seem to be 3-4” max wheel dictated by fixed motor rpm + pulley arrangement.
So without this sounding too much like buyers remorse, just wondering if anyone else has some uplifting applications to share?
(I hope this link comes across) but this is one of my favorite posts of some of Jim Allen’s prior model engine work.
.90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
2 popular commercial TPGs
Themac Themac | Tool Post Grinders | Lathe Grinders
Dumore Tools - Dumore Corp
I’ll kick things off with my own (very limited) experience on the TPG front & some random pictures. Maybe like others I initially cobbled a Dremel type tool in a holder. If the parts were tiny, at best one could say they came out shiny as long as the magic smoke stayed in the motor. But dimensional accuracy was….hit & miss. What was I expecting with $10 bearings sitting in a plastic housing? Something approaching a die grinder was a bit better, but not really. Most suffered by lack of clamping rigidity IMO. I pondered some offshore integrated motor/spindles, some with handy ER collet shaft ends & variable speed typically used on homebrew cnc routers. But generally low-ish watts and/or rpm, seemed a bit more like a speculation buy & looking more & more like a 'real' grinder.
I hummed & hawed about building my own spindle. And soon recognized I knew little about specialized bearings, temperature compensation, specific spindle oils, motors or VFD’s, requisite lightweight belts to achieve low vibration shaft speeds between say 10-40K rpm, … Eventually I decided to get a Themac J35 when the $ opportunity presented. On the plus side it seems like high quality spindle, longstanding industry history, USA manufacture, variable spindle height positioning to suit different lathe swings, a range of OD & ID accessories, relatively easy mount to lathe compound with T-plate adapter.
On the negative side - cost! I suspect commensurate with what is involved to manufacture them. Although there are some used deals out there. The Themac also has kind of an oddball proprietary shank taper that inserts into the spindle cartridge & must be an accurate fit. Eventually I figured out dimensions & made a few arbors of my own. Grinding wheels of specific size & grit & abrasive are not exactly common either, at east in my part of the world.
The bigger rub with TPG’s in general (and maybe this is more my own naivety) is they don’t seem to play well with applications where work is supported by a tailstock (TS) center. The TPG spindle wants to be pretty much exactly where the TS is. And I would think TS support represents many if not the majority of typical precision grinding applications. One can extend the TS quill to full length which helps by presenting the smaller OD, but not really enough & also loss of support. Theoretically you could mount a large diameter wheel which would bring the spindle back out of the way of TS. But I run out of table extension if towards operator. One could mount the TPG at an angle & dress the wheel at an angle, but now you might be limited in terms of approaching a raised boss on the shaft & certainly couldn’t grind in between a small gap like crankshaft journals between neighboring counterweights. I had this idea to mount the TPG on my cross slide on the opposite slide to give more room & would also help stabilize the motor base. But most TPGs I’ve seen seem to be 3-4” max wheel dictated by fixed motor rpm + pulley arrangement.
So without this sounding too much like buyers remorse, just wondering if anyone else has some uplifting applications to share?
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